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Cranial nerves are nerves that emerge from the brainstem instead of the spinal cord.

Names of nerves

In human anatomy, there are 12 pairs of cranial nerves, 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.

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).

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.

• Oliver the optimistic octopus trots triumphantly about facing audiances 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!"
• On Old Olympus' Towering Top A Fin And German Viewed Some Hops
  • (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.)
• 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 Money Mmatters, 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
• Small Ships Make Money, But My Brother Says Big Boats Make More.

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.

See also

• Cranial nerve nucleus

External links

• Examination of the cranial nerves - University of Toronto Medicine
• Diagram of all 12 cranial nerves
• Modalities and pathways

• Peripheral nervous system
• Cranial nerves