MEID 936 NEUROSCIENCE LABORATORY SYLLABUS BY JOHN B. GELDERD, Ph.D. The illustrations within the text of this laboratory syllabus were created by Joan Quarles. Selected illustrations within the syllabus were modified from published illustrations by Frank Netter, MD with the permission of Novartis Medical Education, Whippany, NJ.
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The illustrations within the text of this laboratory syllabus were created by Joan Quarles.
Selected illustrations within the syllabus were modified from published illustrations byFrank Netter, MD with the permission of Novartis Medical Education, Whippany, NJ.
The purpose of this syllabus is to assist and guide the student through the
neuroanatomy laboratory portion of Medical Neuroscience (MEID 936) in a systematic
fashion. It has been prepared specifically for the curriculum at the Texas A&MUniversity Health Science Center College of Medicine. The ultimate goal of the
laboratory portion of this course is to provide a "hands on" experience in learning and
understanding the FUNCTIONAL anatomy of the human central nervous system (CNS).
To assist you in this endeavor, this syllabus will be used in conjunction with the
following laboratory materials:
1. The Medical Neuroscience Laboratory Manual (downloaded from
Blackboard9 (https://elearning.tamhsc.edu/) under MEID 936 Neuroscience
Phase II. This file contains the Neuroscience Manual & Slide Set.
2. Two brain buckets (shared by a MDL group) containing:
#: Whole and Half Brain
#A: Horizontally and Coronally Sectioned Brains
3. An atlas of Neuroanatomy. Each laboratory group will receive one copy of
the Atlas of the Human Brain and Spinal Cord (Fix, J., 2nd ed.). It is
strongly recommended that your laboratory group use your atlas in eachlaboratory session. Moreover, it will be of value in all phases of this course to
help you in understanding the 3 - dimensional anatomy of the human nervous
system.
There is also an additional item that the student should download from
Blackboard9. This includes a set of annotated Neuroscience slides (Neuroscience Lab
Manual Supplement). The file of labeled slides contains representative spinal cord and
NOTE: UNDER NO CIRCUMSTANCES ARE THE BRAIN SPECIMENS TO BE
REMOVED FROM THE LABORATORY AT ANY TIME.
To assist you in learning the neuroanatomical structures discussed in this
laboratory syllabus, there is an “Objectives” statement at the beginning of each
laboratory section. Further, the important structures and/or concepts for each
laboratory are in bold print or are underlined. In addition, questions pertinent to the
area being studied are interspersed throughout each laboratory session in italicized
print .
Laboratory Demonstrations -- Typically, there will be laboratory demonstrations
during each laboratory session. These will consist of models and/or pre-dissected wet
specimens. Since these demonstrations will be "fair game" for laboratory practicalexams, it is recommended that you take the time to view them when they are displayed
during normal laboratory hours.
Finally, it will be useful to read through each laboratory assignment, using your
brain atlas, prior to the laboratory session. This should help make both lecture and
Objectives: 1. Understand the directional terminology of the CNS.
2. Learn the names and locations of the gross anatomical structures of
the brain.
Before we begin, it is important to understand the directional terminology or
nomenclature as it relates to the brain. Below is a diagram (Fig. 1) to assist you in
understanding this terminology. It is important that you understand it, since we will be
using this terminology in lecture and laboratory throughout the course to describe the
relative locations of various CNS structures.
In this laboratory session, we will be studying what could be called "lump and
bump" anatomy. That is, we will be identifying and briefly discussing the gross externaland internal anatomy of the brain. The purpose of this laboratory is simply to
acquaint you with the appearance and location of structures that we will be
revisiting in detail as the course progresses. These structures will also be used
as landmarks to locate and identify other anatomical features of the brain. Use
your atlas to assist in the identification of the structures listed in this and all
future laboratory sessions.
Whole and Half Brain Specimens
We will begin by identifying the features of the major subdivisions of the brain,
using the whole and half brain specimens in your brain buckets. The brain is organized
from rostral to caudal as follows: 1) telencephalon, 2) diencephalon, 3) mesencephalon
[midbrain], 4) metencephalon [pons], 5) myelencephalon [medulla oblongata] and 6)
cerebellum. Items 2 through 5 above are collectively called the brainstem.
The telencephalon is composed of the cerebral hemispheres and portions of the
basal ganglia. The latter will be studied in a subsequent laboratory session. Thecerebral hemispheres are the large, external, convoluted mantles of nervous tissue
that overlie the brainstem. The superficial region of the cerebral hemispheres is
composed of gray matter. Immediately deep to the gray matter is a relatively thick
layer of white matter. To confirm this, look at selected horizontal and coronal sections.
How does this compare to what is seen in spinal cord? The cerebral hemispheres are
divided into right and left halves at the midline by the prominent interhemispheric
(longitudinal cerebral) fissure. The raised areas, or convolutions, on the surface of
the cerebral hemispheres are called gyri (sing. - gyrus). The corresponding grooves
or depressions are collectively called sulci (sing. - sulcus). The larger, deeper
grooves are usually referred to as fissures.
Each cerebral hemisphere is divided into lobes (Fig. 2). Observe the brain from
a lateral view. From this perspective, it resembles a catcher's mitt with the "thumb"
portion located in a ventrolateral position. This "thumb" portion is the temporal lobe. It
is separated from the more dorsal aspect of the brain by a deep groove called the
lateral (Sylvian) fissure. Locate the horizontally arranged superior, middle and
inferior temporal gyri. The middle temporal gyrus is separated from the superior and
inferior temporal gyri by the superior and middle temporal sulci. Immediately dorsalto the lateral fissure is the frontal lobe, which extends from the rostral pole (end) of the
brain caudally to the central sulcus (of Rolando). This sulcus separates the frontal
lobe from the parietal lobe. Two important gyri lie immediately rostral (precentral
gyrus) and caudal (postcentral gyrus) to the central sulcus. Immediately rostral to the
precentral gyrus is the precentral sulcus. Rostral to the precentral sulcus lie three
horizontally arranged gyri. These are, from dorsal to ventral, the superior, middle and
inferior frontal gyri. The middle frontal gyrus is separated from the superior and
inferior frontal gyri by the superior and inferior frontal sulci.
The caudal extent of the parietal and temporal lobes is delineated by an
imaginary line drawn from the parietooccipital sulcus dorsally to the preoccipital
notch ventrally (Fig. 2). The remaining region of brain from the "imaginary line"
caudally is called the occipital lobe. The caudalmost extent of the occipital lobe is
called the occipital pole. The parietal lobe is separated from the temporal lobe by
drawing an imaginary horizontal line that extends caudally from the Sylvian fissure to
the previous "imaginary line" between the parietooccipital sulcus to the preoccipital
notch.
The caudal end of the Sylvian fissure turns dorsally to terminate and is
surrounded by the supramarginal gyrus. Deep to the Sylvian fissure lies a region of
cortex called the insula (insular lobe). Identify this structure on your horizontal andcoronal brain slices. In some instances, the insula can be seen on the whole or half
brain by GENTLY separating the frontal and temporal lobes. If you are unable to see
the insula on your whole or half brain specimens, this structure can be seen clearly on
demonstration. DO NOT FORCE THE LOBES APART BY TEARING BRAIN TISSUE.
Now turn the whole brain over to view the ventral surface. Beginning on the
lateral aspect of the temporal lobe and working medially, find the inferior temporal
sulcus, occipitotemporal (fusiform) gyrus, collateral sulcus and parahippocampal
gyrus. Near the rostral end of the parahippocampal gyrus is a small, medially directed
protuberance of cortical tissue called the uncus.
To complete our survey of the cerebral hemispheres, observe the medial surface
of the half brain. Find the central sulcus as it winds its way onto the dorsal aspect of
the medial surface of the cerebral cortex to terminate. Surrounding the termination of
the central sulcus is the paracentral lobule, which is a fusion of pre- and postcentral
gyri. Immediately caudal to the paracentral lobule is a region of cortex called the
precuneus. It is bounded caudally by the vertically oriented parietooccipital sulcus.
From the occipital pole, the calcarine sulcus runs rostrally to join the parietooccipital
sulcus. The calcarine sulcus divides the occipital lobe into a dorsal region called thecuneus and a ventral region called the lingula.
Located at the approximate center of the medial surface of the half brain is a
sickle shaped structure, the corpus callosum. This is a massive interhemispheric
nerve fiber pathway that provides reciprocal communication between the two cerebral
hemispheres. It is divided into parts from rostral to caudal as follows: rostrum, genu,
body and splenium. The corpus callosum is surrounded by cerebral cortex that
contributes to a structure we will study in detail later called the limbic lobe. From
rostral to caudal, the visible structures of the limbic lobe include the subcallosal gyrus
(located immediately ventral to the rostrum of the corpus callosum), the cingulate
gyrus (surrounding the dorsal aspect of the corpus callosum), the isthmus of the
cingulate gyrus (located immediately ventral to the splenium of the corpus callosum)
and the parahippocampal gyrus of the temporal lobe. The limbic lobe also includes
the hippocampal formation and dentate gyrus (hidden from view within the temporal
lobe). These latter structures will be seen in a subsequent laboratory. Immediately
dorsal to the cingulate gyrus, find the cingulate sulcus.
Hanging from the ventral surface of the corpus callosum is a membrane called
the septum pellucidum. Along the free ventral border of the septum pellucidum is afiber bundle called the fornix. Follow the fornix as it arches rostrally. In the region just
rostral to where the fornix dives out of site is a small interhemispheric fiber bundle called
the anterior commissure. This structure interconnects portions of the temporal lobes
and components of the olfactory system. Immediately rostral and ventral to the anterior
commissure is a thin membrane called the lamina terminalis. This structure spans the
midline. As such, it has been cut on your half brain specimens. The closure of what
embryological structure gives rise to the lamina terminalis? Follow the lamina terminalis
ventrally to the optic chiasm. Note that the optic chiasm is continuous with the optic
nerves rostrally and the optic tracts caudally. Just caudal to the optic chiasm is the
infundibulum (pituitary stalk). Arching caudally from this structure is another thin
sheet of tissue called the tuber cinereum, which leads to the paired mammillary
bodies. (NOTE: since you are viewing the half brain, there will be only one mammillary
body). The region of brain roughly between the lamina terminalis and the caudal aspect
of the mammillary bodies is the hypothalamus. It is separated from the dorsally
located, egg-shaped thalamus by a rostrocaudal groove called the hypothalamic
sulcus. Running along the dorsomedial aspect of the thalamus is a threadlike elevation
of tissue called the stria medullaris thalami. At the caudal end of this structure are the
habenula, pineal gland and posterior commissure.On the half brain, the medial surface of the thalamus typically reveals a severed
medial protrusion of thalamic tissue. This is the remnants of the massa intermedia
(interthalamic adhesion) which, when present, connects the left and right thalami.
The hypothalamus, thalamus, habenula and pineal gland constitute the major portion of
the diencephalon, the most rostral extent of the brainstem. Another structure, the
subthalamus, is also a part of the diencephalon and will be seen at a later date. It
should be noted at this point that there is a space between diencephalic structures on
the left and right sides. This centrally located space is the third ventricle.
At the juncture of the thalamus and midbrain (mesencephalon), there is a
ventral flexure of the brainstem. This is called the cephalic flexure. Proceeding
caudally (inferiorly) from the mammillary body on the half brain, there is a relatively
deep longitudinal furrow in the midline. This is the interpeduncular fossa. Just lateral
to the interpeduncular fossa, observe one of the paired cerebral peduncles (crus
cerebri). These are important structures that carry descending nerve fibers from the
cerebral cortex to other regions of the brain and spinal cord. If intact, the oculomotor
nerve (CN III) can be seen emerging from the medial surface of the cerebral peduncle.
On the dorsal surface of the midbrain immediately caudal to the posterior commissurelie two rounded protuberances. The more rostral one is the superior colliculus, which
is associated with the visual system. The more caudal one is the inferior colliculus,
which is associated with the auditory system. On the whole brain, each of these colliculi
are paired structures (i.e., two superior colliculi, two inferior colliculi [see demonstration])
and are collectively referred to as the corpora quadrigemina or tectum of the
midbrain. Separating the tectum and the more ventrally located tegmentum of the
midbrain is a small rostro-caudal channel called the cerebral aqueduct (of Sylvius).
The large ventral convexity caudal to the cerebral peduncles is the pons
(metencephalon). If an imaginary line is drawn from the inferior aspect of the inferior
colliculus ventrally to the junction of the cerebral peduncles with the pons, this roughly
represents the caudal extent of the midbrain. Follow the pons dorsolaterally. Just
caudal to where the trigeminal nerve (CN V) emerges, there is a thick band of nerve
fibers connecting the pons with the overlying cerebellum. This is the middle
cerebellar peduncle (brachium pontis). (NOTE: there are also inferior and
superior cerebellar peduncles that can be seen on demonstration). The "potbellied"
pons ends caudally where it meets the medulla (myelencephalon) at the medullary-
pontine junction. This can be seen as a horizontal groove from which the abducens
(CN VI), facial (CN VII) and vestibulocochlear (CN VIII) nerves emerge from medialto lateral respectively. The lateral-most portion of this groove where CN VII and VIII
emerge is called the cerebellopontine angle. This is where the pons, medulla and
cerebellum join together. The large cerebellum can be seen sitting astride the dorsal
surfaces of the pons and medulla. Two thin sloping membranes (superior & inferior
medullary vellae) usually can be seen stretching between the cerebellum and the
dorsal surface of the brainstem. The more rostral is the superior medullary velum.
The inferior medullary velum is often difficult to see. This structure stretches from the
inferior aspect of the cerebellum to the medulla. Both vellae form a triangular space
with the pons and medulla, called the fourth ventricle.
The ventral surface of the medulla is best seen on the whole brain. On either
side of the midline on the medulla just caudal to the medullary-pontine junction is a pair
of rounded ridges. These are the pyramids and are caused by the underlying
pyramidal (corticospinal) tract. Immediately lateral to the pyramids at this level are a
pair of egg-shaped swellings called the inferior olives. The inferior olivary nuclei
reside deep to these structures. The groove between the inferior olive and the pyramid
on each side is the preolivary sulcus. Filaments of the hypoglossal nerve (CN XII)
can be seen emerging from this sulcus. Dorsolateral to the inferior olive is anothergroove called the postolivary sulcus from which the glossopharyngeal (CN IX),
vagus (CN X) and the bulbar portion of the spinal accessory (CN XI) nerves
emerge.
Identify the following structures on the whole brain: lamina terminalis, optic
Objectives: 1. Review the layers of the meninges as learned in Gross Anatomy.
2. Locate and be able to name the subarachnoid cisterns which surround
the brain and spinal cord.
The meninges consist of 3 concentric membranous layers of tissue that surround the
brain and spinal cord.
Dura mater -- The outermost, thick, fibrous layer of meninges is called the dura
mater. As many of you know from gross anatomy, the dura mater is made up of 2
layers (an inner [meningeal] and outer [endosteal/periosteal] layer) that are typically
fused together. The outer layer is, in turn, fused to the inner surface of the skull. Assuch, there will not be any dura mater on the brains in your buckets. It should be noted
that the periosteal layer of dura mater passes through the foramen magnum to fuse with
the periostium on the external surface of the skull. Consequently, the only layer of dura
mater covering the spinal cord is the meningeal layer. Since the dura mater is
intimately involved in the drainage of blood from the brain, there will be a demonstration
of the dura mater and its reflections within the skull later in this laboratory session
during our review of the blood supply to the CNS.
Arachnoid mater -- This intermediate layer of the meninges usually remains at
least partially intact on the surface of the brain after it is removed from the skull. On the
whole or half brain, look on the lateral surfaces of the cerebral hemispheres. If the
arachnoid mater is present, it will appear as a transparent membrane that spans the
sulci and fissures of the cerebral cortex. Immediately deep to the arachnoid mater lies
an important region called the subarachnoid space. It contains: 1) cerebrospinal fluid
and 2) the major blood vessels of the brain. Using a blunt probe, slip the tip through an
existing gap or tear in the arachnoid mater and gently elevate the arachnoid layer to
demonstrate the subarachnoid space. Over most of the surface of the brain, thesubarachnoid space is relatively shallow. However, in those regions where there are
wide and/or deep depressions on the surface of the brain, the arachnoid layer stretches
across these depressions, resulting in enlargement of the subarachnoid space. These
Using both the whole and half brains, four of the major cisterns can be
demonstrated. On the ventral surface of the midbrain, locate the cerebral peduncles.
The depression in the midline between the cerebral peduncles is the interpeduncular
fossa. If the arachnoid is present, it will be seen stretching across the interpeduncular
fossa between the cerebral peduncles. The space formed between the floor of the
fossa and the overlying arachnoid layer is called the interpeduncular cistern.
Immediately caudal to the cerebral peduncles lies the convex protuberance of the
ventral pons. The pontine cistern lies immediately ventral to the pons and extends
caudally to enlarge and terminate at the junction of the pons and medulla (medullary-
pontine junction). Now turn the brains over to view the dorsal surface. The two
cisterns on this surface of the brain lie either immediately rostral or caudal to the
cerebellum. The more rostral of these, the superior cerebellar (quadrigeminal)
cistern is best seen on the medial surface of the half brain. It is roughly bounded bythe splenium of the corpus callosum superiorly, the superior and inferior colliculi
(corpora quadrigemina) ventrally and the superior surface of the cerebellum inferiorly.
The remaining cistern of note, the cerebellomedullary cistern (cisterna magna), lies
at the inferior surface of the cerebellum and the dorsal surface of the medulla where the
arachnoid layer reflects from the cerebellum to the medulla. Use your blunt probe to
gently explore the extent of these cisterns. It is important to note that another clinically
important cistern is located immediately caudal to the termination of the spinal cord.
What is the name of this cistern? At what vertebral level does the spinal cord
terminate?
