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Prima Eriawan Putra 1102012212 1
1. Anatomi Nervus Cranialis dan Jaras Motorik serta Sensorik
1.1. Nervus Kranialis
The cranial nerves are analogous in some ways to the spinal
nerves, having both sensory and motor functions. Also, like the
spinal cord, motor cranial nerve nuclei are located more ventrally,
while sensory cranial nerve nuclei are located more dorsally.
However, cranial nerve sensory and motor functions are more
specialized because of the unique anatomy of the head and neck.
Brainstem I: Surface Anatomy and Cranial Nerves 495
Learning the cranial nerves initially requires some
memorization. Over time,however, they become very familiar because
of their important clinical rele-vance. The numbers, names, and
main functions of the cranial nerves are listedin Table 12.1. Note
that the cranial nerves have both sensory and motor func-tions. To
learn the cranial nerves and their functions, two different
reviewstrategies are useful. In one, the cranial nerves are listed
in numerical sequenceand the sensory and motor functions of each
nerve are discussed (see Table12.4). In the second, the different
sensory and motor cranial nerve nuclei arelisted, and the functions
and cranial nerves subserved by each nucleus are dis-cussed (see
Table 12.3). Both approaches are clinically relevant, and we will
useboth strategies at various points in these brainstem chapters to
integrate knowl-edge of the peripheral and central course of the
cranial nerves.
Surface Features of the BrainstemThe brainstem consists of the
midbrain, pons, and medulla (see Figure 12.1). Itlies within the
posterior fossa of the cranial cavity. The rostral limit of the
brain-stem is the midbraindiencephalic junction (see Figure 12.1).
Here the brainstemmeets thalamus and hypothalamus at the level of
the tentorium cerebelli. Mid-brain joins pons at the
pontomesencephalic junction, and pons meets medullaat the
pontomedullary junction. The caudal limit of the brainstem is the
cervi-comedullary junction, at the level of the foramen magnum and
pyramidal de-cussation (see Figure 12.1, Figure 12.2A; see also
Figure 6.8). The cerebellum isattached to the dorsal surface of the
pons and upper medulla (see Figure 12.1).Although some authors have
included the cerebellum or thalamus in the termbrainstem, we adopt
common clinical usage here and take brainstem toimply only
midbrain, pons, and medulla. We discuss the thalamus and
cere-bellum at greater length elsewhere (see Chapters 7 and
15).
On the dorsal surface of the midbrain are two pairs of bumps
called the su-perior colliculi and inferior colliculi (Figure
12.2B). Together, these form the tec-
TABLE 12.1 Cranial Nerve Names and Main Functions
CN NAME MAIN FUNCTION(S)
CN I Olfactory nerve OlfactionCN II Optic nerve VisionCN III
Oculomotor nerve Eye movements; pupil constrictionCN IV Trochlear
nerve Eye movementsCN V Trigeminal nerve Facial sensation; muscles
of
masticationCN VI Abducens nerve Eye movementsCN VII Facial nerve
Muscles of facial expression;
taste; lacrimation; salivationCN VIII Vestibulocochlear nerve
Hearing; equilibrium senseCN IX Glossopharyngeal nerve Pharyngeal
muscles; carotid body
reflexes; salivationCN X Vagus nerve Parasympathetics to most
organs;
laryngeal muscles (voice);pharyngeal muscles (swallowing);
aortic arch reflexes
CN XI Spinal accessory nerve Head turning (trapezius and
ster-nomastoid muscles)
CN XII Hypoglossal nerve Tongue movement
REVIEW EXERCISECover the two right columns in Table 12.1. For
each numberedcranial nerve, provide its name andmain functions.
REVIEW EXERCISEFor the midbrain, pons and medullain Figure 12.1
or 12.2C, point in therostral, caudal, dorsal, ventral, supe-rior,
inferior, posterior and anteriordirections (see definitions in
Figure2.4). How do these differ for pointsabove the
midbraindiencephalicjunction?
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500 Chapter 12
Sensory and Motor Organization of the Cranial NervesThe cranial
nerves are analogous in some ways to the spinal nerves, havingboth
sensory and motor functions. Also, like the spinal cord, motor
cranialnerve nuclei are located more ventrally, while sensory
cranial nerve nucleiare located more dorsally (Figure 12.4).
However, cranial nerve sensory andmotor functions are more
specialized because of the unique anatomy of thehead and neck.
During embryological development, the cranial nerve nucleilie
adjacent to the ventricular system (see Figure 12.4A). As the
nervous sys-
REVIEW EXERCISECover the right column in Table 12.2.Name the
exit foramina for each cra-nial nerve (see Figure 12.3).
TABLE 12.2 Cranial Nerve Exit Foramina
CN NAME EXIT FORAMEN
CN I Olfactory nerves Cribriform plateCN II Optic nerve Optic
canalCN III Oculomotor nerve Superior orbital fissureCN IV
Trochlear nerve Superior orbital fissureCN V Trigeminal nerve V1:
Superior orbital fissure
V2: Foramen rotundumV3: Foramen ovale
CN VI Abducens nerve Superior orbital fissurea
CN VII Facial nerve Auditory canal (stylomastoid foramen)
CN VIII Vestibulocochlear nerve Auditory canalCN IX
Glossopharyngeal nerve Jugular foramenCN X Vagus nerve Jugular
foramenCN XI Spinal accessory nerve Jugular foramen (enters
skull
via foramen magnum)CN XII Hypoglossal nerve Hypoglossal foramen
(canal)
aThe abducens nerve first exits the dura through Dorellos canal
(see Figure 12.3) and thentravels a long distance before exiting
the skull at the superior orbital fissure.
(A)
Sulcus limitans
Fourthventricle
Visceral motornuclei
Somatic motornuclei
Motor nucleiSensory nuclei
Key
(B)
Somaticsensorynuclei
Visceralsensorynuclei
Sulcuslimitans
Branchialmotor: SVE(nucleusambiguus)
Parasympathetic:GVE (dorsalmotor nucleusof CN X)
Somatic motor:GSE (hypoglossal nucleus)
Visceral sensory:SVA and GVA(nucleussolitarius)
Special somaticsensory: SSA(vestibularnuclei)
General somaticsensory: GSA(spinaltrigeminalnucleus)
FIGURE 12.4 Development of CranialNerve Nuclei Sensory and Motor
Lon-gitudinal Columns (A) Cross section ofhuman myelencephalon at
45 daysshowing locations of sensory and motorcranial nerve nuclei
functional columns.(B) Adult medulla, with locations offunctional
columns indicated. Examplesof the nuclei in these columns for a
sec-tion at this level are indicated in paren-theses. (A after
Tuchman-Duplessis H,Auroux M, and Haegel P. 1974. IllustratedHuman
Embryology. Volume 3. NervousSystem and Endocrine Glands. Masson
&Company, Paris. B after Martin JH. 1996.Neuroanatomy: Text and
Atlas. 2nd Ed.McGraw-Hill, New York.)
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Prima Eriawan Putra 1102012212 2
Cranial Nerve I: Olfactory Nerves
Olfactory stimuli are detected by specialized chemoreceptors on
bipolar primary sensory neurons in the olfactory neuroepithelium of
the upper nasal cavities. Axons of these neurons travel via short
olfactory nerves that traverse the cribriform plate of the ethmoid
bone to synapse in the olfactory bulbs. From the olfactory bulbs,
information travels via the olfactory tracts, which run in the
olfactory sulcus between the gyrus rectus and orbital frontal gyri
to reach olfactory processing aeas. Note that although the
olfactory bulbs and tracts are sometimes called CN I, these
structures are actually not nerves, but are part of the central
nervous system.
828 Chapter 18
ecule usually activates several olfactory receptors, enabling a
virtually infinitenumber of different odors to be identified
through combinatorial processing.Olfactory receptor neurons send
unmyelinated axons in the olfactory nervesthrough the cribriform
plate to reach the olfactory bulb (Figure 18.5). The olfac-tory
bulb is part of the central nervous system that lies in a groove
called theolfactory sulcus, between the gyrus rectus and
orbitofrontal gyri (Figure 18.6).In the glomeruli of the olfactory
bulb (see Figure 18.5), olfactory receptor neu-rons synapse onto
mitral cells and tufted cells, both of which have long axonsthat
enter the olfactory tract to reach the olfactory cortex.
Collaterals in the ol-factory tract synapse onto scattered neurons,
forming the anterior olfactory nu-
Mitral cell
Glomerulus
Olfactorynerve
Olfactoryreceptorneuron
Olfactory tract
To contralateral olfactory bulb
To olfactory areas (piriformcortex, periamygdaloid
cortex,olfactory tubercle, amygdala)
Anteriorolfactorynucleus
Tufted cell
Cribriformplate
Olfactorymucosa
Olfactory bulb
FIGURE 18.5 Principal Neurons and Pathways of Olfactory
Nervesand Olfactory Bulbs For simplicity, the periglomerular cells
and gran-ule cells (the major interneurons of the olfactory bulb)
are not shown.
Collateral sulcus
Parahippocampalgyrus
Parahippocampalcortex
Rhinal sulcus
Occipitotemporal(fusiform gyrus)
Orbitofrontalolfactory area
Amygdala(seen throughcortex)
Perirhinal cortex
Lateral olfactorystria
Medial olfactorystria
Entorhinal cortex
Piriform and periamygdaloidcortex (primary olfactory cortex)
Olfactory sulcus
Olfactory bulb
Olfactory tract
Orbital frontal cortex
Anterior perforatedsubstance
Gyrus rectus
Inferior temporal sulcus
FIGURE 18.6 Central Olfactory Structures and Other Components of
the Parahippocampal Gyrus Inferior view.
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Brainstem I: Surface Anatomy and Cranial Nerves 501
tem matures, three motor columns and three sensory columns of
cranial nervenuclei develop that run in an interrupted fashion
through the length of thebrainstem (see Figure 12.4 and Figure
12.5). Each column subserves a differ-ent motor or sensory cranial
nerve function, which can be classified as shownin Table 12.3. The
color codes for each column used in Figures 12.4 and 12.5and in
Table 12.3 will remain constant throughout this chapter. In another
setof terminology described toward the end of this section (and
listed in the fig-ures and tables) each column can also be
described as general vs. special, so-matic vs. visceral, and
afferent vs. efferent. Lets review each of these columnsin more
detail, moving from medial to lateral.
