Chapter 12 & 13 The Brain and Cranial Nerves

• CNS: Composed of the brain and spinal cord

• Brain: Control center for many of body’s functions

• Is composed of wrinkled, pinkish gray tissue

• Consists of • Cerebrum• Cerebellum• Diencephalon• Brain stem

• During the first 26 days of development:

– Ectoderm thickens forming the neural plate

– The neural plate invaginates, forming the neural groove

– The neural groove fuses dorsally and forms the neural tube

• The anterior end of the neural tube expands and constricts to form the three primary brain vesicles

– Prosencephalon – the forebrain

– Mesencephalon – the midbrain

– Rhombencephalon – hindbrain

• In week 5 of embryonic development, secondary brain vesicles form

– Telencephalon and diencephalon arise from the forebrain

– Mesencephalon remains undivided

– Metencephalon and myelencephalon arise from the hindbrain

• Secondary brain vesicles develop into:– Telencephalon – cerebrum

– Diencephalon – thalamus, hypothalamus, and epithalamus

– Mesencephalon – brain stem: midbrain

– Metencephalon – Cerebellum and brain stem: pons

– Myelencephalon – brain stem: medulla oblongata

• Largest portion of brain

• Composed of right and left hemispheres

• Contain numerous folds, increase surface area - Gyri

• Shallow grooves or valleys between gyri - Sulci

• Contain deeper grooves - Fissures

• Sulci and Fissures:

– Longitudinal fissure: separates the two hemispheres

– Lateral fissure: separates temporal lobe from frontal and parietal lobes

– Central sulcus: separates frontal and parietal lobes

• Each Cerebral hemisphere is divided into different lobes:

• Frontal lobe: Imp. in voluntary motor function, motivation, aggression, sense of smell, mood

• Parietal lobe: Center for reception and evaluation of sensory information except smell, hearing, and vision

• Occipital lobe: reception and integration of visual input

• Temporal lobe: reception and evaluation for smell and hearing; memory, abstract thought, judgment

• Insula: Insula is present within temporal lobe – 5th lobe

• Play important role in emotions, regulate body`s homeostasis

• Cerebral cortex: outer surface, consists of gray matter

• Contains no. of neurons such as fusiform cells, stellate cells, pyramidal cells

• Gray matter deep inside the brain – Basal nuclei

• Cerebral Medulla: White matter between the cortex & nuclei

• Consists of deep myelinated fibers and their tracts

• It is responsible for communication between: – Cerebral cortex & lower CNS

center & areas of the cerebrum

• Three types of fibers in cerebral white matter:

• Association fibers: connect area of cerebral cortex within the same hemisphere

• Commissural fibers: connect one hemisphere to the other hemisphere – Corpus callosum

• Projection fibers: connect cerebrum to other parts of the brain and spinal cord

• Masses of gray matter found deep within the white matter of cerebrum, diencephalon & midbrain– basal nuclei

• Nuclei are involved in the control of motor functions

• Nuclei in cerebrum are called Corpus Striatum and composed of:

– Caudate nucleus– Lentiform nucleus

• Lentiform nucleus composed of: - putamen - Globus pallidus

• Basal Nuclei are largest nuclei of brain and occupy largest part of cerebrum

• Sub thalamic nucleus : Present in diencephalon

• Substantia nigra : Present in midbrain

• Both these nuclei function with cerebrum basal nuclei to control movement

• Part of cerebrum and diencephalon

• Plays a role in Emotions and basic survival functions such as memory, reproduction, nutrition

• Limbic system consists of :

• Cingulate gyrus & Para hippocampal gyrus of Cerebrum

• Various nuclei of the thalamus

• Part of the basal nuclei, hypothalamus, olfactory cortex

• Located between brainstem and cerebrum

• Consists of – • Thalamus• Sub thalamus• Hypothalamus• Epithalamus

• 80% of diencephalon

• Two large gray masses connected by interthalamic adhesion or intermediate mass

• Many nuclei

• Sensory relay center of the brain

• All sensory neurons except olfactory neurons first synapse in thalamus

• Then thalamus directs information to proper cortical region

• Regulate emotions, moods, motor activities, learning, and memory

• Small area inferior to thalamus

• Contains several ascending and descending nerve tracts

• Contains sub thalamic nuclei,

parts of red nuclei and substantia nigra

• Involved in controlling motor function

• Most inferior portion of diencephalon, located below the thalamus

• Contains several nuclei and tracts

• Mammillary bodies– Small, paired nuclei bulging anteriorly

from the hypothalamus – Relay station for olfactory pathway

– Response to odor

• Infundibulum – stalk of the hypothalamus; connects hypothalamus to posterior pituitary gland