Pia mater -- This innermost layer of the meninges is, for the most part, closely
adhered to the surface of the brain and spinal cord. As such, it is difficult to
demonstrate with the naked eye. At the microscopic level, small blood vessels that
penetrate the parenchyma (substance) of the brain are surrounded by pial sleeves that
penetrate variable distances into the brain. There are two obvious occasions where the
pia mater separates from the surface of the CNS and can be readily seen. What is the
site of each of these pial separations and what they are called? (Think Gross Anatomy).
Objectives: 1. Learn the major arteries of the CNS and the areas they supply.
2. Review the dural venous sinuses and learn the major veins that drain
the CNS.
Vascular injury and/or vascular disease constitute a major source of nervous
system pathology. The CNS is critically dependent upon glucose and oxygen, neither of
which is stored in significant amounts by the CNS. Consequently, should the blood
supply to the CNS be disrupted, even for a relatively brief period, destruction of CNS
parenchyma occurs with the resultant permanent loss of function, or death.
Brain -- The blood supply to the brain is typically described as being provided bytwo arterial systems: an anterior system which is composed of the internal carotid
arteries and their branches; and the posterior (vertebral - basilar) system, composed of
the vertebral arteries. These two systems are demonstrated in Fig. 4 below.
Posterior System -- Using the whole brain, turn it over to view the ventral
surface. Note the paired vertebral arteries ascending on the ventrolateral surface of
the medulla. As you may recall, the vertebral arteries arise from the subclavian arteries
and ascend through the foramina transversaria of cervical vertebrae C-6 through C-1 to
enter the skull through the foramen magnum. Typically, the vertebral arteries fuse at
the midline at the level of the medullary-pontine junction to form the unpaired basilar
artery. Before the vertebral arteries fuse, they give rise to three pairs of arteries. The
first of these are the posterior spinal arteries that descend on the dorsal surface of the
spinal cord just medial to the dorsal spinal roots. Since the posterior spinal arteries are
typically the first to arise from the vertebral arteries as they traverse the foramen
magnum, they are often absent due to the level where the vertebral arteries were cut
when the brain was removed from the skull. The second branches are the posterior
inferior cerebellar arteries (PICA) which wrap around the medulla to ramify on theinferior surface of the cerebellum. As these arteries wrap around the medulla to gain
access to the cerebellum, they send small branches to supply the lateral region of the
medulla. Occasionally, the posterior spinal arteries may arise from PICA. The anterior
spinal arteries, which descend to fuse as a single artery at the midline on the ventral
surface of the medulla, are usually the last branches from the vertebral arteries before
they fuse to become the basilar artery. Shortly after the basilar artery is formed, it
gives rise to the paired anterior inferior cerebellar arteries (AICA), which ramify on
the ventral surface of the cerebellum. Immediately rostral to these arteries, the basilar
artery gives rise to the paired labyrinthine (internal auditory) arteries each of which
follow their respective vestibulocochlear nerve (CN VIII) into the internal auditory
meatus. Often, these arteries arise from AICA rather than the basilar artery. As the
basilar artery ascends on the ventral surface of the pons, it gives rise to many small
pontine arteries. Just prior to terminating, the basilar artery gives rise to the paired
superior cerebellar arteries, which wrap around and supply the midbrain on their way
to ramify on the superior surface of the cerebellum. The basilar artery terminates at the
level of the midbrain by dividing into the large, paired posterior cerebral arteries. At
this time, look for the oculomotor nerve (CN III) which acts as a landmark by emergingbetween the posterior cerebral and superior cerebellar arteries.
Anterior System -- After entering the skull through the carotid canal, each
internal carotid artery passes through the cavernous sinus, then turns 180°
caudalward to gain access to the ventral surface of the brain. It is at this point that the
internal carotid arteries are cut to remove the brain from the skull. On the ventral
surface of the whole brain, find the severed ends of the internal carotid arteries as
they lie just lateral to the optic chiasm. There are four branches that can be readily
seen arising from the internal carotid arteries: two from the main trunk and two terminal
branches. The first small branch is the posterior communicating artery. This artery
passes caudally to anastomose with the posterior cerebral artery. The next branch is
the anterior choroidal artery. This small artery passes caudally to disappear deep to
the temporal lobe. As the name implies, this artery supplies blood to the choroid
plexus (and other structures). Shortly after the internal carotid artery gives rise to the
anterior choroidal artery, it divides into its two terminal branches, the anterior and
middle cerebral arteries. The latter is larger and considered by some to be the
continuation of the internal carotid artery. Immediately distal to the origin of the anterior
choroidal artery, a variable number (2-5) of small threadlike arteries can be seen arisingfrom the middle cerebral artery and immediately diving into brain parenchyma. These
are the lateral striate (lenticulostriate) arteries. Although small, these arteries supply
critical areas of the brain. The middle cerebral artery continues laterally to dive into the
lateral sulcus (Sylvian fissure) deep to the rostral pole of the temporal lobe.
Branches of the middle cerebral artery can be seen on the lateral surface of the brain as
they emerge from the lateral sulcus.
On the ventral surface of the whole brain, the anterior cerebral arteries can be
seen coursing medially to pass dorsal (deep) to the optic nerves. As they approach the
midline just rostral to the optic chiasm, these arteries are joined together by an
anastomotic channel called the anterior communicating artery. Both anterior cerebral
arteries then disappear by diving into the interhemispheric fissure, each one
supplying ipsilateral structures on the medial and dorsal surface of the brain.
On your half brains, follow the course of an anterior cerebral artery as it
courses along the rostral and dorsal surfaces of the genu of the corpus callosum. At
this point, the anterior cerebral artery typically divides into its two terminal branches, the
pericallosal and callosomarginal arteries. The pericallosal artery runs caudally
along the dorsal surface of the body of the corpus callosum. The callosomarginalartery takes a more dorsal path caudally by running in or near the cingulate sulcus.
Note the branches of these arteries and the general areas they supply.
Now turn your attention back to the ventral surface of the whole brain. The
the brain surrounding the hypothalamus, infundibulum and optic chiasm. Notice the
many small arteries arising from the internal surfaces of the circle of Willis. They are
generally referred to as central or ganglionic arteries. The circle of Willis is clinically
important in that if a blood vessel should be occluded on one side of the circle, blood
can be shunted to bypass the obstruction. However, since this arterial circle is variable,
i.e. branches small or missing, this is not an iron clad rule.
Another issue to consider is that although there is some overlap along the
periphery of the territory for adjacent arteries supplying the brain, these arteries are
functionally "end" arteries, in that they are the sole blood supply to the vast majority of a
given area of brain. Consequently, permanent or prolonged occlusion of a single artery
results in necrosis of the brain area supplied by that artery.
Venous Return -- Unlike the arterial supply to the brain, venules emerge fromthe substance of the brain as fine pial plexuses that coalesce to form larger visible veins
that reside in the subarachnoid space. A good example of this can be seen on the
surface of the cerebral hemispheres. An exception to this general rule can be seen on
the medial surface of a half brain. Look along the dorsomedial aspect of the thalamus
on a half brain. A relatively large vein can usually be seen running in a rostrocaudal
direction. This vein drains deeper brain structures and is called the internal cerebral
vein. As this vein reaches the subarachnoid space at the caudal aspect of the
thalamus, you may be able to see another vein joining the internal cerebral vein from
the ventral side. This is the basal vein (of Rosenthal). If you do not have this vein on
your specimen, it can be seen on demonstration. After it is joined by the basal vein, the
internal cerebral vein becomes the great cerebral vein (of Galen). Into what venous
structure does the great cerebral vein empty? What is the name of the subarachnoid
compartment where the above veins join together?
Although the larger veins within the subarachnoid space on the surface of the
brain generally run in parallel with their arterial counterparts, these veins soon depart
from the arteries to drain into specialized endothelium-lined venous channels between
the meningeal and periosteal layers of the dura mater called dural venous sinuses. These sinuses are formed in certain regions of the cranial cavity where the inner
(meningeal) layer of dura mater separates from the outer (periosteal) layer to form: 1)
horizontal septae that divide the cranial cavity into compartments, or 2) vertical septae
that occupy the fissures between the left and right cerebral and cerebellar hemispheres.
straight sinus, confluence of sinuses, transverse sinus, sigmoid sinus and
cavernous sinus. What subdivision of the brainstem occupies the region within the
tentorial notch? At this time, you should review the direction of normal blood flow in
these dural venous sinuses. What major vein receives blood from the sigmoid sinus?
Spinal Cord -- The spinal cord has a longitudinal and segmental blood supply.
The longitudinal supply is provided by the anterior (one) and posterior (two) spinal
arteries arising from the vertebral arteries. However, these small arteries alone are
only sufficient to supply upper cervical segments. Consequently, the primary source of
spinal cord blood supply is provided by segmental arteries at cervical, thoracic andupper lumbar levels of the vertebral column. These arteries gain access to the spinal
cord through intervertebral foramina where they reinforce the longitudinal supply
through anastomotic channels.
The spinal branches of the segmental arteries enter the intervertebral foramina
and then divide and ramify along the dorsal and ventral spinal roots to reinforce the
longitudinal blood supply to the spinal cord. Spinal veins generally follow their arterial
counterparts on the surface of the spinal cord. However, these veins drain into the
internal vertebral venous plexus, which in turn has connections with the external
vertebral venous plexus and segmental veins. Do these venous plexuses have valves?
What other venous plexuses interconnect with the vertebral venous plexuses?
DEMONSTRATIONS
Skull showing dural reflections and venous channels: falx cerebri, superior
Objectives: 1. Identify and locate all parts the ventricular system of the brain.
2. Identify the brain structures that form the walls/boundaries of the
various parts of the ventricular system.
3. Understand the normal flow of CSF through the ventricles and the
general consequences of ventricular system blockage.
The brain is not a solid mass of nervous tissue. There are four interconnected
cavities, called ventricles, deep within the brain. The two lateral ventricles are located
within the cerebral hemispheres. The third ventricle is situated in the midline between
the left and right portions of the thalamus and hypothalamus. The fourth ventricle is
located between the cerebellum dorsally, and the pons and medulla ventrally. Eachventricle contains structures called choroid plexuses, which produce cerebrospinal fluid
(CSF). The normal flow of CSF within the ventricular system is as follows:
lateral→third→fourth. After it flows through the ventricles, it then gains access to the
subarachnoid space through small foramina in the fourth ventricle. The CSF then
percolates throughout the subarachnoid space surrounding the brain and spinal cord to
eventually empty into the venous system. In addition to supporting and protecting the
brain, CSF is important in the metabolic processes of the brain. The CSF also serves
as an important diagnostic tool for a variety of neurological problems, and can be
aspirated for analysis. It is critical that the CSF has unobstructed flow through the
ventricular system of the brain and into the subarachnoid space. If blockage should
occur, there will be a subsequent expansion of "upstream" ventricles (hydrocephalus),
causing compression of surrounding brain tissue.
Half Brain; Horizontal and Coronal Sections
Observe the medial surface of your half brain specimen. Find the thalamus and
hypothalamus. The area between (medial to) these structures and their contralateralcounterparts is the third ventricle. Find the anterior commissure and the rostral
pole of the thalamus. Between these structures is a small hole called the
interventricular foramen (of Monro). There are two of these foramina, one on each
side. They interconnect the lateral ventricles with the third ventricle. If present, the
choroid plexus can be seen hanging from the roof of the third ventricle. At the caudal
end of the thalamus, the third ventricle narrows into a small passageway, the cerebral
aqueduct (of Sylvius), which courses through the midbrain to open into the fourth
ventricle. The fourth ventricle is continuous caudally with the central canal of the spinal
cord. Attempt to find the choroid plexus in the fourth ventricle. It is from the fourth
ventricle that the CSF enters the subarachnoid space. It does so by passing through: 1)
a single midline foramen (foramen of Magendie) located at the caudal extent of the
fourth ventricle where the inferior medullary velum contacts the medulla, and 2) two
lateral foramina (foramina of Luschka) located at the lateral extremes of the fourth
ventricle. The foramina of Luschka can be found by GENTLY insinuating the tip of your
blunt probe into the lateral reaches of the fourth ventricle. If you have done this
correctly, the tip of the probe will pass through the foramen of Luschka to appear
externally in the cerebellopontine angle. It is common for the choroid plexus of the
fourth ventricle to extend through the foramina of Luschka to reside in the subarachnoidspace. Look in the region of the cerebellopontine angle for tufts of this structure as it
emerges.
The lateral ventricles are located in the cerebral hemispheres. They can be
viewed in their entirety by using both horizontal and coronal brain slices. Using your
brain atlas, begin at the dorsal aspect of the horizontally sliced brain and remove slices,
observing both surfaces of each slice, until the lateral ventricles are exposed. Note that
the fibers of the corpus callosum form the roof, as well as the anterior and posterior
boundaries of the lateral ventricles. Identify the genu, body and splenium of the
corpus callosum. Find the choroid plexus within the lumen of the lateral ventricles
and third ventricle. Identify the frontal (anterior) horn, body and occipital (posterior)
horn of the lateral ventricles. Note the rounded mass that forms the lateral boundary
of the frontal horns. This is the head of the caudate nucleus, a component of the
basal ganglia. Find the thalamus, septum pellucidum and fornix, and determine their
spatial relationships to the lateral ventricles. As you progress from dorsal to ventral,
there is a region near the caudal end of the lateral ventricles where the body, occipital
horn and temporal (inferior) horn meet. This is called the trigone (atrium) of the
lateral ventricle. Follow subsequent sections ventrally into the temporal horn.Find a horizontal section similar to that in the Fix Atlas: Plate 56 (p. 112) and
identify the interventricular foramina (of Monro). As previously stated, these two
foramina are the openings from the lateral ventricles into the centrally located third
ventricle. Just lateral to each of these foramina is a "V" shaped region of white matter
called the internal capsule, an important bidirectional pathway between the cortex and
the brainstem and spinal cord. The apex of each "V" is called the genu of the internal
capsule. Each genu is directed medially and points at the interventricular foramen on
each side, thus serving as a landmark. The genu is continuous with the anterior and
posterior limbs of the internal capsule.
Using your coronal brain slices and your brain atlas, identify the same
structures you found in the horizontal sections. Compare and contrast these
structures as they appear in horizontal and coronal section. The purpose of this
important exercise is to begin to appreciate and understand the three dimensional
anatomy of the brain, which is critical if you are to be successful in this course.
NEURORADIOLOGY
You should now have a basic understanding of the gross anatomy of the brain. Ifthis assumption is correct, you should have little difficulty transferring this knowledge to
interpret the variety of radiologic techniques that are used to visualize the various parts
of the brain.
Using your Neuroscience slide set, look at slide 45. This is a mid-sagittal section
of the brain as visualized by magnetic resonance imaging (MRI). This particular MRI is
a T1 weighted image. What are the visual characteristics of a T1 weighted MRI of the
brain such as the one seen on this slide? Identify the following brain structures/regions
on this slide with the help of your half brain specimens: corpus callosum (all
(all parts), trigone (atrium) of lateral ventricle, splenium of corpus callosum,
lateral fissure and insula.
Slide 51 shows two T2 weighted horizontal images of the brain. In the left
image, note the occipital horn of the lateral ventricle on the left side of the slide. Is
this the patient’s left side? Which of the two images is higher (more superior)? The
midbrain and the middle and posterior cerebral arteries can also be seen. Note the
location and orientation of the cerebral peduncles. Verify this by comparing this image
with a similar horizontal section from your brain specimens. The right image shows the
frontal horns of the lateral ventricles, third ventricle, and trigone (atrium) of the
lateral ventricles. How does the appearance of the ventricles on this slide differ from
that seen in a T1 weighted image? Slide 65 is a similar MRI, showing anterior, middle
and posterior cerebral arteries as well as mammillary bodies, third ventricle,
rostral midbrain and uncus. Is this a T1 or T2 weighted image? Look at slide 64. Is this section rostral or caudal to slide 65 ? What blood vessels can be seen in this view?
INTRODUCTION TO THE MACROSCOPIC ANATOMY OF THE NEURAXIS
Objective: 1. Be able to quickly identify representative levels of the neuraxis,
including the salient internal and external features of these
representative levels as viewed on the stained, macroscopic sections
in your slide sets.
As part of the learning aids in the neuroanatomy laboratory, you have been
provided with a set of slides showing the macroscopic anatomy of the human brain and
spinal cord. The slides are arranged in ascending order, beginning with the sacral
spinal cord (slide #1) and ending with telencephalic structures. Those sections of
the neuraxis from spinal cord through midbrain are cut in horizontal section. Because of
the flexure of the brain between the diencephalon and midbrain, subsequent slidesshow brain sections cut in varying planes between horizontal and coronal. The purpose
of the following exercise is to familiarize you with the "typical" appearance of the various
levels of the spinal cord and brainstem so that you can instantly recognize these levels
when you see them. Make extensive use of your brain atlas, and the supplementary
labeled slides (available on Bb9 and at the back of this laboratory syllabus), to assist
you in the identification of internal and external landmarks at these various levels.
Spinal Cord (slides 1-5)
Observe slide 1. This is a cross section of the sacral spinal cord. The dorsal
surface of this section, as well as subsequent sections of spinal cord and brainstem
through the midbrain, is located at the top of the slide. The vast majority of the sections
on the slides are stained with one of two types of nerve fiber stains. Slide 1 is stained
by the luxol fast blue method for myelinated nerve fibers, and counterstained with
hematoxylin and eosin to reveal neuronal cell bodies. Note the dark blue staining of the
white matter (nerve fibers) around the periphery, and the relative absence of blue
staining in the centrally located gray matter, which contains neuronal cell bodies. Thebutterfly shaped gray matter is divided into dorsal (posterior) and ventral (anterior)
horns with an intervening lateral horn. The latter will be seen more clearly in
subsequent sections. Note the gray commissure that connects the left and right sides
of the gray matter. Observe the small purple dots scattered throughout the ventral horn.
