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A nerve is an enclosed, cable-like bundle of fibers (called
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s) in the
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ...
. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the
electrochemical Electrochemistry is the branch of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying ...
nerve impulses called
action potential In physiology Physiology (; ) is the scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the are ...

action potential
s that are transmitted along each of the
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s to peripheral organs or, in the case of
sensory nerve A sensory nerve, or afferent nerve, is a general anatomic term for a nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and ...
s, from the periphery back to the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
. Each axon, within the nerve, is an extension of an individual
neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...

neuron
, along with other supportive cells such as some
Schwann cell Schwann cells or neurolemmocytes (named after German physiologist Theodor Schwann Theodor Schwann (; 7 December 181011 January 1882) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic gr ...
s that coat the axons in
myelin Myelin is a lipid-rich (fatty) substance that surrounds nerve cell axons (the nervous system's "wires") to Insulator (electricity), insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the a ...
. Within a nerve, each axon is surrounded by a layer of connective tissue called the
endoneurium The endoneurium (also called endoneurial channel, endoneurial sheath, endoneurial tube, or Henle's sheath) is a layer of delicate connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epi ...
. The axons are bundled together into groups called fascicles, and each fascicle is wrapped in a layer of connective tissue called the
perineurium The perineurium is a protective sheath that surrounds a nerve fascicle A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and Brit ...
. Finally, the entire nerve is wrapped in a layer of connective tissue called the
epineurium The epineurium is the outermost layer of dense irregular connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the m ...
. Nerve cells are called neurons, which are further classified as sensory, mixed, or motor neurons. In the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
, the analogous structures are known as
nerve tract A nerve tract is a bundle of nerve fibers (axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a long, slender projectio ...
s.


Structure

Each nerve is covered on the outside by a dense sheath of
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
, the
epineurium The epineurium is the outermost layer of dense irregular connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the m ...
. Beneath this is a layer of fat cells, the
perineurium The perineurium is a protective sheath that surrounds a nerve fascicle A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and Brit ...
, 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 the
endoneurium The endoneurium (also called endoneurial channel, endoneurial sheath, endoneurial tube, or Henle's sheath) is a layer of delicate connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epi ...
. 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 receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called sy ...
s. The endoneurium consists of an inner sleeve of material called the
glycocalyx The glycocalyx, also known as the pericellular matrix, is a glycoprotein Glycoproteins are protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residue ...

glycocalyx
and an outer, delicate, meshwork of
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
fibres. Nerves are bundled and often travel along with
blood vessels The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a comp ...

blood 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 called endoneurial fluid. This acts in a similar way to the
cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochor ...
in the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
and constitutes a ''blood-nerve barrier'' similar to the blood-brain barrier. Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerve
edema Edema, also spelled oedema, and also known as fluid retention, dropsy, hydropsy and swelling, is the build-up of fluid in the body's tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a ...

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 Bilateral Split Sciatic Nerve Magnetic resonance neurography (MRN) is the direct imaging of nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve f ...
, 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 A sensory nerve, also called an afferent nerve, is a nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English ...
s'' conduct signals from
sensory neuron Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called sy ...
s to the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
, for example from the
mechanoreceptors A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neuron, sensory neurons that convert mechanical pressure into electrical signals tha ...
in
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have differ ...

skin
. * ''
Efferent nerve Efferent nerve fibers refer to axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a long, slender projection of a ne ...
s'' conduct signals from the central nervous system along
motor neuron A motor neuron (or motoneuron or efferent neuron) is a whose is located in the , or the , and whose (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly s and s. There are ...
s to their target
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
s and
gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), offic ...

gland
s. * ''Mixed nerves'' contain both afferent and efferent axons, and thus conduct both incoming 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 A spinal nerve is a , which carries motor, sensory, and autonomic signals between the and the body. In the there are 31 pairs of spinal nerves, one on each side of the . These are grouped into the corresponding , , , and regions of the spine. ...

Spinal nerve
s'' innervate (distribute to/stimulate) much of the body, and connect through the
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...

vertebral column
to the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, high ...

spinal cord
and thus to the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
. They are given letter-number designations according to the
vertebra In the vertebrate spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform c ...
through which they connect to the spinal column. * ''
Cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
'' innervate parts of the head, and connect directly to the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
(especially to the
brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and anima ...

brainstem
). They are typically assigned
Roman numerals Roman numerals are a that originated in and remained the usual way of writing numbers throughout Europe well into the . Numbers in this system are represented by combinations of letters from the . Modern style uses seven symbols, each with a ...
from 1 to 12, although
cranial nerve zero The terminal nerve, also known as cranial nerve 0, or simply as CN 0, was discovered by German scientist Gustav Fritsch in 1878 in the brain A brain is an organ (anatomy), organ that serves as the center of the nervous system in all vertebrate ...
is sometimes included. In addition, cranial nerves have descriptive names.


