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A nerve is an enclosed, cable-like bundle of peripheral
Peripheral nervous system

The peripheral nervous system resides or extends outside the central nervous system , which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs....
 axon
Axon

An axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts action potentialaway from the neuron's cell body or soma....
s (the long, slender projections of neuron
Neuron

Neurons are responsive cell in the nervous system that process and transmit information by electrochemical Signal . They are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves....
s). A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons.






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Nerves of the Left Upper Extremity
A nerve is an enclosed, cable-like bundle of peripheral
Peripheral nervous system

The peripheral nervous system resides or extends outside the central nervous system , which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs....
 axon
Axon

An axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts action potentialaway from the neuron's cell body or soma....
s (the long, slender projections of neuron
Neuron

Neurons are responsive cell in the nervous system that process and transmit information by electrochemical Signal . They are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves....
s). A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system
Peripheral nervous system

The peripheral nervous system resides or extends outside the central nervous system , which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs....
. In the central nervous system
Central nervous system

The central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms....
, the analogous structures are known as tracts. Neurons are sometimes called nerve cells, though this term is technically inaccurate since many neurons do not form nerves, and nerves also include non-neuronal Schwann cells that coat the axons in myelin
Myelin

Myelin is an electrically-insulating dielectric material that forms a layer, the myelin sheath. Usually, myelin surrounds only the axon of a neuron....
.

Anatomy

Nerves are categorized into three groups based on the direction that signals are conducted:
  • Afferent nerve
    Afferent nerve

    In the nervous system, afferent neurons , carry action potential from receptor s or sense organs toward the central nervous system. This term can also be used to describe relative connections between structures....
    s conduct signals from sensory neuron
    Sensory neuron

    Sensory neurons or also known as afferent neurons are neurons that are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord....
    s to the central nervous system
    Central nervous system

    The central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms....
    , for example from the mechanoreceptors in skin
    Skin

    The skin is the outer covering of the body, also known as the epidermis. It is the largest organ of the integumentary system made up of multiple layers of epithelial biological tissue, and guards the underlying muscles, bones, ligaments and organ s....
    .
  • Efferent nerve
    Efferent nerve

    In the nervous system, efferent nerves ? otherwise known as motoneuron or effector neurons ? carry action potential away from the central nervous system to effectors such as muscles or glands ....
    s conduct signals from the central nervous system along motor neuron
    Motor neuron

    In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles....
    s to their target muscle
    MUSCLE

    MUSCLE is public domain, multiple sequence alignment software for protein and nucleotide sequences.MUSCLE is integrated into UGENE bioinformatics tool as a plugin....
    s and gland
    Gland

    A gland is an Organ in an animal's body that synthesizes a substance for release such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface ....
    s.
  • Mixed nerves contain both afferent and efferent axons, and thus conduct both incoming sensory
    Sense

    Senses are the physiological methods of perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception....
     information and outgoing muscle commands in the same bundle.
Nerves can be categorized into two groups based on where they connect to the central nervous system:
  • Spinal nerve
    Spinal nerve

    The term spinal nerve generally refers to the mixed spinal nerve, which is formed from the dorsal and ventral roots that come out of the spinal cord....
    s     innervate much of the body, and connect through the spinal column to the spinal cord
    Spinal cord

    The spinal cord is a long, thin, tubular bundle of neuron and glia that extends from the brain. The brain and spinal cord together make up the central nervous system....
    . They are given letter-number designations according to the vertebra
    Vertebra

    A vertebra is an individual bone in the flexible column that defines vertebrate animals. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis....
     through which they connect to the spinal column.
  • Cranial nerves
    Cranial nerves

    Cranial nerves are nerves that emerge directly from the brain stem in contrast to spinal nerves which emerge from segments of the spinal cord. Although thirteen cranial nerves in human anatomy fit this description, twelve are conventionally recognized....
     innervate parts of the head, and connect directly to the brainstem. They are typically assigned Roman numerals
    Roman numerals

    Roman numerals are a numeral system of ancient Rome based on letters of the alphabet, which are combined to signify the sum of their values. The system is decimal but not directly Positional notation and does not include a zero....
     from 1 to 12, although cranial nerve zero
    Cranial nerve zero