These are the ventral motor horn cells that give rise to the majority of axons within the
ventral roots. Observe the arched pink region near the top of the dorsal horn. This is
the substantia gelatinosa, an important sensory relay nucleus. The gray matter
immediately ventral to the substantia gelatinosa contains the less distinct nucleus
proprius (proper sensory nucleus). The white matter dorsal to the substantia
gelatinosa is Lissaur's tract, an intersegmental spinal pathway. The white matter can
be roughly divided into funiculi (columns) based on their position in the spinal cord.
The dorsal funiculus is located between the dorsal horns of the gray matter. The
lateral funiculus occupies the region between the dorsal and ventral roots. The
ventral funiculus lies between the midline and the emergence of the ventral roots.
Immediately ventral to the gray commissure is the anterior white commissure, which
contains nerve fibers that cross the midline. The prominent dorsal and ventral median
fissures serve to vertically divide the white matter at the midline. Each of the above
funiculi contain important ascending and descending nerve fiber tracts that will beidentified at a later date. Note the single anterior spinal artery and the multiple
posterior spinal arteries and their location immediately external to the pia mater.
Now look at slide 2. This is a cross section of the lower lumbar spinal cord
stained with the Weil stain. This histological procedure stains the myelin of nerve fibers
black. The gray matter (cell bodies) remains unstained with the exception of those
regions where myelinated nerve fibers traverse it. Observe the large lateral projections
of the ventral horns. What is the purpose of these lateral extensions? Why would you
expect to see them at this spinal level? In slide 2, and all subsequent slides of the
spinal cord (slides 3-5), identify all of the structures you found on slide 1 .
Slide 3 (upper lumbar) appears similar to slide 2. Dentate (denticulate)
ligaments can be clearly seen in this section. What tissue layer comprises the dentate
ligaments? What are their functions?
Slide 4 (thoracic) reveals an egg shaped bulge at the medial base of the dorsal
horn. This is the dorsal nucleus of Clarke (nucleus dorsalis), an important relay
nucleus for information transmitted to the cerebellum. This slide also displays a
prominent intermediolateral gray (cell) column. What specific cell type is contained
within this column? Why is the gray matter so small in the thoracic region? Slide 5 & slide 6 reveal the transition between the cervical spinal cord and
medulla. Slide 5 is at the C-1 spinal level. Although it resembles the thoracic spinal
cord, there are some distinct differences. The intermediolateral cell column has been
replaced by the spinal accessory nucleus. How would the gray matter differ if this
were a lower cervical section? In addition, at this level the substantia gelatinosa is
being replaced by the spinal nucleus of V (spinal trigeminal nucleus). A roughly
circular region of white matter in the lateral funiculus near the base of the dorsal horn is
separated into multiple fascicles. This is the lateral corticospinal tract, an important
descending pathway which has just decussated (crossed the midline) at more rostral
levels to assume its position within the lateral funiculus. Slide 6 is slightly more rostral
than slide 5, and represents the most caudal extent of the medulla. Find the spinal
accessory nucleus. Also note that the substantia gelatinosa has now expanded to
become the large spinal nucleus of V. The caudal extent of the decussation of the
fibers forming the lateral corticospinal tract can also be seen.
Compare the relative appearances of the spinal cord in slides 1 (sacral), 2
(lower lumbar), 3 (upper lumbar), 4 (thoracic), 5 (cervical) and be able to recognize
and identify each level by listing their major differences.
Medulla (slides 7,12)
Slide 7 is a representative section of the caudal medulla. This level is
occasionally referred to as the "closed" portion of the medulla, since it is caudal to the
fourth ventricle and thus reveals a "closed" central canal surrounded by nuclei and
fiber tracts. The prominent spinal nucleus of V can be seen. The dorsal column
region now displays a nucleus (nucleus gracilis) on either side of the midline. What
external feature reveals the location of these nuclei? The thick, black "X" at the midline
ventrally is the decussation of the pyramids, a crossing of descending nerve fibers
that will form the previously mentioned lateral corticospinal tracts seen in slide 5 & slide
6.
Slide 12 is a typical representation of the rostral medulla. This level is also
called the "open" medulla because the central canal has "opened" to form the floor of
the fourth ventricle. This level is immediately recognizable by the pyramids ventrally,
the coiled appearance of the inferior olivary nuclei, the well-defined inferior
cerebellar peduncles, fourth ventricle and overlying cerebellum. The medial
lemniscus, an important ascending sensory pathway, can be seen in the midlinesandwiched between the left and right inferior olivary nuclei. Also note the choroid
plexus within the fourth ventricle and its extension through the foramina of Luschka
to lie externally within the cerebellopontine angle. The inferior olivary nuclei cause an
external bulge, the inferior olive. Immediately ventral and dorsal to the inferior olive
are the pre- and post-olivary sulci, respectively. What cranial nerves emerge from
each of these sulci?
Pons (slides 16,17,19)
Slide 17 is a cross section of the caudal 1/3 of the pons. The characteristic
ventral convexity of the ventral pons can be clearly seen. Imbedded within this region
are the pyramidal tracts, which are surrounded by the pontine nuclei. Dorsal to the
pyramidal tracts is the medial lemniscus. At this level, it is shaped like a handlebar
mustache and forms the dorsal border of the ventral pons. It is located in a region of
the pons called the pontine tegmentum, which extends dorsally to form the floor of the
fourth ventricle. The gently rounded floor of the fourth ventricle on either side of the
midline is called the facial colliculus. The large black areas forming the lateralboundaries of the pons at this level are the middle cerebellar peduncles (brachium
pontis). In the pontine tegmentum just medial to the middle cerebellar peduncles, the
fascicles of the facial nerve (CN VII) can be seen as they traverse the pons to emerge
caudally at the cerebellopontine angle. Identify the above pontine structures on slide
16, where the superior, inferior and middle cerebellar peduncles can be seen
simultaneously. Can you find the emerging fibers of the abducens nerve (CN VI) in
this section? Compare these sections with slide 19 (rostral pons) where you should be
able to identify the superior cerebellar peduncles, superior medullary velum, fourth
ventricle, ventral pons, pyramidal tracts, pontine tegmentum and medial
lemniscus.
Midbrain (slides 21-23)
All levels of the midbrain are characterized by the cerebral peduncles (crus
cerebri) and interpeduncular fossa ventrally, the substantia nigra (the clear region
immediately dorsal to the crus cerebri), and the medial lemniscus (dorsomedial to the
substantia nigra).The level of the inferior colliculus (slide 21) has unique characteristics, which
include the decussation of the superior cerebellar peduncles, the nucleus of CN IV
and the distinct nuclei of the inferior colliculi.
The level of the superior colliculus (slide 23) is characterized by the paired red
nuclei, nuclear complex of the oculomotor nerve (CN III) and the emerging rootlets
of CN III as they pass through the red nuclei to emerge from the midbrain along the
walls of the interpeduncular fossa.
Slide 22 is a slightly more caudal cut through the superior collicular level. In this
slide, the red nucleus is obliterated by the crossed fibers of the superior cerebellar
peduncles as they ascend to the thalamus, and the emerging fibers of CN III. The
nuclear complex of CN III is also present.
Diencephalon (slides 23,29)
In addition to midbrain, slide 23 also contains some of the caudal structures of
the thalamus, a major subdivision of the diencephalon. These structures of the
thalamus are the pulvinar and the medial and lateral geniculate bodies. Slide 29
reveals more of the nuclei of the thalamus, including two important sensory relay nuclei,the ventral posteromedial (VPM) and ventral posterolateral (VPL) nuclei, as well as
the pulvinar. The centromedian nuclei of the thalamus, the habenular nuclei and
the mammillary bodies are also clearly seen. Midbrain structures such as cerebral
peduncles, substantia nigra and red nuclei are also present. Notice how the
cerebral peduncles merge with the posterior limb of the internal capsule. Using your
half brains, figure out the plane of section of slide 29.
NEURORADIOLOGY
Slide 52 is a midsaggital MRI through the cervical region. Identify the spinal
cord, vertebral bodies and intervertebral disks. Can you find anything abnormal or
pathological on this slide?
Slide 53 is a midsaggital MRI through the lumbar and upper sacral region. Find
the subarachnoid space, intervertebral disks and termination of the spinal cord.
Is this a T1 or T2 weighted image? Using your anatomical knowledge from Gross
Anatomy, can you identify, by number, the location of the bodies of the lumbar
vertebrae? Slides 58-65 are axial (cross section) MRI’s of the cervical spinal cord (slide 58)
through the rostral midbrain (slide 65). All of these sections are T2 weighted. Make
note that the orientation of the neuraxis on these slides is opposite to what you saw on
the stained sections (i.e., the dorsal aspect of each scan is toward the bottom of the
slide). On slide 58, note the characteristic dorsoventral flattening of the cervical spinal
cord and the vertebral arteries located in the foramina transversaria. On slide 59
(caudal medulla), note the vertebral arteries, pyramids and central canal. Slide 60
(rostral medulla) clearly shows the inferior cerebellar peduncles, inferior olive and
pyramids as well as the joining of the two vertebral arteries to form the basilar artery.
At this level, the central canal has opened up into the 4th ventricle. Slide 61 (transition
from rostral medulla to caudal pons) shows the 4th ventricle, cerebellar peduncles,
CN’s VII & VIII and the basilar artery. Slide 62 (midpons) shows the middle
cerebellar peduncle, CN V as it emerges from the middle cerebellar peduncle to enter
Meckel’s cave, 4th ventricle and the basilar artery. Slide 63 (rostral pons) shows the
characteristic shape of the ventral pons, as well as the rostral extent of the 4th
ventricle, both middle and superior cerebellar peduncles and the basilar artery.
Slide 64 (caudal midbrain) shows the characteristic outline of the midbrain at this level,
including the cerebral peduncles, interpeduncular fossa and inferior colliculus. Atthis level, the superior cerebellar arteries can be seen arising from the basilar artery
to embrace the midbrain. The internal carotid arteries, uncus and temporal horn of
the lateral ventricles can also be seen in this slide. Slide 65 (rostral midbrain) shows
a number of structures including cerebral peduncles, interpeduncular fossa,
superior colliculi, red nuclei, mammillary bodies, hypothalamus, uncus and third
ventricle. In addition, the posterior cerebral, middle cerebral, anterior cerebral and
internal carotid arteries can be seen as well as the superior cerebellar cistern. To
verify what you are seeing on slide 65, compare it to a similar horizontal wet brain
Objectives: 1. Know the location and function of each of the described pathways at all
levels of the neuraxis, from their origin to their termination in the
cerebral cortex.
2. Be able to determine the clinical signs and symptoms resulting from
lesions of these pathways at any level of the neuraxis.
There are a number of ascending pathways in the neuraxis that carry a variety of
different types (modalities) of sensory information from the body, extremities and head
to the cerebral cortex. Those dealing with the body, extremities and posterior 1/3 of the
head receive their input from the dorsal roots of spinal nerves. Those pathways
transmitting sensory information to the cortex from the anterior 2/3 of the head (i.e. face,nasal cavities, oral cavity, pharynx, larynx, etc.) receive their input from the cranial
nerves. We will concentrate our efforts on those pathways that are clinically relevant
and produce consistent, repeatable sensory deficits when lesioned (injured).
In studying these pathways, it is best to follow each one from its origins in the
spinal cord to its thalamic terminations using the slide set. The final portions of these
pathways from thalamus to cerebral cortex can be seen on demonstration and on your
wet brain specimens.
Ascending sensory pathways from the body and posterior 1/3 of the head -- The
first order (1o) neurons (the first neuron in the pathway) for all sensory pathways from
the body and posterior 1/3 of the head reside in the dorsal root ganglia of the spinal
nerves. The central processes of these cells enter the spinal cord via the dorsal roots to
either synapse in the spinal cord and/or ascend to brainstem levels. The nerve fibers in
these sensory pathways are typically arranged in a somatotopic fashion throughout their
ascent through the spinal cord, brainstem and telencephalon.
a. Lateral spinothalamic tract [Modalities: pain and temperature]
b. Anterior (ventral) spinothalamic tract [Modalities: crude (light) touch]
c. Spinotectal tract [Modality: pain]. This pathway is the afferent limb of
a “reflex pathway that results in reflexive turning of the head and eyes
in response to a painful (nociceptive) stimulus.
These pathways will be studied together since they occupy similar positions
within the neuraxis. They are listed above in descending order of their relative clinical
importance. The lateral spinothalamic tract is, by far, the most important pathway.
First order neurons for these pathways lie in the dorsal root ganglia in all spinal nerves
(except C-1). The central processes of these cells terminate in the dorsal horn ineither the substantia gelatinosa or the nucleus proprius. Second order (2o) neurons
in these nuclei give rise to axons that cross the midline in the anterior white
commissure to assume a peripheral position in the ventral aspect of the contralateral
lateral funiculus as the lateral spinothalamic tract (and spinotectal tract), or a slightly
more ventral position (anterior spinothalamic tract) (slide 1, slide 2, slide 3, slide 4,
and slide 5). As these fibers cross the midline, they ascend 1-3 spinal segments before
they join their respective contralateral fiber tracts. How would this bit of knowledge
affect the level of sensory deficits if a lesion of the lateral spinothalamic tract occurred at
the T-10 spinal level?
These three pathways ascend together throughout the spinal cord and
brainstem, and are often referred to collectively as the spinal lemniscus or
anterolateral system. When they enter the medulla, they can be found in the lateral
aspect just dorsal to the inferior olivary nucleus (slide 9, slide 10, slide 12, slide 13,
slide 14 and slide 15). In the pons and midbrain (slide 16, slide 17, slide 18, slide 19,
slide 20, slide 21, slide 22, slide 23 and slide 24) the spinal lemniscus can be found
lateral to the medial lemniscus. At the level of the superior colliculus, the fibers of the
spinotectal tract peel off to terminate in the superior colliculus. The remaining lateraland anterior spinothalamic tracts continue rostrally to terminate in the ipsilateral ventral
posterolateral (VPL) nucleus of the thalamus, where third order (3o) neurons in the
VPL then project their axons to the postcentral gyrus via the posterior limb of the
internal capsule (slide 28, slide 29, slide 30 and slide 31) and corona radiata (see
demonstration). NOTE: As the spinal lemniscus proceeds rostrally through the
This ascending pathway is anatomically and functionally separated into two distinct
pathways within the dorsal columns of the spinal cord. Central processes from dorsal
root ganglia (1o neurons) gain access to the spinal cord and ascend in the ipsilateral
dorsal column without synapsing in the spinal cord. Information coming into the spinal
cord from the lower extremities up to approximately the T-7 spinal level, form a singleipsilateral pathway in the dorsal columns called the fasciculus gracilis (slide 1, slide
2, slide 3 and slide 4). The central processes for dorsal root ganglion cells from T-6 up
through C-2 (C-1 spinal nerves have no dorsal root ganglia) ascends in the dorsal
columns lateral to the fasciculus gracilis as the fasciculus cuneatus (slide 5 and slide
6). As these pathways ascend into the medulla, they remain in their same relative
positions (slide 6). The fasciculus gracilis terminates in the nucleus gracilis, which
appears at more caudal levels, while the fasciculus cuneatus continues to ascend to
more rostral levels of the medulla to terminate in the nucleus cuneatus (slide 7 and
slide 8). What external features signal the underlying presence of the nucleus gracilis
and nucleus cuneatus? Second order (2o) neurons in the nucleus gracilis and nucleus
cuneatus give rise to axons that sweep ventrally as the internal arcuate fibers to cross
the midline (sensory decussation/decussation of the medial lemniscus) and form
the contralateral ascending fiber bundle called the medial lemniscus (slide 8). These
axons then ascend somatotopically within the medial lemniscus through the medulla
Objectives: 1. Know the location and function of each of the described pathways from
their origin to their termination in the cerebral cortex.
2. Be able to determine the clinical signs and symptoms resulting from
lesions of these pathways at any point in their travels.
The vast majority of sensation from the external surface of the anterior 2/3 of the
head is provided by the trigeminal nerve (CN V) by way of its three divisions
(ophthalmic, maxillary and mandibular) with the remainder being supplied by CN VII, IX
and X. With the exception of proprioception and possibly pressure, the 1o neurons in
each of the following pathways are located in the sensory ganglia associated with each
of the above nerves. For the modalities of pain and temperature, the central processesfrom each of these sensory ganglia have the same central pathways.
1. Pain and Temperature -- On your whole brain specimens, find CN V, VII, IX and X
and make careful note of where they emerge from the brainstem. Now observe slide
16 and slide 17. These are cross sections through the caudal pons. On both slides,
note the facial colliculus forming the floor of the fourth ventricle, and the fascicles of
CN VII as they arch through the pontine tegmentum to exit the brainstem ventrally.
Just lateral to the axons of CN VII in the pontine tegmentum is a relatively clear, oval
area, the spinal nucleus of V. Surrounding the spinal nucleus of V on the lateral side
is a kidney-shaped area of nerve fibers, the descending (spinal) tract of V. Note the
close proximity of the spinal tract of V to the emerging fibers of CN VII. The descending
tract of V contains descending fibers of the central processes of the ipsilateral trigeminal
ganglion. As this tract descends, it picks up descending nerve fibers from the sensory
ganglia of CN VII, IX and X. These nerve fibers terminate in the spinal nucleus of V.
Both the spinal tract and nucleus of V are present from this point (i.e., caudal pons)
caudally to the level of the upper cervical spinal cord, where they are replaced by
Lissaur's tract and the substantia gelatinosa, respectively. Verify this by followingthe spinal tract and nucleus of V caudally (slide 17, slide 16, slide 15, slide 14, slide
13, slide 12, slide 10, slide 9, slide 8, slide 7, slide 6 and slide 5). As you do this,
note the close proximity of the spinal tract of V to the emerging fibers of CN VII (slide
17 and slide 16), CN IX (slide 14) and CN X (slide 12). 2o neurons in the spinal
nucleus of V give rise to axons that cross the midline obliquely and ascend as the
ventral trigeminothalamic tract (trigeminal lemniscus). (NOTE: There is still some
uncertainty as to the exact location of this tract in humans. The brain atlases show this
tract scattered dorsolateral to the medial lemniscus in the medulla. For the sake of your
collective sanity, we will not require you to know the location of the trigeminal lemniscus
in the medulla). As the medial lemniscus flattens dorsoventrally in the pons (slide 16,
slide 17, slide 18, slide 19 and slide 20), the trigeminal lemniscus remains on its
dorsal aspect, sandwiched between the medial lemniscus and the overlying central
tegmental tract.