Motor nuclei Sensory nuclei
EdingerWestphal nucleus (GVE: CN III)
Oculomotor nucleus (GSE: CN III)
Trochlear nucleus (GSE: CN IV)
Superior salivatory nucleus (GVE: CN VII)
Abducens nucleus (GSE: CN VI)
Trigeminal motor nucleus (SVE: CN V)
Trigeminal nuclei (GSA: CN V, VII, IX, X):
Mesencephalic nucleus of CN V
Chief sensory nucleus of CN V
Spinal trigeminal nucleus
Dorsal and ventral cochlear nuclei(SSA: CN VIII)
Vestibular nuclei (SSA: CN VIII)
Nucleus solitarius, rostral portion(SVA: CN VII, IX, X)
Nucleus solitarius, caudal portion(GVA: CN IX, X)
Facial nucleus (SVE: CN VII)
Dorsal motor nucleus of CN X (GVE: CN X)
Nucleus ambiguus (SVE: CN IX, X)
Hypoglossal nucleus (GSE: CN XII)
Spinal accessory nucleus (SVE: CN XI)
Branchial motor column = SVE Special somatic sensory column =
SSA
General somatic sensory column = GSA
Visceral sensory column = SVA and GVA
Parasympathetic column = GVE
Somatic motor column = GSE
Inferior salivatory nucleus (GVE: CN IX)
FIGURE 12.5 FunctionalColumns of Brainstem Sensoryand Motor
Cranial Nerve NucleiLongitudinal schematic. GSA,general somatic
afferent; GSE,general somatic efferent; GVA,general visceral
afferent; GVE,general visceral efferent; SSA, spe-cial somatic
afferent; SVA, specialvisceral afferent; SVE, special vis-ceral
efferent.
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Prima Eriawan Putra 1102012212 3
The first neuron of the olfactory pathway is the bipolar ol-
factory cell; the second neurons are the mitral and tufted cells of
the olfactory bulb. The neurites of these cells form the olfactory
tract (2nd neuron), which lies adjacent to and just below the
frontobasal (orbitofrontal) cortex. The ol- factory tract divides
into the lateral and medial olfactory striae in front of the
anterior perforated substance; another portion of it terminates in
the olfactory trigone, which also lies in front of the anterior
perforated substance. The fibers of the lateral stria travel by way
of the limen insulae to the amygdala, semilunar gyrus, and ambient
gyrus (prepyriform area). This is the site of the 3rd neuron, which
projects to the anterior portion of the parahippocampal gyrus
(Brod- mann area 28, containing the cortical projection fields and
association area of the olfactory system). The fibers of the medial
stria terminate on nuclei of the septal area below the genu of the
corpus callosum (subcallosal area) and in front of the anterior
commissure. Fibers emerging from these nuclei project, in turn, to
the opposite hemisphere and to the limbic system. The olfactory
pathway is the only sensory pathway that reaches the cerebral
cortex without going through a relay in the thalamus. Its central
connections are complex and still incompletely known.
Cranial Nerve II: Optic Nerves The optic nerve carries visual
information from the retina to the lateral geniculate nucleus of
the thalamus and to the extrageniculate pathways. The retinal
ganglion cells are actually part of the central nervous system, so
the optic nerves are, strictly speaking, tracts and not nerves.
Nevertheless, by widely accepted convention the portion of the
visual pathway in front of the optic chiasm is called the optic
nerve, and beyond this point it is referred to as the optic tract.
The optic nerves travel from the orbit to the intracranial cavity
via the optic canal
4129
Subcallosal area
Medial olfactorystria
Striae medullaresof the thalamus
Longitudinal striae
Habenulo-interpedun-
cular tract
Habenularnucleus
Inter-peduncular
nucleus
Medialforebrain
bundle
Tegmentalnuclei
Dorsallongitudinal
fasciculus
Reticularformation
Prepiriformarea
Area 28(entorhinalarea)
Uncus withamygdaloid body
Lateralolfactorystria
Olfactory epithelium
Olfactory bulb
bipolar olfactory cells
Olfactory bulb Olfactory tract
Temporal pole
Prepiriformarea
Amygdaloidbody
Diagonalband ofBroca
Uncus
Semilunargyrus
Ambient gyrus
Anteriorperforated substance
Medial olfactorystria
Limen insulae
Lateral olfactorystria
Fig. 4.7 The olfactory nerve and tract and the olfactory
pathway
Fig. 4.8 The olfactory nerve and tract as seen from below
Cranial Nerves
Baehr, Duus' Topical Diagnosis in Neurology 2005 ThiemeAll
rights reserved. Usage subject to terms and conditions of
license.
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Prima Eriawan Putra 1102012212 4
Visual information is transmitted centrally as follows. An
object located in the left visual field gives rise to images on the
nasal half of the left retina and the temporal half of the right
retina. Optic nerve fibers derived from the nasal half of the left
retina cross to the left side in the optic chiasm to join the
fibers from the temporal half of the right retina in the right
optic tract. These fibers then pass to a relay station in the right
lateral geniculate body, and then by way of the right optic
radiation into the right visual cortex. The right visual cortex is
thus re- sponsible for the perception of objects in the left visual
field; in analogous fash- ion, all visual impulses relating to the
right visual field are transmitted through the left optic tract and
radiation into the left visual cortex
Cranial Nerve III, IV, VI (Oculomotor, Trochlear, Abducens
Nerves) These nerves, which are responsible for controlling the
extraocular muscles, will be discussed in detail in Chapter 13.
Briefly, CN VI abducts the eye later- ally in the horizontal
direction; CN IV acts through a trochlea, or pulley-like, structure
in the orbit, to rotate the top of the eye medially and move it
down- ward; and CN III subserves all other eye movements. The
oculomotor (CN III) and trochlear (CN IV) nuclei are located in the
midbrain, and the abducens (CN VI) nucleus is in the pons (see
Figures 12.5, 14.3, and 14.4C). Recall that CN III exits the
brainstem ventrally in the interpeduncular fossa, CN IV exits
dorsally from the inferior tectum, and CN VI exits ventrally at the
pontomedullary junc- tion (see Figure 12.2). CN III, IV, and VI
then traverse the cavernous sinus (see Figure 13.11), and exit the
skull via the superior orbital fissure (see Figure 12.3A,C; Table
12.2) to reach the muscles of the orbit. CN III also carries
parasympathetics to the pupillary constrictor and to the ciliary
muscle of the lens. The preganglionic parasympathetic neurons are
located in the EdingerWest- phal nucleus in the midbrain (see
Figure 12.5). They synapse in the ciliary gan- glion located in the
orbit (Figure 12.6). Postganglionic parasympathetic fibers then
continue to the pupillary constrictor and ciliary muscles.
28 Visual pathways
296
ON
OT
OR
OT
OC
SC
PVC
LGB
ON
OR
Shield
Left eye
Meyers loop
Target cortex ofMeyers loop
Figure 28.10 Pathway from the visual field of the left eye to
the primary visual cortex. T denotes the temporal (outer) half of
the left visual field; N denotes the nasal (inner) half of the left
visual field.
In the left retina and optic nerve (ON), the neural
representation of the image is reversed side to side. It is also
inverted top to bottom. The right retina and optic nerve are
inactive because this eye is shielded.
At the optic chiasm (OC) the axons forming the nasal half of the
left optic nerve cross the midline and form the medial half of the
right optic tract (OT). Those forming the lateral half of the nerve
form the lateral half of the left optic tract. Each set synapses in
the corresponding lateral geniculate body (LGB).
The optic radiations (OR) are fan-like (cf. Figure 28.7), with
the axons carrying the foveal input initially in the middle of the
fan.As they approach the occipital pole, the foveal axons (red) in
both hemispheres move to the back and enter the posterior part
of the primary visual cortex (PVC). Note the striped pattern of
delivery to the cortex on both sides. The blank intervals between
are the same width and contain the axons and cortex responsible for
the visual field of the right eye. SC, superior colliculus.
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Prima Eriawan Putra 1102012212 5
The nuclei of the oculomotor and trochlear nerves lie in the
midbrain teg- mentum, while the nucleus of the abducens nerve lies
in the portion of the pontine tegmentum underlying the floor of the
fourth ventricle.
Cranial Nerve V: Trigeminal Nerve
The name trigeminal was given to this nerve because it has three
major branches: the ophthalmic division (V1), maxillary division
(V2), and mandibular division (V3) (Figure 12.7). The trigeminal
nerve provides sensory innervation to the face and should be
distinguished from the facial nerve, which controls the muscles of
facial expression
506 Chapter 12
KEY CLINICAL CONCEPT
12.1 ANOSMIA (CN I)Patients with unilateral anosmia, or
olfactory loss, are rarely aware of thedeficit because olfaction in
the contralateral nostril can compensate. There-fore, when testing
olfaction, the examiner must test each nostril separately(see
neuroexam.com Video 24). Patients are often aware of bilateral
anosmiaand may complain of decreased taste because of the important
contributionof olfaction to the perception of flavor.
Loss of the sense of smell can be caused by head trauma, which
damagesthe olfactory nerves as they penetrate the cribriform plate
of the ethmoid. Inaddition, viral infections can damage the
olfactory neuroepithelium. Ob-struction of the nasal passages can
impair olfaction. Bilateral anosmia is alsocommon in patients with
certain neurodegenerative conditions such asParkinsons disease and
Alzheimers disease.
Intracranial lesions that occur at the base of the frontal lobes
near the olfac-tory sulci can interfere with olfaction. Possible
lesions in this location includemeningioma, metastases, basal
meningitis or less commonly, sarcoidosis, a gran-ulomatous
inflammatory disorder that occasionally involves the nervous
sys-tem, often causing cranial neuropathies. As we will discuss in
KCC 19.11, frontallobe deficits are often difficult to detect
clinically, especially with small lesions.Therefore, lesions at the
base of the frontal lobes can sometimes grow to a verylarge size,
causing little obvious dysfunction other than anosmia. Large
lesionsof the olfactory sulcus region (typically meningiomas) can
also sometimes pro-duce a condition called Foster Kennedy syndrome,
in which there is anosmia to-gether with optic atrophy in one eye
(caused by ipsilateral tumor compression)and papilledema in the
other eye (caused by elevated intracranial pressure).!
CN II: Optic Nerve
FUNCTIONAL CATEGORY FUNCTION
Special somatic sensory Vision
As we discussed in Chapter 11, the optic nerve carries visual
informationfrom the retina to the lateral geniculate nucleus of the
thalamus and to theextrageniculate pathways (see Figures 11.6,
11.15, and 12.2A). The retinalganglion cells are actually part of
the central nervous system, so the opticnerves are, strictly
speaking, tracts and not nerves. Nevertheless, by widelyaccepted
convention the portion of the visual pathway in front of the
opticchiasm is called the optic nerve, and beyond this point it is
referred to as theoptic tract. The optic nerves travel from the
orbit to the intracranial cavityvia the optic canal (see Figure
12.3A,C; Table 12.2). The anatomy and disor-ders of visual pathways
are discussed in greater detail in Chapter 11.
