cranial nerves The 12 paired nerves that origi-
nate within the cranium (skull). The cranial
nerves convey sensory signals and control motor
functions primarily for the head, neck, and face.
Cranial NERVE X, also called the vagus nerve,
serves the organs of the trunk as well. The cranial
nerves and the SPINAL NERVEScombined make up
the PERIPHERAL NERVOUS SYSTEM, dividing and
branching throughout the body to extend nerve
fibers to all tissues.
When identifying the cranial nerves numeri-
cally, neurologists generally use Roman numerals
I through XII to refer to the cranial nerves or des-
ignate them as first cranial nerve, second cranial
nerve, and so forth. The numbers of the cranial
nerves designate the cranial nerves in order from
the front to the back of the BRAINand brainstem.
Cranial nerve IV, the trochlear nerve, is the small-
est, and cranial nerve V, the trigeminal nerve, is
the largest of the cranial nerves. Cranial nerve X
has the most diverse and extensive functions.
Types of Cranial NervesCranial nerves are sensory, motor, or mixed (con-
vey both sensory and motor signals). Sensory
nerves or nerve components are afferent; they
conduct signals from the body to the brain and
may be general (conveying sensory information
such as temperature and PAIN) or special (convey-
ing sensory information for sight, hearing, taste,
or smell). Motor nerves or nerve components are
efferent; they conduct signals from the brain to
the body and may be somatic (serving striated or
voluntary MUSCLE), visceral (serving the smooth
muscle of internal organs), or branchial (serving
the structures that arise from the embryonic gill
arches, which are primarily the structures of the
lower face, jaw, and THROAT). The cranial nerves
cross before leaving the brain and brainstem, serv-
ing structures on the opposite side of the body.
Origins and Paths of the Cranial Nerves
Cranial nerves I and II are special sensory nerves.
The first cranial nerves, the olfactory nerves, origi-
nate in the olfactory bulbs at the front of the
brain, in a region sometimes called the rhinen-
cephalon. The olfactory nerve fibers unify to
become the olfactory tracts as they pass along the
underside of the brain and terminate in the olfac-
tory epithelium, a structure of hairlike extensions
which respond to scent molecules that enter the
NOSE. The second cranial nerves, the optic nerves,
originate at the front of the brain near the olfac-
tory bulbs. The OPTIC NERVE fibers converge into
the optic tracts that terminate as they enter the
retinas of the eyes.
Cranial nerves VIII, the vestibulocochlear
nerves, are the third purely sensory pair of cranial
nerves and arise from the brainstem near the
juncture of the pons and the medulla oblongata.
The vestibulocochlear nerve travels parallel to cra-
nial nerve VII, the facial nerve, through a portion
of the fallopian channel (a narrow tunnel through
the cranium) as each leaves the brain. Each
vestibulocochlear nerve has two branches, one
that terminates within the cochlea, responsible for
hearing, and one that terminates within the struc-
tures of the vestibule, responsible for balance.
Cranial nerve pairs III through VII and IX
through XII originate from clusters of cells within
structures of the brainstem. Cranial nerves III
(oculomotor nerves), IV, VI (abducens nerves), XI
(spinal accessory nerves), and XII (hypoglossal
nerves) have purely motor functions. The remain-
ing cranial nerves—V, VII, IX (glossopharyngeal),
and X—have mixed sensory and motor functions.Conditions That Can Affect the Cranial Nerves
Damage to the cranial nerves may result from
trauma, INFLAMMATION, INFECTION, AUTOIMMUNE DIS-
ORDERS, and tumors. Depending on the nerve
affected the consequences may be disturbances of
sensory perceptions, such as altered taste or
diminished smell, or disturbances of motor func-
tion, such as the facial PARALYSIS of BELL’S PALSY
(damage to cranial nerve VII). HERPES ZOSTER
(shingles) frequently affects cranial nerve V, caus-
ing extreme pain along the involved DERMATOME
(pattern of nerves) of the face. ACOUSTIC NEUROMA
is a noncancerous, slow-growing tumor affecting
cranial nerve VIII that causes progressive HEARING
LOSS.
For further discussion of the cranial nerves
within the context of the structures and functions
of the NERVOUS SYSTEM, please see the overview sec-
tion “The Nervous System.”
See also CENTRAL NERVOUS SYSTEM; SMELL AND
TASTE DISORDERS; SPINAL CORD.246 The Nervous System