    The terminal nerve, or cranial nerve zero, was discovered by German scientist Gustav Fritsch in 1878 in the brains of sharks. It was first found in humans in 1913, although its presence in humans remains controversial....
     is sometimes included. In addition, cranial nerves have descriptive names.Each nerve is covered externally by a dense sheath of connective tissue
    Connective tissue

    Connective tissue is a form of fibrous biological tissue.It is one of the four types of tissue in traditional classifications .Collagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% of the total protein content....
    , the epineurium
    Epineurium

    The epineurium is the outermost layer of connective tissue surrounding a peripheral nerve. It includes the blood vessels supplying the nerve. It consists of adipose tissue and fibrocollagenous tissues....
    . Underlying this is a layer of flat cells, the perineurium
    Perineurium

    In the spinal cord, nerve fibers are each wrapped in a protective sheath known as the endoneurium. These are bundled together into groups known as Nerve fascicle, each surrounded by a protective sheath known as the perineurium....
    , which forms a complete sleeve around a bundle of axons. Perineurial septae extend into the nerve and subdivide it into several bundles of fibers. Surrounding each such fibre is the endoneurium
    Endoneurium

    The endoneurium is a layer of delicate connective tissue that encloses the myelin sheath of a nerve fiber within a funiculus.It is continuous with septa which pass inward from the innermost layer of the perineurium, and shows a ground substance in which are imbedded fine bundles of fibrous connective tissue, primarily collagen, running for...
    . This forms an unbroken tube which extends from the surface of the spinal cord to the level at which the axon synapses with its muscle fibers, or ends in sensory receptor
    Sensory receptor

    In a sensory system, a sensory receptor is a sensory nerve ending that recognizes a stimulus in the internal or external environment of an organism....
    s. The endoneurium consists of an inner sleeve of material called the glycocalyx
    Glycocalyx

    Glycocalyx is a general term referring to extracellular polymeric material produced by some bacteria, epithelia and other cells. The slime on the outside of a fish is considered a glycocalyx....
     and an outer, delicate, meshwork of collagen
    Collagen

    Collagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content....
     fibers. Nerves are bundled along with blood vessels, since the neurons of a nerve have fairly high energy requirements.
Within the endoneurium, the individual nerve fibers are surrounded by a low protein liquid called endoneurial fluid. The endoneurium has properties analogous to the blood-brain barrier
Blood-brain barrier

The blood-brain barrier is a metabolic or cellular structure in the central nervous system that restricts the passage of various chemical substances and microscopic objects between the bloodstream and the neural tissue itself, while still allowing the passage of substances essential to metabolism function ....
, in that it prevents certain molecules from crossing from the blood into the endoneurial fluid. In this respect, endoneurial fluid is similar to cerebro-spinal fluid in the central nervous system
Central nervous system

The central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms....
.During the development of nerve edema
Edema

File:Oedema.jpgEdema or Oedema , formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin, or in one or more cavities of the body....
 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 neurography

Magnetic resonance neurography is the direct imaging of nerves in the body using special modifications of magnetic resonance imaging. The technique obtains a true detailed image of a nerve in which the resonance signal arises from in the nerve itself rather than from surrounding tissues or from fat in the nerve lining....
, and thus MR neurography can identify nerve irritation and/or injury.

Physiology

A nerve conveys information in the form of electrochemical impulses (known as nerve impulses or action potential
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
s) carried by the individual neurons that make up the nerve. These impulses are extremely fast, with some myelinated neurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing a synapse, the message is converted from electrical
Electrical synapse

An electrical synapse is a mechanical and electrically conductor link between two abutting neuron cell s that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction....
 to chemical
Chemical synapse

Chemical synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells such as those in neuromuscular junctions or glands....
 and then back to electrical.

Nerves can be categorized into two groups based on function:
  • Sensory nerves
    Sensory nerves

    Sensory nerves are nerves that receive sensory stimuli, such as how something feels and if it is painful.They are made up of nerve fibers, called sensory fibers ....
     conduct sensory information from their receptors to the central nervous system, where the information is then processed. Thus they are synonymous with afferent nerves.
  • Motor nerves conduct signals from the central nervous system to muscles. Thus they are synonymous with efferent nerves.