CN III, IV, and VI: Oculomotor, Trochlear, and Abducens
Nerves
NERVE FUNCTIONALCATEGORY FUNCTION
CN III Somatic Levator palpebrae superior and all
extraocularmotor muscles, except for superior oblique and
lateral
rectusParasym- Parasympathetics to pupil constrictor and
ciliary
pathetic muscles for near visionCN IV Somatic motor Superior
oblique muscle; causes depression motor
and intorsion of the eyeCN VI Somatic motor Lateral rectus
muscle; causes abduction of the eye
Olfaction
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Brainstem I: Surface Anatomy and Cranial Nerves 507
These nerves, which are responsible for controlling the
extraocular muscles,will be discussed in detail in Chapter 13.
Briefly, CN VI abducts the eye later-ally in the horizontal
direction; CN IV acts through a trochlea, or pulley-like,structure
in the orbit, to rotate the top of the eye medially and move it
down-ward; and CN III subserves all other eye movements. The
oculomotor (CN III)and trochlear (CN IV) nuclei are located in the
midbrain, and the abducens (CNVI) nucleus is in the pons (see
Figures 12.5, 14.3, and 14.4C). Recall that CN IIIexits the
brainstem ventrally in the interpeduncular fossa, CN IV exits
dorsallyfrom the inferior tectum, and CN VI exits ventrally at the
pontomedullary junc-tion (see Figure 12.2). CN III, IV, and VI then
traverse the cavernous sinus (seeFigure 13.11), and exit the skull
via the superior orbital fissure (see Figure12.3A,C; Table 12.2) to
reach the muscles of the orbit. CN III also carriesparasympathetics
to the pupillary constrictor and to the ciliary muscle of the
lens.The preganglionic parasympathetic neurons are located in the
EdingerWest-phal nucleus in the midbrain (see Figure 12.5). They
synapse in the ciliary gan-glion located in the orbit (Figure
12.6). Postganglionic parasympathetic fibersthen continue to the
pupillary constrictor and ciliary muscles.
Other cranial nerve parasympathetics are also summarized in
Figure 12.6.
Pupillaryconstrictor
Otic ganglion
Sphenopalatineganglion
Ciliaryganglion
CN III
CN IX
CN X
EdingerWestphalnucleus
Superiorsalivatory nucleus
Greater petrosalnerve
Parotid gland
Submandibular ganglion
Submandibular gland
Sublingual gland
Lesser petrosalnerve
Chorda tympani
Inferiorsalivatory nucleus
CN IX
CN X(to parasympatheticganglia of thoracoabdominalviscera)
Dorsal motornucleus of CN X
Lacrimal glands
CN VII
Ciliary muscleof lens
FIGURE 12.6 Summary of CranialNerve Parasympathetic Pathways
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508 Chapter 12
CN V: Trigeminal Nerve
FUNCTIONAL CATEGORY FUNCTION
General somatic sensory Sensations of touch, pain, temperature,
joint position,and vibration for the face, mouth, anterior
two-thirds of tongue, nasal sinuses, and meninges
Branchial motor Muscles of mastication and tensor tympani
muscle
The name trigeminal was given to this nerve because it has three
majorbranches: the ophthalmic division (V1), maxillary division
(V2), and mandibulardivision (V3) (Figure 12.7). The trigeminal
nerve provides sensory innervationto the face and should be
distinguished from the facial nerve, which controlsthe muscles of
facial expression. The trigeminal nerve also has a small
Trigeminal ganglionin Meckels cave
Trigeminalsensory nuclei
Trigeminalmotor nucleus
Temporalismuscle
V1
V2
C2
C3
Superior orbital fissure
(A)
(ophthalmic)
(maxillary)
MassetermuscleMassetermuscle
Tensor tympanimuscle
Foramenrotundum
Foramenovale
Anterior belly ofdigastric muscle
Motor root of CN V
(B)
V1
V2
V3
C3
CN VII, IX, X
C2
V3(mandibular)
FIGURE 12.7 Trigeminal Nerve (CN V)(A) Summary of trigeminal
sensory andmotor pathways. (B) General somaticsensory innervation
to the face is pro-vided by the trigeminal nerve as well asby CN
VII, IX, and X. These inputs alltravel to the trigeminal nuclei
(see Figure12.8). The occiput and neck, on the otherhand, are
supplied by cervical nerveroots C2 and C3. Sensory innervation
ofthe supratentorial dura is provided byCN V (not shown).
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Prima Eriawan Putra 1102012212 6
Trigeminal somatosensory functions
The trigeminal nuclei (Figure 12.8 and Figure 12.9) receive
general somatic sen- sory inputs from CN V and other cranial nerves
(see Table 12.3). The main in- puts are carried by CN V and, as we
just mentioned, provide sensation for the face, mouth, anterior
two-thirds of the tongue, nasal sinuses, and supratentor- ial dura.
Smaller inputs from CN VII, IX, and X provide sensation for part of
the external ear (see Figure 12.7B; Table 12.4). In addition, CN IX
provides sen- sation to the middle ear, posterior one-third of the
tongue, and pharynx. CN X additionally provides sensation for the
infratentorial dura and probably also contributes to pharyngeal
sensation (see Table 12.4).
Trigeminal motor functions
The trigeminal motor nucleus mediates the branchial motor
functions of the trigeminal nerve (see Figure 12.7). This nucleus
is located in the upper-to- mid pons (see Figures 12.5 and 14.4B),
near the level where the trigeminal nerve exits the brainstem. The
branchial motor root of the trigeminal nerve runs inferomedial to
the trigeminal ganglion along the floor of Meckels cave and then
joins V3 to exit via the foramen ovale (see Figure 12.3A). It then
supplies the muscles of mastication (see neuroexam.com Video 38),
including the masseter, temporalis, and medial and lateral
pterygoid muscles, as well as several smaller muscles, such as the
tensor tympani, tensor veli palatini, mylohyoid, and anterior belly
of the digastric. The upper motor neuron con- trol reaching the
trigeminal motor nucleus is predominantly bilateral, so uni-
lateral lesions in the motor cortex or corticobulbar tract usually
cause no deficit in jaw movement. Bilateral upper motor neuron
lesions, however, can cause hyperreflexia manifested in a brisk jaw
jerk reflex
508 Chapter 12
CN V: Trigeminal Nerve
FUNCTIONAL CATEGORY FUNCTION
General somatic sensory Sensations of touch, pain, temperature,
joint position,and vibration for the face, mouth, anterior
two-thirds of tongue, nasal sinuses, and meninges
Branchial motor Muscles of mastication and tensor tympani
muscle
The name trigeminal was given to this nerve because it has three
majorbranches: the ophthalmic division (V1), maxillary division
(V2), and mandibulardivision (V3) (Figure 12.7). The trigeminal
nerve provides sensory innervationto the face and should be
distinguished from the facial nerve, which controlsthe muscles of
facial expression. The trigeminal nerve also has a small
Trigeminal ganglionin Meckels cave
Trigeminalsensory nuclei
Trigeminalmotor nucleus
Temporalismuscle
V1
V2
C2
C3
Superior orbital fissure
(A)
(ophthalmic)
(maxillary)
MassetermuscleMassetermuscle
Tensor tympanimuscle
Foramenrotundum
Foramenovale
Anterior belly ofdigastric muscle
Motor root of CN V
(B)
V1
V2
V3
C3
CN VII, IX, X
C2
V3(mandibular)
FIGURE 12.7 Trigeminal Nerve (CN V)(A) Summary of trigeminal
sensory andmotor pathways. (B) General somaticsensory innervation
to the face is pro-vided by the trigeminal nerve as well asby CN
VII, IX, and X. These inputs alltravel to the trigeminal nuclei
(see Figure12.8). The occiput and neck, on the otherhand, are
supplied by cervical nerveroots C2 and C3. Sensory innervation
ofthe supratentorial dura is provided byCN V (not shown).
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Prima Eriawan Putra 1102012212 7
Cranial Nerve VII: Facial Nerves
The main function of the facial nerve is to control the muscles
of facial ex- pression (see neuroexam.com Video 40); however, it
has several other impor- tant functions as well. The main nerve
trunk carries the branchial motor fibers controlling facial
expression, while a smaller branch called the nervus intermedius
carries fibers for the parasympathetic (tears and salivation), vis-
ceral sensory (taste), and general somatosensory functions (Figure
12.10; see also Figures 12.6 and 12.14).
The facial nucleus is located in the branchial motor column,
more caudally in the pons than the trigeminal motor nucleus (see
Figure 12.5; see also Figure 14.4B,C). The fascicles of the facial
nerve loop dorsally around the abducens nucleus, forming the facial
colliculus on the floor of the fourth ventricle
Brainstem I: Surface Anatomy and Cranial Nerves 513
CN VII: Facial Nerve
FUNCTIONAL CATEGORY FUNCTION
Branchial motor Muscles of facial expression, stapedius muscle,
and part of digastric muscle
Parasympathetic Parasympathetics to lacrimal glands, and to
sublingual, submandibular, and all other salivary glands except
parotid
Visceral sensory (special) Taste from anterior two-thirds of
tongueGeneral somatic sensory Sensation from a small region near
the external
auditory meatus
The main function of the facial nerve is to control the muscles
of facial ex-pression (see neuroexam.com Video 40); however, it has
several other impor-tant functions as well. The main nerve trunk
carries the branchial motorfibers controlling facial expression,
while a smaller branch called the nervusintermedius carries fibers
for the parasympathetic (tears and salivation), vis-ceral sensory
(taste), and general somatosensory functions (Figure 12.10; seealso
Figures 12.6 and 12.14).
The facial nucleus is located in the branchial motor column,
more caudally inthe pons than the trigeminal motor nucleus (see
Figure 12.5; see also Figure14.4B,C). The fascicles of the facial
nerve loop dorsally around the abducensnucleus, forming the facial
colliculus on the floor of the fourth ventricle (see Fig-ure 12.2B
and Figure 12.11). The nerve then exits the brainstem
ventrolaterallyat the pontomedullary junction (see Figure 12.2A,C).
Upper motor neuron con-trol of the facial nucleus is discussed in
KCC 12.3 (see Figure 12.13). Briefly, le-sions in the cortex or
corticobulbar tracts cause contralateral face weakness thatspares
the forehead, while lesions of the facial nucleus, nerve fascicles
in thebrainstem, or peripheral nerve cause ipsilateral weakness of
the entire face.
