Nerve

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This article is about neural pathways of the peripheral nervous system. Nerve_sentence_0

For other uses, see Nerve (disambiguation). Nerve_sentence_1

Nerve_table_infobox_0

NerveNerve_header_cell_0_0_0
DetailsNerve_header_cell_0_1_0
SystemNerve_header_cell_0_2_0 Nervous systemNerve_cell_0_2_1
IdentifiersNerve_header_cell_0_3_0
LatinNerve_header_cell_0_4_0 nervusNerve_cell_0_4_1
TA98Nerve_header_cell_0_5_0 Nerve_cell_0_5_1
TA2Nerve_header_cell_0_6_0 Nerve_cell_0_6_1
FMANerve_header_cell_0_7_0 Nerve_cell_0_7_1

A nerve is an enclosed, cable-like bundle of nerve fibres called axons, in the peripheral nervous system. Nerve_sentence_2

A nerve transmits electrical impulses and is the basic unit of the peripheral nervous system. Nerve_sentence_3

A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system. Nerve_sentence_4

Each axon within the nerve is an extension of an individual neuron, along with other supportive cells such as some Schwann cells that coat the axons in myelin. Nerve_sentence_5

Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium. Nerve_sentence_6

The axons are bundled together into groups called fascicles, and each fascicle is wrapped in a layer of connective tissue called the perineurium. Nerve_sentence_7

Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium. Nerve_sentence_8

In the central nervous system, the analogous structures are known as nerve tracts. Nerve_sentence_9

Structure Nerve_section_0

Each nerve is covered on the outside by a dense sheath of connective tissue, the epineurium. Nerve_sentence_10

Beneath this is a layer of fat cells, the perineurium, which forms a complete sleeve around a bundle of axons. Nerve_sentence_11

Perineurial septae extend into the nerve and subdivide it into several bundles of fibres. Nerve_sentence_12

Surrounding each such fibre is the endoneurium. Nerve_sentence_13

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 in sensory receptors. Nerve_sentence_14

The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer, delicate, meshwork of collagen fibres. Nerve_sentence_15

Nerves are bundled and often travel along with blood vessels, since the neurons of a nerve have fairly high energy requirements. Nerve_sentence_16

Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid called endoneurial fluid. Nerve_sentence_17

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. Nerve_sentence_18

Molecules are thereby prevented from crossing the blood into the endoneurial fluid. Nerve_sentence_19

During the development of nerve edema from nerve irritation (or injury), the amount of endoneurial fluid may increase at the site of irritation. Nerve_sentence_20

This increase in fluid can be visualized using magnetic resonance neurography, and thus MR neurography can identify nerve irritation and/or injury. Nerve_sentence_21

Categories Nerve_section_1

Nerves are categorized into three groups based on the direction that signals are conducted: Nerve_sentence_22

Nerve_unordered_list_0

Nerves can be categorized into two groups based on where they connect to the central nervous system: Nerve_sentence_23

Nerve_unordered_list_1

Terminology Nerve_section_2

Main article: Anatomical terms of neuroanatomy Nerve_sentence_24

Specific terms are used to describe nerves and their actions. Nerve_sentence_25

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. Nerve_sentence_26

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_sentence_27

Development Nerve_section_3

Nerve growth normally ends in adolescence, but can be re-stimulated with a molecular mechanism known as "Notch signaling". Nerve_sentence_28

Regeneration Nerve_section_4

If the axons of a neuron are damaged, as long as the cell body of the neuron is not damaged, the axons would regenerate and remake the synaptic connections with neurons with the help of guidepost cells. Nerve_sentence_29

This is also referred to as neuroregeneration. Nerve_sentence_30

The nerve begins the process by destroying the nerve distal to the site of injury allowing Schwann cells, basal lamina, and the neurilemma near the injury to begin producing a regeneration tube. Nerve_sentence_31

Nerve growth factors are produced causing many nerve sprouts to bud. Nerve_sentence_32

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_sentence_33

Nerve regeneration is very slow and can take up to several months to complete. Nerve_sentence_34

While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect. Nerve_sentence_35

Function Nerve_section_5

A nerve conveys information in the form of electrochemical impulses (as nerve impulses known as action potentials) carried by the individual neurons that make up the nerve. Nerve_sentence_36

These impulses are extremely fast, with some myelinated neurons conducting at speeds up to 120 m/s. Nerve_sentence_37

The impulses travel from one neuron to another by crossing a synapse, where the message is converted from electrical to chemical and then back to electrical. Nerve_sentence_38

