Cranial Nerves pdf - Neuroanatomy

Lecture

Cranial Nerves

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Cranial Nerves

There are 12 pairs of cranial nerves, which leave the brain and pass through
foramina and fissures in the skull. All the nerves are distributed in the head
and neck, except cranial nerve X, which also supplies structures in the
thorax and abdomen. The cranial nerves are named as follows:

Olfactory

Optic

Oculomotor

Trochlear

Trigeminal

Abducent

Facial

Vestibulocochlear

Glossopharyngeal

10.

Vagus

11.

Accessory

12.

Hypoglossal

Organization of the Cranial Nerves

The olfactory, optic, and vestibulocochlear nerves are entirely sensory.

The oculomotor, trochlear, abducent, accessory, and hypoglossal nerves are
entirely motor.

The trigeminal, facial, glossopharyngeal, and vagus nerves are both sensory
and motor nerves.

Olfactory Nerves (Cranial Nerve I)

The olfactory nerves arise from the olfactory receptor nerve cells in the
olfactory mucous membrane located in the upper part of the nasal cavity
above the level of the superior concha

Each receptor cell consists of a small bipolar nerve cell.

olfactory hairs

: a number of short cilia arise from the coarse peripheral process

of the bipolar cell

olfactory nerve fibers

: Bundles of these nerve fibers pass through the openings

to enter the olfactory bulb. The

bone

ethmoid

cribriform plate of the

of the
olfactory nerve fibers are unmyelinated and are covered with Schwann cells

Olfactory Bulb

This ovoid structure possesses several types of nerve cells, the largest of which

tufted cells and granular cells

, Smaller nerve cells, called

the mitral cell

is
The olfactory bulb, in addition, receives axons from the contralateral
olfactory bulb through the olfactory tract.

Olfactory Tract

narrow band of white matter runs from the posterior end of the olfactory bulb
beneath the inferior surface of the frontal lobe of the brain

It consists of the central axons of the mitral and tufted cells of the bulb and
some centrifugal fibers from the opposite olfactory bulb

As the olfactory tract reaches the anterior perforated substance, it divides into

carries the axons to the

stria

. The lateral

striae

medial and lateral olfactory

and

periamygdaloid

olfactory area of the cerebral cortex, namely, the

The medial

(primary olfactory cortex)

areas often known as the

prepiriform
olfactory stria carries the fibers that cross the median plane in the anterior
commissure to pass to the olfactory bulb of the opposite side

The entorhinal area (area 28) of the parahippocampal gyrus, which receives
numerous connections from the primary olfactory cortex, is called the

(secondary olfactory cortex)

The primary olfactory cortex sends nerve fibers to many other centers within
the brain to establish connections for emotional and autonomic responses
to olfactory sensations.

A: Distribution of
olfactory
nerves on the
lateral wall of
the nose.

B: Connections
between the
olfactory cells
and the neurons
of the olfactory
bulb.

C: Connections
between the
olfactory cell
and the rest of
the olfactory
system.

Optic Nerve (Cranial Nerve II)

Origin of the Optic Nerve

The fibers of the optic nerve are the axons of the cells in the ganglionic layer of
the retina. They converge on the optic disc and exit from the eye, about 3 or

optic nerve

mm to the nasal side of its center, as the

The fibers of the optic nerve are myelinated, but the sheaths are formed from
oligodendrocytes rather than Schwann cells, since the optic nerve is
comparable to a tract within the central nervous system.

The optic nerve leaves the orbital cavity through the optic canal and unites with
the optic nerve of the opposite side to form the

optic chiasma

Optic Chiasma

In the chiasma, the fibers from the nasal (medial) half of each retina, including the
nasal half of the macula, cross the midline and enter the optic tract of the
opposite side, while the fibers from the temporal (lateral) half of each retina,
including the temporal half of the macula, pass posteriorly in the optic tract of
the same side

Optic Tract

The optic tract emerges from the optic chiasma and passes posterolaterally around
the cerebral peduncle. Most of the fibers now terminate by synapsing with nerve

, which is a small projection from the posterior

lateral geniculate body

cells in the

nucleus and the

pretectal

part of the thalamus. A few of the fibers pass to the

and are concerned with light reflexes

of the midbrain

colliculus

superior

Lateral Geniculate Body

The lateral geniculate body is a small, oval swelling projecting from the pulvinar of
the thalamus. It consists of six layers of cells, on which synapse the axons of the
optic tract. The axons of the nerve cells within the geniculate body leave it to
form the

optic radiation

Optic Radiation:

The fibers of the optic radiation are the axons of the nerve cells of the

lateral geniculate body. The tract passes posteriorly through the retrolenticular part

of the internal capsule and terminates in the visual cortex (area 17),

Optic Pathway

Visual field defects associated with
lesions of the optic pathways.