The facial nerve exits the brainstem ventrolaterally at the
pontomedullaryjunction, lateral to CN VI in a region called the
cerebellopontine angle (see Fig-ure 12.2A,C). It then traverses the
subarachnoid space and enters the internalauditory meatus (see
Figure 12.3A; see also Figure 4.13C) to travel in the audi-tory
canal of the petrous temporal bone together with the
vestibulocochlearnerve (see Figure 12.14). At the genu of the
facial nerve, the nerve takes a turnposteriorly and inferiorly in
the temporal bone to run in the facial canal, justmedial to the
middle ear (see Figures 12.10 and 12.14). The geniculate
ganglionlies in the genu and contains primary sensory neurons for
taste sensation inthe anterior two-thirds of the tongue, and for
general somatic sensation in aregion near the external auditory
meatus (see Table 12.5; Figure 12.7B). Themain portion of the
facial nerve exits the skull at the stylomastoid foramen
(seeFigures 12.3B and 12.10). It then passes through the parotid
gland and dividesinto five major branchial motor branches to
control the muscles of facial ex-pression: the temporal, zygomatic,
buccal, mandibular, and cervical branches(see Figure 12.10). Other
smaller branchial motor branches innervate thestapedius (see
Figures 12.10 and 12.15), occipitalis, posterior belly of the
digas-tric, and stylohyoid muscles. The cranial nerves controlling
the middle earmuscles can be recalled by the mnemonic Trigeminal
for Tensor Tympani andSeventh for Stapedius. Both the tensor
tympani and the stapedius dampenmovements of the middle ear
ossicles (see the section on CN VIII later in thischapter),
providing feedback modulation of acoustic signal intensity.
The preganglionic parasympathetic fibers of the facial nerve
originate in thesuperior salivatory nucleus (see Figure 12.10) and
are carried by two smallbranches off the main trunk of the facial
nerve. The greater petrosal nerve takesoff at the genu of the
facial nerve (see Figure 12.14) to reach the sphenopalatine
Facial muscles
MNEMONIC
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514 Chapter 12
(pterygopalatine) ganglion, where postganglionic parasympathetic
cells projectto the lacrimal glands and nasal mucosa (see Figure
12.10). The chorda tympanileaves the facial nerve just before the
stylomastoid foramen and travels backupward to traverse the middle
ear cavity before exiting the skull at thepetrotympanic fissure
(see Figures 12.3B and 12.10), just medial and posterior tothe
temporomandibular joint. The chorda tympani then joins the lingual
nerve(a branch of CN V3) to reach the submandibular ganglion (also
called the sub-maxillary ganglion), where postganglionic
parasympathetics arise to supplythe submandibular (submaxillary)
and sublingual salivary glands as well asother minor salivary
glands aside from the parotid. Note that the majority(~70%) of
saliva production arises from the submandibular salivary
glands.
The lingual nerve and chorda tympani also carry special visceral
sensoryfibers mediating taste sensation (see neuroexam.com Video
41) for the ante-rior two-thirds of the tongue (see Figure 12.10).
The primary sensory tastefibers have their cell bodies in the
geniculate ganglion (Figure 12.12; see alsoFigure 12.14 and Table
12.5). These cells synapse onto secondary sensory
Sphenopalatineganglion
Sublingualgland
Submandibulargland
Submandibularganglion
Lacrimalglands
Taste, anterior2/3 of tongue
Greater petrosalnerve
Stapediusmuscle
GeniculateganglionGeniculateganglion
Internalacousticmeatus
Nervusintermedius
Superior salivatorynucleus
Mandibular branchMandibular branch
Cervical branchCervical branch
Buccal branchBuccal branch
Zygomatic branchZygomatic branch
TemporalbranchTemporalbranch
Chorda tympani
Facial nucleus
Nucleus solitarius
Spinal trigeminalnucleus
Stylomastoidforamen
Petrotympanicfissure
Lingual nerve
Posterior auricularbranch
FIGURE 12.10 Facial Nerve(CN VII) Summary of facialnerve sensory
and motorpathways.
Taste
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Prima Eriawan Putra 1102012212 8
Facial Nerve Lesion
Cranial Nerve VIII: Vestibulocochlear Nerves This nerve carries
the special somatic sensory functions of hearing and vestibu- lar
sense from the structures of the inner ear. The vestibulocochlear
nerve exits the brainstem at the pontomedullary junction just
lateral to the facial nerve, in a region called the
cerebellopontine angle
Auditory informa- tion throughout these pathways is
tonotopically organized. Primary sensory neurons in the spiral
ganglion send their axons in the cochlear division of CN VIII to
reach the dorsal and ventral cochlear nuclei, which are wrapped
around the lateral aspect of the inferior cerebellar peduncle at
the pontomedullary junc- tion (see Figures 12.16 and 12.17C). The
hearing pathways then ascend through the brainstem bilaterally
through a series of relays to reach the inferior colliculi, medial
geniculate nuclei, and, ultimately, the auditory cortex. Because
auditory information from each ear ascends bilaterally in the
brainstem, with decussations oc- curring at multiple levels,
unilateral hearing loss is not seen in le- sions in the central
nervous system proximal to the cochlear nuclei.
Brainstem I: Surface Anatomy and Cranial Nerves 517
tion, patients may suffer from dry eye, resulting from decreased
lacrimationwith parasympathetic involvement (see Figure 12.10).
Neurologic examinationis notable for unilateral lower motor
neurontype facial weakness, sometimesassociated with loss of taste
on the ipsilateral tongue (test with mustard orsugar applied with a
cotton swab; see neuroexam.com Video 41). The remain-der of the
exam should be normal in Bells palsy. The presence of hand
weak-ness, sensory loss, dysarthria, or aphasia suggests an upper
motor neuron le-sion. In clinically typical cases, imaging studies
are usually normal, however,most practitioners will order an MRI
scan to exclude a structural lesion andblood studies including a
blood count, glucose, and Lyme titer.
Treatment of Bells palsy has been controversial, but recent
evidence sug-gests that a 10-day course of oral steroids started
soon after onset improveschances for full recovery. The possible
role of antiviral agents in treating Bellspalsy remains uncertain.
Incomplete eye closure and decreased tearing cancause corneal
ulcerations. Therefore, patients should be given lubricating
eye-drops and instructions to tape the eye shut at night. About 80%
of patients re-cover fully from Bells palsy within 3 weeks,
although some are left with vari-able degrees of residual weakness.
During recovery, regenerating facial nerve
R L
R L
Region of weakness
R L
Contralateralprimary motorcortex
Ipsilateralprimary motorcortex
Lesion A
Lower motorneurontypefacial weakness
Lesion B
Pons
CN VII
Lower motorneuron
Upper motorneuron
A
B
Upper motorneurontypefacial weakness
FIGURE 12.13 Upper Motor Neuron versus Lower MotorNeuron Facial
Weakness With an upper motor neuron le-sion (Lesion A), the upper
face is spared because both hemi-spheres contribute to movement of
the upper face, and the unaffected hemisphere can compensate. With
a lower motorneuron lesion (Lesion B), the entire face is affected
on one side.
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Prima Eriawan Putra 1102012212 9
Nervus Cranialis IX: Glossopharyngeal Nerves
The branchial motor portion of the nerve supplies one muscle,
the sty- lopharyngeus (Figure 12.20), which elevates the pharynx
during talking and swallowing and contributes (with CN X) to the
gag reflex. The branchial motor component of CN IX arises from the
nucleus ambiguus in the medulla. The general visceral sensory
portion of the glossopharyngeal nerve con- veys inputs from
baroreceptors and chemoreceptors in the carotid body. These aerents
travel to the caudal nucleus solitarius of the medulla, also known
as the cardiorespiratory nucleus (see Figure 12.20).
Glossopharyngeal special visceral sensation mediates taste for the
posterior one-third of the tongue, which reaches the rostral
nucleus solitarius, or gustatory nucleus (see Figures 12.5, 12.12,
and 12.20). General somatic sensory functions of CN IX are the
sensation of touch, pain, and temperature from the posterior one-
third of the tongue, pharynx, middle ear, and a region near the
external au- ditory meatus
Brainstem I: Surface Anatomy and Cranial Nerves 521
nerve (see Figures 12.14 and 12.15). The hair cells of the
cochlea, togetherwith their supporting cells, are called the organ
of Corti. There is a tonotopicrepresentation determined by
structural width and stiffness along the lengthof the organ of
Corti such that higher-frequency sounds activate hair cellsnear the
oval window, while lower-frequency sounds activate hair cells
nearthe apex of the cochlea (see Figure 12.15).
Lets follow the pathways for hearing centrally, from the
cochlear nuclei tothe primary auditory cortex (Figure 12.16 and
Figure 12.17). Auditory informa-tion throughout these pathways is
tonotopically organized. Primary sensoryneurons in the spiral
ganglion send their axons in the cochlear division of CNVIII to
reach the dorsal and ventral cochlear nuclei, which are wrapped
aroundthe lateral aspect of the inferior cerebellar peduncle at the
pontomedullary junc-tion (see Figures 12.16 and 12.17C). The
hearing pathways then ascend through
Superior temporal gyrus
Heschls transversegyri (area 41)
Insula
CN VIII
Ventral cochlearnucleus
Superior olivarynuclear complex
Trapezoid body
Inferior cerebellarpeduncles
Lateral lemniscus
Brachium ofinferior colliculus
Medial geniculatenucleus (MGN)
Dorsalcochlearnucleus
Spiralganglioncells
A
B
C
Inferior colliculus Cochlea
FIGURE 12.16 Central Auditory Pathways Mainnuclei and pathways
are shown from the cochlearnerve to the auditory cortex. Levels of
sections for Figure 12.17 are indicated.
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530 Chapter 12
CN IX: Glossopharyngeal Nerve
FUNCTIONAL CATEGORY FUNCTION
Branchial motor Stylopharyngeus muscleParasympathetic
Parasympathetics to parotid glandGeneral somatic sensory Sensation
from middle ear, region near the external
auditory meatus, pharynx, and posterior one-third of tongue
Visceral sensory (special) Taste from posterior one-third of
tongueVisceral sensory (general) Chemoreceptors and baroreceptors
of carotid body
The glossopharyngeal nerve was named for its role in sensation
for the pos-terior tongue and pharynx; however, it has additional
functions as well. Itexits the brainstem as several rootlets along
the upper ventrolateral medulla,just below the pontomedullary
junction and just below CN VIII, between theinferior olive and the
inferior cerebellar peduncle (see Figure 12.2A,C). Thenerve
traverses the subarachnoid space to exit the skull via the jugular
fora-men (see Figure 12.3A,B; Table 12.2).