In the midbrain, the trigeminal lemniscus assumes a position along the medial
concave surface of the medial lemniscus as the latter fans out laterally and dorsally
(slide 21, slide 22, slide 23 and slide 24) to assume a more vertical orientation. The
ascending 2o axons in the trigeminal lemniscus terminate in the ventral posteromedial
nucleus (VPM) of the thalamus (slide 29 and slide 30). 3o
neurons in the VPM giverise to axons that travel through the posterior limb of the internal capsule (slide 28,
slide 29, slide 30 and slide 31) and through the corona radiata (see demonstration) to
terminate in the postcentral gyrus near the Sylvian fissure.
2. Fine Touch, Vibration and Pressure -- The central processes of the trigeminal
ganglion cells course into the mid pons via CN V and terminate ipsilaterally on the
enlarged rostral extension of the spinal nucleus of V, called the chief (principal)
sensory nucleus of V. This nucleus lies in the lateralmost aspect of the pontine
tegmentum just medial to the middle cerebellar peduncle (slide 18). The 2o neurons
in this nucleus give rise to axons that cross the midline obliquely and ascend to join the
trigeminal lemniscus at midbrain levels (slide 21, slide 22, slide 23 and slide 24).
These nerve fibers then follow the exact synaptic pathway to the cortex as described
above for pain and temperature fibers for the anterior 2/3rds of the head. How would
the symptoms differ in a lesion of the trigeminal lemniscus in the caudal pons, as
opposed to a lesion of this structure at the superior collicular level?
3. Proprioception [and pressure (?)] -- In the case of proprioception, and possiblypressure, the 1o neurons in this pathway do not lie in the trigeminal ganglia, but instead
lie within the mesencephalic nucleus of V, which resides in the brainstem from the
mid pons to the superior collicular level of the midbrain. In the mid pons, this nucleus
lies dorsomedial to the chief sensory nucleus of V (slide 18). The peripheral processes
of these cells join CN V to be distributed with the three divisions of this nerve. They
Objectives: 1. Know the location and function of each of the described pathways at all
levels of the neuraxis from their origin in the cerebral cortex to their
termination in the brainstem and/or spinal cord.
2. Be able to determine the clinical signs and symptoms resulting from
lesions of these pathways at any level of the neuraxis in which they are
located.
The term "pyramidal system" refers to the direct (through internuncial cells),
volitional (voluntary) motor pathways from the cerebral cortex to the motor nuclei that
control the voluntary muscles of the body, extremities and head. Conversely, the term
"extrapyramidal system" (to be studied later) refers to motor pathways from the cerebralcortex that form loops with the basal ganglia and thalamus, and function in more
stereotypic movements and maintenance of posture.
Although the terms "pyramidal system" and "extrapyramidal system" are old
terms that have come into disrepute in recent years as our knowledge of the functioning
of the nervous system grows, they still cling to life among both basic scientists and
clinicians. Classically, the pyramidal system is subdivided into two parts: 1) the
corticobulbar tract, which terminates on cranial nerve motor nuclei within the
brainstem and thus controls the voluntary muscles of the head (and some in the neck),
and 2) the corticospinal (pyramidal) tract, which provides descending nerve fibers
from the cortex to the motor cell columns in the spinal cord for voluntary movement of
the body and extremities. Since these two pathways have identical functions in their
respective areas of innervation, and follow similar paths through brainstem levels, they
will be studied together.
Whole and Half Brains; Horizontal and Coronal Sections
Although there are significant contributions from other areas of the cerebralcortex (Brodmann's areas 6,8,3,1,2,5), both the corticobulbar and corticospinal tracts
begin primarily in the precentral gyrus (Brodmann's area 4). Find these regions on
your whole and/or half brains. The cortical neurons that reside in the above regions are
called upper motor neurons. These upper motor neurons are located somatotopically
within the precentral gyrus. The neurons for the corticobulbar tract are located near the
Sylvian fissure, whereas the neurons for the corticospinal tract reside somatotopically
in the remainder of the precentral gyrus as it arches dorsally and medially. Study
the motor homunculus on your lecture handout and make sure you understand this
concept. Axons arising from these upper motor neurons descend through the corona
radiata, which converges into the relatively compact internal capsule (observe these
structures on both your horizontal and coronal sections). The corticobulbar fibers
assume a compact position within the genu of the internal capsule, which resides at
the rostral pole of the thalamus. The corticospinal axons reside in a compact,
somatotopic fashion toward the caudal extent of the posterior limb of the internal
capsule. Can you think of a possible negative consequence related to the compact
nature of the corticobulbar and corticospinal fibers as they descend through the internal
capsule? As we descend from diencephalic to midbrain levels, each internal capsule is
continuous inferiorly with the ipsilateral cerebral peduncle. Compare and contrast theappearance and relationship of the internal capsule and cerebral peduncles on the
horizontal and coronal slices. This relationship can often be seen particularly well on
your coronal slices where the posterior limb of the internal capsule blends inferiorly
with the cerebral peduncles. Observing both coronal and horizontal slices, what
structure always resides immediately medial to the posterior limb of the internal
capsule?
As the corticobulbar and corticospinal axons descend into the midbrain, they are
classically described as residing in the middle 3/5 of the cerebral peduncles, with the
corticobulbar fibers occupying the medial portion and the corticospinal fibers arranged
somatotopically with lower extremity fibers most lateral. The cerebral peduncles can
usually be seen cut in cross section on the most inferior horizontal brain slice.
Now look on the ventral surface of your whole brain and note that the cerebral
peduncles terminate by disappearing caudally into the convexity of the ventral pons.
The pyramidal (corticospinal) tract emerges caudally in the medulla as the pyramids.
"What happened to the corticobulbar tract?", you may ask. Good question! (Try to
figure this out before you read on). The following is a good answer. As the
corticobulbar tract descends through the midbrain and pons, nerve fibers from this tractare peeling off to synapse on the motor nuclei of CN III, IV, V, VI and VII. The
remaining fibers for the medullary motor nuclei [nucleus ambiguus (motor nucleus for
CN IX, X and XI) and the hypoglossal nucleus] have also begun to separate from the
corticospinal fibers in the caudal pons to arch dorsally to synapse on these nuclei.
Consequently, the vast majority of the nerve fibers in the pyramids are those of the
pyramidal (corticospinal) tract. Find the longitudinal median fissure between the
pyramids in the rostral medulla. As you follow this distinct fissure caudally, it will blur, or
fill in, at the level of the caudal medulla. This is caused by the pyramidal decussation,
the crossing of the pyramidal tract to form the lateral corticospinal tract within the
spinal cord. Although a small percentage of fibers do not decussate, they are typically
not clinically relevant.
Slide Set (Don't put your brains away yet!)
Now, using your slide set and your atlas, follow the respective pathways of the
corticospinal and corticobulbar tracts. Begin with slide 35, which is a horizontal section
through the diencephalon at the level of the interventricular foramina of Monro. The
rostral direction is toward the top of the slide. Identify the anterior limb, genu and posterior limb of the internal capsule. Also note the columns of the fornix, third
ventricle and thalamus. Now, find a slice from your horizontally sectioned wet brain
specimen that compares to slide 35 and find the same structures as discussed above.
Note, on both the slide and the brain slice, the anatomical relationship between the
posterior limb of the internal capsule and the thalamus. It is strongly suggested that you
remember this relationship, since it is a consistent one that serves to identify these
structures in a variety of planes.
Now view slide 32. This is roughly a coronal section through the mid thalamus.
Find the thalamus, posterior limb of the internal capsule, third ventricle and
massa intermedia. What is the space immediately dorsal to the massa intermedia?
As you did previously, find a comparable coronal slice from your wet brain specimens
and compare it to this slide while you identify the above structures.
Slide 31, slide 30 and slide 29 show the transition between the posterior limb
of the internal capsule and the cerebral peduncles. Identify these structures, as well
as the thalamus and third ventricle. Slide 28 is particularly interesting, since it
provides a roughly coronal view of the diencephalon and oblique views of both midbrain
and pons. Using your half brain or one of your coronal sections for comparison, figureout the plane of section of this slide.
Using slide 24, slide 23, slide 22 and slide 21, follow and identify the
corticospinal and corticobulbar tracts caudally through the midbrain, noting the general
location and somatotopic arrangement of these tracts in the cerebral peduncles. On
slide 20, slide 19, slide 18, slide 17 and slide 16, notice how these tracts are
separated into loosely arranged fascicles in the ventral pons. The fascicles of the
corticobulbar tract are represented in the dorsomedial region of these fascicles. Slide
15 illustrates the medullary-pontine junction. The caudal remnants of the ventral
pons can be seen at the bottom of the slide. The pyramidal tracts have coalesced to
form two distinct fiber bundles, the pyramids, ventral to the inferior olivary nuclei. At
this level, only a few fascicles of the corticobulbar tracts remain.
Slide 14, slide 13, slide 12, slide 10 and slide 9 show the typical, consistent
appearance and location of the pyramids from rostral to caudal medulla. Slide 8
illustrates the beginning of the pyramidal (motor) decussation. The median fissure
between the pyramids is displaced to the right at its apex and the dorsomedial region of
the right pyramid is beginning to move dorsally. The remaining rostrocaudal extent of
the pyramidal decussation is illustrated in slide 7 and slide 6, which demonstrate the
fibers of the pyramidal tract crossing the midline to assume a more dorsolateral location.Once these fibers attain their new position contralaterally, they are called the lateral
corticospinal tract. Slide 5 (spinal cord level C-1) shows the location of the lateral
corticospinal tracts as loose fascicles of nerve fibers tucked into the concavity along
the lateral surface of the dorsal and ventral horns. The lateral corticospinal tract
maintains this position throughout the spinal cord (slide 4, slide 3, slide 2 and slide 1).
Would the symptoms be the same in an individual with a lesion of the right pyramid and
another individual with a lesion of the right lateral corticospinal tract? Can you explain
Objectives: 1. Be able to locate and identify the cranial nerves on the wet brains, and
understand the various functions of each.
2. Demonstrate a general understanding of “functional components” as
described in this section.
3. Be able to determine the clinical signs and symptoms resulting from
lesions involving all components of CN’s I, III, IV, V, VI, VII, IX, X, XI,
XII.
There are twelve pairs of cranial nerves. A lesion to any one of these nerves
results in clinically demonstrable deficits. As such, any general neurological exam
should include an assessment of cranial nerve function. The purpose of this laboratorysession is twofold: 1) to (re)acquaint you with the gross anatomy of the cranial nerves,
and 2) to study the internal macroscopic anatomy of selected cranial nerves (CN I, III,
IV, V, VI, VII, IX, X, XI and XII). The remaining cranial nerves (CN II and VIII) will be
studied, in detail, in subsequent laboratory sessions.
In the classic description of cranial nerves (and spinal nerves), each type of
nerve fiber contained within a given nerve (i.e. sensory, motor, autonomic, etc.) is
assigned a unique descriptive nomenclature to describe their general function. Each of
these descriptive terms is called a functional component. Although the importance of
the concept of functional components is dwindling, they provide an organized way to
categorize nerve fiber function. The following is a list of the functional components
found in the cranial nerves:
1. General Somatic Afferent (GSA) -- sensory nerve fibers that serve the skin as well
as the oral and nasal cavities and include "general" sensations such as touch, pain,
7. Special Visceral Efferent (SVE) -- motor nerve fibers that innervate muscles of
branchiomeric origin (muscles of mastication, facial expression, pharynx, larynx,
etc.).
Whole and Half Brains
Using your whole and half brains, turn them over to view the ventral surface and
observe the following:
CN I (olfactory nerve) -- This nerve provides us with our sense of smell (SVA). In
addition to its unique sensory function, the primary olfactory pathway is also unique
among the sensory pathways in that it does not have a relay through the thalamus to
the cerebral cortex, but instead sends projections directly to phylogenetically older
cortical areas (paleocortex). Another unique aspect of the olfactory pathway is the lack
of a decussation, i.e. the pathway is ipsilateral. The 1o cell bodies in this pathway are
located in the upper reaches of the nasal cavity. The actual filaments (axons) of CN I
extend through the cribriform plate of the ethmoid bone as the olfactory nerves, whichpenetrate the olfactory bulbs to synapse with the 2o neurons located there.
On the ventral surface of the brain, the olfactory bulbs can be seen on the
surface of the frontal lobes near the midline. The olfactory tract (stalk) extends
caudally from the olfactory bulb, carrying the axons of 2o neurons. As the olfactory tract
approaches the anterior perforated substance, it divides into medial and lateral
olfactory stria, each stria forming the sides of a triangular region called the olfactory
trigone. The medial olfactory stria crosses the midline via the anterior commissure to
supply interconnections between the left and right olfactory bulbs. Consequently, the
lateral olfactory stria is the principal central projection pathway for the olfactory system.
Follow the lateral olfactory stria as it heads toward the region of the uncus, where it
synapses in the primary olfactory cortex (periamygdaloid cortex, piriform cortex) and
the amygdala, a cluster of subcortical nuclei located deep to this paleocortex.
Secondary connections from these areas project to the rostral region of the
parahippocampal gyrus (entorhinal cortex), hippocampus (not seen here) and
hypothalamus. It should also be noted that the hypothalamus, thalamus,
parahippocampal gyrus and amygdala form part of the limbic system, the "emotional"
part of our brain.
CN II (optic nerve) -- This nerve is for vision (SSA). The optic nerves are seen in the
midline just caudal to the olfactory tracts. The left and right optic nerves meet at the
midline and fuse, forming the optic chiasm. Note the relationship of the infundibulum
and the optic chiasm. Two diverging nerve bundles, the optic tracts, can be seen
arching laterally and posteriorly from the optic chiasm to interconnect with the
diencephalon and midbrain.
CN III (oculomotor nerve) -- This nerve supplies motor innervation (GSE) to theextraocular muscles with the exception of the superior oblique and lateral rectus
muscles. In addition, it provides preganglionic parasympathetic fibers (GVE) to the
ciliary ganglion, which, in turn, provides postganglionic parasympathetic innervation to
the sphincter pupillae muscles and the muscles of the ciliary body that control the shape
of the lens. The oculomotor nerve can be seen emerging from the walls of the
interpeduncular fossa between the cerebral peduncles of the midbrain. As it
emerges, it passes between the posterior cerebral and superior cerebellar arteries
before eventually passing into the orbit via the superior orbital fissure.
CN IV (trochlear nerve) -- This small, threadlike nerve supplies motor innervation to the
superior oblique muscle of the orbit (GSE). If present, it can be seen along the lateral
aspect of the cerebral peduncles near their junction with the ventral pons. It is the
only cranial nerve to emerge from the dorsal aspect of the brainstem. NOTE: Do not
attempt to view this nerve on your brain specimens as it emerges from the dorsal
midbrain. This can be seen on demonstration. After emerging just caudal to the
inferior colliculi, it arches ventrally around the caudal midbrain to dive between the
layers of dura mater forming the anterolateral border of the tentorial notch. It gainsaccess to the orbit via the superior orbital fissure.
CN V (trigeminal nerve) -- This nerve supplies sensory innervation (GSA) to the
anterior 2/3 of the head, oral and nasal cavities and soft palate. In addition to supplying
motor (SVE) and proprioceptive innervation (GSA) to the muscles of mastication, it also
supplies motor (SVE) and proprioceptive (GSA) innervation to the myelohyoid, anterior
belly of digastric, tensor tympani and tensor veli palatini muscles. It is also thought to
supply proprioceptive fibers to the extraocular muscles. This large nerve emerges from
the rostral border of the middle cerebellar peduncle along the lateral aspect of the
pons to enter Meckel's cave, where the trigeminal (semilunar) ganglion resides. Distal
to the ganglion, CN V separates into its three divisions (ophthalmic, maxillary and
mandibular).
CN VI (abducens nerve) -- This nerve supplies motor innervation (GSE) to the lateral
rectus muscle of the orbit. It can be found near the midline, emerging from the inferior
pontine sulcus at the medullary-pontine junction. It courses anteriorly to gain access
to the orbit via the superior orbital fissure.
CN VII (facial nerve) -- This complex nerve gives motor innervation (SVE) to the
muscles of facial expression as well as the stapedius, posterior belly of the digastric and
stylohyoid muscles. In addition, preganglionic parasympathetic innervation (GVE) is
supplied (via parasympathetic ganglia containing postganglionic neurons) to the lacrimal
gland, the submandibular and sublingual salivary glands, as well as the mucous
membranes of the hard palate, soft palate and nasal cavities. It also contains sensory
nerve fibers that convey taste (SVA) from the anterior 2/3 of the tongue (and soft and
hard palates) and general sensation (GSA) from the external ear. This nerve can be
seen as it emerges at the cerebellopontine angle. What foramen does it traverse to
gain access to the facial canal?
CN VIII (vestibulocochlear nerve) -- This sensory nerve conducts auditory information
(SSA) from the cochlea and information for equilibrium from the semicircular canals. It
emerges just lateral to the facial nerve in the cerebellopontine angle and enters the
same foramen as the facial nerve.
CN IX (glossopharyngeal nerve) -- This nerve provides motor innervation (SVE) to thestylopharyngeus muscle. It also contains preganglionic parasympathetic nerve fibers
(GVE) destined for the otic ganglion. Postganglionic fibers from this ganglion are
secretomotor to the parotid gland. The sensory nerve fibers in this nerve provide taste
(SVA) and general sensation (GVA) from the posterior 1/3 of the tongue. It also
provides general sensation from the palatine tonsils (GVA), middle ear (GVA) and
external ear (GSA), and information from the carotid body and sinus (GVA). This nerve
emerges from the postolivary sulcus immediately caudal to CN VIII to pass into the
jugular foramen.