• Hypothalamus controls endocrine system

• Regulates Pituitary gland

– Initiates physical expression of emotions (linked to the limbic system)

– Regulates thirst, food intake, body temp., sexual behavior, pleasant and painful feelings, pleasure, fear, rage, sleep-wake cycles

– Regulates autonomic centers in the brain stem controlling B.P., digestive rate, respiration rate

• Small area superior and posterior to thalamus

• Consists of Pineal body and Habenular nucleus

• Pineal gland – Secretes melatonin– Melatonin helps regulate

sleep-wake cycles– Pineal gland may play a role

in controlling the onset of puberty

• Habenular nucleus: Involved in emotional and visceral responses to odors

• Consists of three regions – • Midbrain• Pons• Medulla Oblongata

• Connects spinal cord to the remainder of brain

• Associated with 10 of the 12 pairs of cranial nerves

• Damage to brainstem causes death

• Because many reflexes essential for survival are integrated in brainstem

• Located between the diencephalon and the pons

• Midbrain structures include:– Cerebral peduncles – two

bulging structures, consists of descending tracts

– Which carry motor information from cerebrum to Brain Stem & Spinal cord

– Cerebral aqueduct – narrow canal that connects the third and fourth ventricle

– Various nuclei

• Nuclei that control cranial nerves III (oculomotor), IV (trochlear) and V (trigeminal)

• Corpora quadrigemina – four dome shaped nuclei on the dorsal midbrain

- Superior colliculi – 2 Superior nuclei, visual reflex centers, coordinate head and eye movements

- Inferior colliculi – 2 Inferior nuclei, auditory relay centers, Act in reflexive responses toAct in reflexive responses to soundsound

• Substantia nigra – Interconnected with basal nuclei of cerebrum, involve in coordinating movements

• Contains melanin pigment

• Precursor of dopamine neurotransmitter

• Degeneration of dopamine-releasing neurons – Parkinson`s disease

• Red nucleus – Reddish due to rich blood supply and iron content

• Aid in unconscious regulation and coordination of motor activities

• Superior to the medulla oblongata

• Contains ascending & descending tracts• Many Nuclei

– Pontine nuclei: anterior portion of pons, relay information between cerebrum and cerebellum

– Nuclei for cranial nerves V-IX: located in posterior pons

– Pons nuclei are the part of - Sleep center - Help medulla oblongata to control respiratory movements

• Most inferior part of brain & continuous with SC

• Has both ascending and descending nerve tracts

• Many medullary nuclei functions as:

• Regulate heart rate, blood vessel diameter, respiration, swallowing, vomiting, hiccupping, coughing, and sneezing

• Pyramids: 2 anterior bulges

• Contains descending nerve tracts on the anterior surface. Inferiorly fibers decussate (cross over); thus each half of the brain controls the opposite half of the body

• Olives: 2 rounded structure; protrude from anterior surface. Nuclei within help regulate balance, coordination, modulation of sound from inner ear

• Nuclei of cranial nerves V, IX-XII

• Attached to brainstem posterior to pons

• Three paired fiber tracts that connect the cerebellum to the brain stem

• Superior peduncles connect the cerebellum to the midbrain

• Middle peduncles connect the pons to the cerebellum

• Inferior peduncles connect the medulla oblongata to the cerebellum

• Two bilaterally symmetrical hemispheres connected medially by the vermis

• Folia – Ridges of Cerebellar cortex

• Arbor vitae – White matter with a tree- like appearance

• Cerebral cortex contains several types of neurons including:

• stellate, basket, granule, Golgi cells,• Purkinje cells – largest cell in CNS

• Cerebellar cortex contains more than 1012 neurons, more neurons than entire Cerebral cortex

