Nerve

Nerve
Nerve
	Nerves (yellow) in the arm
Details
System	Nervous system
Identifiers
Latin	nervus
TA98	A14.2.00.013
TA2	6154
FMA	65132
	Anatomical terms of neuroanatomy [edit on Wikidata]

Anerveis an enclosed, cable-like bundle of nerve fibers (calledaxons) in theperipheral nervous system.

Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for theelectrochemicalnerve impulses calledaction potentialsthat are transmitted along each of theaxonsto peripheral organs or, in the case ofsensory nerves,from the periphery back to thecentral nervous system.Each axon, within the nerve, is an extension of an individualneuron,along with other supportive cells such as someSchwann cellsthat coat the axons inmyelin.

Within a nerve, 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.Finally, the entire nerve is wrapped in a layer of connective tissue called theepineurium.Nerve cells (often called neurons) are further classified assensory,motor,ormixed nerves.

In thecentral nervous system,the analogous structures are known asnerve tracts.^[1]^[2]

Structure

Cross-section of a nerve

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 of axons. Perineurial septae 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 the spinal cord to the level where the axon synapses with its muscle fibres, or ends insensory receptors.The endoneurium consists of an inner sleeve of material called theglycocalyxand an outer, delicate, meshwork ofcollagenfibres.^[2]Nerves are bundled and often travel along withblood vessels,since the neurons 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 fluidin thecentral nervous systemand constitutes ablood-nerve barriersimilar to theblood–brain barrier.^[3]Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerveedemafrom 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 neurography,and thus MR neurography can identify nerve irritation and/or injury.

Terminology

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 toinnervatethat 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.

Development

Nerve growth normally ends in adolescence, but can be re-stimulated with a molecular mechanism known as "Notch signaling".^[4]

Regeneration

If the axons of aneuronare damaged, as long asthe cell bodyof 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 nervedistalto 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]

Function

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 somemyelinatedneurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing asynapse,where the message is converted fromelectricaltochemicaland then back to electrical.^[2]^[1]

Nerves can be categorized into two groups based on function:

Anafferent nerve fiberconducts sensory information from asensory neuronto the central nervous system, where the information is then processed. Bundles of fibres oraxons,in the peripheral nervous system are called nerves, and bundles of afferent fibers are known assensory nerves.^[1]^[2]
Anefferent nerve fiberconducts signals from amotor neuronin the central nervous system to muscles. Bundles of these fibres are known asefferent nerves.

Nervous system

Thenervous systemis the part of ananimalthat coordinates its actions by transmittingsignalsto 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,including thebrainstem,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 transmit signals from the CNS are calledmotororefferentnerves, while those nerves that transmit information from the body to the CNS are calledsensoryorafferent.Spinal nervesserve both functions and are calledmixednerves. The PNS is divided into three separate subsystems, thesomatic,autonomic,andentericnervous systems. Somatic nerves mediate voluntary movement.

The autonomic nervous system is further subdivided into thesympatheticand theparasympatheticnervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control thegastrointestinalsystem. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are calledcranial nerveswhile those exiting from the spinal cord are calledspinal nerves.

Clinical significance

Micrographdemonstratingperineural invasionofprostate cancer.H&E stain.

Cancercan spread by invading the spaces around nerves. This is particularly common inhead and neck cancer,prostate cancerandcolorectal cancer.

Nerves can be damaged by physical injury as well as conditions likecarpal tunnel syndrome(CTS) andrepetitive strain injury.Autoimmune diseasessuch asGuillain–Barré syndrome,neurodegenerative diseases,polyneuropathy,infection,neuritis,diabetes,or failure of the blood vessels surrounding the nerve all causenerve damage,which can vary in severity.

Multiple sclerosisis a disease associated with extensive nerve damage. It occurs when themacrophagesof an individual's own immune system damage the myelin sheaths that insulate the axon of the nerve.

Apinched nerveoccurs when pressure is placed on a nerve, usually from swelling due to an injury, or pregnancy and can result inpain,weakness, numbness or paralysis, an example being CTS. Symptoms can be felt in areas far from the actual site of damage, a phenomenon calledreferred pain.Referred pain can happen when the damage causes altered signalling to other areas.

Neurologistsusually diagnose disorders of nerves by aphysical examination,including the testing ofreflexes,walkingand other directed movements,muscle weakness,proprioception,and the sense oftouch.This initial exam can be followed with tests such asnerve conduction study,electromyography(EMG), andcomputed tomography(CT).^[8]

Other animals

A neuron is calledidentifiedif 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 cellsof fish.^[11]^: 38–44Every 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 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]^: 112Such neurons appear most commonly in the fast escape systems of various species—thesquid giant axonandsquid 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 netsserve 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,CtenophoraandEchinodermata.

History

Herophilos(335–280 BC) described the functions of theoptic nervein sight and theoculomotor nervein eye movement. Analysis of the nerves in thecraniumenabled him to differentiate betweenblood vesselsand nerves (Ancient Greek:νεῦρον (neûron)"string, plant fiber, nerve" ).