CN X (vagus nerve) -- This nerve provides motor innervation (SVE) to the muscles of
the pharynx (except stylopharyngeus), larynx and soft palate (except tensor veli
palatini). In addition, it provides preganglionic parasympathetic fibers (GVE) to
parasympathetic ganglia that innervate smooth muscles and glands in the larynx,
pharynx and all thoracic and abdominal viscera down to the left colic flexure. It also
provides sensory innervation to these same structures (GVA) as well as taste (SVA) to
the epiglottis and sensory innervation to the external ear (GSA). This nerve emerges
from the postolivary sulcus immediately caudal to CN IX as several compact rootlets
arranged in a rostrocaudal fashion and exits the skull via the jugular foramen.
CN XI (spinal accessory nerve) -- This nerve provides motor innervation (GSE) to the
trapezius and sternocleidomastoid muscles. It will not be present on your specimens,
since it arises from the lateral aspect of the upper cervical spinal cord to ascend through
the foramen magnum and assume a position along the lateral aspect of the medulla
before it exits the skull through the jugular foramen.
CN XII (hypoglossal nerve) -- This nerve provides motor innervation (GSE) to the
intrinsic and extrinsic muscles of the tongue. It exits the brainstem as a series of
loosely arranged rostrocaudal rootlets arising from the preolivary sulcus. These
rootlets converge to exit the skull via the hypoglossal canal.
Slide Set
CN III (slides 24-22) -- Begin with slide 24. This is a section through the rostral
midbrain. The gray matter that lies dorsomedial to the red nuclei contains the
oculomotor nuclear complex. Within this region are separate nuclei that control eachof the extraocular muscles innervated by CN III (Don't panic! You will not be asked to
identify each of these individual nuclei). In addition, this nuclear complex contains two
small, almond shaped nuclei that lie on its dorsal aspect. These are the Edinger-
Westphal nuclei. What specific cell type is contained in the Edinger-Westphal nuclei?
Although close to the midline, each of the nuclei within the oculomotor nuclear complex
gives rise to axons that remain ipsilateral to form CN III on their respective sides. The
boomerang shaped white matter immediately lateral to the oculomotor nuclei is the
medial longitudinal fasciculus (MLF). This important fiber tract contains axons that
interconnect the motor nuclei of CN III, IV and VI. Can you speculate why these nuclei
should be interconnected?
On slide 23, find the above structures, including the emerging fibers of CN III
along the lateral walls of the interpeduncular fossa. As these fibers emerge from the
oculomotor nuclear complex, they fan out laterally and ventrally to penetrate the red
nucleus before they swerve medially to exit the midbrain ventrally. This phenomenon
can be seen clearly on slide 22.
CN IV (slides 21,20) -- On slide 21, the trochlear nucleus replaces the Edinger-
Westphal nucleus just ventral to the periaqueductal gray. The small fascicle of axonsarising laterally from each nucleus is CN IV as it begins its journey to the dorsal aspect
of the pons by arching laterally and dorsally. The medial longitudinal fasciculus can
be seen just ventral to the trochlear nuclei and as a thin horizontal strip across the
midline. What specific level of the midbrain is represented in this slide?
Slide 20 (rostral pons) is caudal to the trochlear nucleus, a structure of the
midbrain. However, it shows the fibers of CN IV in two perspectives: 1) as the axons
of CN IV arise from the trochlear nucleus, they turn caudally to form a tight fascicle of
fibers in the lateral reaches of the periaqueductal gray, and 2) upon reaching the
rostral pons, the fibers of CN IV cross the midline within the superior medullary velum
as the decussation of CN IV to emerge from the dorsum of the brainstem as the
contralateral CN IV. With this in mind, what clinical symptom(s) would you expect to
see following a lesion of the right trochlear nucleus? Also note the medial longitudinal
fasciculus along the floor of the periaqueductal gray.
CN V (slides 23-20,18-12,10-6) -- Many of the major central components of this nerve
have been covered previously under the section entitled "Sensory Pathways for the
Anterior 2/3 of the Head" (pp. 34-36). Go back and review that section now, then comeback to this section for additional information.
Slide 18 (middle 1/3 of pons) shows all the central sensory and motor structures
of V with the exception of the spinal nucleus and tract of V, which reside at more caudal
levels. Reacquaint yourselves with the middle cerebellar peduncle, chief sensory
nucleus of V, and the mesencephalic tract and nucleus of V. Just medial to the
chief sensory nucleus of V is a small fascicle of nerve fibers, the motor root of V.
These fibers originated from the clear, egg shaped area, the motor nucleus of V,
located just medial to the motor root of V. This motor nucleus supplies the muscles
innervated by CN V. Do you remember what they are? Find the MLF on this slide.
CN VI and CN VII (slides 17-15) -- These three slides show the caudal 1/3 of the pons.
Slide 17 and slide 16 are essentially at the same level and show many of the same
structures. On each of these slides, find the following: fourth ventricle, middle
cerebellar peduncles, medial lemniscus and pyramidal tracts. The floor of the
fourth ventricle shows a groove at the midline, flanked by gently sloping mounds called
the facial colliculi. The clear oval areas immediately subjacent to the facial colliculi are
the left and right nuclei of CN VI. Is this a motor or sensory nucleus? Arising from the
medial surface of the nucleus of CN VI, and coursing ventrally through the mediallemniscus, are the small fascicles of axons forming CN VI. Just medial to the nucleus of
CN VI lie two fasciculi, the more dorsal one is the (internal) genu of CN VII. The other
is the MLF. Note the close proximity of the MLF and the nucleus of CN VI. The fascicle
of axons arching ventrolaterally from the lateral aspect of the floor of the fourth ventricle
is CN VII. Just medial to CN VII in the ventrolateral reaches of the pontine tegmentum
is an oval area, the motor nucleus of CN VII. What nucleus and its associated
pathway lie just lateral to CN VII in the pontine tegmentum? CN VII can also be seen
externally as it emerges just lateral to the ventral pons (slide 16 also shows CN VIII).
At this point, it is critical to understand the internal path of axons arising from the
motor nucleus of CN VII. After arising from the nucleus, these axons travel
dorsomedially (cannot be seen here) to pass just caudal to the nucleus of CN VI and lie
at the floor of the fourth ventricle near the midline. At this point, they bend rostrally
(internal genu of CN VII) to ascend to the rostral pole of the nucleus of CN VI. They
then arch over the nucleus of CN VI and proceed ventrally and caudally to exit the
brainstem. How does this compare to the internal path of CN IV?
It should also be noted (but not seen) that the superior salivatory nucleus lies
on the dorsomedial aspect of each facial nucleus. This important autonomic nucleussupplies preganglionic parasympathetic nerve fibers (GVE) via CN VII that are
secretomotor to the lacrimal gland (via the pterygopalatine ganglion), and to the
submandibular and sublingual salivary glands (via the submandibular ganglion).
Externally, just medial to CN VIII, some fascicles of CN VII can be seen.
Slide 15 shows the medullary-pontine junction. The spinal nucleus of V can
be seen just medial to the inferior cerebellar peduncle. On the right side, the small
nuclear area immediately dorsomedial to the spinal nucleus of V, and just medial to the
speckled region, is the rostral extent of the nucleus solitarius. The thin rim of white
matter immediately lateral to this nucleus is the fasciculus (tractus) solitarius. This
nucleus and its related tract conduct primarily taste information from CN VII, IX and X
(taste pathway will be discussed shortly). The small oval nuclear region just medial to
the nucleus solitarius contains the rostral extent of the inferior salivatory nucleus
(GVE). This nucleus is associated with CN IX. Externally, CN VII can be seen just
lateral to the postolivary sulcus on the left side. A lesion of this nerve would cause what
clinical symptom(s)?
CN IX (slides 29,19-12,10,9) -- Begin with slide 14 (rostral medulla). On the right side,CN IX can be clearly seen emerging from the postolivary sulcus. The pale area
immediately dorsomedial to the concavity of the postolivary sulcus contains the nucleus
ambiguus. This diffuse motor nucleus (SVE) resides in the rostral medulla and
contributes axons to both CN IX and X. It supplies innervation to the muscles of
branchiomeric origin supplied by these two cranial nerves. If you are having difficulty
finding this nucleus, do not despair. It lives up to its name in that it is difficult, at best, to
pinpoint in any given section. You will have your best luck finding it in slide 12 and
slide 15 (right side).
In addition to SVE innervation, CN IX also supplies GVE fibers in the form of
preganglionic parasympathetic axons to the otic ganglion for salivary secretions from
the parotid gland. These fibers are contributed by the inferior salivatory nucleus. This
small nucleus lies just medial to the nucleus solitarius (don’t worry about trying to find
it, just know that it resides in this area).
Slide 13, slide 12, slide 10 and slide 9 illustrate the location of the nucleus and
tractus solitarius. These bilateral structures gradually approach each other to fuse at
the midline caudal to the obex. What is the obex and at what level of the neuraxis is it
located? Although these structures extend into the caudal medulla, it should be notedthat 2o taste (gustatory) efferents from this nucleus arise from the rostral-most portion of
the nucleus.
Most of the efferent nerve fibers from the nucleus solitarius form the primary
ascending taste pathway, which is located primarily in the ipsilateral central tegmental
tract. In the rostral medulla, the central tegmental tract is located immediately dorsal to
the inferior olivary nuclei (slide 9, slide 12 and slide 13). In the caudal half of the pons,
it is located just dorsal to the medial lemniscus (slide 16, slide 17, slide 18 and slide
19). As the central tegmental tract ascends through the rostral pons and into the
midbrain it migrates dorsally to lie on the posterior (dorsal) surface of the red nucleus
just lateral to the medial longitudinal fasciculus. Taste fibers in the central tegmental
tract project to the ventral posteromedial nucleus of the thalamus (slide 29) which,
in turn, sends 3o axons to the postcentral gyrus near the Sylvian fissure (area 43)
[gustatory neocortex] and the insula (see demonstration).
CN X (slides 10-8) -- The vagus nerve contains: 1) the shared contributions from the
nucleus ambiguus (SVE) as previously described [see CN IX above], 2) taste and
general sensation from visceral structures that use the tractus and nucleus solitarius
(SVA, GVA), and 3) general sensation from the region of the external ear (GSA) thatenters the spinal tract of V. In addition, it uniquely contains preganglionic
parasympathetic nerves fibers (GVE) that supply viscera from the head down to the left
colic flexure. These axons arise from the dorsal motor (efferent) nucleus of X. The
rostral extent of this nucleus can be seen on slide 10. Find the tractus and nucleus
solitarius. The clear area dorsomedial to these structures is the dorsal motor
nucleus of X. Now follow this nucleus to its caudal extent (slide 9 and slide 8).
CN XI (slides 6,5) -- The motor nerve fibers innervating the trapezius and
sternocleidomastoid muscles (GSE) arise from a special nucleus located in the upper 5
or 6 cervical spinal segments, called the (spinal) accessory nucleus. This nucleus
can be seen as a lateral extension of the ventral horn on slide 6 and slide 5. On slide
6, observe the rootlets of the spinal accessory nerve lateral to the spinal cord.
CN XII (slides 10-8) -- This nerve contains motor nerve fibers to the intrinsic and
extrinsic muscles of the tongue (GSE). They arise from the hypoglossal nucleus, which
is located adjacent to the dorsal motor nucleus of X throughout its rostrocaudal extent.
On slide 10, locate the dorsal motor nucleus of X. Just ventromedial to thisnucleus is a round gray structure, the hypoglossal nucleus. On slide 9, axons can be
seen arising from the hypoglossal nucleus and traveling ventrally just lateral to the
medial lemniscus to emerge at the preolivary sulcus. Slide 8 shows the caudal
extent of the hypoglossal nucleus and a rootlet of CN XII emerging from the preolivary
into a single, broad based, loop circuit as follows: All areas of the cerebral cortex →
via internal & external capsule → striatum (caudate and putamen) and
subthalamic nucleus → globus pallidus and substantia nigra → VA, VL and DM
nuclei of thalamus → via internal capsule → all areas of cerebral cortex.
Although the basal ganglia per se do not project to the spinal cord, they connect
with structures that do (cortex, red nucleus, RF). Through these connections, the basal
ganglia function to modulate and integrate somatic motor activity. Through its input
from virtually all areas of the cerebral cortex to the striatum and subthalamic nucleus,
and its subsequent output from the substantia nigra and globus pallidus to the thalamus
and back to the cerebral cortex, the basal ganglia act in concert with the cerebellum as
an interface between our sensory and motor systems.
Lesions of the various nuclei of the basal ganglia result in relatively discrete
motor deficits collectively called dyskinesias (abnormal involuntary movements).REMINDER: Don’t forget that you should be able to name the discrete motor deficits
that arise as a result of lesions to different regions of the basal ganglia as described in
lecture.
Horizontal and Coronal Sections
Corpus Striatum (caudate, putamen and globus pallidus) and substantia nigra --
The caudate nucleus forms an incomplete ring around the dorsolateral and ventrolateral
aspect of the thalamus and is divided into three parts: head, body and tail. The large
head is located rostral to the thalamus, the body along the dorsolateral aspect of the
thalamus and the tail curves ventrolaterally to reside in the roof of the inferior horn of the
lateral ventricle. The tail terminates at the amygdaloid nucleus. The putamen and
globus pallidus reside in the concavity of the caudate nucleus, with the putamen located
lateral to the globus pallidus (see Fig. 5 on next page). The substantia nigra is located
in the midbrain tegmentum just dorsomedial to the cerebral peduncles. The
subthalamic nucleus lies ventral to the thalamus at the junction of the midbrain and
diencephalon (this structure will be seen on slides).
On your coronally sliced brain specimen, select a section just rostral to the
anterior commissure. The nuclear mass forming the lateral wall of the lateral ventricles
is the head of the caudate nucleus. Just ventrolateral to the anterior limb of the
internal capsule is the putamen. On the next section caudally, the head of the caudatenucleus remains in the same position if the thalamus is not present. However, the
putamen is typically joined medially at this level by the globus pallidus. The next
section caudally should contain the thalamus. If so, the head of the caudate nucleus
has been replaced by the body of the caudate nucleus. If the hippocampal formation
can be seen in the inferior horn of the lateral ventricle in the temporal lobe, find the
small tail of the caudate nucleus in the roof of the inferior horn of the lateral ventricle.
Follow and identify the body and tail of the caudate nucleus, putamen and globus
pallidus in subsequent sections. On some of your specimens, the coronal cuts may
go far enough caudal that the midbrain has been cut in frontal section. If so, try to find a
black pigmented region in the midbrain tegmentum. This is the substantia nigra.
What causes the black pigmentation of this region?
Select a horizontal section just ventral to the body of the corpus callosum.
(NOTE: As you go through your horizontal sections, correlate and compare what you
see with the same structures in the coronal sections. This will help you get a better
understanding of the 3-dimensional anatomy of the basal ganglia). Identify the head
and body of the caudate nucleus on this section, if possible. On more ventral
sections, identify the head and tail of the caudate nucleus, putamen and globus
pallidus. As you view more ventral sections, you may be able to observe the head of
the caudate nucleus fuse with the putamen at the rostral extent of the anterior limb of
the internal capsule. In addition, if the most ventral cut goes through the midbrain, the
substantia nigra can be seen as a black pigmented region just dorsomedial to the
cerebral peduncles. If your specimen does not show this, look on your neighbor's
specimen and/or look at the demonstrations.
Slide Set
Begin with slide 21. The substantia nigra can be seen at this and all midbrain
levels as a pale nuclear region dorsomedial to the cerebral peduncles. Follow the
substantia nigra rostrally (slide 22, slide 23, slide 24, slide 25, slide 26 and slide 28).
Lesioning the substantia nigra produces what clinical malady? Note the body and tail
of the caudate nucleus in slide 26 and slide 28, and a close-up of the tail of the
caudate nucleus in the roof of the inferior horn of the lateral ventricle on slide 27.
As the transition zone between the midbrain and diencephalon is reached (slide
25, slide 29, slide 30 and slide 31), the subthalamic nuclei appear dorsal to the
substantia nigra as fusiform tapered structures resembling cat's eyes. A lesion of the
right subthalamic nucleus would produce what SPECIFIC clinical symptom(s)? On
slide 31, two of the output pathways from the globus pallidus to the thalamus can be
seen. The thin dark line of fibers on the dorsal surface of the subthalamic nucleus on
the right side is the lenticular fasciculus. Near its medial extent, the fibers of the
lenticular fasciculus arch dorsolaterally (not easily seen here) to join the large bundle of
nerve fibers just dorsal to the lenticular fasciculus, called the thalamic fasciculus. The
thalamic fasciculus contains axons from the cerebellum, globus pallidus (via the
lenticular fasciculus and ansa lenticularis) and the substantia nigra, that are destined forthe VA, VL and DM nuclei of the thalamus. Also find the putamen and body of the
caudate nucleus on this slide and note the relationship between the globus pallidus
and internal capsule.
Slide 32 (left side) and slide 33 (right side) show the prominent ansa
lenticularis as it arises from, and travels ventral to, the medial segment of the globus
superior surface. These are branches of the superior cerebellar arteries and their
corresponding veins.
In contrast to the superior surface, the inferior surface of the hemispheres is
convex. The midline vermis is depressed and hidden from view, forming the floor of a
deep crevice, the posterior median fissure. The falx cerebelli resides in this fissure
when the brain is in the skull. Near the midline inferiorly, the cerebellum surrounds the
dorsolateral aspect of the medulla with two swellings, the cerebellar tonsils.
Occasionally, the cerebellar tonsils are useful in diagnosing elevated intracranial
pressure, since they tend to herniate through the foramen magnum as a result of this
condition. Two named blood vessels supply the inferior surface of the cerebellum.
