Editing Nerve (section)

==Structure==
Each nerve is covered on the outside by a dense sheath of [[connective tissue]], the [[epineurium]]. Beneath this is a layer of fat cells, the [[perineurium]], which forms a complete sleeve around a bundle of [[Axon|axons]]. Perineurial [[Septum|septae]] extend into the nerve and subdivide it into several bundles of fibres. Surrounding each such fibre is the [[endoneurium]]. This forms an unbroken tube from the surface of the [[spinal cord]] to the level where the axon [[Synapse|synapses]] with its muscle fibres, or ends in [[sensory receptor]]s. The endoneurium consists of an inner sleeve of material called the [[glycocalyx]] and an outer delicate meshwork of [[collagen]] fibres.<ref name="Marieb"/> Nerves are bundled and often travel along with [[blood vessels]], since the [[Neuron|neurons]] of a nerve have fairly high energy requirements.

Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid called '''endoneurial fluid'''. This acts in a similar way to the [[cerebrospinal fluid]] in the [[central nervous system]] and constitutes a '''blood-nerve barrier''' similar to the [[blood–brain barrier]].<ref>{{cite journal|last1=Kanda|first1=T|title=Biology of the blood-nerve barrier and its alteration in immune mediated neuropathies|journal= Journal of Neurology, Neurosurgery & Psychiatry|date=Feb 2013|volume=84|issue=2|pages=208–212|pmid=23243216|doi=10.1136/jnnp-2012-302312|s2cid=207005110}}</ref> Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerve [[edema]] from nerve irritation (or injury), the amount of endoneurial fluid may increase at the site of irritation. This increase in fluid can be visualized using [[magnetic resonance neurography|magnetic resonance (MR) neurography]], and thus MR neurography can identify nerve irritation and/or injury.

===Categories===
Nerves are categorized into three groups based on the direction that signals are conducted:

* [[Afferent nerve fiber|Afferent nerve]]s conduct sensory information from [[sensory neuron]]s to the [[central nervous system]], for example from the [[mechanoreceptors]] in [[skin]]. Bundles of afferent fibers are known as ''sensory nerves''.<ref name="Purves" /><ref name="Marieb" />
* [[Efferent nerve fiber|Efferent nerve]]s conduct signals from the central nervous system along [[motor neuron]]s to their target [[muscle]]s and [[gland]]s. Bundles of these fibres are known as ''efferent nerves.''
* [[Mixed nerve]]s contain both afferent and efferent axons, and thus conduct both incoming [[sense|sensory]] information and outgoing muscle commands in the same bundle. All spinal nerves are mixed nerves, and some of the cranial nerves are also mixed nerves.
Nerves can be categorized into two groups based on where they connect to the central nervous system:
* [[Spinal nerve]]s innervate (distribute to/stimulate) much of the body, and connect through the [[vertebral column]] to the [[spinal cord]] and thus to the [[central nervous system]]. They are given letter-number designations according to the [[vertebra]] through which they connect to the spinal column.
* [[Cranial nerves]] innervate parts of the head, and connect directly to the [[brain]] (especially to the [[brainstem]]). They are typically assigned [[Roman numerals]] from 1 to 12, although [[cranial nerve zero]] is sometimes included. In addition, cranial nerves have descriptive names.

===Terminology===
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{{Main|Anatomical terms of neuroanatomy}}

Specific terms are used to describe nerves and their actions. A nerve that supplies information to the brain from an area of the body, or controls an action of the body is said to ''innervate'' that section of the body or organ. Other terms relate to whether the nerve affects the same side ("ipsilateral") or opposite side ("contralateral") of the body, to the part of the brain that supplies it.