Cranial nerves

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Cranial nerves are nerves that emerge directly from the brain in contrast to spinal nerves which emerge from segments of the spinal cord. There are conventionally recognised twelve cranial nerves in humans. 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.

1 Names of nerves
2 Cranial nerves in non-human vertebrates
3 Mnemonic devices
4 Thirteen cranial nerves?
5 See also
6 External links

[edit] Names of nerves

The 12 pairs of cranial nerves are traditionally abbreviated by the corresponding Roman numerals. They are numbered according to where their nucleii lie in the brain stem, i.e 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.

All cranial nerves except for the olfactory and optic nerves belong to the peripheral nervous system, having axons that are myelinated by Schwann cells (myelinating cells of the peripheral nervous system) rather than oligodendrocytes (myelinating cells of the central nervous system). The olfactory and optic nerves are named as such, but are technically not nerves, and are continuations of the central nervous system.

Note: technically the accessory nerve splits into the spinal accessory nerve and the cranial accessory nerve, but the latter quickly combines with the vagus nerve.

#	Name	Nuclei	Function
I	Olfactory nerve	Anterior olfactory nucleus	Sense of smell
II	Optic nerve	Lateral geniculate nucleus	Transmits visual information to the brain
III	Oculomotor nerve	Oculomotor nucleus, Edinger-Westphal nucleus	Controls most of the eye movements
IV	Trochlear nerve	Trochlear nucleus	Rotates the eye away or down from the nose
V	Trigeminal nerve	Principal sensory trigeminal nucleus, Spinal trigeminal nucleus, Mesencephalic trigeminal nucleus, Trigeminal motor nucleus	Gives sensations to the face
VI	Abducent nerve (or abducens nerve)	Abducens nucleus	Controls each eye's ability to move away from the midline
VII	Facial nerve (VII)	Facial nucleus, Solitary nucleus, Superior salivary nucleus	Controls facial expression and taste to two-thirds of the tongue, salivary glands
VIII	Vestibulocochlear nerve (or auditory-vestibular nerve)	Vestibular nuclei, Cochlear nuclei	Senses sound, rotation and gravity (essential for balance & movement)
IX	Glossopharyngeal nerve	Nucleus ambiguus, Inferior salivary nucleus, Solitary nucleus	Controls various sensations, glands, and muscles
X	Vagus nerve	Nucleus ambiguus, Dorsal motor vagal nucleus, Solitary nucleus	Supplies motor parasympathetic fibers to nearly all internal organs
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	Controls most of the tongue muscles as well as others

[edit] Cranial nerves in non-human vertebrates

Human cranial nerves are evolutionarily homologous to those found in many other vertebrates. The reptiles including birds, and mammals). 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 0 (as it exits the brain before the first cranial nerve).

[edit] Mnemonic devices

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.

OLd OPie OCCasionally' TRies TRIgonometry And Feels VEry GLoomy, VAGUe Acutely HYPOactive
On Old Olympus' Towering Top A Finely Vested German Viewed A Hawk
On Old Olympus' Towering Top A Finn And German Viewed Some Hops
- (Note that the Vestibulocochlear nerve is referred to by its former name, Auditory, in this mnemonic.)
Oliver the optimistic octopus trots triumphantly about facing audiences glossily vaguely spinning hippos.
- (Note that the accessory nerve is referred to by its alternate name Spinal accessory nerve, and the Vestibulocochlear nerve by its former name, Auditory, in this mnemonic.)
Old Opticians Occasionally Too Try And Feel Audrey Glossop's Vagina AcH!
Oh, Oh, Oh, To Touch And Feel Virgin Girl's Vagina And Hymen.
Oh, Oh, Oh, To Touch And Feel A Girl's Vagina And Hymen.
- (Note that the Vestibulocochlear nerve is referred to by its former name, Auditory, in this mnemonic.)
Ooh, Ooh, Ooh To Touch And Feel Very Good Velvet. Such Heaven!
- (Note that the accessory nerve is referred to by its alternate name Spinal accessory nerve in this mnemonic.)
Ooh, Ooh, Ooh Topless Tiffany And Fat Valerie Got Vaginitis And Hepatitis
Oh Once One Takes The Anatomy Final Very Good Vacations Are Heavenly
OOO Truely There Are Five Very Gorgeous Vixens Awaiting Him

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.
Some Say Marry Money, But My Brother Says Big Brains Matter More.
Some Say Marry Money, But My Brother Says Big Boobs Matter More.
Some Say Marry Money, But My Brother Says Big Butts Matter More.
Small Ships Make Money, But My Brother Says Big Boats Make More.
Some Says Marry Money But My Brother Says Bad Business Marry Money

[edit] Thirteen cranial nerves?

Although twelve nerves are classically described in humans, there is a theory that finds favour with some authors, that humans really have thirteen cranial nerves (Andy Lelli , 1999). If the C1 spinal nerve were considered the thirteenth cranial nerve, C2 through C8 would be renamed as the first through seventh cervical nerves (though anatomists might change the term 'cervical' to 'nuchal' or some other term to avoid confusion between the two nomenclatures). This would make the numbering system consistent all the way through the spinal column - every nerve would exit below its corresponding vertebra, and the number of vertebrae would equal the number of spinal nerves.

Alternatively, C1 could be considered the spinal root of the hypoglossal nerve (CN XII). This would, again, make the numbering system consistent.