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 ofpsychologicalworry or hesitancy, as in the common traditional phrases "my poor nerves",^[14] "high-strung",and"nervous breakdown".^[15]

References

^^a ^b ^cPurves, 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.ISBN 978-0-87893-697-7.
^^a ^b ^c ^dMarieb EN, Hoehn K (2007).Human Anatomy & Physiology(7th ed.). Pearson. pp. 388–602.ISBN 978-0-8053-5909-1.
^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.PMID 23243216.S2CID 207005110.
^Yale Study Shows Way To Re-Stimulate Brain Cell Growth ScienceDaily Archived2017-07-07 at theWayback Machine(Oct. 22, 1999) — Results Could Boost Understanding Of Alzheimer's, Other Brain Disorders
^Kunik, D (2011)."Laser-based single-axon transection for high-content axon injury and regeneration studies".PLOS ONE.6(11): e26832.Bibcode:2011PLoSO...626832K.doi:10.1371/journal.pone.0026832.PMC3206876.PMID 22073205.
^Burnett, Mark; Zager, Eric."Pathophysiology of Peripheral Nerve Injury: A Brief Review: Nerve Regeneration".Medscape Article.Medscape.Archivedfrom the original on 2011-10-31.Retrieved2011-10-26.
^Tortora, G.J., Derrickson, B. (2016).Principles of Anatomy and Physiology (15th edition).J. Wiley.ISBN 978-1-119-34373-8.{{cite book}}:CS1 maint: multiple names: authors list (link)
^Weinberg.Normal computed tomography of the brain.p. 109.^{[full citation needed]}
^Hoyle G, Wiersma CA (1977).Identified neurons and behavior of arthropods.Plenum Press.ISBN 978-0-306-31001-0.
^"Wormbook: Specification of the nervous system".Archivedfrom the original on 2011-07-17.
^^a ^bStein, PSG (1999).Neurons, Networks, and Motor Behavior.MIT Press.ISBN 978-0-262-69227-4.
^Simmons PJ, Young D (1999).Nerve Cells and Animal Behaviour.Cambridge University Press. p.43.ISBN 978-0-521-62726-9.
^ Pickering, Neil (2006).The Metaphor of Mental Illness.International perspectives in philosophy and psychiatry. Oxford University Press. p. 99.ISBN 9780198530879.Retrieved27 May2023.[...] 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.ISBN 9780674049161.Retrieved27 May2023.'Mr. Bennet, how can you abuse your own children in such a way? You take delight in ve xing 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.ISBN 9780198530879.Retrieved27 May2023.[...] in everyday English we find [...] lay terms such as 'nervous breakdown' that relate to mental illness as a whole [...]

External links

Library resourcesabout
Nerve

Resources in your library

List of nerves
The Nervous Systemat Wikibooks (human)
Nervous Systemat Wikibooks (non-human)
Parsons, Frederick Gymer (1911)."Nerve".Encyclopædia Britannica.Vol. 19 (11th ed.). pp. 394–400.

[Purves-1] Purves, 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.ISBN 978-0-87893-697-7.

[Marieb-2] Marieb EN, Hoehn K (2007).Human Anatomy & Physiology(7th ed.). Pearson. pp. 388–602.ISBN 978-0-8053-5909-1.

[3] 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.PMID 23243216.S2CID 207005110.

[4] Yale Study Shows Way To Re-Stimulate Brain Cell Growth ScienceDaily Archived2017-07-07 at theWayback Machine(Oct. 22, 1999) — Results Could Boost Understanding Of Alzheimer's, Other Brain Disorders

[5] Kunik, D (2011)."Laser-based single-axon transection for high-content axon injury and regeneration studies".PLOS ONE.6(11): e26832.Bibcode:2011PLoSO...626832K.doi:10.1371/journal.pone.0026832.PMC3206876.PMID 22073205.

[6] Burnett, Mark; Zager, Eric."Pathophysiology of Peripheral Nerve Injury: A Brief Review: Nerve Regeneration".Medscape Article.Medscape.Archivedfrom the original on 2011-10-31.Retrieved2011-10-26.

[7] Tortora, G.J., Derrickson, B. (2016).Principles of Anatomy and Physiology (15th edition).J. Wiley.ISBN 978-1-119-34373-8.{{cite book}}:CS1 maint: multiple names: authors list (link)

[Weinberg-8] Weinberg.Normal computed tomography of the brain.p. 109.^{[full citation needed]}

[Hoyle-9] Hoyle G, Wiersma CA (1977).Identified neurons and behavior of arthropods.Plenum Press.ISBN 978-0-306-31001-0.

[Wormbook-10] "Wormbook: Specification of the nervous system".Archivedfrom the original on 2011-07-17.

[Stein-11] Stein, PSG (1999).Neurons, Networks, and Motor Behavior.MIT Press.ISBN 978-0-262-69227-4.

[Simmons43-12] Simmons PJ, Young D (1999).Nerve Cells and Animal Behaviour.Cambridge University Press. p.43.ISBN 978-0-521-62726-9.

[13] Pickering, Neil (2006).The Metaphor of Mental Illness.International perspectives in philosophy and psychiatry. Oxford University Press. p. 99.ISBN 9780198530879.Retrieved27 May2023.[...] 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.

[14] For example: Austen, Jane(2010) [1813].Spacks, Patricia Meyer(ed.).Pride and Prejudice: An Annotated Edition.Cambridge, Massachusetts: Harvard University Press. p. 32.ISBN 9780674049161.Retrieved27 May2023.'Mr. Bennet, how can you abuse your own children in such a way? You take delight in ve xing 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.'

[15] Pickering, Neil (2006).The Metaphor of Mental Illness.International perspectives in philosophy and psychiatry. Oxford University Press. p. 1.ISBN 9780198530879.Retrieved27 May2023.[...] in everyday English we find [...] lay terms such as 'nervous breakdown' that relate to mental illness as a whole [...]

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