What Are the Functions of the Cranial Nerves?

There are twelve pairs of cranial nerves, all but two of which originate in the brain stem. These nerves facilitate operation of the senses and some muscles' of sense organs.

Cranial Nerves I and II

• Cranial nerves I and II are the olfactory and optic nerves, respectively. These are the only two cranial nerves that do not originate in the brain stem. The olfactory nerve originates in the part of the brain called the olfactory bulb. It functions in controlling the sense of smell (olfaction). The optic nerve originates in the eye and carries information to the brain for vision. These nerves can be tested very simply. In the case of cranial nerve I, a simple test to see if the subject can identify certain distinct smells like coffee, garlic or lemon. Cranial nerve II can be tested using the standard eye chart.

Cranial Nerves III, IV and VI

• These are the nerves that control movement. Cranial nerve III, the oculomotor nerve, is the nerve that controls eye movement and pupil constriction. Cranial nerves IV and VI also control eye movement. They are called, in order, the Trochlear nerve and the Abducens nerve. All three may be tested by holding one finger up and asking the subject to keep his head still and track the movement of your finger from side to side and up and down. To check pupil constriction, look at the pupils in bright and then in dim light. The pupils should be smaller in bright light and larger in dim.

Cranial Nerve V

• Cranial nerve V is the trigeminal nerve and controls the touch and pain sensations from the face and head. The trigeminal nerve also controls the muscles used for chewing. To test this nerve, ask the subject to close his jaws as if biting down on gum. To test facial sensation, touch different parts of her face with something soft or blunt and ask her to name the places you are touching. Do not place anything in the mouth.

Cranial Nerves VII and VIII

• These cranial nerves are the facial nerve (VII) and vestibulocochlear nerve (VIII). The facial nerve controls taste on the front two-thirds of the tongue. It also receives information from the ear and controls the muscles of facial expression. This can be tested by asking the subject to smile or frown. Taste can be tested with something sweet or salty placed on the front part of the tongue. The vestibulocochlear nerve controls hearing and balance. To test, determine how far away the subject can hear a particular sound. It is not safe to test balance except in an actual clinical test.

Cranial Nerves IX and X

• The glossopharyngeal nerve (IX) controls some functions of swallowing as well as taste on the back third of the tongue. It conducts information from the tongue, tonsils and pharynx (the cavity that connects the mouth and nasal passages with the esophagus). Observe the subject as he drinks and swallows water. Taste may be tested in the previously described fashion except the salty or sweet substance should be placed on the back of the tongue. The vagus nerve (X) controls pain/touch sensations of digestion, heart rate and some glands. Do not test the vagus nerve.

Cranial Nerve XI

• The spinal accessory nerve controls the muscles of head movement. In order to test cranial nerve XI, place your hands lightly on the sides of the subjects head and ask her to move it in all directions.

Cranial Nerve XII

• The hypoglossal nerve serves to control the muscles of the tongue. To test, ask the subject to stick out his tongue and wiggle it in all directions.