Cranial Nerves Explained: Functions, Anatomy, and Clinical Assessment

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.