The branchial motor portion of the nerve supplies one muscle,
the sty-lopharyngeus (Figure 12.20), which elevates the pharynx
during talking andswallowing and contributes (with CN X) to the gag
reflex. There is evidencethat the glossopharyngeal nerve may
provide some innervation to otherpharyngeal muscles; however, most
pharyngeal muscles are supplied pri-marily by the vagus (see the
next section). The branchial motor componentof CN IX arises from
the nucleus ambiguus in the medulla (see Figure 12.20).Ambiguus is
Latin for ambiguous, and this name can be rememberedbecause the
nucleus is difficult to discern on conventional stained
sections(see Figure 14.5A,B). Parasympathetic preganglionic fibers
in the glossopha-ryngeal nerve arise from the inferior salivatory
nucleus in the pons (see Fig-ure 12.20). These parasympathetic
fibers leave the glossopharyngeal nervevia the tympanic nerve and
then join the lesser petrosal nerve to synapse inthe otic ganglion,
providing postganglionic parasympathetics to the parotidgland.
The general visceral sensory portion of the glossopharyngeal
nerve con-veys inputs from baroreceptors and chemoreceptors in the
carotid body.These afferents travel to the caudal nucleus
solitarius of the medulla, alsoknown as the cardiorespiratory
nucleus (see Figure 12.20). Glossopharyngealspecial visceral
sensation mediates taste for the posterior one-third of thetongue,
which reaches the rostral nucleus solitarius, or gustatory nucleus
(seeFigures 12.5, 12.12, and 12.20). General somatic sensory
functions of CN IXare the sensation of touch, pain, and temperature
from the posterior one-third of the tongue, pharynx, middle ear,
and a region near the external au-ditory meatus (see Figure 12.7B).
The glossopharyngeal nerve has two sen-sory ganglia located within
or just below the jugular foramen (see Table12.5). General and
special visceral sensation are conveyed by primary sen-sory neurons
in the inferior (petrosal) glossopharyngeal ganglion. General
so-matic sensation is conveyed by primary sensory neurons in both
the inferiorand superior (jugular) glossopharyngeal ganglion.
MNEMONIC
REVIEW EXERCISEWhich cranial nerve contributes tothe greater
petrosal nerve? Whichcontributes to the lesser petrosalnerve? (See
Figure 12.6.)
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Prima Eriawan Putra 1102012212 10
Nervus Cranialis X: Vagus Nerves
The largest part of the vagus nerve provides parasympathetic
innervation to the heart, lungs, and digestive tract, extending
nearly to the splenic flexure (see Figures 6.13 and 12.21).
Parasympathetic preganglionic fibers arise from the dorsal motor
nucleus of CN X, which runs from the rostral to the caudal medulla
(see Figure 14.5A,B). The dorsal motor nucleus of CN X forms the
vagal trigone on the floor of the fourth ventricle, just lateral to
the hypoglos- sal trigone, near the obex (see Figure 12.2B).
The branchial motor component of the vagus (Figure 12.21)
controls nearly all pharyngeal and upper esophageal muscles
(swallowing and gag reflex) and the muscles of the larynx (voice
box). The nucleus ambiguus supplies branchial motor fibers that
travel in the vagus nerve to the muscles of the palate, pharynx,
upper esophagus, and larynx, and in the glossopharyngeal nerve (CN
IX) to the stylopharyngeus (see Figure 12.20).
A branch of the vagus called the recurrent laryngeal nerve (see
Figure 12.21) loops back upward from the thoracic cavity to control
all intrinsic la- ryngeal muscles except for the cricothyroid,
which is innervated by another branch of the vagus, the superior
laryngeal nerve. The fibers in the recurrent laryngeal nerve arise
from the caudal portion of the nucleus ambiguus. After they exit
the brainstem, these fibers travel briefly with CN XI before
joining CN X (see the next section).
General somatic sensory fibers of the vagus (see Figure 12.21)
supply the pharynx, larynx, meninges of the posterior fossa, and a
small region near the external auditory meatus (see Figure 12.7B).
Note that below the larynx and pharynx, conscious (general somatic)
sensation from the viscera is carried by spinal, not cranial,
nerves. However, unconscious, general visceral
Brainstem I: Surface Anatomy and Cranial Nerves 531
Oticganglion
Carotid sinus
Carotid body
Parotid gland
Inferior salivatorynucleus
Superiorglosso-pharyngealganglion
Rostralnucleus solitarius(gustatory nucleus)
Caudal nucleussolitarius(cardiorespiratorynucleus)
Lesserpetrosalnerve
Inferiorglosso-pharyngealganglion Spinal trigeminal
nucleus
Nucleus ambiguus
Sensation from middleear and external ear
Sensation frompharynx and posteriorone-third of tongue
Stylopharyngeus muscle
Jugular foramen
Taste, posteriorone-third of tongue
CN IX
CN IX
FIGURE 12.20 Glossopharyngeal Nerve (CN IX)Summary of
glossopharyngeal nerve sensory andmotor pathways.
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532 Chapter 12 CN X: Vagus Nerve
FUNCTIONAL CATEGORY FUNCTION
Branchial motor Pharyngeal muscles (swallowing) and laryngeal
muscles (voice box)
Parasympathetic Parasympathetics to heart, lungs, and digestive
tractdown to the splenic flexure
General somatic sensory Sensation from pharynx, meninges, and a
small region near the external auditory meatus
Visceral sensory (special) Taste from epiglottis and
pharynxVisceral sensory (general) Chemoreceptors and baroreceptors
of the aortic arch
The vagus nerve derives its name from the wandering course it
takes in pro-viding parasympathetic innervation to organs
throughout the body(vagus means wandering in Latin). Other
important functions are alsoserved by the vagus, as we will discuss
here. The vagus nerve exits the ven-trolateral medulla as several
rootlets just below CN IX, between the inferiorolive and the
inferior cerebellar peduncle (see Figure 12.2A,C). It crosses
thesubarachnoid space and then leaves the cranial cavity via the
jugular fora-men (see Figures 12.3A,B and 12.21).
The largest part of the vagus nerve provides parasympathetic
innervationto the heart, lungs, and digestive tract, extending
nearly to the splenic flexure(see Figures 6.13 and 12.21).
Parasympathetic preganglionic fibers arise fromthe dorsal motor
nucleus of CN X, which runs from the rostral to the caudalmedulla
(see Figure 14.5A,B). The dorsal motor nucleus of CN X forms
thevagal trigone on the floor of the fourth ventricle, just lateral
to the hypoglos-sal trigone, near the obex (see Figure 12.2B).
Postganglionic parasympatheticneurons innervated by the vagus are
found in terminal ganglia located withinor near the effector
organs. Recall that parasympathetics to the gastrointesti-nal tract
beyond the splenic flexureand to the urogenital systemare pro-vided
by parasympathetic nuclei in the sacral spinal cord (see Figure
6.13).
The branchial motor component of the vagus (Figure 12.21)
controls nearlyall pharyngeal and upper esophageal muscles
(swallowing and gag reflex)and the muscles of the larynx (voice
box). The nucleus ambiguus suppliesbranchial motor fibers that
travel in the vagus nerve to the muscles of thepalate, pharynx,
upper esophagus, and larynx, and in the glossopharyngealnerve (CN
IX) to the stylopharyngeus (see Figure 12.20).
A branch of the vagus called the recurrent laryngeal nerve (see
Figure12.21) loops back upward from the thoracic cavity to control
all intrinsic la-ryngeal muscles except for the cricothyroid, which
is innervated by anotherbranch of the vagus, the superior laryngeal
nerve. The fibers in the recurrentlaryngeal nerve arise from the
caudal portion of the nucleus ambiguus. Afterthey exit the
brainstem, these fibers travel briefly with CN XI before joiningCN
X (see the next section). Some texts consider these caudal fibers
of thenucleus ambiguus to be part of CN XI and refer to the caudal
nucleus am-biguus as the cranial nucleus of CN XI. However, we
include these fiberswith CN X because they spend the majority of
their course traveling withCN X, not CN XI. Upper motor neuron
innervation to the nucleus ambiguuscontrolling the voice and
voluntary swallowing is from bilateral motor cor-tex (see Figure
6.2), except for the palate, which receives unilateral innerva-tion
from the contralateral cortex (for example, see Case 6.5).
General somatic sensory fibers of the vagus (see Figure 12.21)
supply thepharynx, larynx, meninges of the posterior fossa, and a
small region near theexternal auditory meatus (see Figure 12.7B).
Note that below the larynx andpharynx, conscious (general somatic)
sensation from the viscera is carried byspinal, not cranial,
nerves. However, unconscious, general visceral sensationfrom
chemoreceptors and baroreceptors of the aortic arch,
cardiorespiratorysystem, and digestive tract is carried to the
brainstem by the vagus nerve.
REVIEW EXERCISEList the branchial motor, parasympa-thetic,
general somatic sensory, andvisceral sensory functions of CN IXand
CN X. Name the nucleus sub-serving each function. (See Figures12.20
and 12.21.)
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Prima Eriawan Putra 1102012212 11
sensation from chemoreceptors and baroreceptors of the aortic
arch, cardiorespiratory system, and digestive tract is carried to
the brainstem by the vagus nerve.
Cranial Nerve XI: Accessory Nerves The spinal accessory nucleus
(also known as the accessory spinal nu- cleus) protrudes laterally
between the dorsal and ventral horns of the spinal cord central
gray matter (see Figure 14.5D), providing branchial motor* fibers
to this nerve. CN XI then exits the cranium again via the jugular
foramen to supply the sternomastoid and upper portions of the
trapezius muscle. The sternomastoid muscle turns the head toward
the opposite side, and the trapezius is involved in elevating the
shoulder. The lower portions of the trapez- ius are usually
supplied mainly by cervical nerve roots C3 and C4.
Cranial Nerve XII: Hypoglossal Nerves
The hypoglossal nerve exits the ventral medulla as multiple
rootlets be- tween the pyramid and inferior olivary nucleus (see
Figure 12.2A,C). This nerve exits through its own foramen, the
hypoglossal foramen (see Figure 12.3A,B), and provides somatic
motor innervation to all intrinsic and extrin- sic tongue muscles
except for the palatoglossus, which is supplied by CN X (see
neuroexam.com Video 47). The hypoglossal nucleus is located near
the midline on the floor of the fourth ventricle in the medulla
(see Figure 14.5A,B), forming the hypoglossal trigone, just medial
to the dorsal nucleus of CN X (see Figures 12.2B, 12.4B, and
12.5).