1. Right-sided circumferential
blindness due to retrobulbar
neuritis.

2. Total blindness of the right eye due
to division of the right optic nerve.

3. Right nasal hemianopia due to a
partial lesion of the right side of
the optic chiasma.

4. Bitemporal hemianopia due to a
complete lesion of the optic
chiasma.

5. Left temporal hemianopia and right
nasal hemianopia due to a lesion
of the right optic tract.

6. Left temporal and right nasal
hemianopia due to a lesion of the
right optic radiation.

7. Left temporal and right nasal
hemianopia due to a lesion of the
right visual cortex.

Neurons of the Visual Pathway

Four neurons conduct visual impulses to the visual cortex:

(1)

rods and cones, which are specialized receptor neurons in the retina;

(2)

bipolar neurons, which connect the rods and cones to the ganglion cells;

(3)

ganglion cells, whose axons pass to the lateral geniculate body; and

(4)

neurons of the lateral geniculate body, whose axons pass to the cerebral
cortex

Visual Reflexes

Direct and Consensual Light Reflexes

If a light is shone into one eye, the pupils of both eyes normally constrict.

The constriction of the pupil on which the light is shone is called the direct

light reflex; the constriction of the opposite pupil, even though no light fell on

that eye, is called the consensual light reflex

Accommodation Reflex

When the eyes are directed from a distant to a near object, contraction of

the medial recti brings about convergence of the ocular axes; the lens

thickens to increase its refractive power by contraction of the ciliary muscle;

and the pupils

Corneal Reflex

Light touching of the cornea or conjunctiva results in blinking of the eyelids

Visual Body Reflexes

The automatic scanning movements of the eyes and head that are made

when reading, the automatic movement of the eyes, head, and neck toward

the source of the visual stimulus, and the protective closing of the eyes and

even the raising of the arm for protection

Pupillary Skin Reflex

The pupil will dilate if the skin is painfully stimulated by pinching

Oculomotor Nerve (Cranial Nerve III)

The oculomotor nerve has two motor nuclei:

(1)

the main motor nucleus

(2)

the accessory parasympathetic nucleus.

Course of the Oculomotor Nerve

The oculomotor nerve emerges on the anterior surface of the midbrain. It
passes forward between the posterior cerebral and the superior cerebellar
arteries. It then continues into the middle cranial fossa in the lateral wall of
the cavernous sinus. Here, it divides into a superior and an inferior ramus,

superior orbital fissure

which enter the orbital cavity through the

the

nerve supplies the following extrinsic muscles of the eye:

oculomotor

The

, superior rectus, medial rectus, inferior rectus,

superioris

palpebrae

levator

ciliary

. It also supplies, through its branch to the

and inferior oblique
ganglion and the short ciliary nerves, parasympathetic nerve fibers to the
following intrinsic muscles: the constrictor pupillae of the iris and ciliary
muscles.

Therefore, the oculomotor nerve is entirely motor and is responsible for lifting
the upper eyelid; turning the eye upward, downward, and medially;
constricting the pupil; and accommodating the eye.

The distribution of the oculomotor nerve

Trochlear Nerve (Cranial Nerve IV)

The trochlear nucleus is situated in the anterior part of the gray matter that
surrounds the cerebral aqueduct of the midbrain

The nerve fibers, after leaving the nucleus, pass posteriorly around the central
gray matter to reach the posterior surface of the midbrain

The trochlear nerve, the most slender of the cranial nerves and the only one to
leave the posterior surface of the brainstem emerges from the midbrain and
immediately decussates with the nerve of the opposite side. The trochlear
nerve passes forward through the middle cranial fossa in the lateral wall of

superior orbital

the cavernous sinus and enters the orbit through the

muscle of the eyeball. The

superior oblique

. The nerve supplies the

fissure
trochlear nerve is entirely motor and assists in turning the eye downward
and laterally.

