Cranial nerve

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Gray's Fig. 686 - Upper part of medulla spinalis and hind- and mid-brains; posterior aspect, exposed in situ. (Illustrates several, though not all, of the cranial nerves.)

Cranial nerves are nerves that emerge from the brainstem instead of the spinal cord. Cranial nerves I and II are named as such, but are technically not nerves, as they are continuations of the central nervous system.

[edit] Names of nerves

In human anatomy, there are exactly 12 pairs of them, traditionally abbreviated by the corresponding Roman numerals:

Olfactory nerve (I)
Optic nerve (II)
Oculomotor nerve (III)
Trochlear nerve (IV)
Trigeminal nerve (V), subdivided into
- Ophthalmic nerve (V1)
- Maxillary nerve (V2)
- Mandibular nerve (V3)
Abducens nerve (VI), sometimes called the abducent nerve
Facial nerve (VII), subdivided into
- Intermediate nerve, sensory/motor portion
- Facial nerve, motor portion
Vestibulocochlear nerve (VIII), sometimes called the auditory nerve
Glossopharyngeal nerve (IX)
Vagus nerve (X)
Accessory nerve (XI), sometimes called the cranial accessory nerve or spinal accessory nerve.
Hypoglossal nerve (XII)

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.

[edit] Nerves and nuclei

As well as the visible nerves outside of the brain, most of the cranial nerves have associated nuclei within the brainstem. These nuclei are areas of grey matter, and damage to them can have a similar affect to the severing of an actual nerve. Axons to (and from) cranial nerves synapse first at the nuclei.

[edit] Arrangement of the nuclei

Just as grey matter in the ventral (closer to front of a human) spinal cord tends to be efferent (motor) fibers, and the dorsal horn tends to contain sensory neurons, nuclei in the brainstem are arranged in an analogous way.

Close to the midline are the somatic efferent nuclei, such as the oculomotor nucleus, which control skeletal muscle. Just lateral to this are the autonomic (or visceral) efferent nuclei (for instance, the Edinger-Westphal nucleus, which controls tears).

There is a separation, called the sulcus limitans, and lateral to this are the sensory nuclei. Near the sulcus limitans are the visceral afferent nuclei, namely the solitary tract nucleus.

More lateral, but also less posterior, are the general somatic afferent nuclei. This is the trigeminal nucleus. Back at the dorsal surface of the brainstem, and more lateral are the special somatic afferents, this handles sensation such as balance.

Another area, not on the dorsum of the brainstem, is where the branchial efferent nuclei reside. These formed from the branchial arches, in the embryo. This area is a bit below the autonomic motor nuclei, and includes the nucleus ambiguus, facial nerve nucleus, as well as the motor part of the trigeminal nerve nucleus.

[edit] Cranial nerves in non-human vertebrates

Human cranial nerves are evolutionarily homologous to those found in many other vertebrates. The first ten pairs of cranial nerves arose in the common ancestor of tetrapods (amphibians, 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.

One such mnemonic is as follows: On Old Olympus' Towering Top A Fin And German Viewed Some Hops (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.)
Another such mnemonic is: Ooh, Ooh, Ooh To Touch And Feel Virgin Girls Vaginas, Ah Heaven! There are many politically correct variations of this, including 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.)
A popular clean mnemonic is: Oh Once One Takes The Anatomy Final Very Good Vacations Are Heavenly
Another to help remember the types of information these nerves carry (sensory, motor, or both) is thus: Some Say Marry Money, But My Brother Says Big Brains Matter More.

See mnemonic for more mnemonic devices to aid remembering the cranial nerves.

[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.