1.2. Jaras Motorik
Functional mapping and lesion studies have demonstrated that the
primary motor and somatosensory cortices are somatotopically
organized (Figure 6.2). That is, adjacent regions on the cortex
correspond to adjacent areas on the body surface. The cortical maps
are classically depicted by a motor homunculus and a sensory
homunculus (homunculus means little man in Latin)
Many of these general visceral afferents reach the caudal
nucleus solitarius(cardiorespiratory nucleus; see Figures 12.5 and
14.5B). The vagus nerve alsocontains a small number of special
visceral sensory fibers that carry taste sen-sation from the
epiglottis and posterior pharynx to the rostral nucleus solitar-ius
(gustatory nucleus) (see Figures 12.5 and 14.5A).
The primary sensory neuron cell bodies for CN X general and
special vis-ceral sensation are located in the inferior (nodose)
vagal ganglion (Table 12.5),located just below the jugular foramen.
Cell bodies for general somatic sen-sation are located in both the
inferior vagal ganglion and the superior (jugu-lar) vagal ganglion,
which lies within or just below the jugular foramen.
Jugular foramen
Aortic archreceptors
To thoracoabdominalviscera (vagus nerve)
Recurrentlaryngealnerve
Superiorlaryngealnerve
Cricothyroidmuscle
Taste, epiglottis
Pharyngeal nerve(sensory andmotor plexus)
Soft palate
Sensation,meninges ofposterior fossa
Spinal trigeminalnucleus
Nucleus solitarius
Dorsal motornucleus of CN X
Nucleus ambiguus
Superior vagalganglion
Inferior vagalganglion
Sensation,external ear
CN X
FIGURE 12.21 Vagus (CN X)Summary of vagal sensory andmotor
pathways.
Brainstem I: Surface Anatomy and Cranial Nerves 533
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Prima Eriawan Putra 1102012212 12
Corticospinal Tract and Other Motor Pathways 225
Premotor cortex
Supplementarymotor area
Primary motorcortex
Central sulcus Primarysomatosensorycortex
Parietalassociationcortex
Secondarysomatosensoryarea (in parietaloperculum)
Supplementarymotor area
Primary motorcortex
Primarysomatosensorycortex
Parietalassociationcortex
(A)
(B)
66
65, 7
4
4 3, 1, 2
3,1,2
5, 7
FIGURE 6.1 Motor and Somatosen-sory Cortical Areas(A) Lateral
view showing primary andassociation cortical areas of the
sensoryand motor cortex and reciprocal connec-tions between these
regions. Numbersindicate corresponding Brodmann areas(see Figure
2.15). (B) Medial view.
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226 Chapter 6
somatotopic organization along their entire length, which can be
traced fromone level to the next in the nervous system.
A useful generalization for remembering the somatotopic
representationsis: The arms are medial to the legs with two
exceptions: the primary sensorimotorcortices and the posterior
columns (see, for example, Figures 6.2, 6.10, and 7.3).
Basic Anatomy of the Spinal CordThe spinal cord contains a
butterfly-shaped central gray matter surrounded byascending and
descending white matter columns, or funiculi (Figure 6.3A).
Sen-sory neurons in the dorsal root ganglia have axons that
bifurcate. One branchconveys sensory information from the
periphery, and the other carries thisinformation through the dorsal
nerve root filaments into the dorsal aspect ofthe spinal cord. The
central gray matter has a dorsal (posterior) horn that is in-volved
mainly in sensory processing, an intermediate zone that contains
in-terneurons and certain specialized nuclei (Table 6.2), and a
ventral (anterior)horn that contains motor neurons. Motor neurons
send their axons out of thespinal cord via the ventral nerve root
filaments. The spinal gray matter can alsobe divided into nuclei
or, using a different nomenclature, into laminae namedby Bror Rexed
(Figure 6.3B; see also Table 6.2), with different functions
that
MNEMONIC
Leg
Hip
Neck
HeadArmElbow
ForeamHandFingersThumb
EyeNose
Face
Lips
Teeth
Gums
Tongue
Jaw
Pharynx
Abdome
n
Genitals Toes
Motor cortexSomatosensory cortex
Midline
Trunk Kne
e
Hip
Shou
lder
Arm
Elbo
w
Wris
tH
and
Fing
ers
Thum
b
Eye
Neck
Brow
Face
Lips
TonguePharynxLarynx
Jaw
Trun
k
Left Right
FIGURE 6.2 Somatosensory and Motor Homunculi Note that the size
ofeach region of the homunculi is relatedto its importance in
sensory or motorfunction, resulting in a distorted-appear-ing map
(see also Figure 10.1).
REVIEW EXERCISEIs a patient with face and arm weak-ness more
likely to have a lesion onthe lateral cortical surface or in
theinterhemispheric fissure? Is a patientwith leg weakness more
likely tohave a lesion on the lateral corticalsurface or in the
interhemisphericfissure?
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Prima Eriawan Putra 1102012212 13
Anatomy of Spinal Cord The spinal cord contains a
butterfly-shaped central gray matter surrounded by ascending and
descending white matter columns, or funiculi (Figure 6.3A). Sen-
sory neurons in the dorsal root ganglia have axons that bifurcate.
One branch conveys sensory information from the periphery, and the
other carries this information through the dorsal nerve root
filaments into the dorsal aspect of the spinal cord. The central
gray matter has a dorsal (posterior) horn that is in- volved mainly
in sensory processing, an intermediate zone that contains in-
terneurons and certain specialized nuclei (Table 6.2), and a
ventral (anterior) horn that contains motor neurons. Motor neurons
send their axons out of the spinal cord via the ventral nerve root
filaments. The spinal gray matter can also be divided into nuclei
or, using a dierent nomenclature, into laminae named by Bror
Rexed
Corticospinal Tract and Other Motor Pathways 227
we will discuss in this chapter and in Chapter7. The spinal cord
white matter consists of dor-sal (posterior) columns, lateral
columns, and ven-tral (anterior) columns (see Figure 6.3A).
The spinal cord does not appear the same atall levels (Figure
6.4). The white matter is thick-est in the cervical levels (see
Figure 6.4C), wheremost ascending fibers have already entered
thecord and most descending fibers have not yetterminated on their
targets, while the sacral cordis mostly gray matter (see Figure
6.4F). In ad-dition, the spinal cord has two enlargements(see
Figure 6.4A). The cervical enlargement andthe lumbosacral
enlargement give rise to thenerve plexuses for the arms and legs.
The spinalcord has more gray matter at the cervical andlumbosacral
levels (see Figure 6.4C,E,F) than
(A)
Intermediate zone
Ventral (anterior) horn
Spinalnerve
Sensoryneuron
Ventral root(motor)
Dorsal root(sensory)
Motor neuron
Ventralcolumn
Dorsalcolumn
Dorsal (posterior) horn
Lateralcolumn
Dorsal rootganglion
I IIIII
IV
V
VI
VII
VIII
IX
(B)
Rexeds laminae:
Medial and lateralmotor nuclei
Nuclei:
Intermediatezone
Nucleusproprius
Substantiagelatinosa
Marginalzone
X
FIGURE 6.3 Basic Spinal CordAnatomy (A) Gray matter, white
matter, and dorsal and ventral roots. (B) Spinal cord nuclei (left)
and Rexedslaminae (right). (See also Table 6.2.) (B from DeArmond
SJ, Fusco MM, May-nard MD. 1989. Structure of the HumanBrain: A
Photographic Atlas. 3rd Ed. Ox-ford, New York.)
TABLE 6.2 Nuclei and Laminae of the Spinal Cord
REGION NUCLEI REXEDS LAMINAE
Dorsal horn Marginal zone IDorsal horn Substantia gelatinosa
IIDorsal horn Nucleus proprius III, IVDorsal horn Neck of dorsal
horn VDorsal horn Base of dorsal horn VIIntermediate zone Clarkes
nucleus, VII
intermediolateral nucleusVentral horn Commissural nucleus
VIIIVentral horn Motor nuclei IXGray matter Grisea centralis X
surrounding central canal
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3 60
Precentral gyrus
From area 8
Caudatenucleus
(tail)
Thalamus
Lentiformnucleus
Internal capsule
Caudate nucleus (head)
Cortico-mesencephalic tract
Corticonuclear tract
Corticospinal(pyramidal) tract
Pyramid
Decussation of the pyramids
Anterior corticospinaltract (uncrossed)
Midbrain
Corticopontine tract
Cerebral peduncle( = crus cerebri)
Pons
Medulla
Lateral corticospinaltract (crossed)
Motor end plate
T
IIIIV
V
VI
VII
IXXXIIXI
C1
Multiform layer
Molecular layer
Externalgranular layer
Externalpyramidal layer
Internalgranular layer
Internalpyramidal layer
Fig. 3.3 Microarchitectureof the motor cortex(Golgi stain)
Fig. 3.4 Course of the pyramidal tract
3 Motor System
Baehr, Duus' Topical Diagnosis in Neurology 2005 ThiemeAll
rights reserved. Usage subject to terms and conditions of
license.
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Prima Eriawan Putra 1102012212 14
Corticospinal Tract The corticospinal tractmore specifically,
the lateral corticospinal tractis the most clinically important
descending motor pathway in the nervous system. This pathway
controls movement of the extremities, and lesions along its course
produce characteristic deficits that often enable precise clinical
localiza- tion. Because of its clinical importance, we will discuss
the corticospinal tract in greater detail than the other descending
motor systems.
Over half of the corticospinal tract fibers originate in the
primary motor cor- tex (Brodmanns area 4) of the precentral gyrus.
The remainder arise from the premotor and supplementary motor areas
(area 6) or from the parietal lobe (ar- eas 3, 1, 2, 5, and 7)
(Figure 6.9A). The primary motor cortex neurons contribut- ing to
the corticospinal tract are located mostly in cortical layer 5 (see
Figure 2.14B).
This tract originates in the motor cortex and travels through
the cerebral white matter (corona radiata), the posterior limb of
the internal capsule (where the fibers lie very close together),
the central portion of the cerebral peduncle (crus cerebri), the
pons, and the base (i.e., the anterior portion) of the medulla,
where the tract is externally evident as a slight protrusion called
the pyramid. The medullary pyramids (there is one on either side)
give the tract its name. At the lower end of the medulla, 80-85% of
the pyramidal fibers cross to the other side in the so-called
decussation of the pyramids. The fibers that do not cross here
descend the spinal cord in the ipsilateral anterior funiculus as
the anterior corticospinal tract; they cross farther down (usually
at the level of the segment that they supply) through the anterior
commissure of the spinal cord (cf. Fig. 3.6). At cervical and
thoracic levels, there are probably also a few fibers that remain
uncrossed and innervate ipsilateral motor neurons in the anterior
horn, so that the nuchal and truncal musculature receives a
bilateral cortical inner- vation.