Distribution of the trochlear nerve

Trigeminal Nerve (Cranial Nerve V)

is the largest cranial nerve and contains both sensory and motor fibers. It is the
sensory nerve to the greater part of the head and the motor nerve to several
muscles, including the muscles of mastication

Trigeminal Nerve Nuclei

(1)

the main sensory nucleus,

(2)

the spinal nucleus,

(3)

the mesencephalic nucleus

(4)

the motor nucleus

Course of the Trigeminal Nerve

The trigeminal nerve leaves the anterior aspect of the pons as a small motor
root and a large sensory root

The nerve passes forward out of the posterior cranial fossa and rests on the
upper surface of the apex of the petrous part of the temporal bone in the
middle cranial fossa. The large sensory root now expands to form the
crescent-shaped trigeminal ganglion, which lies within a pouch of dura
mater called the

trigeminal or Meckel cave

Branches arise from the anterior border or the ganglion:

The ophthalmic nerve (V1)

contains only sensory fibers and leaves the skull

to enter the orbital cavity.

superior orbital fissure

through the

The maxillary nerve (V2)

also contains only sensory fibers and leaves the skull

rotundum

foramen

through the

The mandibular nerve (V3)

contains both sensory and motor fibers and leaves

ovale

foramen

the skull through the

Distribution of the trigeminal nerve.

Sensory nerve supply to the skin of the head and
neck. Note that the skin over the angle of the jaw is
supplied by the great auricular nerve (C2 and C3)
and not by branches of the trigeminal nerve.

The sensory fibers to
the skin of the face
from each division
supply a distinct
zone, there being
little or no overlap
of the dermatomes
(compare with the
overlap of the
dermatomes of the
spinal nerves).

As noted previously,
the motor fibers in
the mandibular
division are mainly
distributed to
muscles of
mastication

Abducent Nerve (Cranial Nerve VI)

The abducent nerve is a small motor nerve that supplies the lateral rectus
muscle of the eyeball

The small motor nucleus is situated beneath the floor of the upper part of
the fourth ventricle, close to the midline and beneath the colliculus facialis

Course:

The fibers of the abducent nerve pass anteriorly through the pons and
emerge in the groove between the lower border of the pons and the medulla
oblongata It passes forward through the cavernous sinus, lying below and
lateral to the internal carotid artery. The nerve then enters the orbit through

superior orbital fissure.

the

The abducent nerve is entirely a motor nerve and supplies the lateral rectus
muscle and, therefore, is responsible for turning the eye laterally.

Distribution of the abducent nerve

Lecture

Cranial Nerves

–12

)

Facial Nerve (Cranial Nerve VII)

The facial nerve has three nuclei:

(1)

the main motor nucleus

(2)

the parasympathetic nuclei

(3)

the sensory nucleus

1- Main Motor Nucleus

lies deep in the reticular formation of the lower part of the pons

The part of the nucleus that supplies the muscles of the upper part of the face
receives corticonuclear fibers from both cerebral hemispheres

The part of the nucleus that supplies the muscles of the lower part of the face
receives only corticonuclear fibers from the opposite cerebral hemisphere

2- Parasympathetic Nuclei

lie posterolateral to the main motor nucleus

They are the superior salivatory and lacrimal nuclei

3- Sensory Nucleus

The sensory nucleus is the upper part of the nucleus of the tractus solitarius

Facial nerve nuclei and their central connections

Course of the Facial Nerve

abducent

travel posteriorly around the medial side of the

fibers of the motor root
nucleus. pass around the nucleus beneath the colliculus facialis in the floor of
the fourth ventricle then emerge from the brainstem

) is formed of the central processes of the

intermediu

nervus

The sensory root (
unipolar cells of the geniculate ganglion. It also contains the efferent
preganglionic parasympathetic fibers from the parasympathetic nuclei

Two roots (sensory and motor) form the facial nerve that emerge from the
anterior surface of the brain between the pons and the medulla oblongata

enter the internal acoustic meatus in the petrous part of the temporal bone
laterally with the vestibular nerve

nerve enters the facial canal and runs laterally through the inner ear

On medial wall of the tympanic cavity, the nerve expands to form the sensory
geniculate ganglion

the facial nerve turns downward on the medial side of the aditus of the mastoid
antrum, descends behind the pyramid, and emerges from the stylomastoid
foramen

Branches of the facial nerve within the petrous part of the temporal bone;
the taste fibers are shown in black. The glossopharyngeal nerve is also
shown

Distribution of the facial nerve

Distribution of the Facial Nerve

supplies the muscles of facial expression, the auricular

The motor nucleus

muscles, the stapedius, the posterior belly of the digastric, and the
stylohyoid muscles

nucleus supplies the submandibular and sublingual

salivatory

The superior

salivary glands and the nasal and palatine glands.

nucleus supplies the lacrimal gland.

The lacrimal

thirds of the

receives taste fibers from the anterior two

The sensory nucleus

tongue, the floor of the mouth, and the palate.

