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cranial nerves

cranial nerves are those that carry information directly to and from the brain, entering or emerging through openings in the cranium (skull). There are twelve pairs, known by Roman numerals according to the sequence in which they are attached to the brain. Those that serve the special senses of smell and vision are purely sensory, and differ from the rest in being essentially extensions of the brain itself. The others are part motor, part sensory; for these, the relevant motor neurons (analogous to those in the spinal cord) are in collections of cells (nuclei) in the brain stem, and the cell bodies of the sensory neurons are in ganglia outside the brain stem (analogous to those in the dorsal roots of spinal nerves). Also, nerve fibres belonging to the cranial component of the parasympathetic division of the autonomic nervous system arise from neurons in the brain stem and form a part of some of the cranial nerves.

The 12 pairs of cranial nerves

I

Olfactory

Special sensory, for smell. Consists of small bundles of fibres passing from the nerve endings in the olfactory epithelium, through perforated bone at the top of the nose, to enter the olfactory bulb, underneath the frontal lobe.

II

Optic

Special sensory, for vision. Made up of fibres that converge from the whole of the retina. Pass backwards to the base of the brain from the back of the eyeballs.

III

Oculomotor

Mainly motor to small muscles that move the eyeball. Also carry autonomic (parasympathetic) nerve fibres that constrict the pupil.

IV

Trochlear

Mainly motor to the muscle that turns the eyeball downwards and outwards.

V

Trigeminal

The largest of the cranial nerves, with 3 main divisions: ophthalmic, maxillary, and mandibular. Mainly sensory, from most of the tissues of the head, face, and mouth; motor to the muscles that move the lower jaw.

VI

Abducens

Motor to the muscle that moves the eyeball outwards.

VII

Facial

Motor to the facial muscles; sensory, for taste, from the front part of the tongue; also parasympathetic nerve fibres to salivary glands.

VIII

Auditory (vestibulo-cochlear)

Mainly sensory, for hearing and balance; enter the brain from the inner ear (cochlea and vestibule).

IX

Glossopharyngeal

Sensory and motor, for the mouth, neck, including pharynx, larynx, and tongue (taste from the back part); also transmit sensory information concerning blood gases and blood pressure from the neck arteries; parasympathetic fibres to salivary glands.

X

Vagus

Carry visceral sensory information from thoracic and abdominal organs; motor to the larynx (speech) and oesophagus (swallowing); parasympathetic to heart, lungs, and to muscle and glands of the alimentary tract as far as the middle of the colon.

XI

Accessory

Shares the functions of X, and joins with uppermost spinal nerves to innervate muscles that move the head and shoulders.

XII

Hypoglossal

Motor to the tongue muscles.


Sheila Jennett

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cranial nerves

cranial nerves Ten to twelve pairs of nerves in vertebrates that emerge directly from the brain. They supply the sense organs and muscles of the head, neck, and viscera. Examples of cranial nerves include the optic nerve (II) and the vagus nerve (X). With the spinal nerves, the cranial nerves form an important part of the peripheral nervous system.

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cranial nerves

cranial nerves (kray-niăl) pl. n. the 12 pairs of nerves that arise directly from the brain and leave the skull through separate apertures. Compare spinal nerves.

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