The majority of pyramidal tract fibers cross in the decussation
of the py- ramids, then descend the spinal cord in the
contralateral lateral funiculus as the lateral corticospinal tract.
This tract shrinks in cross-sectional area as it travels down the
cord, because some of its fibers terminate in each segment along
the way. About 90 % of all pyramidal tract fibers end in synapses
onto interneurons, which then transmit the motor impulses onward to
the large motor neurons of the anterior horn, as well as to the
smaller motor neurons
Corticospinal Tract and Other Motor Pathways 231
the spinal cord. Lateral motor systems travel in the lat-eral
columns of the spinal cord and synapse on themore lateral groups of
ventral horn motor neuronsand interneurons (Figure 6.7). Medial
motor systemstravel in the anteromedial spinal cord columns
tosynapse on medial ventral horn motor neurons andinterneurons.
The two lateral motor systems are the lateral corti-cospinal
tract and the rubrospinal tract (Table 6.3). These
FIGURE 6.7 Somatotopic Organization of Medial and Lat-eral Motor
System Projections to Anterior Horn Cells Lateralmotor systems
(corticospinal and rubrospinal tracts) project to lateralanterior
horn cells, while medial motor systems (anterior
corticospinal,vestibulospinal, reticulospinal, and tectospinal
tracts) project to medial an-terior horn cells. Lateral anterior
horn cells control distal muscles of the ex-tremities, while medial
anterior horn cells control proximal trunk muscles. (Spinal cord
section from DeArmond SJ, Fusco MM, Maynard MD. 1989. Structure of
the Human Brain: A Photographic Atlas. 3rd Ed. Oxford University
Press, New York.)
TABLE 6.3 Lateral and Medial Descending Motor Systems
SITE OF DECUSSATION LEVELS OF TRACT SITE OF ORIGIN (WHERE
RELEVANT) TERMINATION FUNCTION
LATERAL MOTOR SYSTEMS
Lateral corticospinal tract Primary motor cortex Pyramidal
decussation, Entire cord (pre- Movement ofand other frontal and at
the cervicomedullary dominantly at contralateral parietal areas
junction cervical and limbs
lumbosacral enlargements)
Rubrospinal tract Red nucleus, Ventral tegmental decus- Cervical
cord Movement ofmagnocellular sation, in the midbrain contralateral
division limbs (function
is uncertain in humans)
MEDIAL MOTOR SYSTEMS
Anterior corticospinal tract Primary motor cortex Cervical and
Control of and supplementary upper thoracic bilateral axial motor
area cord and girdle
musclesVestibulospinal tracts Medial VST: medial Medial VST:
Medial VST:
(VSTs)a and inferior vestibular Cervical and positioning
ofnuclei; lateral VST: upper thoracic head and neck;lateral
vestibular cord; Lateral lateral VST:nucleus VST: entire cord
balance
Reticulospinal tracts Pontine and medullary Entire cord
Automatic reticular formation posture and
gait-related movements
Tectospinal tract Superior colliculus Dorsal tegmental Cervical
cord Coordination of decussation, in head and eye the midbrain
movement
(uncertain in humans)
aDespite their names, both medial and lateral VSTs are medial
motor systems.
Dorsal
Ventral
Medial motor systems
Anterior horncells for proximalmuscles
Anterior horncells for distalmuscles
Lateral motor systems
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Prima Eriawan Putra 1102012212 15
Corticospinal Tract and Other Motor Pathways 233
Lateralgeniculatenucleus
Frontopontine and othercorticofugal fibers
Anterior thalamic radiation
Anterior limb
Corticospinal andcorticobulbar tractsCorticospinal
andcorticobulbar tracts
6
3,1,2
5,7
4
Pyramid
Posterior limbof internal capsule
Basis pedunculi
(A)
(B)
Posterior limbCorticopontine and othercorticofugal fibers
Superior thalamic radiation(includes somatosensoryradiation)
Corticospinal tract
Auditory radiation(inferior thalamicpeduncle)
Optic radiation(posterior thalamicpeduncle)
Thalamus
Medialgeniculatenucleus
Head ofcaudatenucleus
Corticobulbartract
Genu
Glob
us
pallid
us
Putam
en
AT
L
F
Lateralcorticospinaltract (crossed)
Dorsal
Ventral
Pyramidal (motor) decussation
Anteriorcorticospinaltract (uncrossed)
Pyramidaldecussation
Pyramid
FIGURE 6.9 Internal Capsule (A) Three-dimensional representation
of the internalcapsule. Corticospinal and corticobulbarfibers
arising from the primary motor cor-tex and adjacent regions form
part of the in-ternal capsule. Corticobulbar fibers projectto lower
motor neurons in the brainstem.About 85% of corticospinal fibers
cross overat the pyramidal decussation to form thelateral
corticospinal tract, while the remain-ing fibers form the anterior
corticospinaltract. (B) Horizontal section through inter-nal
capsule showing the anterior limb,genu, and posterior limb in
relation to thethalamus, head of the caudate, andlentiform nucleus
(putamen and globus pal-lidus). Major fiber pathways of the
internalcapsule are indicated.
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Neuroanatomy Overview and Basic Definitions 33
Motor neurons that project from the cortex down to the spinal
cord orbrainstem are referred to as upper motor neurons (UMNs).
UMNs formsynapses onto the lower motor neurons (LMNs), which are
located in the ante-rior horns of the central gray matter of the
spinal cord (see Figure 2.16B) or inbrainstem motor nuclei. The
axons of LMNs project out of the CNS via theanterior spinal roots
or via the cranial nerves to finally reach muscle cells inthe
periphery. Lesions affecting UMNs and LMNs have certain distinct
clini-cal features, which we will learn about in Chapter 3.
(A) (B)
Precentral gyrus(motor cortex)
Primarymotor cortex
Posterior limbof internalcapsule
Pyramidaldecussation
Cervical
Thoracic
Lumbar
Sacral
Lateral corticospinal tract
Anterior horn
Skeletalmuscle
Upper motor neuron
Lower motor neuron
FIGURE 2.16 Overview of Cortico-spinal Tract (A) Pathway of
upper mo-tor neuron from motor cortex to lowermotor neuron in
contralateral spinalcord. (B) Representative sections throughthe
cerebral cortex, pyramidal decussa-tion, and spinal cord showing
corti-cospinal tract.
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Prima Eriawan Putra 1102012212 16
Corticonuclear (corticobulbar) Tract Some of the fibers of the
pyramidal tract branch o from the main mass of the tract as it
passes through the midbrain and then take a more dorsal course
toward the motor cranial nerve nuclei (Figs. 3.4 and 4.54, p. 212).
The fibers supplying these brainstem nuclei are partly crossed and
partly uncrossed (for further details, cf. Chapter 4, section 4.4
Cranial Nerves). The nuclei receiving pyramidal tract input are the
ones that mediate voluntary movements of the cranial musculature
through cranial nerves V (the trigeminal nerve), VII (the facial
nerve), IX, X, and XI (the glossopharyngeal, vagus, and accessory
nerves), and XII (the hypoglossal nerve).
236 Chapter 6
Pons
Midbrain
Rostralmedulla
Cervicomedullaryjunction(decussation)
Spinal cord
Pyramid
Basispedunculi
Pyramidaldecussation Lateral cortico-
spinal tract
Internal capsule(posterior limb)
Cortex
(A) Lateral corticospinal tract
Basispontis
Pons
Midbrain
Rostralmedulla
Caudalmedulla
Spinal cord
Cortex
(B) Rubrospinal tract
Red nucleus(magnocellulardivision)
Ventral tegmentaldecussation
Lateral column
Lateral intermediatezone and lateralmotor nuclei
Lateral inter-mediate zoneand lateralmotor nuclei
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Prima Eriawan Putra 1102012212 17
1.3. Jaras Sensorik
The term somatosensory generally refers to bodily sensations of
touch, pain, tem- perature, vibration, and proprioception (limb or
joint position sense). There are two main pathways for somatic
sensation (see Table 7.1 and Figures 7.1 and 7.2):
The posterior columnmedial lemniscal pathway conveys
proprioception, vibration sense, and fine, discriminative touch
(see Figure 7.1).
The anterolateral pathways include the spinothalamic tract and
other asso- ciated tracts that convey pain, temperature sense, and
crude touch (see Figure 7.2). Since some aspects of touch sensation
are carried by both pathways, touch sen- sation is not eliminated
in isolated lesions to either pathway. Four types of sensory neuron
fibers are classified according to axon diam- eter (Table 7.2).
These dierent fiber types have specialized peripheral recep- tors
that subserve dierent sensory modalities. Larger-diameter,
myelinated axons conduct faster than smaller-diameter or
unmyelinated axons. Sensory neuron cell bodies are located in the
dorsal root ganglia (see Figures 7.1 and 7.2). Each dorsal root
ganglion cell has
Corticospinal Tract and Other Motor Pathways 237
Pons
Midbrain
Medulla
Cervicalspinal cord
(C) Anterior corticospinal tract (D) Vestibulospinal tracts
Basispedunculi
Pyramid
Ventral column
Medial intermediate zoneand medial motor nuclei
Pons
Midbrain
Rostralmedulla
Cervicalspinal cord
Medialvestibularnucleus
Medialvestibulospinaltracts
Lateralvestibulospinaltract
Lateral vestibularnucleus
Medial intermediatezone and medialmotor nuclei
Cortex Cortex
FIGURE 6.11 Descending Motor Pathways Illustrated on this page,
overleaf, andpage 238 (see also Table 6.3).
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276 Chapter 7
IN CHAPTER 6, WE DISCUSSED the anatomy of the corticospinal
tract and otherdescending motor pathways. In this chapter we will
discuss the other twomajor long tracts of the nervous system (Table
7.1). These are the so-matosensory pathways: the posterior
columnmedial lemniscal system and theanterolateral systems. Like
the corticospinal tract, these pathways are somato-topically
organized (see Figure 6.2). Understanding the functions and
pointsof decussation of the three major long tracts (see Table 7.1)
is fundamental toclinical neuroanatomical localization.
In the sections that follow, we will learn to use the anatomy of
the three ma-jor long tracts to localize lesions in the nervous
system. We will discuss com-mon disorders of the spinal cord and
other locations that affect these pathways.In addition, brainstem
and spinal cord mechanisms of pain modulation willbe addressed. The
organization of the thalamus, serving as the major relayfor sensory
and other information traveling to the cortex, will be reviewed
aswell. Finally, we will discuss the roles of sensory and motor
pathways in bowel,bladder, and sexual function.