Clinical importance

Damage to nerves can be caused by physical injury, swelling (e.g. carpal tunnel syndrome
Carpal tunnel syndrome

Carpal tunnel syndrome , or median neuropathy at the wrist, is a medical condition in which the median nerve is compressed at the wrist, leading to paresthesias, numbness and muscle weakness in the hand....
), autoimmune diseases (e.g. Guillain-Barré syndrome
Guillain-Barré syndrome

Guillain-Barr? syndrome is an acute inflammatory demyelinating polyneuropathy , an Autoimmune Disease disease affecting the peripheral nervous system, usually triggered by an acute infectious process....
), infection (neuritis
Neuritis

Neuritis is the general inflammation of the peripheral nervous system....
), diabetes or failure of the blood vessels surrounding the nerve. A pinched nerve occurs when pressure is placed on a nerve, usually from swelling due to an injury or pregnancy. Nerve damage or pinched nerves are usually accompanied by pain
Pain

Pain, in the sense of physical pain, is a typical sensory experience that may be described as the unpleasant awareness of a noxious stimulus or bodily harm....
, numbness, weakness, or paralysis
Paralysis

Paralysis is the complete loss of muscle function for one or more muscle groups. Paralysis can cause loss of feeling or loss of mobility in the affected area....
. Patients may feel these symptoms in areas far from the actual site of damage, a phenomenon called referred pain
Referred pain

Referred pain is a term used to describe the phenomenon of pain perceived at a site adjacent to or at a distance from the site of an injury's origin....
. Referred pain occurs because when a nerve is damaged, signalling is defective from all parts of the area from which the nerve receives input, not just the site of the damage. Neurologists
Neurology

Neurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the Central nervous system, Peripheral nervous system, and autonomic nervous systems, including their coverings, blood vessels, and...
 usually diagnose disorders of the nerves by a physical examination
Physical examination

File:Reeve 978.jpgPhysical examination or clinical examination is the process by which a health care provider investigates the body of a patient for sign of disease....
, including the testing of reflex
ReFLEX

ReFLEX is a wireless protocol developed by Motorola which is used for two-way paging.The Motorola PageWriter released in 1996 was one of the first devices to use the ReFLEX network protocol....
es, walking
Walking

Walking is the main form of animal locomotion on Earth, distinguished from running and crawling . When carried out in shallow waters, it is usually described as wading and when performed over a steeply rising object or an obstacle it becomes scrambling or climbing....
 and other directed movements, muscle
MUSCLE

MUSCLE is public domain, multiple sequence alignment software for protein and nucleotide sequences.MUSCLE is integrated into UGENE bioinformatics tool as a plugin....
 weakness, proprioception
Proprioception

Proprioception ; from Latin proprius, meaning "one's own" and perception) is the sense of the relative position of neighbouring parts of the body....
, and the sense of touch
Somatosensory system

The somatosensory system is a diverse sensory system comprising the receptors and processing centres to produce the sensory modality such as touch, temperature perception, proprioception , and nociception ....
. This initial exam can be followed with tests such as nerve conduction study
Nerve conduction study

A nerve conduction study is a test commonly used to evaluate the function, especially the ability of electrical conduction, of the motor nerve and sensory nerves of the human body....
 and electromyography
Electromyography

Electromyography is a technique for evaluating and recording the activation signal of muscles. EMG is performed using an medical instrument called an electromyograph, to produce a record called an electromyogram....
 (EMG).

Nerve Growth & stimulation

Nerve growth normally ends in adolescence, but can be re-stimulated with a molecular mechanism known as "Notch signaling", working on a Notch receptor:

Yale Study Shows Way To Re-Stimulate Brain Cell Growth ScienceDaily (Oct. 22, 1999) — Results Could Boost Understanding Of Alzheimer's, Other Brain Disorders http://www.sciencedaily.com/releases/1999/10/991022005127.htm

See also

  • Nervous system
    Nervous system

    The nervous system is a Neural network of specialized cells that communicate information about an animal's surroundings and itself. It processes this information and causes reactions in other parts of the body....
  • Dermatome (Anatomy)
  • Nerve injury
    Nerve injury

    There is no single classification system that can describe all the many variations of nerve injury. Most systems attempt to correlate the degree of injury with symptoms, pathology and prognosis....


Additional images