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 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.


Development

Nerve growth normally ends in adolescence, but can be re-stimulated with a molecular mechanism known as "
Notch signaling The Notch signaling pathway is a highly conserved cell signaling In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular bi ...

Notch signaling
".


Regeneration

If the axons of a
neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...

neuron
are damaged, as long as the cell body of the neuron is not damaged, the axons can regenerate and remake the synaptic connections with neurons with the help of guidepost cells. This is also referred to as
neuroregeneration Neuroregeneration refers to the regrowth or repair of nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex p ...
. The nerve begins the process by destroying the nerve
distal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe something in its standard anatomical position. This ...
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.


Function

A nerve conveys information in the form of electrochemical impulses (as nerve impulses known as
action potential In physiology Physiology (; ) is the scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the are ...

action potential
s) carried by the individual neurons that make up the nerve. These impulses are extremely fast, with some
myelinated Myelin is a lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, phys ...
neurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing a
synapse In the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiol ...

synapse
, where the message is converted from
electrical Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon in which an object changes its positio ...
to
chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which ...
and then back to electrical. Nerves can be categorized into two groups based on function: * An
afferent nerve fiber Afferent nerve fibers refer to axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from th ...
conducts sensory information from a
sensory neuron Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called sy ...
to the central nervous system, where the information is then processed. Bundles of fibres or
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s, in the peripheral nervous system are called nerves, and bundles of afferent fibers are known as ''sensory nerves''. * An
efferent nerve fiber Efferent nerve fibers refer to axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a long, slender projection of a ne ...
conducts signals from a
motor neuron A motor neuron (or motoneuron or efferent neuron) is a whose is located in the , or the , and whose (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly s and s. There are ...
in the central nervous system to muscles. Bundles of these fibres are known as ''efferent nerves''.


Nervous system

The
nervous system In biology, the classical doctrine of the nervous system determines that it is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals ...

nervous system
is the part of an
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
that coordinates its actions by transmitting to and from different parts of its body. In vertebrates it consists of two main parts, the
central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

central nervous system
(CNS) and the
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ...
(PNS). The CNS consists of the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
and
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, high ...

spinal cord
. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called ''
motor An engine or motor is a machine A machine is a man-made device that uses power to apply forces and control movement to perform an action. Machines can be driven by animals and people A people is a plurality of person A person ( ...
'' or '' efferent'' nerves, while those nerves that transmit information from the body to the CNS are called ''sensory'' or ''afferent''.
Spinal nerve A spinal nerve is a , which carries motor, sensory, and autonomic signals between the and the body. In the there are 31 pairs of spinal nerves, one on each side of the . These are grouped into the corresponding , , , and regions of the spine. ...

Spinal nerve
s serve both functions and are called ''mixed'' nerves. The PNS is divided into three separate subsystems, the
somatic Somatic may refer to: * Somatic (biology), referring to the cells of the body in contrast to the germ line cells ** Somatic cell, a non-gametic cell in a multicellular organism * Somatic nervous system, the portion of the vertebrate nervous syste ...
, , and
enteric The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syst ...
nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the
parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulating t ...
nervous 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 the
gastrointestinal The gastrointestinal tract, (GI tract, GIT, digestive tract, digestion tract, alimentary canal) is the tract from the mouth to the anus which includes all the organs of the digestive system The human digestive system consists of the human ...
system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called
cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
while those exiting from the spinal cord are called
spinal nerves A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the ...
.


Clinical significance

Cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumor A benign tumor is a mass of cells Cell most often refers to: * Cell (biolo ...

Cancer
can spread by invading the spaces around nerves. This is particularly common in head and neck cancer, and prostate and colorectal cancer. Nerves can be damaged by physical injury as well conditions like
carpal tunnel syndrome Carpal tunnel syndrome (CTS) is a medical condition due to compression of the median nerve The median nerve is a nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), ...

carpal tunnel syndrome
and
repetitive strain injury A repetitive strain injury (RSI) is an injury to part of the musculoskeletal The human musculoskeletal system (also known as the human locomotor system, and previously the activity system) is an organ system An organ system is a biologica ...
.
Autoimmune disease An autoimmune disease is a condition arising from an abnormal immune response An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of differe ...

