Anerve is an enclosed, cable-like bundle of nerve fibers (calledaxons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for theelectrochemical nerve impulses calledaction potentials that are transmitted along each of theaxons to peripheral organs or, in the case ofsensory nerves, from the periphery back to thecentral nervous system. Each axon is an extension of an individualneuron, along with other supportive cells such as someSchwann cells that coat the axons inmyelin.
Each axon is surrounded by a layer of connective tissue called theendoneurium. The axons are bundled together into groups calledfascicles, and each fascicle is wrapped in a layer of connective tissue called theperineurium. The entire nerve is wrapped in a layer of connective tissue called theepineurium. Nerve cells (often called neurons) are further classified as eithersensory ormotor.
Each nerve is covered on the outside by a dense sheath ofconnective tissue, theepineurium. Beneath this is a layer of fat cells, theperineurium, which forms a complete sleeve around a bundle ofaxons. Perineurialsepta extend into the nerve and subdivide it into several bundles of fibres. Surrounding each such fibre is theendoneurium. This forms an unbroken tube from the surface of thespinal cord to the level where the axonsynapses with its muscle fibres, or ends insensory receptors. The endoneurium consists of an inner sleeve of material called theglycocalyx and an outer delicate meshwork ofcollagen fibres.[2] Nerves are bundled and often travel along withblood vessels, since theneurons of a nerve have fairly high energy requirements.
Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid calledendoneurial fluid. This acts in a similar way to thecerebrospinal fluid in thecentral nervous system and constitutes ablood-nerve barrier similar to theblood–brain barrier.[3] Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerveedema from nerve irritation (or injury), the amount of endoneurial fluid may increase at the site of irritation. This increase in fluid can be visualized usingmagnetic resonance (MR) neurography, and thus MR neurography can identify nerve irritation and/or injury.
Efferent nerves conduct signals from the central nervous system alongmotor neurons to their targetmuscles andglands. Bundles of these fibres are known asefferent nerves.
Mixed nerves contain both afferent and efferent axons, and thus conduct both incomingsensory 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 nerves innervate (distribute to/stimulate) much of the body, and connect through thevertebral column to thespinal cord and thus to thecentral nervous system. They are given letter-number designations according to thevertebra through which they connect to the spinal column.
Cranial nerves innervate parts of the head, and connect directly to thebrain (especially to thebrainstem). They are typically assignedRoman numerals from 1 to 12, althoughcranial nerve zero is sometimes included. In addition, cranial nerves have descriptive names.
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 toinnervate 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.
Nerve growth normally ends in adolescence but can be re-stimulated with a molecular mechanism known as "notch signaling".[4] If the axons of aneuron are damaged, as long asthe cell body of the neuron is not damaged, the axons can regenerate and remake the synaptic connections with neurons with the help ofguidepost cells. This is also referred to asneuroregeneration.[5] The nerve begins the process by destroying the nervedistal to the site of injury allowing Schwann cells, basal lamina, and the neurilemma near the injury to begin producing a regeneration tube. Nerve growth factors are produced causing many nerve sprouts to bud. When one of the growth processes finds the regeneration tube, it begins to grow rapidly towards its original destination guided the entire time by the regeneration tube. Nerve regeneration is very slow and can take up to several months to complete. While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect.[6]
A nerve conveys information in the form of electrochemical impulses (as nerve impulses known asaction potentials) carried by the individual neurons that make up the nerve. These impulses are extremely fast, with somemyelinated neurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing asynapse, where the message is converted fromelectrical tochemical and then back to electrical.[2][1]
Thenervous system is the part of ananimal that coordinates its actions by transmittingsignals to and from different parts of its body.[7] In vertebrates it consists of two main parts, thecentral nervous system (CNS) and theperipheral nervous system (PNS). The CNS consists of thebrain andspinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers oraxons, that connect the CNS to all remaining body parts. Nerves that exit from the cranium are calledcranial nerves while those exiting from the spinal cord are calledspinal nerves.
This sectionmay need to be rewritten to comply with Wikipedia'squality standards, as "neuron" does not coincide with "nerve". Relevant discussion may be found on thetalk page.You can help. The talk page may contain suggestions.(April 2025)
A neuron is calledidentified if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has exactly one neuron with the same set of properties.[9] In vertebrate nervous systems, very few neurons are "identified" in this sense. Researchers believe humans have none—but in simpler nervous systems, some or all neurons may be thus unique.[10]
In vertebrates, the best known identified neurons are the giganticMauthner cells of fish.[11]: 38–44 Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has an axon that crosses over, innervating (stimulating) neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into aC-shape, then straightens, thereby propelling itself rapidly forward. Functionally of this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not the only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response all by itself, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response.
Mauthner cells have been described ascommand neurons. A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior all by itself.[11]: 112 Such neurons appear most commonly in the fast escape systems of various species—thesquid giant axon andsquid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances.[12]
In organisms ofradial symmetry,nerve nets serve for the nervous system. There is no brain or centralised head region, and instead there are interconnected neurons spread out in nerve nets. These are found inCnidaria,Ctenophora andEchinodermata.
Modern research has not confirmedWilliam Cullen's 1785 hypothesis associating mental states with physical nerves,[13] although popular or lay medicine may still invoke "nerves" in diagnosing or blaming any sort ofpsychological worry or hesitancy, as in the common traditional phrases "my poor nerves",[14]"high-strung", and "nervous breakdown".[15]
^abcPurves, Dale; Augustine, George J.; Fitzpatrick, David; Hall, William C.; LaMantia, Anthony-Samuel; McNamara, James O.; White, Leonard E. (2008).Neuroscience (4 ed.). Sinauer Associates. pp. 11–20.ISBN978-0-87893-697-7.
^Kanda, T (Feb 2013). "Biology of the blood-nerve barrier and its alteration in immune mediated neuropathies".Journal of Neurology, Neurosurgery & Psychiatry.84 (2):208–212.doi:10.1136/jnnp-2012-302312.PMID23243216.S2CID207005110.
^Tortora, G.J., Derrickson, B. (2016).Principles of Anatomy and Physiology (15th edition). J. Wiley.ISBN978-1-119-34373-8.{{cite book}}: CS1 maint: multiple names: authors list (link)
^Pickering, Neil (2006).The Metaphor of Mental Illness. International perspectives in philosophy and psychiatry. Oxford University Press. p. 99.ISBN978-0-19-853087-9. Retrieved27 May 2023.[...] William Cullen [...] as early as 1785 [...] postulated that certain mental disorders were the result of some unknown physical change in the nerves, for which he coined the term neurosis. This term has since quite altered its meaning, as it now refers not to a state of the nerves but to a nervous state.
^For example:Austen, Jane (2010) [1813].Spacks, Patricia Meyer (ed.).Pride and Prejudice: An Annotated Edition. Cambridge, Massachusetts: Harvard University Press. p. 32.ISBN978-0-674-04916-1. Retrieved27 May 2023.'Mr. Bennet, how can you abuse your own children in such a way? You take delight in vexing me. You have no compassion on my poor nerves.' [...] 'You mistake me, my dear. I have a high respect for your nerves. They are my old friends. I have heard you mention them with consideration these twenty years at least.'
^Pickering, Neil (2006).The Metaphor of Mental Illness. International perspectives in philosophy and psychiatry. Oxford University Press. p. 1.ISBN978-0-19-853087-9. Retrieved27 May 2023.[...] in everyday English we find [...] lay terms such as 'nervous breakdown' that relate to mental illness as a whole [...]
