cranial nerve

n.
Any of several nerves that arise in pairs from the brain stem and reach the periphery through openings in the skull. There are 12 such pairs in mammals, birds, and reptiles and usually 10 pairs in amphibians and fish.

Any peripheral nerve which has its central nervous system connection with the brain, as opposed to the spinal cord, and reaches the brain through a hole (foramen) in the skull. Nerve fibers are sensory if they carry information from the periphery to the brain, and motor if they carry information from the brain to the periphery. Sensory fibers are classified as (1) somatic sensory (ss) if they come from the skin or muscle sense organs, (2) visceral sensory (vs) if they come from the viscera, and (3) special sensory (sp.s) if they come from special sense organs such as the eye and ear. Motor fibers are classified as (1) somatic motor (sm) if they carry information to somatic striated muscles, (2) general visceral motor (gvm) if they carry information to glands, smooth muscle, or cardiac muscle, and (3) special visceral motor (svm) if they carry information to visceral striated muscle. A cranial nerve may have only one fiber type or several; cranial nerves with several fiber types are called mixed nerves.

In mammals, 12 pairs of cranial nerves, numbered I through XII, are usually described. However, mammals also have an anterior unnumbered nerve, the terminal nerve. Many vertebrates also have two pairs of lateral line nerves (unnumbered), and lack discrete nerves XI and XII. The table summarizes the cranial nerves of vertebrates. See also Brain.

Cranial nerves of vertebrates

No.	Name (type)	Peripheral origin or destination
—	Terminal (ss)	Anterior nasal epithelium
I	Olfactory (sp.s)	Olfactory mucosa
—	Vomeronasal (sp.s)	Vomeronasal mucosa
II	Optic (sp.s)	Retina of eye
III	Oculomotor (sm)	Four extrinsic eye muscles
IV	Trochlear (sm)	One extrinsic eye muscle
V	Trigeminal (svm)	Muscles of mandibular arch derivative
	(ss)	Most of head
VI	Abducens (sm)	One extrinsic eye muscle
—	Anterior lateral line (sp.s)	Lateral line organs of head
VII	Facial (svm)	Muscles of hyoid arch derivative
	(gvm)	Salivary glands
	(ss)	Small part of head
	(vs)	Anterior pharynx
	(sp.s)	Taste, anterior tongue
VIII	Vestibulocochlear (sp.)	Inner ear
—	Posterior lateral line (sp.s)	Lateral line organs of trunk
IX	Glossopharyngeal (svm)	Muscles of third branchial arch
	(gvm)	Salivary gland
	(ss)	Skin near ear
	(vs)	Part of pharynx
	(sp.s)	Taste, posterior tongue
X	Vagus (svm)	Muscles of arches 4–6
	(gvm)	Most viscera of entire trunk
	(vs)	Larynx and part of pharynx
	(sp.s)	Taste, pharynx
XI	Spinal accessory (svm)	Some muscles of arches 4–6
XII	Hypoglossal (sm)	Muscles of tongue and anterior throat

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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.

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.

The human brain viewed from below to show the cranial nerves

— Sheila Jennett

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