Main Somatosensory PathwaysThe term somatosensory generally
refers to bodily sensations of touch, pain, tem-perature,
vibration, and proprioception (limb or joint position sense). There
aretwo main pathways for somatic sensation (see Table 7.1 and
Figures 7.1 and 7.2):
The posterior columnmedial lemniscal pathway conveys
proprioception,vibration sense, and fine, discriminative touch (see
Figure 7.1).
The anterolateral pathways include the spinothalamic tract and
other asso-ciated tracts that convey pain, temperature sense, and
crude touch (seeFigure 7.2).
Since some aspects of touch sensation are carried by both
pathways, touch sen-sation is not eliminated in isolated lesions to
either pathway.
Four types of sensory neuron fibers are classified according to
axon diam-eter (Table 7.2). These different fiber types have
specialized peripheral recep-tors that subserve different sensory
modalities. Larger-diameter, myelinatedaxons conduct faster than
smaller-diameter or unmyelinated axons.
Sensory neuron cell bodies are located in the dorsal root
ganglia (see Figures7.1 and 7.2). Each dorsal root ganglion cell
has a stem axon that bifurcates, re-sulting in one long process
that conveys sensory information from the periph-
ery and a second process that carries informationinto the spinal
cord through the dorsal nerve roots.A peripheral region innervated
by sensory fibersfrom a single nerve root level is called a
dermatome.The dermatomes for the different spinal levels forma map
over the surface of the body (see Figure 8.4)that can be useful in
localizing lesions of the nerveroots or spinal cord. In Chapters 8
and 9 we will dis-cuss localization based on dermatome and
periph-eral nerve patterns of sensory and motor loss. In
thischapter we will focus on the central course of the
so-matosensory pathways in the spinal cord and brain.Just as our
knowledge that the corticospinal tractcrosses over at the pyramidal
decussation helps uslocalize CNS lesions (see Figure 6.11A), it is
equally
ANATOMICAL AND CLINICAL REVIEW
TABLE 7.1 Main Long Tracts of the Nervous System
NAME (AND LEVEL) PATHWAY(S) FUNCTION OF DECUSSATION
Lateral Motor Pyramidal decus-corticospinal sation
(cervico-tract medullary junction)
Posterior column Sensory (vibration, Internal arcuate fibers
medial lemniscal joint position, (lower medulla)pathway fine
touch)
Anterolateral Sensory (pain, Anterior commissure pathways
temperature, (spinal cord)
crude touch)
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Prima Eriawan Putra 1102012212 18
a stem axon that bifurcates, re- sulting in one long process
that conveys sensory information from the periph- ery and a second
process that carries information into the spinal cord through the
dorsal nerve roots. A peripheral region innervated by sensory
fibers from a single nerve root level is called a dermatome.
Individual somatosensory fibers enter the spinal cord at the
dorsal root entry zone (DREZ; also called the RedlichOber- steiner
zone) and then give o numerous collaterals that make synaptic con-
tact with other neurons within the cord. Fibers subserving dierent
sensory modalities occupy dierent positions in the spinal cord
(Fig. 2.15). It is impor- tant to note that the myelin sheaths of
all aerent fibers become considerably thinner as the fibers
traverse the root entry zone and enter the posterior horn. The type
of myelin changes from peripheral to central, and the myelinating
cells are no longer Schwann cells, but rather oligodendrocytes.
Somatosensory information is sequentially transmitted over three
types of neurons: first- order neurons, which transmit information
from sensory receptors to dorsal horn neurons; second-order CNS
association neurons, which communicate with various reflex circuits
and transmit information to the thalamus; and third- order neurons,
which forward the information from the thalamus to the sensory
cortex.
227
C2C3
C4
C5
C6
C7C8
L1
L 2
L 3
L 4
L5
S1
S1
S2
T2T3T4T5T6T7T8
T9
T10
T11
T12
Ophthalmic n.
Maxillary n.
Mandibular n.
Trigeminal n.C 2
C 3
C 4
C5
C6
C7
C8
T2T3T4T5T6T7T8T9
T 10T 11T 12
L1L 2
L 3
L 4
L 5
L5L4
S 1
S1
S2
S 3
S5S 4
T 1
T1
Fig. 2.6 Segmental innervation of the skin (after
HansenSchliack). a Anterior view. b Posteriorview.
cent roots, a dermatomal deficit of touch is generally hard to
demonstrate,while one of pain and temperature sensation is more
readily apparent. Thus,nerve root lesions can be more sensitively
detected by testing for hypalgesia oranalgesia, rather than
hypesthesia or anesthesia.
Sensory deficits due to peripheral nerve lesions. It is easy to
see why a lesionaffecting a nerve plexus or a peripheral nerve
produces a sensory deficit of anentirely different type than a
radicular lesion. As plexus lesions usually cause a
Peripheral Components of the Somatosensory System and Peripheral
Regulatory Circuits
Baehr, Duus' Topical Diagnosis in Neurology 2005 ThiemeAll
rights reserved. Usage subject to terms and conditions of
license.
320 Chapter 8
IN THE PRECEDING TWO CHAPTERS, we studied the three main motor
andsensory pathways in the central nervous system (see Table 7.1).
Wewill now follow these somatic sensory and somatic motor
pathwaysinto the peripheral nervous system (see Figure 2.1) to
explore theanatomy and clinical disorders of the peripheral nerves.
In this chap-ter, we will concentrate on the anatomy of spinal
nerve roots and theirrelation to the vertebral structures, regions
of innervation, and commonclinical disorders. Peripheral autonomic
functions were already dis-cussed in Chapter 6. In Chapter 9, we
will follow the nerves further intothe periphery, and we will
discuss the brachial and lumbosacral plexusesand peripheral nerve
branches.
Segmental Organization of the Nervous SystemLike their
invertebrate ancestors, humans retain some degree of segmen-tal
organization, especially in the spinal cord. There are 8 cervical
(C1C8),12 thoracic (T1T12), 5 lumbar (L1L5), 5 sacral (S1S5), and 1
coccygeal(Co1) spinal segments (Figure 8.1). During development,
the bones ofthe spine continue to grow after the spinal cord has
reached its fullsize. Therefore, in adults the spinal cord normally
ends with the conusmedullaris at the level of the L1 or L2
vertebral bones. The nerve roots (seeFigure 8.1; see also Figure
6.3) travel downward to reach their exit pointsat the appropriate
level. Below the L1 or L2 vertebral bones, the spinalcanal contains
nerve roots with no spinal cord, forming the cauda equina,meaning
horses tail (see Figures 8.1A and 8.3C). The conus medullaristapers
into the filum terminale, a thin strand of connective tissue
runningin the center of the cauda equina. The roots of the cauda
equina are or-ganized such that the most centrally located roots
are from the most cau-dal segments of the spinal cord (see Figure
8.3C).
8
C1(A)
C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L3
L4
S1S2
S3
S4S5
Co1
L5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
12L12345
11
10
9
8
7
6
5
4
3
2
T1
76
5
4
3
2
1
L1
L2
L3
L4
L5
S
C2
C3
C4
C5
C6
C7
Conus medullaris
Cauda equina
Lumbar cord
Sacral cord
Cervical cord
Thoracic cord
(B)
Dorsal median septum
Central canal
Intermediate zone
Ventral (anterior) horn
White matter
Gray matter
Spinalnerve
Ventral root(motor)
Dorsal root(sensory)
Ventral median fissure
Ventralcolumn
Dorsalcolumn
Dorsal (posterior) horn
Lateralcolumn
C1
FIGURE 8.1 Spinal Cord and Nerve Roots in Relation to the
Vertebral Spinal Canal(A) Sagittal view. The cervical (C5T1) and
lumbosacral (L1S3) spinal cord enlarge-ments supply nerves to the
arms and legs, respectively. At the level of the L1 or L2
ver-tebral bones the spinal cord ends, and nerve roots continue as
the cauda equina. (B) Mo-tor (ventral) roots and sensory (dorsal)
roots join at each segment to form spinal nerves.
ANATOMICAL AND CLINICAL REVIEW
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Somatosensory SystemsSensory systems can be conceptualized as a
series of neu-rons consisting of first-order, second-order, and
third-order neurons. First-order neurons transmit
sensoryinformation from the periphery to the CNS.
Second-orderneurons communicate with various reflex networks
andsensory pathways in the spinal cord and travel directly tothe
thalamus. Third-order neurons relay informationfrom the thalamus to
the cerebral cortex (Fig. 35-1).
This organizing framework corresponds with thethree primary
levels of neural integration in thesomatosensory system: the
sensory units, which containthe sensory receptors; the ascending
pathways; and thecentral processing centers in the thalamus and
cerebralcortex. Sensory information usually is relayed andprocessed
in a cephalad (toward the head) direction bythe three orders of
neurons. Many interneurons processand modify the sensory
information at the level of thesecond- and third-order neurons, and
many more par-ticipate before coordinated and appropriate
learned-movement responses occur. The number of
participatingneurons increases exponentially from the
primarythrough the secondary and the secondary through thetertiary
levels.
The Sensory UnitThe somatosensory experience arises from
informationprovided by a variety of receptors distributed
throughoutthe body. These receptors monitor four major types
ormodalities of sensation: discriminative touch, which isrequired
to identify the size and shape of objects andtheir movement across
the skin; temperature sensation;sense of movement of the limbs and
joints of the body;and nociception, or pain.
Each of the somatosensory modalities is mediated bya distinct
system of receptors and pathways to the brain;however, all
somatosensory information from the limbs
and trunk shares a common class of sensory neuronscalled dorsal
root ganglion neurons. Somatosensoryinformation from the face and
cranial structures is trans-mitted by the trigeminal sensory
neurons, which func-tion in the same manner as the dorsal root
ganglionneurons. The cell body of the dorsal root ganglion neu-ron,
its peripheral branch (which innervates a small areaof periphery),
and its central axon (which projects to theCNS) form what is called
a sensory unit.
The fibers of different dorsal root ganglion neurons con-duct
impulses at varying rates, ranging from 0.5 to 120 m/second. This
rate depends on the diameter of the nervefiber. There are three
types of nerve fibers that transmitsomatosensory information: types
A, B, and C. Type Afibers, which are myelinated, have the fastest
rate of con-duction.1,2 Type A fibers convey cutaneous pressure
andtouch sensation, cold sensation, mechanical pain, and heatpain.
Type B fibers, which also are myelinated, transmitinformation from
cutaneous and subcutaneous mechano-receptors. The unmyelinated type
C fibers have the small-est diameter and the slowest rate of
conduction. Theyconvey warmhot sensation and mechanical and
chemi-cal as well as heat- and cold-induced pain sensation.
Dermatomal Patt