The 12 pairs of cranial nerves emerge directly from the base of
the brain and relay information between the brain and the head and neck regions;
except for the cranial nerve X, which also communicates with internal organs.
Cranial nerves are numbered according to the order they exit the brain, from front to back.
Each nerve of a pair innervates one side of the head or body.
Cranial nerves can be solely sensory, solely motor, or mixed. Some mixed nerves
are predominantly motor or predominantly sensory. - Cranial nerve I, also known as olfactory nerve,
is a sensory nerve responsible for the sense of smell. It originates in olfactory mucosa of
the nasal cavity and terminates in olfactory bulb at the base of frontal lobe. Olfactory
nerve function is assessed as the ability to smell, and is done for each nostril separately.
- Cranial nerve II is optic nerve responsible for vision. It originates in the retina of the eye and
ends in the thalamus. Optic nerve damage leads to partial or total blindness. Vision acuity is
tested to assess nerve damage, one eye at a time. - Cranial nerve III, or oculomotor nerve,
is predominantly motor. It controls most of the eye movements, as well as opening of
eyelid and constriction of pupil. It originates in the midbrain and contains both somatic and
parasympathetic fibers. Somatic fibers innervate several extraocular, extrinsic eye muscles;
while parasympathetic fibers terminate inside the eyeball and supply intrinsic eye muscles
responsible for movement of the lens and pupil. Cranial nerves III are classified as
predominantly motor because they also contain a small number of sensory fibers that provide
the brain with feedback information about eye movements and location, known as proprioception.
Oculomotor nerve palsy results in drooping eyelid, dilated pupil,
loss of accommodation reflex, double vision, and inability to move eye in certain directions.
A characteristic sign is the “down and out” deviation, where the affected eye drifts
downward and outward. Additional assessment tests include pupillary response to light,
and ability to tract moving objects. - Cranial nerve IV, also called trochlear nerve,
is the smallest cranial nerve and the only one that exits from the dorsal side of the brainstem.
It originates in the midbrain and terminates in the superior oblique muscle of the eye.
Damage to this nerve leads to double vision and eye deviation upward. The affected eye
is unable to move down when looking to the direction of the normal eye. Patients
often adopt a characteristic head tilt forward (chin tuck in) and toward the normal eye side.
- Cranial nerve V, or trigeminal nerve, connects the pons of the brainstem and the face.
It has 3 divisions: - the ophthalmic division
conveys sensory information from the upper face, including the surface of eyeball,
superior nasal mucosa, and frontal and ethmoid sinuses. Loss of sensation is tested by touching
the eyeball with a cotton wisp. To note, however, that a “no blinking” response may also result from
facial muscle weakness due to 7th cranial nerve damage, in which case the patient can
feel the cotton wisp but fails to blink. - the maxillary division relays sensory
information from the middle section of the face, including the inferior nasal mucosa,
maxillary sinus, palate, and upper teeth and gums. - the mandibular division is a mixed nerve. Its
sensory component transmits sensation from the lower face, including the anterior two
thirds of the tongue, but excluding taste buds, and lower teeth and gums. The motor component
controls the muscles of mastication, or chewing. Impaired motor function can be detected as a
deviation of the jaw to the side of weakened muscles when the patient clenches the teeth.
- Cranial nerve VI, or abducens nerve, is a predominantly motor nerve responsible for
lateral eye movement. It originates in the lower pons and terminates in the lateral
rectus muscle of the eye. Damage to this nerve results in inability to move eye laterally.
The affected eye turns inward at rest. The defect is more noticeable when the patient looks toward
the affected side or fixates at faraway objects. - Cranial nerves VII, also known as facial nerve,
is a mixed nerve with many branches and diverse functions:
- It controls the muscles of facial expression, including those involved
in eye blinking and closing; - It conveys taste sensations
from the anterior two-thirds of the tongue; - and it carries parasympathetic nerve impulses
to tear glands and salivary glands. The motor division has 5 branches,
each provides input to a group of facial muscles. There are also motor fibers to
the stapedius muscle of the middle ear. Damage to facial nerve results in facial
muscle weakness, which typically manifests as asymmetry of facial movements, especially when the
patient smiles or grimaces. Other symptoms include drooping of mouth, drooling, inability to close
one eye, facial pain or abnormal sensation, distorted sense of taste, mostly for sweet
and salty foods, and intolerance to loud noise. - Cranial nerve VIII, or vestibulocochlear nerve,
consists of 2 nerves: vestibular nerve responsible for equilibrium, and cochlear nerve responsible
for hearing. The vestibular nerve originates in the vestibule of the inner ear and terminates in
the pons; while cochlear nerve originates in the cochlea of the inner ear, and ends in the medulla.
Damage to cranial nerve VIII results in impaired hearing, vertigo, tinnitus, and involuntary
rhythmic eye movements known as nystagmus. - Cranial nerve IX, also known as glossopharyngeal
nerve, is a mixed nerve that provides sensory, motor, and parasympathetic functions:
- It conveys sensory information from the upper pharynx, middle and outer ear, and the posterior
third of the tongue, including taste buds. - It carries visceral sensory signals from
baroreceptors in the carotid sinus, and chemoreceptors in the carotid body,
providing inputs for regulation of blood pressure and monitoring
of blood oxygen, respectively. - It provides parasympathetic
innervation to the parotid salivary gland; - and controls the stylopharyngeus muscle
responsible for elevation of the larynx, pharynx, as well as dilation of pharynx
during speech and swallowing. Damage to glossopharyngeal
nerve results in difficulty swallowing, speaking, and distorted sense of taste,
especially for bitter and sour tastants. - Cranial nerve X, or vagus nerve,
is the longest cranial nerve with diverse functions, many of which are critical:
- It is the major parasympathetic nerve regulating pulmonary,
cardiovascular and digestive activities; - It controls most muscles of the pharynx,
larynx, and some muscles of the soft palate and tongue, and thus
plays an important role in swallowing and speech; - It conveys sensory information from the pharynx,
larynx, and thoracic and abdominal areas, including baroreceptors and
chemoreceptors in the aorta, for regulation of blood pressure and blood oxygen level.
- Minor functions include general sensation from the outer ear, and taste sensation
from the pharynx, palate, and epiglottis. Damage to vagus nerve results in hoarseness
or loss of voice, difficulty swallowing, impaired gag reflex, reduced gastrointestinal motility,
and increased heart rate. The effect is fatal if both nerves are damaged.
The 9th and 10th cranial nerves are usually evaluated together. In addition to observing
any speech or swallowing problems, patients are tested for symmetry of the gag reflex and
symmetry of palate elevation when saying "ah". - Cranial nerve XI, or accessory nerve, is an
unusual cranial nerve that has both cranial and spinal roots. The cranial roots originate from the
medulla and exit the skull as the internal branch, which merges shortly with the vagus nerve. This
part of accessory nerve is thought to innervate muscles of the palate, pharynx, and larynx.
The spinal roots exit as the external branch and control the sternocleidomastoid and trapezius
muscles. There are also some sensory fibers carrying sensory and nociceptive signals.
People with accessory nerve damage typically experience shoulder discomfort, weakness;
and the affected shoulder may sag. Patients may also have difficulty turning the head to
the opposite side of the affected muscle. - Cranial nerve XII, or hypoglossal nerve,
is a predominantly motor nerve controlling extrinsic and intrinsic muscles of the tongue.
It is responsible for various tongue movements and shapes required for
normal swallowing and speech production. Damage to this nerve results in speech
and swallowing difficulties. The tongue typically deviates toward the affected side.