Vestibulocochlear Nerve (Cranial Nerve VIII)

the vestibular nerve

- the cochlear nerve

the vestibular nerve

The vestibular nerve conducts nerve impulses from the utricle and saccule that
provide information concerning the position of the head; the nerve also
conducts impulses from the semicircular canals that provide information
concerning movements of the head.

They enter the anterior surface of the brainstem in a groove between the lower
border of the pons and the upper part of the medulla oblongata

The Vestibular Nuclear Complex

(1)

the lateral vestibular nucleus,

(2)

the superior vestibular nucleus

(3)

the medial vestibular nucleus, and

(4)

the inferior vestibular nucleus

Vestibular nerve nuclei and their central connections

Cochlear Nerve

The cochlear nerve conducts nerve impulses concerned with sound from the organ
of Corti in the cochlea. The fibers of the cochlear nerve are the central
processes of nerve cells located in the spiral ganglion of the cochlea

They enter the anterior surface of the brainstem at the lower border of the pons on
the lateral side of the emerging facial nerve and are separated from it by the
vestibular nerve

Cochlear Nuclei

Anterior and posterior cochlear nuclei

Course of the Vestibulocochlear Nerve

The vestibular and cochlear parts of the nerve leave the anterior surface of the
brain between the lower border of the pons and the medulla oblongata

They run laterally in the posterior cranial fossa and enter the internal acoustic
meatus with the facial nerve. The fibers are then distributed to the different
parts of the internal ear

Cochlear nerve

nuclei and

their central
connections

Distribution of the vestibulocochlear nerve

Glossopharyngeal Nerve (Cranial Nerve IX)

The glossopharyngeal nerve has three nuclei:

(1)

the main motor nucleus: supply the stylopharyngeus muscle

(2)

the parasympathetic nucleus

(3)

the sensory nucleus: is part of the nucleus of the tractus solitarius

Course

It leaves the anterolateral surface of the upper part of the medulla oblongata
as a series of rootlets in a groove between the olive and the inferior
cerebellar peduncle

jugular foramen

leaves the skull through the

The nerve then descends through the upper part of the neck in company with
the internal jugular vein and the internal carotid artery to reach the posterior

. The nerve then

muscle, which it supplies

stylopharyngeus

border of the
passes forward between the superior and middle constrictor muscles of the

sensory branches to the mucous membrane of the pharynx

pharynx to give
and the posterior third of the tongue

Nerve (Cranial Nerve X)

Vagus

The vagus nerve has three nuclei:

the main motor nucleus: is formed by the nucleus ambiguus The efferent fibers supply
the constrictor muscles of the pharynx and the intrinsic muscles of the larynx

the parasympathetic nucleus:

are distributed to the involuntary muscle of the bronchi, heart, esophagus,

efferent fibers

stomach, small intestine, and large intestine as far as the distal one-third of the
transverse colon

nerve (carotid

glossopharyngeal

from the hypothalamus and from the

afferent fibers

sinus reflex).

the sensory nucleus

Nerve

Vagus

of the

Course

Leave medulla oblongata as a series of rootlets in a groove between the olive and the inferior
cerebellar peduncle

jugular foramen

leaves the skull through the

two sensory ganglia, a rounded superior ganglion, and a cylindrical inferior ganglion

the cranial root of the accessory nerve joins the vagus nerve and is distributed mainly in its
pharyngeal and recurrent laryngeal branches

with the internal

the carotid sheath

nerve descends vertically in the neck within

vagus

The
jugular vein and the internal and common carotid arteries

The right vagus nerve The left vagus nerve

Distribution
of the vagus
nerve

Accessory Nerve (Cranial Nerve XI)

The accessory nerve is a motor nerve that is formed by the union of a cranial
and a spinal root.

The cranial root (part) is formed from the axons of nerve cells of the nucleus
ambiguus

The spinal root (part) is formed from axons of nerve cells in the spinal nucleus

Hypoglossal Nerve (Cranial Nerve XII)

The hypoglossal nerve is a motor nerve that supplies all the intrinsic muscles of
the tongue as well as the styloglossus, the hyoglossus, and the genioglossus
muscles.

The hypoglossal nerve fibers emerge on the anterior surface of the medulla
oblongata between the pyramid and the olive

hypoglossal canal

leaves the skull through the

In the upper part of its course, the hypoglossal nerve is joined by C1 fibers to
from the cervical plexus

the hypoglossal nerve controls the movements and shape of the tongue

Distribution of the accessory nerve

Distribution of hypoglassal nerve