Overview of the Cranial Nerves: Cranial Nerve Disorders: Merck Manual Home Edition

Brain, Spinal Cord, and Nerve Disorders

Cranial Nerve Disorders

Overview of the Cranial Nerves

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Overview of the Cranial Nerves

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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).

Viewing the Cranial Nerves

Twelve pairs of cranial nerves emerge from the underside of the brain, pass through openings in the skull, and lead to parts of the head, neck, and trunk.

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 oculomotor palsy, trigeminal neuralgia, Bell palsy, and hemifacial spasm.

Did You Know...

Some cranial nerve disorders cause problems with eye movement.
Others cause brief, intermittent attacks of excruciating facial pain.

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 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
Cranial Nerve Number	Name	Function	Test
1st	Olfactory	Smell	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.
2nd	Optic	Vision	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.
2nd	Optic	Detection of light	The ability to detect light is tested by shining a bright light (as from a flashlight) into each pupil in a darkened room.
3rd	Oculomotor	Eye movement upward, downward, and inward	The ability to move each eye up, down, and inward is tested by asking the person to follow a target moved by the examiner.
		Narrowing (constriction) or widening (dilation) of the pupil in response to changes in light	The pupils' response to light is checked by shining a bright light (as from a flashlight) into each pupil in a darkened room.
		Raises the eyelids	The upper eyelid is checked for drooping (ptosis).
4th	Trochlear	Eye movement downward and inward	The ability to move each eye down and inward is tested by asking the person to follow a target moved by the examiner.
5th	Trigeminal	Facial sensation	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.
5th	Trigeminal	Chewing	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.
6th	Abducens	Eye movement outward	The ability to move each eye outward beyond the midline is tested by asking the person to look to the side.
7th	Facial	Facial expression, taste in the front two thirds of the tongue, production of saliva and tears, and controls a muscle involved in hearing	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 (aspirinSome Trade Names BAYER , quinineSome Trade Names QUALAQUIN , or aloes).
8th	Auditory (vestibulocochlear)	Hearing	Hearing is tested with a tuning fork or with headphones that play tones of different frequencies (pitches) and loudness (audiometry).
8th	Auditory (vestibulocochlear)	Balance	Balance is tested by asking the person to walk a straight line.
9th	Glossopharyngeal	Swallowing, the gag reflex, and speech	Because both the 9th and 10th cranial nerves control swallowing and the gag reflex, 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 (another test of palate movement).
10th	Vagus	Swallowing, the gag reflex, and speech
10th	Vagus	Control of muscle in some internal organs and the heart rate	This function is not tested as part of the cranial nerve examination.
11th	Accessory	Neck turning and shoulder shrugging	The person is asked to turn the head and to shrug the shoulders against resistance provided by the examiner.
12th	Hypoglossal	Tongue movement	The person is asked to stick out the tongue, which is observed for deviation to one side or the other.

Testing Cranial Nerves

Cranial Nerve Number	Name	Function	Test
1st	Olfactory	Smell	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.
2nd	Optic	Vision	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.
2nd	Optic	Detection of light	The ability to detect light is tested by shining a bright light (as from a flashlight) into each pupil in a darkened room.
3rd	Oculomotor	Eye movement upward, downward, and inward	The ability to move each eye up, down, and inward is tested by asking the person to follow a target moved by the examiner.
		Narrowing (constriction) or widening (dilation) of the pupil in response to changes in light	The pupils' response to light is checked by shining a bright light (as from a flashlight) into each pupil in a darkened room.
		Raises the eyelids	The upper eyelid is checked for drooping (ptosis).
4th	Trochlear	Eye movement downward and inward	The ability to move each eye down and inward is tested by asking the person to follow a target moved by the examiner.
5th	Trigeminal	Facial sensation	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.
5th	Trigeminal	Chewing	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.
6th	Abducens	Eye movement outward	The ability to move each eye outward beyond the midline is tested by asking the person to look to the side.
7th	Facial	Facial expression, taste in the front two thirds of the tongue, production of saliva and tears, and controls a muscle involved in hearing	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 (aspirinSome Trade Names BAYER , quinineSome Trade Names QUALAQUIN , or aloes).
8th	Auditory (vestibulocochlear)	Hearing	Hearing is tested with a tuning fork or with headphones that play tones of different frequencies (pitches) and loudness (audiometry).
8th	Auditory (vestibulocochlear)	Balance	Balance is tested by asking the person to walk a straight line.
9th	Glossopharyngeal	Swallowing, the gag reflex, and speech	Because both the 9th and 10th cranial nerves control swallowing and the gag reflex, 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 (another test of palate movement).
10th	Vagus	Swallowing, the gag reflex, and speech
10th	Vagus	Control of muscle in some internal organs and the heart rate	This function is not tested as part of the cranial nerve examination.
11th	Accessory	Neck turning and shoulder shrugging	The person is asked to turn the head and to shrug the shoulders against resistance provided by the examiner.
12th	Hypoglossal	Tongue movement	The person is asked to stick out the tongue, which is observed for deviation to one side or the other.

Last full review/revision September 2012 by Michael Rubin, MDCM

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