• Cerebellum consists of 3 parts:

• Flocculonodular lobe: a small inferior part

• Vermis: narrow central worm like structure

• Two lateral hemispheres

• Flocculonodular lobe: Control balance and eye movements

• Vermis and medial portion of hemispheres: Control posture, locomotion, fine motor coordination

• Lateral hemispheres, major portion: works with cerebrum to plan, practice, learn complex movements

• Three connective tissue membranes lie external to the CNS – dura mater, arachnoid mater, and pia mater

• Functions of the meninges

– Cover and protect the CNS

– Protect blood vessels and enclose venous sinuses

– Contain cerebrospinal fluid (CSF)

– Form partitions within the skull

• Outermost, strong meninx composed of two fibrous connective tissue layers

• The two layers separate in certain areas and form Dural venous sinuses

• All veins drain blood from brain

empty into Dural venous sinuses

• Dura mater extends inward in some areas forming septa to anchor the brain to the skull

• Three Dural septa extend into major brain fissures and limit excessive movement of the brain

– Falx cerebri – fold that dips into the longitudinal fissure

– Falx cerebelli – runs along the vermis of the cerebellum

– Tentorium cerebelli – horizontal Dural fold extends into the transverse fissure

• The middle meninx, which forms a loose brain covering

• It is separated from the dura mater by the subdural space

• Pia mater: thin, delicate C.T. membrane closely attached to the surface of the brain

• Subarachnoid space: Present between pia mater and subarachnoid mater

• Contains blood vessels, and cerebrospinal fluid

Brain ventricles arise from expansions of the lumen of embryonic neural tube

Interconnected chambers within brain

Ventricular chambers are filled with CSF and lined by ependymal cells Lateral ventricles: large cavities within cerebral hemispheres

Third ventricle: within diencephalon

Interventricular foramina join lateral ventricles with third ventricle

Fourth ventricle:

Dorsal to pons and medullaDorsal to pons and medulla

4th ventricle connected to 3rd ventricle by the cerebral aqueduct

Opens into central canal of Opens into central canal of spinal cordspinal cord

And also connected to the subarachnoid space by two lateral & one medial apertures

• Similar to serum, but most protein removed

• Flows around and in the brain and spinal cord

• Protective cushion around CNS

• Prevents the brain from crushing under its own weight

• Protects the CNS from blows and other trauma

• Nourishes the brain

• 99% water

• Also sugar (glucose), chlorides, few proteins, ions, vitamin C

• Total volume of 150 ml (replaced every 8 hrs.)

• 500 ml formed daily

• Formed by choroid plexuses in the brain ventricles

– Choroid plexuses are composed of ependymal cells, their support tissue, and associated blood vessels

• Endothelial cells (lining all capillaries) have tight junctions between them

• Astrocytes have foot processes that

influence capillary permeability

• Basement membrane of endothelium

• Excludes many potentially harmful Excludes many potentially harmful substancessubstances

• Ineffective against some substancesIneffective against some substances– Fats and fat soluble moleculesFats and fat soluble molecules– Respiratory gasesRespiratory gases– AlcoholAlcohol– NicotineNicotine– AnesthesiaAnesthesia

• Brain – Requires a tremendous amount of

blood

– Receives 15-20% of blood pumped by heart

– Interruption can cause unconsciousness and irreversible brain damage

– High metabolic rate; dependent upon constant supply of oxygen and glucose

– Receives blood through arteries: internal carotids and vertebral arteries. The vertebral arteries join to form the basilar artery. Carotids plus basilar form the cerebral arterial circle (Circle of Willis).

Olfactory nerve (I)Olfactory nerve (I)

Optic nerve (II)Optic nerve (II)

optic chiasmaoptic chiasma

Oculomotor nerve (III)Oculomotor nerve (III)Trochlear nerve (IV)Trochlear nerve (IV)

Trigeminal nerve (V)Trigeminal nerve (V) Abducens nerve (VI)Abducens nerve (VI)

Facial nerve (VII)Facial nerve (VII) Vestibulocochlear nerve Vestibulocochlear nerve (VIII)(VIII)

Glossopharyngeal nerve (IX)Glossopharyngeal nerve (IX)Vagus nerve (X)Vagus nerve (X)