Their origin and distribution can be highly variable and considerable overlap of
territories is not uncommon. Their typical distribution is described here. The posterior
inferior cerebellar arteries typically arise from the vertebral arteries. They archdorsally around the medulla, giving off small branches to the lateral medullary
region, then continue to ramify on the inferior surface of the cerebellum posterior to the
tonsils. The anterior inferior cerebellar arteries typically arise from the basilar artery
to pass laterally over the cerebellopontine angle and ramify on the inferior surface of
the cerebellum anterior to the tonsils. Just lateral to the cerebellopontine angle is a
slender lateral projection of cerebellar tissue, the flocculus (part of the flocculonodular
lobe). A fissure extends laterally from the posterior aspect of the flocculus. This is the
posterolateral fissure, which separates the flocculonodular lobe from the posterior
lobe.
Two of the three cerebellar peduncles can be seen on the ventral surface of your
whole brain specimens. The large middle cerebellar peduncle (brachium pontis) lies
rostral to the flocculus. Medial to the flocculus, find the inferior olive and postolivary
sulcus. The rounded mass dorsal to the postolivary sulcus is the inferior cerebellar
peduncle (restiform body). The above two peduncles transmit primarily afferent nerve
Now look at the medial surface of your half brain specimen (see Fig. 6 below).
The cut was made through the midline, bisecting the vermis.
The cut surface of the cerebellum has the appearance of trees with leaves
sprouting along the extent of their branches. Each one of these "leaves" is called a
folium (pl. folia). Like the cerebral cortex, the neurons of the cerebellar cortex lie at
the surface in the folia. Close inspection reveals "branches" of white matter converging
in the deeper regions of the cerebellum to form "trunks." The bases of the trunks merge
to form the deep white matter that constitutes the roof of the fourth ventricle.
Imbedded in this deep white matter are the deep cerebellar nuclei (to be seen on
slides). The cerebellar cortex at the vermis is separated into nine lobules, based
roughly on the "tree" analogy as described above (i.e. each "tree" is approximately
equal to one lobule). We will learn only those lobules bordering the primary and
posterolateral fissures. Find the culmen and the declive along the superior surface of
the vermis. The deep groove between these two lobules is the primary fissure, which
separates the anterior and posterior lobes. Being careful not to tear any tissue, follow
this fissure onto the superior surface of the cerebellum. Now find the nodule (the
central part of the flocculonodular lobe) and the uvula along the inferior surface of thevermis. The groove between these two lobules is the posterolateral fissure. Note the
cerebellar tonsil just inferior and lateral to the uvula.
Now find the superior medullary velum, which forms the roof of the fourth
ventricle at this level. The relatively thick wall of the fourth ventricle at this level is
1. Spinal cord -- In contrast to the dorsal column and spinal lemniscus
pathways which transmit conscious information, spinal cord pathways to
the cerebellum transmit the unconscious modalities of proprioception,
touch and pressure impulses to the vermal and paravermal regions of
the cerebellum. They do so by ascending along the superficial surfaces of
the lateral funiculi of the spinal cord and enter the cerebellum primarily
through the inferior cerebellar peduncle. We will not be concerned with
naming or following these pathways on slides.
2. Brainstem -- Brainstem afferents arise primarily from three sources:
A) pontine nuclei, B) vestibular nuclei and C) inferior olivary nuclei.
A. (Cortico)-ponto-cerebellar pathway -- Axons from the cerebral cortex
project to the ipsilateral pontine nuclei via the cerebral peduncles. The
pontine nuclei then project their axons to the contralateral cerebellar
cortex (lateral region of posterior lobe) via the middle cerebellar peduncle.
B. Vestibulocerebellar tract -- From the vestibular nuclei via the
juxtarestiform body to the flocculonodular lobe.
C. (Cortico)-olivo-cerebellar pathway -- Axons from the cerebral cortex
project to the inferior olivary nucleus (complex). Cells in the inferior olivary
complex then project axons contralaterally to all areas of the cerebellar
cortex via the inferior cerebellar peduncle. NOTE: In addition to cortical
input, the inferior olivary complex also receives afferents from the red
nucleus, basal ganglia, RF and spinal cord.
Begin with slide 19. The cerebral cortex provides axons to the ventral pontine
nuclei via the cerebral peduncles. The pontine nuclei within the ventral pons give riseto axons that cross the midline as the pontocerebellar pathway (transverse pontine
fibers). These can be seen as horizontal blue streaks within the ventral pons. These
axons gain access to the cerebellum via the large middle cerebellar peduncle to
terminate in the lateral hemispheres of the posterior lobe. Identify the above structures
There are four pairs of vestibular nuclei (to be studied shortly) within the medulla
and pons. These nuclei provide afferent fibers to the flocculonodular lobe of the
cerebellum via the juxtarestiform body. As the name implies, this relatively small
pathway to the cerebellum lies in juxtaposition (medial) to the restiform body (inferior
cerebellar peduncle), and can be seen on slide 17, slide 16, slide 15 and slide 14.
(It should also be noted that this pathway carries efferent fibers from the cerebellum to
the vestibular nuclei).
The inferior olivary nucleus (slides 15-12,10,9) projects axons to the cerebellum
via the contralateral inferior cerebellar peduncle. Begin with slide 15, which is at the
level of the medullary-pontine junction and observe the following: The left and right
inferior olivary nuclei each project their axons to the contralateral cerebellar cortex by
sending them medially to decussate through the medial lemniscus and continue
through the contralateral inferior olivary nucleus to arch dorsolaterally where theyenter the contralateral inferior cerebellar peduncle. This can be seen clearly on slide
13 and slide 12. These afferents to the cerebellum from the inferior olivary nuclei go
directly to Purkinje cells and are collectively called climbing fibers.
Efferent Connections -- Although the afferent connections/pathways to the cerebellum
are important, they are relatively diffuse from an anatomical standpoint. In contrast, the
vast majority of axons exit the cerebellum via the superior cerebellar peduncle
(exception: juxtarestiform body). To understand this concept, consider that the input to
the cerebellum comes from a wide variety of structures and travels to all regions of the
cerebellar cortex via three peduncles. The output from the cerebellar cortex (Purkinje
cells) is also diffuse. However, the vast majority of these cells do not project their axons
outside the confines of the cerebellum, but instead send them to converge and synapse
on the four pairs of deep cerebellar nuclei, which are located within the white matter in
the roof of the fourth ventricle. This relatively compact set of nuclei gives rise to axons
that pass primarily through the superior cerebellar peduncle. Because of this compact
anatomical arrangement, small lesions to this region (deep cerebellar nuclei, sup.
cerebellar peduncle) can produce profound effects.
Deep cerebellar nuclei (slides 10,11,13,14) -- Look at slide 10. The deep
cerebellar nuclei are located within the deep white matter of the cerebellum
overriding the caudal pons and medulla. They are, from lateral to medial:
1. dentate nucleus -- receives afferents from the Purkinje cells of the lateral
hemispheres.
2. emboliform nucleus
nucleus interpositus; receives afferents fromthe Purkinje cells of the paravermal region.
3. globose nucleus
4. fastigial nucleus -- receives afferents from the Purkinje cells of the vermis
and flocculonodular lobe.
Now find the above nuclei on slide 11, slide 13 and slide 14. The fastigial
nucleus projects its axons to vestibular nuclei via the juxtarestiform body. Axons from
the remaining deep cerebellar nuclei exit the cerebellum via the superior cerebellar
peduncle (slide 16 and slide 19). Slide 20 and slide 21 illustrate how the superior
cerebellar peduncle arches ventromedially to decussate (decussation of the superior
cerebellar peduncle) at the levels of the rostral pons and caudal midbrain. After
decussating, these axons pass through the contralateral red nucleus (slide 22, slide
23, slide 24, slide 25). Some of these axons terminate in the red nucleus which, in
turn, gives rise to axons that decussate immediately and descend to the spinal cord as
the rubrospinal tract. This pathway can be seen as rounded projections from the
ventral surface of the decussation of the superior cerebellar peduncle on slide 21. Therubrospinal tract provides excitatory motor innervation primarily to the flexor muscles of
the upper extremity.
Other axons from the deep cerebellar nuclei pass through the red nucleus and
ascend to terminate in the ventrolateral (VL) nucleus of the thalamus (slide 31 and
slide 32). The VL gives rise to axons that project to the cerebral cortex (Brodmann's
areas 4 and 6). This pathway provides information to the cerebral cortex concerning the
location of the body in space to ensure smooth coordinated movements. Thus, the
cerebral cortex forms loop circuits with the cerebellum as follows: cerebral cortex →
via int. capsule & cerebral peduncles → pontine & inf. olivary nuclei → via middle
& inf. cerebellar peduncles → cerebellum → via sup. cerebellar peduncle → VL of
thalamus → via int. capsule → cerebral cortex. Can you recall a pathway or
pathways involving the basal ganglia that are similar or have common elements to those
Objectives: 1. Find and identify the vestibular nuclei and understand the functional
significance of their connections with extraocular nuclei, cerebellum
and spinal cord.
2. Understand how the vestibular system is tested and the normal
behavioral consequences of the testing procedures.
CN VIII (vestibulocochlear nerve) -- As the name implies, the vestibulocochlear
nerve has two primary functions: equilibrium (vestibular portion) and hearing (cochlear
portion). As such, each division of this nerve has discrete external structures as well as
discrete internal nuclei and pathways within the brainstem that serve each of these
modalities. In this laboratory session, we will identify the central nuclei and pathwaysthat serve the vestibular division of CN VIII.
Peripherally, hair cells within the ampullae of the semicircular canals, utricle and
sacculus are connected with the peripheral processes of the primary vestibular
afferents. The cell bodies for these primary afferents are located in the vestibular
(Scarpa's) ganglion, which resides in the internal auditory meatus. A few of the central
processes of these ganglion cells project directly to the flocculonodular lobe via the
juxtarestiform body. However, the vast majority of these central processes project to
the ipsilateral vestibular nuclei located in the pons and medulla. The vestibular nuclei
give rise to axons that project to spinal cord, cerebellum, extraocular motor nuclei, RF,
contralateral vestibular nuclei and thalamus (small nucleus between VPM and VPL).
Efferents from the vestibular nuclei to the thalamus are relayed to the postcentral and
superior temporal gyri. These projections from the vestibular nuclei to the thalamus and
on to the cerebral cortex are thought to ascend via the brainstem reticular formation or
possibly with ascending auditory fibers to provide cortical awareness of equilibrium.
Vestibular division of CN VIII (slides 18-12,10) -- Beginning with slide 15, locate
the inferior cerebellar peduncle. The "salt and pepper" or speckled regiondorsomedial to the inferior cerebellar peduncle identifies the inferior (spinal)
vestibular nucleus. Medial to the inferior vestibular nucleus lies the medial vestibular
nucleus. Notice on the left side how the fibers of the juxtarestiform body intermingle
with medial and inferior vestibular nuclei. Identify the medial longitudinal fasciculus
(MLF) in the midline at the floor of the fourth ventricle. At this level, the MLF contains:
1) ascending axons from the vestibular nuclei to the extraocular motor nuclei (CN III, IV,
VI) for coordinating eye movements during the process of maintaining equilibrium, and
2) descending bilateral axons from the medial vestibular nuclei for control of somatic
muscles in maintaining equilibrium. The MLF continues inferiorly into the cervical spinal
cord as the medial vestibulospinal tract. Identify, where possible, the above structures
on slide 14, slide 13, slide 12 and slide 10.
Now return to slide 15 and reorient yourselves. Find the inferior vestibular
nucleus. On subsequent rostral slides, this nucleus is replaced by the lateral
vestibular nucleus. The slender dorsolateral extension of gray matter from the lateral
vestibular nucleus is the caudal extent of the superior vestibular nucleus.
Using your brain atlas and slide 16, slide 17 and slide 18, follow the MLF and
the medial, lateral and superior vestibular nuclei rostrally to get a feel for their
location and rostrocaudal extent. As you proceed, also observe the juxtarestiformbody on slide 16. Note that the lateral and medial vestibular nuclei do not extend into
the upper 1/2 of the pons (slide 18). How far rostrally would you expect to find the
MLF, and why?
NEURORADIOLOGY
On slide 61, find CN VII & VIII emerging from the cerebellopontine angle. Also
find the internal auditory meatus and the semicircular canals.
Objectives: 1. Understand the anatomy and function of the central and peripheral
structures that comprise the auditory system.
2. Know and understand the differences in symptoms between central
and peripheral lesions of the auditory system.
Auditory information is transmitted to the CNS via the auditory division of CN VIII
(vestibulocochlear nerve). Peripherally, hair cells within the cochlea are in synaptic
contact with the peripheral processes of the spiral ganglion cells, which are located in
the bony modiolus. The central processes of these cells form the cochlear division of
CN VIII.
Observe slide 40. This is a cross section through the cochlea. The large spaceat the top of the slide is the scala vestibuli (contains perilymph). The diagonal
membrane is called the vestibular (Reissner's) membrane. This structure separates
the scala vestibuli and the scala media (cochlear duct). The latter contains
endolymph. The cavity at the bottom of the slide is the scala tympani which is
continuous with the scala vestibuli at the apex of the cochlea. As such, it contains
perilymph. The large structure on the left (modiolus) forms a shelf-like process called
the bony spiral lamina. At the tip of the shelf, the basilar membrane stretches to the
right to make contact with the spiral ligament which attaches to the outer region of the
bony labyrinth (not seen). Resting on the basilar membrane is the (spiral) organ of
Corti. Above the basilar membrane at approximately its midpoint, a row of three
slender cells can be seen. These are the outer hair cells. Just to the left of these cells
is the fusiform space called the space of Nuel. The space just to the left of the space
of Nuel is the (inner) tunnel of Corti, which is bounded on the left and right by the
inner and outer columns respectively. The inner hair cells reside at the tip of the
inner column. In this plane of section, there are typically one row of inner hair cells and
3 rows of outer hair cells. The next space to the left is the internal spiral sulcus. The
roof of this sulcus is formed by the translucent tectorial membrane, which extends tothe right to make contact with the apical hairs of the inner and outer hair cells. The hair
cells are in contact with the peripheral processes of the spiral ganglion cells, which
gain access to the hair cells via the bony spiral lamina. The spiral ganglion resides in
the modiolus and contains the primary afferent cell bodies in the auditory pathway.
Vibration of the foot of the stapes on the oval window creates displacement of the
basilar membrane. This phenomenon produces a shearing effect between the tectorial
membrane and the hair cells. When the hairs are displaced because of the shearing
forces, the hair cells stimulate the spiral ganglion to transmit impulses into the CNS via
the auditory division of CN VIII.
As a point of interest, there is tonotopic organization within the cochlea. That is,
higher frequencies result in displacement of the basilar membrane at the base of the
cochlea, with lower frequencies displacing the basilar membrane toward the apex of the
cochlea. This marks the beginning of a continuous tonotopic arrangement of cells and
nerve fibers, both peripheral and central, throughout the auditory pathway.
The central auditory pathways can be seen on slide 28 and slides 26 -12. Begin
with slide 15. On the right side, just lateral to the inferior cerebellar peduncle, there
is a large region of mottled gray matter, the ventral cochlear nucleus. Immediatelyventral to this nucleus is the darkly stained region of CN VIII as it emerges from the
cerebellopontine angle. The clear nuclear area along the dorsal aspect of the inferior
cerebellar peduncle is the dorsal cochlear nucleus. Identify, where possible, the
above structures on slide 14, slide 13 and slide 12.
Now follow the auditory pathway as it ascends through the brainstem. Beginning
with slide 15, note the ventral cochlear nucleus and CN VIII on the right side. The
dorsal and ventral cochlear nuclei contain the 2o auditory neurons. Although the
cochlear nuclei project some of their axons ipsilaterally, the majority cross the midline
via the trapezoid body, which is the small bundle of vertically arranged axons located
in the midline between the two medial lemnisci. Some of the crossed fibers of the
ventral cochlear nucleus synapse in the contralateral superior olivary nucleus
(SOLN), which can be seen on slide 16. This nucleus is located just lateral to the
central tegmental tract and is shaped like an inverted "V". The majority of axons
arising from the SOLN enter the ipsilateral lateral lemniscus, which lies just lateral to
the SOLN, where they join the ascending fibers of ipsilateral and contralateral cochlear
nuclei. NOTE: In addition to ascending fibers, some axons from the SOLN project to
the motor nuclei of CN V and VII, others project to the motor nuclei of CN III, IV, VI andthe RF. Can you speculate why these connections would be present? Find the SOLN,
lateral lemniscus and trapezoid body in slide 17. Since the SOLN is found only in
the caudal 1/3 of the pons, it will not be seen in slide 18 and subsequent slides. Follow
the lateral lemniscus rostrally as it fans out to assume a vertical orientation in the lateral
wall of the rostral pons (slide 19 and slide 20). On slide 20, the lateral lemniscus is
split into medial and lateral portions by the nucleus of the lateral lemniscus, another
relay nucleus in the auditory pathway. On slide 21, the nerve fibers of the lateral
lemniscus can be seen sweeping medially to enter the ventrolateral aspect of the
nucleus of the inferior colliculus where they synapse. Axons arising from the
nucleus of the inferior colliculus emerge from the dorsolateral aspect of the nucleus to
ascend along the lateral wall of the rostral midbrain as the brachium of the inferior
colliculus (slide 22). Follow the brachium of the inferior colliculus rostrally as it
enters and synapses within the ipsilateral medial geniculate (body) (slide 23, slide
24, slide 25, slide 26 and slide 28). Axons arising from the medial geniculate body
project to the dorsal aspect of the superior temporal gyrus called the transverse
temporal gyrus (of Heschl) (Brodmann's areas 41,42). NOTE: THIS CAN BE SEEN
ON DEMONSTRATION.
Given the external and internal anatomy of the auditory system, would a unilateral lesion of the lateral lemniscus in the midbrain produce ipsilateral loss of
hearing? If not, what symptom(s) would you expect from this lesion and why? Where
would you lesion the auditory pathway to achieve total loss of hearing in the left ear?
