Cranial Nerves

CRANIAL NERVES
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Anatomy of brain & brainstem
Except for cranial nerve (CN) I (olfactory) and CN II (optic), the anatomy of the CNs
is inextricably linked to that of the brain-stem
OLFACTORY NERVE (CN I)
The first order neurons of the olfactory system are bipolar

sensory cells that lie in the olfactory epithe- lium, which
occupies a small area on the superior nasal concha, upper
nasal septum, and roof of the nose
The olfactory nerve is a sensory nerve with but one function

—smell. The ability to perceive and identify various odors
differs from person to person.
OLFACTORY NERVE (CN I)
OPTIC NERVE (CN II)
The optic nerve is a central nervous system (CNS) fiber pathway connecting the retina and the
brain. The peripheral receptors, retinal rods and cones, are stimulated by light rays that pass
through the cornea, lens, and vitreous
OPTIC NERVE (CN II)
Pupillary afferent fibers from the right eye are crossed and uncrossed and run in both optic tracts. They leave the tract before the
LGB and send projections to the pretectal region bilaterally. The Edinger-Westphal nucleus sends pupillomotor fibers through the
third cranial nerve to the ciliary ganglion, and postganglionic fibers innervate the pupil sphincter. Because of the bilaterality of
the pathways, a light stimulus in the right eye causes pupillary constriction in both eyes.
OPTIC NERVE (CN II)
The optic nerve extends from the retina to the optic chiasm; it is approximately 5 cm long. It is
conventionally divided into four portions: intraocular (1 mm; the disc), intraorbital (about 25
mm), intracan- alicular (about 9 mm), and intracranial (12 to 16 mm).
OCULOMOTOR NERVE (CN III)
The oculomotor, or third cranial nerve (CN III), arises from

the oculomotor nuclear complex in the midbrain and
conveys motor fibers to extraocular muscles, plus
parasympathetic fibers to the pupil and ciliary body.
The extraocular muscles and the third nerve in the orbit.

The yoke muscles control extraocular movement in the

Actions of the extraocular muscles on the left resulting from a six cardinal directions of gaze.
combination of movements of the globe in the eye. Arrows denote
the main directions of action for each muscle, three dimensions.
CN III palsy produces varying degrees and combinations of

extraocular muscle weakness, ptosis, and pupil involvement.
Anatomy of the oculomotor nerve in relation to the major arteries at the base of the
brain. An aneurysm arising from the posterior communicating artery is compressing
and distorting the nerve.
TOCHLEAR NERVE (CN IV)
The trochlear, or fourth cranial nerve (CN IV), is the smallest

CN. It arises from the trochlear nucleus located just anterior to
the aqueduct in the gray matter of the lower mesencephalon at
the level of the inferior colliculus, immediately above the pons
It is the only CN to exit from the brainstem posteriorly, and because of

this extra distance, CN IV has the longest intracranial course of any of the
CNs
TOCHLEAR NERVE (CN IV)
The nerve circles the brainstem, then turns and runs forward,
passing between the posterior cerebral and superior cerebellar
arteries, then courses along the tentorium. It penetrates the dura
just behind and lateral to the posterior clinoid processes and
enters the cavernous sinus in proximity to CN III. In the sinus, it
is located superolaterally, below CN III but above the trigeminal
branches. Leaving the cavernous sinus, it traverses the superior
orbital fissure, enters the orbit, and crosses over CN III to
supply the superior oblique. It does not pass through the annulus
of Zinn.
ABDUCENS NERVE (CN VI)
Sixth nerve palsies are common, and many resolve with

no explanation. With a complete CN VI palsy, the eye
cannot be abducted and often rests in a posi- tion of
adduction
ABDUCENS NERVE (CN VI)
The supranuclear mechanisms that control gaze are designed to ensure that the fovea maintains fixation on the
object of interest despite movements of the object, the eyes, or the head.
A saccade (Fr. “jerk”) is a quick, small-amplitude eye movement used to acquire a target. Smooth pursuit
mechanisms use slower eye movements to track a target once acquired. Saccades are designed to rapidly shift
gaze to the target; pursuit movements are designed to maintain foveation of a moving target.
The CN VI nucleus is the final common pathway controlling horizontal eye movements. The vertical
gaze centers lie in the mid-brain. There are six currently recognized eye movement control systems:
saccadic, smooth pursuit, vergence, fixation, optokinetic, and vestibulo-ocular reflex (VOR).
TRIGEMINAL NERVE (CN V)
The trigeminal, or fifth cranial, nerve (CN V) is the

largest and one of the most complex CNs. It has a
large sensory part (portio major; 170,000 fibers) and a
much smaller motor part (portio minor; 7,700 fibers).
The sensory component has three divisions: the first, or

ophthalmic division (CN V1), the second or maxillary
division (CN V2), and the third or mandibular
division (CN V3). The motor and principal sensory
nuclei are located in the midpons
The sensory portion innervates the face, teeth, oral, and nasal
cavities, the scalp back to the vertex, the intracranial dura, and the
cerebral vasculature, and provides proprioceptive information for
muscles of mastication.
The motor portion innervates the muscles of mastication. CN V has

extensive connections with other CNs. There is a small input to both
the mossy and climbing fiber systems of the cerebellum
(trigeminocerebellar fibers).
The trigeminal nerve and its connections.

