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Twelve pairs of nerves—the cranial nerves—lead directly from the brain to various parts of the head, neck, and trunk. Some of the cranial nerves are involved in the special senses (such as seeing, hearing, and taste), and others control muscles in the face or regulate glands. The nerves are named and numbered (according to their location, from the front of the brain to the back).
A cranial nerve disorder may affect the connections between cranial nerve centers within the brain. An example is internuclear ophthalmoplegia. Or, a disorder may affect only one cranial nerve. Examples are trigeminal neuralgia, Bell's palsy, hemifacial spasm, and glossopharyngeal neuralgia.
Symptoms depend on which nerves are damaged. For example, damage often occurs to nerves that control eye movement. If both eyes have trouble moving in the same direction, people may not be able to look in that direction. If only one eye can look in a certain direction, people may have double vision (two images seen side by side) when they look in that direction.
When doctors suspect a cranial nerve disorder, they test the function of a cranial nerve by asking the person to do simple tasks, such as to follow a moving target with the eyes.
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| Testing Cranial Nerves |
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Cranial Nerve Number
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Name
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Function
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Test
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1st
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Olfactory
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Smell
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The ability to smell is tested by asking the person to identify items with very specific odors (such as soap, coffee, and cloves) placed under the nose. Each nostril is tested separately.
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2nd
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Optic
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Vision
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The ability to see is tested by asking the person to read an eye chart. Peripheral vision is tested by asking the person to detect objects or movement from the corners of the eyes.
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Detection of light
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The ability to detect light is tested by shining a bright light (as from a flashlight) into each pupil in a darkened room.
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3rd
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Oculomotor
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Eye movement upward, downward, and inward
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The ability to move each eye up, down, and inward is tested by asking the person to follow a target moved by the examiner.
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Narrowing (constriction) or widening (dilation) of the pupil in response to changes in light
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The pupils' response to light is checked by shining a bright light (as from a flashlight) into each pupil in a darkened room.
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Raises the eyelids
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The upper eyelid is checked for drooping (ptosis).
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4th
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Trochlear
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Eye movement downward and inward
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The ability to move each eye down and inward is tested by asking the person to follow a target moved by the examiner.
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5th
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Trigeminal
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Facial sensation
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Sensation in areas of the face is tested using a pin and a wisp of cotton. The blink reflex is tested by touching the cornea of the eye with a cotton wisp.
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Chewing
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Strength and movement of muscles that control the jaw are tested by asking the person to clench the teeth and open the jaw against resistance.
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6th
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Abducens
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Eye movement outward
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The ability to move each eye outward beyond the midline is tested by asking the person to look to the side.
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7th
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Facial
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Facial expression, taste in the front two thirds of the tongue, and production of saliva and tears
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The ability to move the face is tested by asking the person to smile, to open the mouth and show the teeth, and to close the eyes tightly. Taste is tested using substances that are sweet (sugar), sour (lemon juice), salty (salt), and bitter (aspirin, quinine, or aloes).
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8th
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Auditory (vestibulocochlear)
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Hearing
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Hearing is tested with a tuning fork or with headphones that play tones of different frequencies (pitches) and loudness (audiometry).
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Balance
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Balance is tested by asking the person to walk a straight line.
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9th
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Glossopharyngeal
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Swallowing, gag reflex, and speech
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Because the 9th and 10th cranial nerves control similar functions, they are tested together. The person is asked to swallow. The person is asked to say “ah-h-h” to check movement of the palate (roof of the mouth) and uvula (the small, soft projection that hangs down at the back of throat). The back of the throat may be touched with a tongue blade, which evokes the gag reflex in most people. The person is asked to speak to determine whether the voice sounds nasal.
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10th
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Vagus
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Swallowing, gag reflex, and speech
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Control of muscle in internal organs (including the heart)
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11th
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Accessory
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Neck turning and shoulder shrugging
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The person is asked to turn the head and to shrug the shoulders against resistance provided by the examiner.
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12th
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Hypoglossal
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Tongue movement
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The person is asked to stick out the tongue, which is observed for deviation to one side or the other.
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Last full review/revision July 2007 by Michael Rubin, MDCM
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