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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. In humans, there are traditionally twelve pairs of cranial nerves. Only the first and the second pair emerge from the cerebrum; the remaining ten pairs emerge from the brainstem.
The cranial nerves are part of the peripheral nervous system (PNS) with the exception of cranial nerve II or 'optic nerve', along with the retina, which is not a true peripheral nerve but a tract of the diencephalon.[1] Cranial nerve ganglia originate in the central nervous system (CNS). The remaining eleven axons extend beyond the brain and are therefore considered part of the PNS.[2]
Human cranial nerves are nerves similar to those found in many other vertebrates. Cranial nerves XI and XII evolved in other species to amniotes (non-amphibian tetrapods), thus totaling twelve pairs. In some primitive cartilaginous fishes, such as the spiny dogfish or mud shark (Squalus acanthias), there is a terminal nerve numbered zero, since it exits the brain before the traditionally designated first cranial nerve.
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.
An example mnemonic sentence for the initial letters "OOOTTAFAGVSH" is "On old Olympus's towering tops, a Finn and German viewed some hops,"[6] and for the initial letters "OOOTTAFVGVAH" is "Oh, oh, oh, to touch and feel very good velvet...ah, heaven."[7] The differences between these depend on "acoustic" versus "vestibulocochlea" and "spinal-accessory" versus "accessory".
A useful mnemonic for remembering which nerves are motor (M), sensory (S), or both (B) is, "Some Say Marry Money But My Brother Says Big Boobs Matter More". There are many more mnemonics from many sources, for example OLd OPie OCcasionally TRies TRIGonometry And Feels VEry GLOomy, VAGUe, And HYPOactive.[8]
3D rendering of cranial nerves (inferior view).
Animations of cranial nerve input to here (University of Liverpool Veterinary School).
M: CNS
anat (n/s/m/p/4/e/b/d/c/a/f/l/g)/phys/devp
noco (m/d/e/h/v/s)/cong/tumr, sysi/epon, injr
proc, drug (N1A/2AB/C/3/4/7A/B/C/D)
M: PNS
anat (h/r/t/c/b/l/s/a)/phys (r)/devp/prot/nttr/nttm/ntrp
noco/auto/cong/tumr, sysi/epon, injr
proc, drug (N1B)
Reflex, Nervous system, Cranial nerves, Brainstem, Moro reflex