Autoimmune disease
s such as
Guillain–Barré syndrome Guillain–Barré syndrome (GBS) is a rapid-onset Paralysis, muscle weakness caused by the immune system damaging the peripheral nervous system. Typically, both sides of the body are involved, and the initial symptoms are changes in sensation or ...
,
neurodegenerative disease A neurodegenerative disease is caused by the progressive loss of structure or function of neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells ...
s,
polyneuropathy Polyneuropathy ( poly- + neuro- + wikt:-pathy, -pathy) is damage or disease affecting peripheral nerves (peripheral neuropathy) in roughly the same areas on both sides of the body, featuring weakness, numbness, and burning pain. It usually begins ...
, infection,
neuritis Neuritis () is inflammation of a nerve or the general inflammation of the peripheral nervous system. Inflammation, and frequently concomitant Demyelinating disease, demyelination, cause impaired conduction of neural signals and leads to aberrant ne ...
,
diabetes Diabetes mellitus, commonly known as just diabetes, is a group of metabolic disorders characterized by a hyperglycemia, high blood sugar level over a prolonged period of time. Symptoms often include frequent urination, Polydipsia, increased th ...

diabetes
, or failure of the blood vessels surrounding the nerve all cause nerve damage, which can vary in severity.
Multiple sclerosis Multiple sclerosis (MS), also known as encephalomyelitis disseminata, is the most common demyelinating disease, in which the Myelin, insulating covers of nerve cells in the Human brain, brain and spinal cord are damaged. This damage disrupts the ...
is a disease associated with extensive nerve damage. It occurs when the
macrophage Macrophages (abbreviated as Mφ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the ...

macrophage
s of an individual's own immune system damage the myelin sheaths that insulate the axon of the nerve. A
pinched nerve Radiculopathy, also commonly referred to as pinched nerve, refers to a set of conditions in which one or more nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerv ...
occurs when pressure is placed on a nerve, usually from swelling due to an injury, or pregnancy and can result in
pain Pain is a distressing feeling often caused by intense or damaging stimuli. The defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage." In medical ...
, weakness, numbness or paralysis, an example being
carpal tunnel syndrome Carpal tunnel syndrome (CTS) is a medical condition due to compression of the median nerve The median nerve is a nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), ...

carpal tunnel syndrome
. Symptoms can be felt in areas far from the actual site of damage, a phenomenon called
referred pain Referred pain, also called reflective pain, is pain perceived at a location other than the site of the painful stimulus. An example is the case of angina pectoris brought on by a myocardial infarction (heart attack), where pain is often felt in the ...
. Referred pain can happen when the damage causes altered signalling to other areas.
Neurologists Neurology (from el, νεῦρον (neûron), "string, nerve" and the suffix -logia, "study of") is a branch of medicine Medicine is the Art (skill), art, science, and Praxis (process) , practice of caring for a patient and managing the d ...
usually diagnose disorders of the nerves by a
physical examination In a physical examination, medical examination, or clinical examination, a medical practitioner examines a patient A patient is any recipient of health care Health care, health-care, or healthcare is the maintenance or improvement of health ...

physical examination
, including the testing of
reflex In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

reflex
es,
walking Walking (also known as ambulation) is one of the main gait Gait is the pattern of Motion (physics), movement of the limb (anatomy), limbs of animals, including Gait (human), humans, during Animal locomotion, locomotion over a solid substrate. M ...

walking
and other directed movements,
muscle weakness Muscle weakness is a lack of muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A gi ...
,
proprioception Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense A sense is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological ...
, and the sense of
touch The somatosensory system is a part of the sensory nervous system The sensory nervous system is a part of the nervous system responsible for processing sense, sensory information. A sensory system consists of sensory neurons (including the sen ...
. This initial exam can be followed with tests such as
nerve conduction study A nerve conduction study (NCS) is a medical diagnostic test commonly used to evaluate the function, especially the ability of electrical conduction Electrical resistivity (also called specific electrical resistance or volume resistivity) is a ...
,
electromyography Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles The skeleton refers to the frames of support of animal bodies. There are several different skeletal types: the exoskeleto ...
(EMG), and
computed tomography A CT scan or computed tomography scan (formerly known as computed axial tomography or CAT scan) is a medical imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of ...

computed tomography
(CT).


