cranial nerve

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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 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 human brain viewed from below to show the cranial nerves

— Sheila Jennett

Cranial nerves

Cranial nerves are nerves that emerge directly from the brain in contrast to spinal nerves which emerge from segments of the spinal cord. Although thirteen cranial nerves in humans fit this description, twelve are conventionally recognized. The nerves from the third onward arise from the brain stem. Except for the tenth and the eleventh nerve, they primarily serve the motor and sensory systems of the head and neck region. However, unlike peripheral nerves which are separated to achieve segmental innervation, cranial nerves are divided to serve one or a few specific functions in wider anatomical territories.

Names of Nerves

The 12 pairs of cranial nerves are traditionally abbreviated by the corresponding Roman numerals. They are numbered according to where their nuclei lie in the brain stem, e.g. Cranial Nerve III (the Oculomotor nerve) leaves the brainstem at a higher position than Cranial nerve XII, whose origin is located more caudally (lower) than the other cranial nerves.

#	Name	Nuclei	Function
0	Cranial nerve zero (CN0 is not traditionally recognized.)[1]	olfactory trigone, medial olfactory gyrus, and lamina terminalis	Still controversial New research indicates CN0 may play a role in the detection of pheromones [2][3]
I	Olfactory nerve	Anterior olfactory nucleus	Transmits the sense of smell
II	Optic nerve	Lateral geniculate nucleus	Transmits visual information to the brain
III	Oculomotor nerve	Oculomotor nucleus, Edinger-Westphal nucleus	Innervates levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique, which collectively perform most eye movements
IV	Trochlear nerve	Trochlear nucleus	Innervates the superior oblique muscle, which depresses, pulls laterally, and intorts the eyeball
V	Trigeminal nerve	Principal sensory trigeminal nucleus, Spinal trigeminal nucleus, Mesencephalic trigeminal nucleus, Trigeminal motor nucleus	Receives sensation from the face and innervates the muscles of mastication
VI	Abducens nerve	Abducens nucleus	Innervates the lateral rectus, which abducts the eye
VII	Facial nerve	Facial nucleus, Solitary nucleus, Superior salivary nucleus	Provides motor innervation to the muscles of facial expression and stapedius, receives the special sense of taste from the anterior 2/3 of the tongue, and provides secretomotor innervation to the salivary glands (except parotid) and the lacrimal gland
VIII	Vestibulocochlear nerve (or auditory-vestibular nerve or statoacustic nerve)	Vestibular nuclei, Cochlear nuclei	Senses sound, rotation and gravity (essential for balance & movement)
IX	Glossopharyngeal nerve	Nucleus ambiguus, Inferior salivary nucleus, Solitary nucleus	Receives taste from the posterior 1/3 of the tongue, provides secretomotor innervation to the parotid gland, and provides motor innervation to the stylopharyngeus
X	Vagus nerve	Nucleus ambiguus, Dorsal motor vagal nucleus, Solitary nucleus	Supplies branchiomotor innervation to most laryngeal and pharyngeal muscles; provides parasympathetic fibers to nearly all thoracic and abdominal viscera down to the splenic flexure; and receives the special sense of taste from the epiglottis
XI	Accessory nerve (or cranial accessory nerve or spinal accessory nerve)	Nucleus ambiguus, Spinal accessory nucleus	Controls muscles of the neck and overlaps with functions of the vagus
XII	Hypoglossal nerve	Hypoglossal nucleus	Provides motor innervation to the intrinsic muscles of the tongue and other glossal muscles

Cranial nerves in non-human vertebrates

Human cranial nerves are evolutionarily homologous to those found in many other vertebrates. Cranial nerves XI and XII evolved in the common ancestor to amniotes (non-amphibian tetrapods) thus totalling twelve pairs. These characters are synapomorphies for their respective clades. In some primitive cartilagenous fishes, such as the dogfish (Squalos acanthos), there is a terminal nerve numbered zero (as it exits the brain before the first cranial nerve).

Mnemonic devices

Main article: List of mnemonics for the cranial nerves

As the list is important to keep in mind during the examination of the nervous system, there are many mnemonic devices in circulation to help remember the names and order of the cranial nerves. Because the mind recalls rhymes well, the best mnemonics often use rhyming schemes. Examples are:

• On Old Olympus Towering Tops, A Finn And German Viewed Some Hops.
• OLd OPie OCcasionally TRies TRIGonometry And Feels VEry GLOomy, VAGUe, And HYPOactive
• On Occasion Our Trusty Truck Acts Funny. Very Good Vehicle Any How.

Another to help remember the types of information these nerves carry (sensory, motor, or both) is thus:

• Some Say Money Matters, But My Brother Says Big Brains Matter More.

See also

• Cranial nerve nucleus

External links

• Examination of the cranial nerves - University of Toronto Medicine
• Norman/Georgetown cranialnerves
• CN Medical Notes on rahulgladwin.com

References

1. ^ Fuller GN, Burger PC. "Nervus terminalis (cranial nerve zero) in the adult human." Clin Neuropathol 9, no. 6 (Nov-Dec 1990): 279-283.
2. ^ Merideth, Michael. "Human Vomeronasal Organ Function." Oxford Journals: Chemical Senses, 2001.
3. ^ Fields, R. Douglas. "Sex and the Secret Nerve." Scientific American Mind, February 2007.

Nerves of head and neck: the cranial nerves
I-IV	olfactory • optic • oculomotor (superior branch, inferior branch) • trochlear
V: trigeminal	trigeminal ganglion • ophthalmic • maxillary • mandibular
VI: abducens	no significant branches
VII: facial	nervus intermedius • geniculate • inside facial canal (greater petrosal, nerve to the stapedius, chorda tympani) • at exit from stylomastoid foramen (posterior auricular, digastric • stylohyoid) • on face (temporal, zygomatic, buccal, mandibular, cervical)
VIII: vestibulocochlear	cochlear (striae medullares, lateral lemniscus) • vestibular (Scarpa's ganglion)
IX: glossopharyngeal	nucleus ambiguus • ganglia (superior, petrous) • tympanic (tympanic plexus, lesser petrosal) • carotid sinus • pharyngeal branches • pharyngeal plexus
X: vagus	ganglia (jugular, nodose) • in the jugular fossa (meningeal branch, auricular branch) • in the neck (pharyngeal branch, superior laryngeal ext and int, recurrent laryngeal, superior cervical cardiac) • in the thorax (inferior cardiac, pulmonary branches, anterior vagal trunk, posterior vagal trunk) • in the abdomen (celiac branches - renal branches - hepatic branches of anterior vagal trunk - anterior gastric branches of anterior vagal trunk - posterior gastric branches of posterior vagal trunk)
XI-XII	accessory • hypoglossal

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