Nerves can be categorized into two groups based on function: Nerve_sentence_39

Nerve_unordered_list_2

  • An afferent nerve fiber conducts sensory information from a sensory neuron to the central nervous system, where the information is then processed. Bundles of fibres or axons, in the peripheral nervous system are called nerves, and bundles of afferent fibers are known as sensory nerves.Nerve_item_2_5
  • An efferent nerve fiber conducts signals from a motor neuron in the central nervous system to muscles. Bundles of these fibres are known as efferent nerves.Nerve_item_2_6

Nervous system Nerve_section_6

Main article: Nervous system Nerve_sentence_40

The nervous system is the part of an animal that coordinates its actions by transmitting signals to and from different parts of its body. Nerve_sentence_41

In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). Nerve_sentence_42

The CNS consists of the brain and spinal cord. Nerve_sentence_43

The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body. Nerve_sentence_44

Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent. Nerve_sentence_45

Spinal nerves serve both functions and are called mixed nerves. Nerve_sentence_46

The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Nerve_sentence_47

Somatic nerves mediate voluntary movement. Nerve_sentence_48

The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. Nerve_sentence_49

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. Nerve_sentence_50

The enteric nervous system functions to control the gastrointestinal system. Nerve_sentence_51

Both autonomic and enteric nervous systems function involuntarily. Nerve_sentence_52

Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves. Nerve_sentence_53

Clinical significance Nerve_section_7

Cancer can spread by invading the spaces around nerves. Nerve_sentence_54

This is particularly common in head and neck cancer, and prostate and colorectal cancer. Nerve_sentence_55

Nerves can be damaged by physical injury as well conditions like carpal tunnel syndrome and repetitive strain injury. Nerve_sentence_56

Autoimmune diseases such as Guillain–Barré syndrome, neurodegenerative diseases, polyneuropathy, infection, neuritis, diabetes, or failure of the blood vessels surrounding the nerve all cause nerve damage, which can vary in severity. Nerve_sentence_57

Multiple sclerosis is a disease associated with extensive nerve damage. Nerve_sentence_58

It occurs when the macrophages of an individual's own immune system damage the myelin sheaths that insulate the axon of the nerve. Nerve_sentence_59

A pinched nerve occurs when pressure is placed on a nerve, usually from swelling due to an injury, or pregnancy and can result in pain, weakness, numbness or paralysis, an example being carpal tunnel syndrome. Nerve_sentence_60

Symptoms can be felt in areas far from the actual site of damage, a phenomenon called referred pain. Nerve_sentence_61

Referred pain can happen when the damage causes altered signalling to other areas. Nerve_sentence_62

Neurologists usually diagnose disorders of the nerves by a physical examination, including the testing of reflexes, walking and other directed movements, muscle weakness, proprioception, and the sense of touch. Nerve_sentence_63

This initial exam can be followed with tests such as nerve conduction study, electromyography (EMG), and computed tomography (CT). Nerve_sentence_64

Other animals Nerve_section_8

A neuron is called identified 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. Nerve_sentence_65

In vertebrate nervous systems, very few neurons are "identified" in this sense. Nerve_sentence_66

Researchers believe humans have none—but in simpler nervous systems, some or all neurons may be thus unique. Nerve_sentence_67

In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish. Nerve_sentence_68

Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right. Nerve_sentence_69

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. Nerve_sentence_70

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 a C-shape, then straightens, thereby propelling itself rapidly forward. Nerve_sentence_71

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. Nerve_sentence_72

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. Nerve_sentence_73

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. Nerve_sentence_74

Mauthner cells have been described as command neurons. Nerve_sentence_75

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. Nerve_sentence_76

Such neurons appear most commonly in the fast escape systems of various species—the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. Nerve_sentence_77

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. Nerve_sentence_78

In organisms of radial symmetry, nerve nets serve for the nervous system. Nerve_sentence_79

There is no brain or centralised head region, and instead there are interconnected neurons spread out in nerve nets. Nerve_sentence_80

These are found in Cnidaria, Ctenophora and Echinodermata. Nerve_sentence_81

History Nerve_section_9

Further information: History of neurology and neurosurgery Nerve_sentence_82

Herophilos 335–280 BCE, described the optic nerve and the oculomotor nerve for sight and eye movement. Nerve_sentence_83

Analysis of the nerves in the cranium allowed him to differentiate between blood vessels and nerves i.e. Ancient Greek: , “string (plant fiber), nerve”. Nerve_sentence_84

See also Nerve_section_10

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Nerve.