Cranial Nerves

Cranial Nerves

The cranial nerve is a part of the peripheral nervous system and primarily serves the head and the neck. There are about 12 pairs of cranial nerves where only one pair extends to the thoracic and the abdominal cavities.

Characteristics of the cranial nerves

1. Cranial nerves are numbered and are in order.
2. The names of the cranial nerve, in most cases, reveal the most significant structure that they contain.
3. Most cranial nerves are mixed nerves, meaning they are both functioning as sensory and for motor function. Only three pairs of cranial nerves are purely sensory in function.
4. To remember the cranial nerves in order take note and memorize this: Oh, Oh, Oh To Touch And Feel Very Good Velvet AH. The first letter of the words (capitalized letter) represents the name of the cranial nerves in order.

The Twelve Cranial Nerves

Cranial Nerve I: Olfactory

The first cranial nerve is purely sensory in function. The olfactory nerve is the one responsible for carrying the impulse for smelling things (sense of smell).  Olfactory receptor fibers arise from the nasal mucosa and synapse with the olfactory bulbs. Olfactory bulb in turn, sends fibers to the adrenal cortex where the interpretation of stimuli is done. To know if this cranial nerve is functioning well, the subject is asked to identify substances by sniffing certain aromatic substances while his or her eyes are covered. Substances such as cloves or vanilla may be used; however, alcohol or irritating substances should be avoided.

Cranial Nerve II: Optic

Optic cranial nerve is the second nerve that is purely sensory in function. This is the nerve that carries the impulse of a person to see things or for vision. The fibers of this nerve surface from the retina of the eyes and form the optic nerve. There are two optic nerves and these two forms the optic chiasma by partial crossover of fibers. These fibers are branching continuously to the optic cortex through the optic tracts. To assess the functioning of this nerve, vision and visual field are tested with an eye chart. This is done by asking and testing the subject’s point in which he or she first saw the object moving into the visual field. The interior of the eye are viewed using special equipment called an opthalmoscope.

Cranial Nerve III: Oculomotor

Oculomotor nerve functions as the supply of motor fibers to four of the six eye muscles. These muscles are the superior, inferior and the middle rectus and the inferior oblique muscles. The eyeball is directed into the eyelid onto the internal eye muscles that controls the shape of the lens and the size of the pupils. The fibers of the oculomotor nerve run from the midbrain to the eye. To examine the functioning of this nerve, the subject’s pupils are examined for size, shape and size equality. Aside from that papillary reflex is tested by using a penlight and noting the change in the size of the pupils. Pupils normally should constrict when it is exposed to light or illuminated. Another area to be assessed is the eye convergence, where the subject is assessed if he or she can follow a moving object.

Cranial Nerve IV: Trochlear

The cranial nerve IV or trochlear nerve supplies motor fibers for one external eye muscle which is the superior oblique muscle. The fibers of this nerve run from the midbrain to the eye. In assessing the functioning of this nerve, the test done is the same with cranial nerve III for determining the ability of the subject to follow a moving object.

Cranial Nerve V: Trigeminal

The trigeminal nerve serves as motor and sensory fibers in the human body. The pair trigeminal nerve serves a sensory function by conducting impulses from the skin of the face and mucosa of the nose anf mouth. The motor fibers that cranial nerve V plays a very important role in eating. With the presence of these motor fibers chewing muscles are activated. These fibers surface from the pons and form three divisions that run along the face. To assess the sensory function of this cranial nerve, sensations of pain, touch and temperature should be tested. This is done by using a safety pin and hot and cold objects. The corneal reflex is tested using a wisp of cotton. To assess the motor function, the subject is asked to open his or her mouth against resistance and move the jaw from side to side.

Cranial Nerve VI: Abducens

Abduces are cranial nerves that leave the pons and runs to the eye. Cranial nerve VI plays a very important role in supplying motor fibers to the lateral rectus muscle of the eye. This enables a paerson to roll the eye laterally. To assess the ability of the eye to move laterally, the test conducted is the same with cranial nerve III.

Cranial Nerve VII: Facial

Cranial nerve VII is responsible for activating the muscles of the facial expression. Also this paired nerve is responsible for stimulating the lacrimal and the salivary glands. Sensory impulse is also carried by this nerve. The taste of the taste buds in the anterior portion of the tongue is due to the working of the facial cranial nerve. The fibers of Cranial Nerve VII leave the pons and moves to the face. To test the sensory function of the tongue, the anterior two-thirds of the tongue is tested for the ability to taste sweet, sour, bitter and salty substances. Tearing is tested with ammonia fumes and facial expression is assessed by asking the subject to close his or her eyes, smile, frown and whistle.

Cranial Nerve VIII: Vestibulocochlear

This is a purely sensory cranial nerve and is responsible for transmitting impulses for the sense of balance through the vestibular branch. The cochlear branch is the one that transmits the impulses for the sense of hearing. The fibers of Cranial Nerve VIII run from the equilibrium and hearing receptors in the inner ear to the brain stem. To check a subject’s hearing, air and bone conduction is assessed using a tuning fork.

Cranial Nerve IX: Glossopharyngeal

The glossopharyngeal cranial nerve is the one supplying the motor fibers to the throat or pharynx. These fibers allow a person to swallow food and produce saliva in the human body. Aside from the motor fibers, cranial nerve IX also carries sensory impulses from the taste buds of the posterior tongue and from the pressure receptors of the carotid artery. Both fibers surface from the medulla which runs along to the throat. In assessing the functioning of this crania nerve the subject’s gag and swallowing reflex is checked. Aside from that, the subject will also be asked to cough and speak. For the sensory function assessment, the posterior tongue is assessed for taste.

Cranial Nerve X: Vagus

The vagus cranial nerve is responsible for carrying sensory impulses from and motor impulses to the pharynx, larynx and the abdominal and thoracic viscera. Most of these motor fibers are parasympathetic fibers that promote digestive activity and help in the regulation of heart activity. The fibers of cranial nerve X surface from the medulla and descend to the thorax and abdominal cavity. The test for the functioning of the vagus nerve is conducted with the cranial nerve IX.

Cranial Nerve XI: Accessory

The accessory cranial nerve arises from the medulla and superior spinal cord. These fibers travel back to the muscles of the neck and back. Motor fibers of the accessory nerve activate the sternocleidomastod and trapezius muscles. Assessment of the nerve involves measuring the strength of the subject’s sternocleidomastoid and trapezius muscles by asking him or her to rotate the head and move the shoulders against resistance.

Cranial Nerve XII: Hypoglossal

Cranial nerve XII carries both motor fibers and sensory fibers. The motor fibers are responsible for controlling tongue movement while the sensory fibers carry the impulses of the tongue. These fibers are moving along from the medulla of the brain to the tongue. During the assessment of the nerve function the subject is asked to stick out his or her tongue and any position abnormalities are noted by the nurse or examiner.

Cranial Nerves Summary