NEURORADIOLOGY
On slide 61, find CN VII & VIII, the internal auditory meatus and the cochlea.
DEMONSTRATIONS
Half brain showing: Auditory cortex.
Dorsal view of brainstem showing: inferior colliculus, brachium of inferior
hypothalamus regulates basic physiologic functions such as temperature
regulation, heart rate, blood pressure and gastrointestinal activity. For
example, the hypothalamus monitors blood temperature, producing both
visceral (blood vessel constriction) and somatic activity (shivering) if the
temperature of the blood circulating through the hypothalamus should
drop. In addition, through its connections with the limbic system, the
hypothalamus regulates emotion-based behavior such as anger, rage and
sexual activity. To produce these global effects, the hypothalamus has
extensive influence via the endocrine system and through synaptic
connections within the CNS, including telencephalon, brainstem and
spinal cord.
Although experimental and clinical evidence indicate that specific regions of thehypothalamus, when lesioned, produce specific deficits or behavioral abnormalities, it
cannot be said with absolute certainty that the hypothalamus is solely responsible for a
given function. For example, there is an important pathway, the medial forebrain
bundle, which travels rostrocaudally in the lateral region of the hypothalamus and
interconnects a variety of forebrain and midbrain structures with the hypothalamus.
Consequently, should the lateral region of the hypothalamus be lesioned, it would
include the medial forebrain bundle. As a result, this pathway must be considered as a
possible source of the resulting clinical symptom(s).
Whole and Half Brains
On the ventral surface of your whole brain specimens, find the optic nerves (CN
II). Gently elevate them and attempt to see the lamina terminalis, which forms both
the rostral wall of the 3rd ventricle and the rostral boundary of the hypothalamus.
Immediately caudal to the optic chiasm, the infundibulum can be seen. This structure
connects the hypothalamus with the pituitary (hypophysis). The region of the
hypothalamus between the infundibulum and the mammillary bodies is the tubercinereum. The rounded swellings of the mammillary bodies form the caudalmost
extent of the hypothalamus and reveal the location of the underlying mammillary nuclei.
On the medial surface of your half brain specimens, find the lamina terminalis,
optic chiasm, tuber cinereum and mammillary body. The midline region
immediately dorsal to the optic chiasm, tuber cinereum and mammillary body is the 3rd
ventricle. The wall of the 3rd ventricle from this level dorsally to the hypothalamic
sulcus is formed by the hypothalamus. Note how the optic chiasm fuses with the
ventral portion of the hypothalamus. It is at this point that axons from the ganglion cells
of the retina enter the hypothalamus to provide regulation of diurnal rhythms.
The hypothalamus can be roughly divided into four regions from rostral to caudal:
1. preoptic area -- the region immediately rostral to the optic chiasm.
2. supraoptic (anterior) region -- the area immediately dorsal to the optic
chiasm.
3. tuberal region -- the tuber cinereum and the area dorsal to it.
4. mammillary (posterior) region -- the region of the mammillary bodies andthe area dorsal to them.
Slide Set
The hypothalamus has many afferent and efferent connections associated with
the limbic system (to be covered later in this laboratory session). In this portion of the
laboratory, we will study the anatomy of the hypothalamus per se and the important
pathways to and from the brainstem and spinal cord.
Begin with slide 9. The central tegmental tract is sandwiched between the
inferior olivary nucleus and the spinal lemniscus. This pathway carries nerve fibers from
a variety of sources, including cortical and basal ganglia input to the brainstem reticular
formation (RF), efferent fibers from red nucleus to inferior olive and visceral (taste)
information from the nucleus solitarius to the hypothalamus. Taste fibers peel off from
the central tegmental tract to terminate in the hypothalamus. The hypothalamus, in
turn, sends descending fibers to the brainstem and spinal cord. This combined
ascending/descending pathway provides an autonomic response (salivation, sweating,
vomiting) to the tastes that are being experienced. Follow the central tegmental tract rostrally through the medulla and pons (slide 10, slide 12, slide 13, slide 14, slide 15,
slide 16, slide 17, slide 18). Through these regions, the central tegmental tract resides
on the dorsal aspect of the medial lemniscus as the latter assumes a horizontal position
in the pons. The dorsal longitudinal fasciculus (DLF) maintains a relatively constant
position at the floor of the 4th ventricle throughout the brainstem and can be best seen
on slide 14 (bilaterally) and slide 15 (right side). This important (primarily) efferent
pathway from the hypothalamus synapses on: 1) brainstem RF, 2) brainstem nuclei
involved with eating, swallowing, vomiting, salivation and digestion and 3) enters the
spinal cord in the lateral funiculus to synapse on cells of the intermediolateral cell
column.
The various regions of the hypothalamus can be seen on slides 24, 25, 29-33, 38
and 39. Begin with slide 24 and slide 25, which reveal the mammillary bodies and
part of the tuberal region of the hypothalamus in horizontal section. Slide 29, slide
30 and slide 31 also show the mammillary bodies. Slide 32 is an oblique section
through the tuberal region of the hypothalamus. The tuberal region is flanked by the
optic tracts. The 3rd ventricle can be seen in the midline. Note the hypothalamic
sulcus delineating the dorsal extent of the hypothalamus. The large, compact fascicle
of axons within the tuberal region is the fornix. This important pathway to thehypothalamus will be discussed in detail as part of the limbic system. At this point, it is
an important landmark that separates the medial and lateral regions of the
hypothalamus. The diffuse gray colored area immediately lateral to the fornix is the
medial forebrain bundle as it travels through the scattered cells of the lateral nucleus
of the hypothalamus. Slide 33 is an oblique section through the infundibulum and
rostral tuberal region. On slide 33, identify the 3rd ventricle, fornix, medial and
lateral nuclear regions, medial forebrain bundle and the optic tracts.
Slide 38 is a relatively high power view of a coronal section through the anterior
commissure and anterior region of the hypothalamus. Unlike the other brain
sections in your slide set, this region is stained with a cellular stain. As such, the nuclei
(cell bodies) will stain darkly with the fiber tracts appearing unstained. Find the 3rd
ventricle. The rounded dark areas within the walls of the 3rd ventricle are the
paraventricular nuclei. The dorsal aspect of the optic chiasm can be seen as it fuses
with the hypothalamus ventrally. The darkly stained region in the lateral concavity
between the optic chiasm and the hypothalamus on each side are the supraoptic
nuclei. What neurotransmitter(s) is (are) secreted by the cells of the paraventricular
and supraoptic nuclei? Can you find the septum pellucidum , lateral ventricles and internal capsule ? What part of the internal capsule is seen here?
Slide 39 is a sagittal section of the brainstem. Is this a midsagittal section? Can
you defend your answer based on sound anatomical evidence? Identify the
mammillary, tuberal and anterior regions of the hypothalamus. Also find the optic
nerve, optic chiasm, anterior commissure and fornix within the hypothalamus.
As our understanding of this complex system has grown, other nuclear structures
and pathways have been added. These include the amygdala, which has direct
connections with the hippocampal formation. In addition, the amygdala has connections
with: 1) wide areas of the hypothalamus and dorsomedial nucleus of the thalamus
(which projects to the frontal lobe) via the stria terminalis and/or ventral amygdalofugal
pathway; 2) septal nuclei via the diagonal band of Broca and; 3) habenular nuclei via
the stria terminalis and stria medullaris. (NOTE: indirect connections to the habenular
nuclei are relayed through the septal nuclei and hypothalamus).
The primary purpose of the above discussion is not necessarily for the
ANATOMY per se, but to allow the student to appreciate the numerous and complexinterconnections between limbic structures as well as the broad interconnections
between the limbic system and the rest of the brain. As one might surmise, these
interconnections are critical to the functional integrity of this complex system.
The cortical components of the limbic system will be identified first, followed by
subcortical components and finally the pathways that connect these components.
Cortical components -- Turn your half brain specimens to observe the medial surface.
Immediately rostral to the anterior commissure and lamina terminalis is a small vertical
strip of cortex called the paraterminal gyrus. Just rostral to the paraterminal gyrus and
immediately ventral to the rostrum of the corpus callosum is the subcallosal gyrus.
These two gyri together are often referred to as the septal area. Follow the subcallosal
gyrus rostrally to the genu of the corpus callosum where the subcallosal gyrus becomes
the cingulate gyrus. The cingulate gyrus extends along the dorsal surface of the
corpus callosum from the genu to the splenium. Just caudal to the splenium of the
corpus callosum the cingulate gyrus narrows and dives ventrolaterally as the isthmus
of the cingulate gyrus. The latter is continuous with the parahippocampal gyrus onthe ventromedial surface of the temporal lobe. Due to an involution of the temporal
cortex, the hippocampal formation is hidden from view within the depths of the temporal
lobe just caudal to the uncus. This region of cortex is best seen on a coronal section.
Select a coronal section immediately caudal to the uncus. Just lateral (deep) to
the medial surface of the temporal lobe is an undulating region of cortex buried within
the temporal lobe. This is the hippocampal formation. In more caudal coronal
sections of the hippocampal formation, this structure has the appearance of a sea horse
from which it derives its name (G. hippokampos = sea horse). It is covered ventrally by
the parahippocampal gyrus. The region where the medial lip of the parahippocampal
gyrus bends 180o to turn dorsally and laterally is called the subiculum, a component
part of the hippocampal formation. The remaining parts of the hippocampal formation
will first be seen on slides. It will then be easier for you to identify them on your
coronal/horizontal sections. The paraterminal gyrus, subcallosal gyrus, cingulate gyrus,
isthmus of the cingulate gyrus, parahippocampal gyrus and hippocampal formation form
a continuous cortical circle or rim called the limbic lobe.
Subcortical nuclear components -- Select a coronal section that includes the uncus.Deep (lateral) to the uncus within the temporal lobe is the rounded subcortical nuclear
mass of the amygdala. What sensory pathway has direct connections with the
amygdala? Note the close proximity of the amygdala and the hippocampal formation.
The anterior and dorsomedial nuclei of the thalamus are also prominent anatomical
components of the limbic system. Find coronal sections that contain these thalamic
nuclei.
Turn again to the medial surface of your half brain specimens. Deep (lateral) to
the septal area and immediately rostral to the anterior commissure are the septal
nuclei (these will be seen on slides). As previously mentioned, the hypothalamus is a
central figure in the limbic system. The habenula, which lies just rostral to the pineal
body, is also typically included as part of the limbic system.
Pathways connecting cortical and subcortical components of the limbic system -- On the
medial surface of the half brain, find the stria medullaris as it arches rostrocaudally
across the medial surface of the thalamus to terminate in the habenula. The stria
medullaris carries afferents to the habenula from the amygdala, septal nuclei and
hypothalamus. Now find the body of the fornix as it arches from caudal to rostralalong the ventral border of the septum pellucidum. The fornix is the major efferent
pathway from the hippocampus. As the fornix approaches the rostral pole of the
thalamus, it dives ventrally as the columns of the fornix (see Fig. 8 below).
The majority of axons in the columns of the fornix pass posterior to the anterior
commissure and penetrate the hypothalamus where they synapse in the mammillary
bodies. Those fibers of the fornix passing rostral to the anterior commissure
(precommissural fornix) terminate primarily in the septal nuclei and anterior
hypothalamus.
As the fornix arises from the hippocampus, it arches caudally and dorsally toward
the splenium of the corpus callosum. This can be seen in both horizontal and coronal
sections. Select a coronal section immediately caudal to the uncus and a horizontal
section looking down on the dorsal aspect of the thalamus just ventral to the body of the
corpus callosum. On the coronal section, note the thin strip of white matter that forms
the lateral wall of the hippocampal formation. This is the alveus and is formed by the
efferent fibers arising from the hippocampus. The alveus courses dorsomedially to form
a free lip of white matter called the fimbria, the initial portion of the fornix. Select amore caudal coronal section through the splenium of the corpus callosum (if possible)
and observe the fimbriae as they arch dorsomedially to form the crura (sing. = crus) of
the fornix. As the crura approach the midline to form the body of the fornix, you may
be able to see a membranous sheet of tissue fusing the left and right fornix. This is the
commissure of the fornix (hippocampal commissure).
At this time, try to find the crura, hippocampal commissure and the body and
columns of the fornix on the horizontal sections. NOTE: If you cannot find the above
structures on your sections due to the "luck of the cut", look on your neighbor's
specimens and/or look at the demonstrations.
Now find a horizontal section just ventral to the body of the corpus callosum and
look at the dorsal aspect of the intact thalamus (if your "luck of the cut" allows this view).
The rounded mass along the dorsolateral aspect of the thalamus is the caudate
nucleus. In the rostrocaudal groove between these two structures is a thin bead of
white matter called the stria terminalis. It may be hidden from view by the
thalamostriate (terminal) vein or choroid plexus. If so, look on your neighbor’s brain or
the demonstrations. What are the origin and termination of axons in the stria
terminalis? Now follow the body of the fornix, stria terminalis and stria medullaris
rostrally on your coronal sections. Note the relationship between the columns of the
fornix and the anterior commissure. In a section containing the anterior thalamic nuclei
and/or the mammillary bodies, you may be able to find the columns of the fornix as
they dive ventrally within the substance of the hypothalamus to terminate in the
mammillary bodies. In the same section(s), try to find the mammillothalamic tract,
which loops dorsally from the mammillary bodies to terminate in the anterior nucleus of
the thalamus. To what cortical region does the anterior nucleus of the thalamus
project? Find the cingulate gyrus on your coronal sections. The white matter
immediately deep to the gray matter of the cingulate cortex is the cingulum, which
contains the efferent axons from the cingulate gyrus to the parahippocampal gyrus.
Slide Set
The following slides (26-36,38,39) will reveal cortical regions and subcortical nuclei as
well as pathways of the limbic system. As you proceed through these slides, attempt to
correlate them with the wet brain specimens.
Begin with slide 26. Immediately dorsal to the pulvinar on both sides lie thecrura of the fornix. Sandwiched between the crura is the caudal extent of the body of
the corpus callosum. Just lateral to the fornix lies the body of the caudate nucleus.
The relatively small brown-stained region just ventral to the caudate nucleus is the stria
terminalis. The lower left side of this slide reveals a coronal section through the
temporal lobe at the level of the hippocampal formation, which is composed of the
hippocampus proper, dentate gyrus and subiculum. The cortical region medially and
ventrally is the parahippocampal gyrus. The region of this gyrus dorsally where it
swerves laterally is called the subiculum. Where the subiculum meets the overlying
dentate gyrus, it becomes the hippocampus proper. The groove between the
hippocampus proper and the overlying dentate gyrus is called the hippocampal
fissure. The darkly stained white matter dorsal to the dentate gyrus is the fimbria of
the fornix, which tapers laterally as the alveus.
A higher power view of the hippocampus proper can be seen on slide 27. On
this slide, identify the subiculum, dentate gyrus, hippocampal fissure,
hippocampus proper, alveus and fimbria of the fornix. Note that the hippocampus
proper extends laterally from the hippocampal fissure and arches dorsally to terminate
by tucking its "head" into the dentate gyrus. Identify the choroid plexus stretchingbetween the fimbria of the fornix and the region just medial to the stria terminalis which
is embedded in the roof of the inferior (temporal) horn of the lateral ventricle. The
nuclear mass in the roof of the lateral ventricle lateral to the stria terminalis is the tail of
the caudate nucleus, part of the basal ganglia. The space medial to the choroid plexus
is the subarachnoid space, whereas the space lateral to the choroid plexus is the
inferior (temporal) horn of the lateral ventricle.
Slide 28 is slightly more rostral and contains the habenular nuclei. Find the
crura and fimbria of the fornix, and the alveus. The stria terminalis can also be
seen between the thalamus and body of the caudate nucleus as well as in the roof of
the inferior horn of the lateral ventricle. Slide 29 also contains the habenular nuclei
in addition to the habenulointerpeduncular tract (fasciculus retroflexus). This output
pathway from the limbic system arches ventrally and caudally to synapse in the RF of
the midbrain tegmentum. Also note the appearance of the mammillary bodies on this
slide.
Slide 30 illustrates the origin of the mammillothalamic tract as it arises from the
medial aspect of the mammillary nuclei. Where does this fiber tract terminate? Within
the lateral aspect of the mammillary nuclei are pale blue vertical strips. These are theterminal fibers of the columns of the fornix. Also note the lightly stained uncus of the
temporal lobes located ventrolateral to the mammillary bodies. The region deep
(lateral) to the uncus is the amygdala.
Slide 31 shows the body and columns of the fornix, stria terminalis, stria
medullaris, mammillothalamic tract, uncus and amygdala. Nerve fibers from the
amygdala can be seen arching dorsomedially as the ventral amygdalofugal pathway
(see the labeled illustration of slide 31 in the atlas at the back of this syllabus), which
terminates primarily in the dorsomedial nucleus of the thalamus. Also note the
compact bundle of nerve fibers in the temporal lobe. These are the fibers of the
anterior commissure as they proceed rostrally before they cross the midline. As such,
follow the fiber bundles of the anterior commissure rostrally on subsequent slides until
they meet at the midline.
Slide 32 and slide 33 illustrate the body and columns of the fornix, stria
medullaris and mammillothalamic tract. The latter can be seen as a tangentially cut
fascicle of axons dorsolateral to the hypothalamic sulcus on slide 32. On slide 33, it is
visible on the right side as a looping fascicle of axons approaching the anterior nuclear
group from the ventral side. Slide 33 also shows the mammillotegmental tract as athin tract of axons just medial and dorsal to the columns of the fornix. This pathway
descends to the RF in the midbrain tegmentum.
Slide 34 shows the columns of the fornix in two different planes. Just ventral to
the body of the corpus callosum, the fibers of the columns of the fornix are cut in
nucleus of the thalamus and dorsomedial nucleus of the thalamus.
NEURORADIOLOGY
On slide 45, find the paraterminal gyrus, cingulate gyrus, isthmus of thecingulate gyrus and the fornix. What region on this slide represents the septal area?
