- Cranial nerves
Cranial nerves are
nerves that emerge directly from the brain stemin 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.
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 synapomorphiesfor their respective clades. In some primitive cartilagenous fishes, such as the dogfish("Squalus acanthias"), there is a terminal nerve numbered zero (as it exits the brain before the first cranial nerve).
As the list is important to keep in mind during the examination of the nervous system, there are many
mnemonicdevices 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. The best known example is, "On old Olympus' Towering Tops, a Finn and German viewed some hops","cite journal |author=Herlevich NE |title=Reflecting on old Olympus' towering tops |journal=Journal of ophthalmic nursing & technology |volume=9 |issue=6 |pages=245–6 |year=1990 |pmid=2254946 |doi= |accessdate=2007-11-06] .
For additional memonics, see
Cranial nerve nucleus
List of mnemonics for the cranial nerves
* [http://icarus.med.utoronto.ca/NeuroExam/ Examination of the cranial nerves] -
University of TorontoMedicine
* [http://www.rahulgladwin.com/blog/2006/07/cranial-nerves-and-cranial.html CN Medical Notes on rahulgladwin.com]
* [http://remus.rutgers.edu/~emtklein/anatomy/cn.html Information about the Cranial Nerves]
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