Cranial nerves
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Cranial nerves are nerves which 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.
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)
- Facial nerve (VII)
- Vestibulocochlear nerve (VIII), sometimes called the auditory nerve
- Glossopharyngeal nerve (IX)
- Vagus nerve (X)
- Accessory nerve (XI), sometimes called the spinal accessory nerve
- Hypoglossal nerve (XII)
As the list is important to keep in mind during the examination of the nervous system, there are many mnemonics in circulation, invariably based on the first letters of the twelve nerves.
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.
Arrangement of the nuclei
Just as grey matter in the ventral (closer to front of a human) spinal cord tends to be efferent (motor) fibres, 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 that 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 posteriorly, are the general somatic afferent nuclei. This is the trigeminal nucleus. Back at the dorsal surface of the brainstem, and more laterally 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.
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.
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