Slide 46 shows the hippocampal formation. Slide 47 shows the crura of the fornix,
while slide 48 and slide 49 illustrate the columns of the fornix.
Objectives: 1. Know the functional anatomy of the globe of the eye.
2. Know the anatomy & function of the primary visual pathway and the
consequences of a lesion to any part of this pathway.
The visual system is a delicately balanced, highly complex system that enables
us to see our environment in great detail, and in color. The act of "seeing" or visualizing
an object occurs in two phases: The first phase involves light reflecting from an object
and passing into the globe of the eye via the cornea. Within the eye, the lens directs
and focuses the light onto a specialized region at the back of the eye called the retina.
The second stage of visualization is the conversion of this light energy into electrical
impulses by the retina. Ganglion cells within the retina give rise to axons that form theoptic nerve. Visual information that has been processed by the retina is transmitted
through the optic nerve to the optic chiasm where some axons enter the hypothalamus
to influence diurnal rhythms. The majority of nerve fibers enter the optic tracts, which
project axons to the primary visual cortex in the occipital lobe via a relay in the lateral
geniculate body of the thalamus. The primary visual cortex then relays this visual
information to other areas of the cerebral cortex for further processing and analysis.
Some of the optic tract fibers bypass the lateral geniculate to project into the rostral
midbrain where they provide the input for visual reflexes (pupillary dilation and
constriction, accommodation and eye movements in response to visual stimuli).
Globe of the eye (slides 41-44) -- We will begin our study of the visual system by
examining the globe of the eye. Slide 44 shows a low power view of a horizontal
section cut through the equator of the eye. Find the cornea, anterior chamber, iris,
lens, posterior chamber, ciliary body and ciliary processes. Now turn to slide 41
and identify these same structures on this higher power view. Also observe the
pigmented layer of the iris on this slide. What effect does contraction of the muscles
of the ciliary body have on the tension applied to the lens? What effect does this have on the shape of the lens? Would a lesion of the superior cervical sympathetic ganglia
have any effect on the shape of the lens? Why (or why not)? Now turn back to slide 44
and identify the vitreous chamber, retina, optic disc, central artery to the retina,
optic nerve and dura mater, arachnoid mater and subarachnoid space. Can you
explain why the optic nerve is surrounded by meninges? Slide 43 is a high power view
of the macula lutea, the region of the retina that produces the best visual acuity, and
the posterior wall of the eye. Note the three visible cell layers of the retina, the visual
receptor cells (rods and cones), bipolar cells and ganglion cells. The depressed
area at the center of the macula lutea is the fovea centralis. What is the unique
feature of this region? Also identify the pigment layer of the retina, the choroid layer
and the sclera. Between what layers does retinal detachment occur? Why is it
important to reattach the retina as soon as possible? Now observe slide 42. This is a
high power view of the optic disk and optic nerve. Why is the optic disk called the
"blind spot"? Where are the cell bodies of origin for the nerve fibers in the optic nerve?
Nerve fibers arising from the nasal (medial) half of the retina cross to the contralateral
side of the brain via the optic chiasm. The remaining nerve fibers from the temporal
(lateral) half of the retina remain ipsilateral.
The remainder of the visual pathway can be seen on slides 23-26,28,29,31-33and on demonstration. To better understand the plane of section on each of these
slides, compare each slide with your half brain specimen. For example, look at slide
33. At the bottom of the slide in the midline is a small part of the infundibulum. Find
this structure on your half brain just posterior to the optic chiasm. Approximately in the
middle of the slide is the massa intermedia. Find this structure on your half brain.
Now draw an imaginary line between the infundibulum and the massa intermedia. This
is the plane of section on slide 33. Note the optic tract (slide 33 and slide 32) just
lateral to the hypothalamus. Follow the optic tract posteriorly as it diverges to lie just
ventral to the cerebral peduncles (slide 31, slide 30 and slide 29). On the left side of
slide 28, the terminal portion of the optic tract can be seen entering the laminated
lateral geniculate. On slide 28, slide 26 and slide 23, the optic radiations can be
seen emerging from the dorsolateral aspect of the lateral geniculate bodies as they
head for the primary visual cortex (area 17). Slide 25 reveals the optic chiasm, optic
tract and lateral geniculate. The fibers of the brachium of the superior colliculus
can be seen after they exit the optic tract to wind around the dorsal aspect of the medial
geniculate just ventral to the pulvinar to enter the pretectal area. Some of these fibers
cross the midline in the posterior commissure. What is the function of this pathway? What is the plane of section in slide 25 ? The brachium of the superior colliculus,
optic tract, medial geniculate, lateral geniculate and pulvinar can also be clearly
Coronal sections -- Starting with the section through the optic chiasm, follow the
optic tracts caudally to the lateral geniculate bodies and note the optic radiations
emerging from the lateral geniculate bodies. At the level of the medial and lateralgeniculate bodies, attempt to find the brachium of the superior colliculus.
Horizontal sections -- Select the section that contains the anterior commissure
and if possible, find the medial and lateral geniculate bodies. This section should
also reveal the optic radiations. It should be noted at this time that axons within the
optic radiations that serve the upper visual fields (lower retinal fields) travel rostrally
from the lateral geniculates to loop around the rostral pole of the temporal horn of the
lateral ventricles before they turn caudally to join the remaining optic radiations and
terminate in the primary visual cortex. Consequently, lesions of the temporal lobe may
result in visual field deficits. This indirect pathway is called Meyer’s loop (see
demonstration). On the section immediately ventral to this one, try to find the optic
tracts as they wind around the brainstem just anterior (ventral) to the cerebral
peduncles. NOTE: Whether you see some of these structures will rely on the "luck of
the cut". If you are unable to see the above structures on your sectioned brains, look on
the brain specimens of one of your neighbors.
Half brain -- On the medial surface, find the parietooccipital sulcus and thecalcarine sulcus. The cortex surrounding the dorsal and ventral lips of the calcarine
sulcus is the primary visual cortex (area 17), which receives afferents from the
ipsilateral lateral geniculate bodies. A good portion of the primary visual cortex is
hidden from view, since the calcarine sulcus extends laterally into the occipital cortex.
The general cortical region dorsal to the calcarine sulcus is the cuneus. Its counterpart
ventral to the calcarine sulcus is the lingula.
NEURORADIOLOGY
Find the optic nerve, optic chiasm, calcarine fissure, cuneus and lingula on
slide 45. Can you find the lateral geniculate bodies on slide 46 ? (Hint: Compare
this slide with your coronal brain specimens). The optic radiations can be clearly seen
on slide 47, slide 48 and slide 49. A lesion of the optic radiations on the right side
Objectives: 1. Revisit those areas of primary cortex previously learned and review
their location and function.
2. Learn & understand other areas of cortex presented here with
particular emphasis on those areas related to speech.
3. Know the clinical symptoms related to lesions of cortical areas as
presented in lecture.
The cerebral cortex is composed of: 1) a convoluted sheet of neurons at the
external surface of the cortex and 2) the subjacent white matter that interconnects the
cells of the cerebral cortex with other cortical cells and with cells in other regions of the
CNS. It is the well-developed cerebral cortex in humans that gives us our uniqueabilities to participate in language and abstract thinking. The cerebral cortex is also
critically involved in our perception of the outside world and our ability to move and
adapt to our environment.
The vast majority of the cerebral cortex (over 90%) is classified as neocortex. As
the name implies, it is the most recent type of cortex to develop. The classification of
the types of cerebral cortex is largely based on the histological cytoarchitecture of the
outer mantle of cortical cells as follows:
1. Neocortex (Isocortex) -- classically described as those regions of
cerebral cortex that contain, either during development or in the adult
stage, 6 layers of cells from superficial to deep.
2. Paleocortex -- contains from 3 - 5 layers of cells, and is restricted to
the primary olfactory cortex on the ventral surface of the brain.
3. Archicortex -- contains 3 layers of cells, and is restricted to the
hippocampal formation.
Through the years, a number of anatomists have attempted to categorize the
cerebral cortex based on neural cytoarchitecture. It was originally thought that the
anatomical differences between different areas of the cortex could be precisely related
to function. Although none of the published classifications have achieved the specificity
that was originally hoped for, overall, Brodmann's (circa 1909) numerical classifications
of 52 cortical regions have emerged as the standard and have generally withstood the
test of time. However, as more information is gathered on brain function using more
sophisticated research techniques, our understanding of the functioning of the various
regions of the cerebral cortex, and the CNS as a whole, is rapidly evolving.
To ease your fears, we will not attempt to identify all of Brodmann's 52 areas of
the cerebral cortex, but instead concentrate on those areas that relate to a specific
modality or function. It should be noted that much of this laboratory session will be a
review of cortical areas we have already studied. However, this laboratory will also give
you the opportunity to review pathways related to a number of cortical regions. For
example, Brodmann's area 4 of the precentral gyrus contributes axons to the
corticospinal and corticobulbar pathways. You should be able to identify the location of
these pathways on both your wet brain specimens and your slide sets.You should perform this exercise on all known ascending and descending pathways.
Whole and Half Brains
On the left lateral surface of the cerebral cortex, identify the central sulcus,
precentral gyrus, and the superior, middle and inferior frontal gyri. Make sure you
understand the somatotopic arrangement of the precentral gyrus. Identify the general
region of the premotor area (area 6) and frontal eye fields (area 8). An ablative
lesion of area 8 on the left side results in what symptom(s)? Find the Sylvian fissure.
The inferior frontal gyrus lies on the superior bank of this fissure just rostral to the
precentral gyrus and is divided into three parts from caudal to rostral. The opercular
part of the inferior frontal gyrus is small and lies just rostral to the precentral gyrus;
the triangular part resembles an inverted triangle. Both the opercular and triangular
portion represent Broca's speech area (areas 44,45). Lesion of this area results in
Broca's aphasia. What are the symptoms of Broca's aphasia? What region of the body
is controlled by the precentral gyrus immediately caudal to Broca's area? The
horizontal gyrus just rostral to the triangular portion of the inferior frontal gyrus is theorbital part of the inferior frontal gyrus. If the Sylvian fissure is gently opened, the
insular cortex can be seen. Can you name two sensory modalities that terminate in
the insular cortex?
On a lateral view of the left temporal lobe, find the superior, middle and inferior
temporal gyri. The dorsal surface of the superior temporal gyrus is hidden by the
frontal and parietal opercula, and contains the transverse gyri of Heschl (areas
41,42). What symptom(s) would you expect to observe if Heschl's gyri were lesioned on
the left side? The lateral surface of the superior temporal gyrus, approximately from the
level of the precentral gyrus rostrally to the posterior portion of the supramarginal
gyrus caudally, contains the auditory association area (area 22). The posterior
portion of area 22 is Wernicke's area, which acts to integrate visual and auditory
information required to comprehend written and spoken language. A lesion of this area
results in Wernicke's aphasia. Can you describe the symptoms of Wernicke's
aphasia?
Immediately caudal to the supramarginal gyrus of the parietal lobe is the
angular gyrus. These two gyri form the inferior parietal lobule. A lesion of the
inferior parietal lobule, but not Wernicke's area, results in a complex series of disorders
which may include any combination of the following: alexia, anomia, constructionalapraxia, agraphia, finger agnosia and confusion or inability to distinguish between the
left and right sides of the body. Can you define the above terms that describe this
lesion? What symptoms would you expect to see in a comparable lesion of the right
cerebral hemisphere? What artery supplies this region? Find the postcentral gyrus
(areas 3,1,2). This is the somatosensory cortex. What pathways terminate along this
somatotopically arranged gyrus?
Find the calcarine fissure at the caudal pole of the occipital lobe. The gyri
forming the upper and lower lips of this fissure are the primary visual cortex (area 17).
The visual association areas (areas 18 and 19), are arranged concentrically around
area 17 on the lateral surface of the occipital lobe. Now follow the calcarine fissure
around to the medial surface of the occipital lobe, where this fissure forms a deep
horizontal groove that projects laterally. Thus, although area 17 can be seen
immediately dorsal (cuneus) and ventral (lingula) to the calcarine fissure, much of area
17 is hidden from view within the depths of the occipital lobe. As on the lateral surface
of the occipital lobe, areas 18 and 19 surround area 17. What symptoms would result
following a lesion of the left primary visual cortex? What major artery supplies this
region? Follow the calcarine fissure rostrally where it is joined by the parietooccipitalsulcus. Find the cingulate gyrus, the isthmus of the cingulate gyrus and the
paracentral lobule. What Brodmann's areas are encompassed by the paracentral
lobule? What part(s) of the body does the paracentral lobule serve? Is the paracentral
lobule sensory or motor? What symptoms would you see if it were lesioned? What
artery supplies the paracentral lobule? Follow the cingulate gyrus as it curves ventrally
around the genu of the corpus callosum to become the subcallosal gyrus. Also
identify the paraterminal gyrus. What diencephalic nucleus projects to the cingulate
gyrus? To what important system does the cingulate gyrus belong?
Turn to the ventral surface of the brain and identify the uncus and
parahippocampal gyrus. What important structure lies deep to the uncus? What
clinical symptoms would you see if this structure was lesioned bilaterally? What is the
classification of cerebral cortex that comprises the uncus?
The remaining areas of cortex come under the broad heading of association
cortices, which correlate the various sensory inputs and deliver them to the appropriate
cortical areas for action.
Intercortical Connections -- Afferent input to the cerebral cortex comes from a variety
of subcortical structures. As you have probably surmised by now, it is the thalamus, viathe internal capsule, that provides the greatest single source of subcortical input to the
cerebral cortex. Similarly, efferents from the cerebral cortex also pass to subcortical
structures primarily through the internal capsule.
With the possible exception of the visual cortex, virtually all areas of the cerebral
cortex interconnect across the midline with comparable cortical areas via subcortical
white matter called commissural fibers (pathways). Ipsilateral connections (within the
same hemisphere) are accomplished by way of association fibers.
Commissural fibers -- Turn to the medial surface of your half brain sections.
Identify the rostrum, genu, body and splenium of the corpus callosum. It is this
massive interhemispheric commissure that provides the vast majority of commissural
fibers between the cerebral hemispheres. The anterior commissure also transmits
interhemispheric fibers between the temporal lobes. Why isn't the posterior commissure
included in this group of commissural fibers?
Association fibers -- The association fiber bundles are difficult to see. However,
their general location can be determined. Select a coronal section midway through thebody of the corpus callosum and identify the location of the following fiber bundles
within the white matter deep to the cellular layers of the cerebral cortex. The white
matter immediately deep (lateral) to the cingulate cortex is the cingulum, which
connects the cingulate cortex with the parahippocampal gyrus and hippocampal
formation. The region of white matter immediately dorsal to the body of the caudate
1. Medial longitudinal fasciculus (MLF)2. Motor root of CN V3. Trigeminal lemniscus4. Lateral lemniscus5. Spinal lemniscus6. Medial lemniscus7. Pontine nuclei8. Corticospinal & corticobulbar tracts9. Superior vestibular nucleus10. Mesencephalic tract (root) of V
surrounded by mesencephalic nucleus of V11. Chief (principal) sensory nucleus of V12. Motor nucleus of V13. Middle cerebellar peduncle14. Central tegmental tract15. Transverse pontine (pontocerebellar) fibers
1. Nucleus of the inferior colliculus2. Periaqueductal gray3. Mesencephalic nucleus of V & tract of V (lateral to nucleus of V)4. Medial longitudinal fasciculus (MLF)5. Trochlear nucleus & CN IV arising laterally6. Decussation of superior cerebellar peduncles7. Substantia nigra8. Corticospinal tract9. Corticobulbar tract10. Fibers forming brachium of inferior colliculus11. Lateral lemniscus
1. Commissure of superior colliculus2. Superior colliculus3. Spinal lemniscus4. Trigeminal lemniscus5. Medial lemniscus6. Central tegmental tract7. Medial longitudinal fasciculus (MLF)8. Oculomotor nuclear complex (& Edinger-Westphal nucleus)
9. Oculomotor n. (CN III)10. Pulvinar of thalamus11. Brachium of the inferior colliculus12. Brachium of the superior colliculus13. Medial geniculate body14. Lateral geniculate body15. Optic tract16. Cerebral peduncle17. Substantia nigra
1. Body of the corpus callosum2. Crus of the fornix3. Body of the caudate nucleus4. Stria terminalis5. Third ventricle6. Habenula7. Posterior commissure
8. Cerebral aqueduct9. Red nucleus10. Medial lemniscus (& cerebello-rubro-thalamic fibers)11. Medial geniculate body12. Lateral geniculate body13. Fimbria of the fornix14. Hippocampal formation15. Posterior limb of the internal capsule16. Dorsomedial nucleus of the thalamus17. Centromedian nucleus of the thalamus
18. Ventral posterolateral (VPL) nucleus of the thalamus19. Ventral posteromedial (VPM) nucleus of the thalamus20. Cerebral peduncle
1. Body of the corpus callosum2. Body of the fornix3. Body of the caudate nucleus4. Stria terminalis5. Insula6. External capsule7. Putamen8. Globus pallidus
9. Posterior limb of the internal capsule10. Cerebral peduncle11. Ventral amygdalofugal pathway12. Anterior commissure13. Amygdala14. Thalamic fasciculus15. Lateral ventricle16. Third ventricle17. Ventrolateral (VL) nucleus of the thalamus
18. Dorsomedial (DM) nucleus of the thalamus19. Lenticular fasciculus20. Subthalamic nucleus
1. Head of the caudate nucleus2. Putamen3. Interventricular foramen of Monro4. Columns of the fornix5. Dorsomedial (DM) nucleus of the thalamus6. Lateral region of the thalamus7. Body of the fornix
8. Anterior limb of the internal capsule9. Septal nuclei10. Globus pallidus11. Genu of the internal capsule12. Anterior nuclear group of the thalamus13. Posterior limb of the internal capsule14. Stria terminalis15. Tail of the caudate nucleus
* = fusion of the putamen and head of the caudate nucleus