Cutaneous distribution of the trigeminal nerve. A. Peripheral distribution. B. Segmental

distribution.
FACIAL NERVE (CN VII)
The facial, or seventh, cranial nerve (CN VII) is a predominantly motor nerve that innervates
the muscles of facial expression and the muscles of the scalp and ear, as well as the
buccinator, platysma, stapedius, stylohyoid, and posterior belly of the digastric.
N.VII carries parasympathetic secretory fibers to the

submandibular and sublingual salivary glands, the lacrimal
gland and to the mucous membranes of the oral and nasal
cavities. It has some sensory functions; the most important is
to mediate taste from the anterior two-thirds of the tongue. It
also conveys exteroceptive sensation from the eardrum and
external auditory canal, proprioceptive sensation from the
muscles it supplies, and general visceral sensation from the
salivary glands and mucosa of the nose and pharynx.
Anatomically the motor division of the nerve is separate

from the sensory and parasympathetic portions. In its
course from its exit from the pons until its terminal
arborizations, several important branches are given off in the
following order: the greater (superficial) petrosal nerve, the
nerve to the stapedius, and the chorda tympani.
VESTIBULOCOCHLEAR/ACOUSTIC (CN VIII)
The vestibulocochlear, acoustic, or eighth cranial nerve (CN

VIII) has two components, the vestibular and the cochlear,
blended into a single trunk.
The cochlear portion subserves hearing; the vestibular nerve

subserves equilibration, coordination, and orientation in space.
Both are classified as special sensory afferent nerves. The two
components originate in separate peripheral receptors and have
distinct central connections.
The acoustic nerve traverses the internal auditory canal (IAC),

where it lies lateral and inferior to the facial nerve. It crosses
the cerebellopontine angle, passes around the inferior
cerebellar peduncle, and enters the upper medulla at its
junction with the pons near the lateral recess of the fourth
ventricle
GLOSSOPHARINGEAL NERVE (CN IX)
The glossopharyngeal, as its name implies, is distributed

principally to the tongue and pharynx.
It conveys general sensory as well as special sensory (taste)

fibers from the posterior third of the tongue.
GLOSSOPHARINGEAL NERVE (CN IX)
• CN IX supplies parasympathetic innervation to the parotid gland and to the mucous membranes of
the posterior inferior mouth and pharynx
• Sensory neurons of CN IX are located in the superior and inferior glossopharyngeal ganglia
• CN IX is difficult to examine because most or all of its functions are shared by other nerves and
because many of the structures it supplies are inaccessible. It is possible to examine pain and touch
sensation of the pharynx, tonsillar region and soft palate, and the gag reflex
•
VAGUS NERVE (CN X)
The vagus (L. “wandering,” because of its wide distribution) is the

longest and most widely distributed the pharynx and larynx, and
parasympathetic axons from the DMNX to the smooth muscles and
glands of the pharynx and larynx and to the thoracic and abdominal
viscera. Its terminal ramifications reach the splenic flexure of the
colon.
• Normal functions mediated by CNs IX and X include swallowing,

phonation, and airway protection and modulation.
• The complex process of swallowing is divided into two stages,
controlled primarily by CNs IX, X, and XII. In the first stage, the
food bolus is driven back into the fauces by tongue action.
VAGUS NERVE (CN X)
SPINAL ACCESSORY NERVE (CN XI)
• The spinal accessory (SA) nerve, cranial nerve XI (CN XI), is

actually two nerves that run together in a common bundle for a
short distance. The smaller cranial portion (ramus internus) is a
special visceral efferent (SVE) accessory to the vagus.
• The major part of CN XI is the spinal portion (ramus externus).

Its function is to innervate the sternocleidomastoid (SCM) and
trapezius muscles
HYPOGLOSSAL NERVE (CN XII)
The hypoglossal nerve (CN XII) is a purely motor

nerve, supplying the tongue. Its cells of origin are in the
hypoglossal nuclei, which are upward extensions of the
anterior gray columns of the spinal cord; they consist of
large, multipolar cells, similar to the anterior horn
motoneurons
HYPOGLOSSAL NERVE (CN XII)

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CRANIAL NERVES

The first order neurons of the olfactory system are bipolar

The olfactory nerve is a sensory nerve with but one function

The oculomotor, or third cranial nerve (CN III), arises from

The extraocular muscles and the third nerve in the orbit.

The yoke muscles control extraocular movement in the

CN III palsy produces varying degrees and combinations of

The trochlear, or fourth cranial nerve (CN IV), is the smallest

It is the only CN to exit from the brainstem posteriorly, and because of

Sixth nerve palsies are common, and many resolve with

The trigeminal, or fifth cranial, nerve (CN V) is the

The sensory component has three divisions: the first, or

The motor portion innervates the muscles of mastication. CN V has

The trigeminal nerve and its connections.

Cutaneous distribution of the trigeminal nerve. A. Peripheral distribution. B. Segmental

N.VII carries parasympathetic secretory fibers to the

Anatomically the motor division of the nerve is separate

The vestibulocochlear, acoustic, or eighth cranial nerve (CN

The cochlear portion subserves hearing; the vestibular nerve

The acoustic nerve traverses the internal auditory canal (IAC),

The glossopharyngeal, as its name implies, is distributed

It conveys general sensory as well as special sensory (taste)

The vagus (L. “wandering,” because of its wide distribution) is the

• Normal functions mediated by CNs IX and X include swallowing,

• The spinal accessory (SA) nerve, cranial nerve XI (CN XI), is

• The major part of CN XI is the spinal portion (ramus externus).

The hypoglossal nerve (CN XII) is a purely motor

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