Accessory nerve (XI)Accessory nerve (XI)Hypoglossal nerve (XII)Hypoglossal nerve (XII)

• 12 pair numbered by their location (from anterior to posterior)

• C.N. (I) and (II) attach to the cerebrum/diencephalon

• C.N. (III) through (XII) attach to the brainstem

• Almost all of the cranial nerves serve the head and neck

• C.N. (X), Vagus, extends into the abdominal cavity

• Cranial nerves can have sensory, motor or parasympathetic neuron

fibers (or all of these)

• C.N. (III), (VII), (IX), and (X) contain parasympathetic fibers

• Old Opie Occasionally Tries Trigonometry And Feels Very

Gloomy, Vague, And Hypoactive

• Sensory only

• Carry smell sensations from nasal cavity

• Originate in nasal cavity

• Pass through the cribriform plate of ethmoid bone to olfactory bulb

• Olfactory nerves olfactory bulb olfactory tract inferior frontal and medial temporal lobes

• Sensory only

• Originate from the retina of the eye

• Optic nerves optic chiasma optic tracts lateral geniculate bodies of thalamus optic radiations visual cortex in occipital lobe

• Function: carry afferent impulses for vision

• Motor, sensory and parasympathetic fibers

• Fibers extend from the ventral midbrain, pass through the superior orbital fissure, and go to the extrinsic eye muscles

• Functions in raising the eyelid, directing the eyeball, constricting the iris, and controlling lens shape

• Motor and sensory

• Primarily a motor nerve that

directs the eyeball

• Fibers emerge from the dorsal midbrain and enter the orbits via the superior orbital fissures; innervate the superior oblique muscle

• Motor, sensory

• Fibers run from the face to the pons via the superior orbital fissure (V1), the foramen rotundum (V2), and the foramen ovale (V3)

• Three branches:Three branches:

– Ophthalmic Branch V1

• Convey sensory impulses from upper eyelid, eye surface, tear glands, nose, scalp and forehead

Maxillary Branch V2

– Convey sensory impulses from upper teeth, gum and lip, palate and skin of cheek and lower eyelid

Mandibular Branch V3

• Motor to muscles of mastication

• Conveys sensory impulses from lower teeth, gum, lip, skin of jaw and part of scalp

• Motor and sensory

• Fibers leave the inferior pons and enter the orbit via the superior orbital fissure to eye

• Motor to lateral rectus muscles, an extrinsic muscle of the eye

• Convey proprioceptor impulses from same muscle to brain

• Motor, sensory and parasympathetic fibers

• Motor to muscles of facial expression

• Taste from anterior 2/3 of tongue

• Parasympathetic innervation of salivary glands (submandibular and sublingual)

• Sensory only

• Two branches– Cochlear Branch

• Afferent fibers from cochlea in inner ear carrying auditory messages

– Vestibular Branch• Afferents from vestibule

and semicircular canals carrying info. on equilibrium

• Motor, sensory and parasympathetic fibers

• Fibers emerge from medulla, leave the skull via the jugular foramen, & run to the throat

• Motor to muscles of the pharynx for swallowing

• Sensory from tongue, tonsils, Eustachian tubes

• Sensory from carotid arteries regarding blood pressure and chemistry

• Parasympathetic innervation of the Parotid salivary glands

• Motor, sensory and parasympathetic fibers

• The only cranial nerve that extends beyond the head and neck

• Fibers emerge from the medulla via the jugular foramen

• Motor to muscles of pharynx and larynx

• Sensory from posterior tongue and pharynx, thoracic and abdominal viscera

• Parasympathetic innervation of heart, lungs, smooth muscles of pharynx, larynx, thoracic and abdominal viscera

• Formed from a cranial root emerging from the medulla & a spinal root arising from the superior region of the spinal cord

• Motor and sensory

• Motor to the muscles of the shoulder, neck (sternocleidomastoid, trapezius) pharynx, larynx

• Proprioceptive afferents from the same muscles

• Mostly motor, some sensory

• Fibers arise from the medulla and exit the skull via the hypoglossal canal

• Motor to intrinsic and extrinsic tongue muscles (move the tongue)

• Proprioceptive afferents back from the same muscles