Other animals

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. 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. In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish. 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 a C-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 as
command neuron A command neuron is a single neuron A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapse SyNAPSE is a DARPA program that aims to develop electronic neuromor ...
s. 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. Such neurons appear most commonly in the fast escape systems of various species—the
squid giant axon The squid giant axon is the very large (up to 1.5 mm in diameter; typically around 0.5 mm) axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re ...
and
squid giant synapse The squid giant synapse is a chemical synapse found in squid. It is the largest chemical junction in nature. Anatomy The squid giant synapse (Fig 1) was first recognized by John Zachary Young in #, 1939. It lies in the stellate ganglion on each si ...
, 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. In organisms of
radial symmetry Symmetry in biology refers to the symmetry observed in organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...
,
nerve net A nerve net consists of interconnected neuron A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapse SyNAPSE is a DARPA program that aims to develop electron ...
s 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 in
Cnidaria Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

Cnidaria
,
Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum of marine life, marine invertebrates, commonly known as comb jellies, that marine habitats, inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly r ...

Ctenophora
and
Echinodermata An echinoderm is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of num ...

Echinodermata
.


History

Herophilos 335–280 BC, described the
optic nerve The optic nerve, also known as cranial nerve II, or simply as CN II, is a paired cranial nerve that transmits visual system, visual information from the retina to the brain. In humans, the optic nerve is derived from optic stalks during the se ...

optic nerve
and the
oculomotor nerve The oculomotor nerve is the third cranial nerve (CN III). It enters the orbit (anatomy), orbit through the superior orbital fissure and innervates extrinsic eye muscles that enable most eye movement, movements of the eye and that raise the eye ...

oculomotor nerve
for sight and eye movement. Analysis of the nerves in the cranium allowed him to differentiate between blood vessels and nerves i.e. grc, νεῦρον (neûron), ''“string (plant fiber), nerve”''.


See also

*
Connective tissue in the peripheral nervous system Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, se ...
*
Dermatome (anatomy) A dermatome is an area of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the ar ...
* List of nerves of the human body *
Nerve injury Nerve injury is injury to nervous tissue. There is no single classification system that can describe all the many variations of nerve injury. In 1941, Peripheral nerve injury classification, Seddon introduced a classification of nerve injuries base ...
*
Nervous system In biology, the classical doctrine of the nervous system determines that it is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals ...

Nervous system
*
Neuropathy Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or n ...
*
Peripheral nerve injury Nerve injury is injury Injury, also known as physical trauma, is damage Damage is any change in a thing, often a physical object, that degrades it away from its initial state. It can broadly be defined as "changes introduced into a system tha ...
*
Peripheral nerve injury classification Classification of peripheral nerve injury Nerve injury is injury Injury, also known as physical trauma, is damage Damage is any change in a thing, often a physical object, that degrades it away from its initial state. It can broadly be define ...


References


Further reading


Nervous system
William E. Skaggs,
Scholarpedia ''Scholarpedia'' is an English-language wiki-based online encyclopedia An online encyclopedia, also called an Internet encyclopedia, or a digital encyclopedia, is an encyclopedia An encyclopedia or encyclopaedia (British English) is a re ...
* * * * Squire, L. ''et al.'' (2012). ''Fundamental Neuroscience, 4th edition''.
Academic Press Academic Press is an academic An academy (Attic Greek: Ἀκαδήμεια; Koine Greek Ἀκαδημία) is an institution of secondary education, secondary or tertiary education, tertiary higher education, higher learning, research, or honor ...
; * * Damasio, A. R. (1994). ''Descartes' Error: Emotion, Reason, and the Human Brain. '' New York,
Avon Books Avon Publications is one of the leading publishers of Romance novel, romance fiction. At Avon's initial stages, it was an American mass market paperback, paperback book and comic book publisher. The shift in content occurred in the early 1970s with ...
. (Hardcover) (Paperback) * Gardner, H. (1976). ''The Shattered Mind: The Person After Brain Damage. '' New York,
Vintage Books Vintage Books is a trade paperback publishing imprint Imprint or imprinting may refer to: Entertainment * Imprint (TV series), ''Imprint'' (TV series), Canadian television series * Imprint (Masters of Horror), "Imprint" (''Masters of Horror''), ...

Vintage Books
, 1976 * Goldstein, K. (2000). ''The Organism. '' New York, Zone Books. (Hardcover) (Paperback) *


External links


List of nerves
* (human) * (non-human) {{Authority control Nerves Peripheral nervous system Neuroanatomy Soft tissue