Nerve

Nerves (yellow)

WikiDoc Resources for Nerve
Articles
Most recent articles on Nerve Most cited articles on Nerve Review articles on Nerve Articles on Nerve in N Eng J Med, Lancet, BMJ
Media
Powerpoint slides on Nerve Images of Nerve Photos of Nerve Podcasts & MP3s on Nerve Videos on Nerve
Evidence Based Medicine
Cochrane Collaboration on Nerve Bandolier on Nerve TRIP on Nerve
Clinical Trials
Ongoing Trials on Nerve at Clinical Trials.gov Trial results on Nerve Clinical Trials on Nerve at Google
Guidelines / Policies / Govt
US National Guidelines Clearinghouse on Nerve NICE Guidance on Nerve NHS PRODIGY Guidance FDA on Nerve CDC on Nerve
Books
Books on Nerve
News
Nerve in the news Be alerted to news on Nerve News trends on Nerve
Commentary
Blogs on Nerve
Definitions
Definitions of Nerve
Patient Resources / Community
Patient resources on Nerve Discussion groups on Nerve Patient Handouts on Nerve Directions to Hospitals Treating Nerve Risk calculators and risk factors for Nerve
Healthcare Provider Resources
Symptoms of Nerve Causes & Risk Factors for Nerve Diagnostic studies for Nerve Treatment of Nerve
Continuing Medical Education (CME)
CME Programs on Nerve
International
Nerve en Espanol Nerve en Francais
Business
Nerve in the Marketplace Patents on Nerve
Experimental / Informatics
List of terms related to Nerve

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

A nerve is an enclosed, cable-like bundle of axons (the long, slender projection of a neuron). Neurons are sometimes called nerve cells, though this term is technically imprecise since many neurons do not form nerves, and nerves also include the glial cells that ensheath the axons in myelin.

Anatomy

Nerves are part of the peripheral nervous system. Afferent nerves convey sensory signals to the central nervous system, for example from skin or organs, while efferent nerves conduct stimulatory signals from the central nervous system to the muscles and glands. Afferent and efferent nerves are often arranged together, forming mixed nerves. The median nerve controls motor and sensory function in the hand.

Each peripheral nerve is covered externally by a dense sheath of connective tissue, the epineurium. Underlying this is a layer of flat cells forming a complete sleeve, the perineurium. Perineurial septa extend into the nerve and subdivide it into several bundles of fibres. Surrounding each such fibre is the endoneurial sheath. This is a tube which extends, unbroken, from the surface of the spinal cord to the level at which the axon synapses with its muscle fibres or ends in sensory endings. The endoneurial sheath consists of an inner sleeve of material called the glycocalyx and an outer, delicate, meshwork of collagen fibres. Peripheral nerves are richly supplied with blood.

Most nerves connect to the middle systems through the spinal cord. The twelve cranial nerves, however, connect directly to parts of the brain. Spinal nerves are given letter-number combinations according to the vertebra through which they connect to the spinal column. Cranial nerves are assigned numbers, usually expressed as Roman numerals from I to XII. In addition, most nerves and major branches of nerves have descriptive names. Inside the central nervous system, bundles of axons are termed tracts rather than nerves.

The signals that nerves carry, sometimes called nerve impulses, are also known as action potentials: rapidly (up to 120 m/s) traveling electrical waves, which begin typically in the cell body of a neuron and propagate rapidly down the axon to its tip or "terminus." The signals cross over from the terminus to the adjacent neurotransmitter receptor through a gap called the synapse. Motor neurons innervate or activate muscles groups. The nerve system runs through the spinal cord.

Clinical importance

Damage to nerves can be caused by physical injury, swelling (e.g. carpal tunnel syndrome), autoimmune diseases (e.g. Guillain-Barré syndrome), infection (neuritis), diabetes, or failure of the blood vessels surrounding the nerve. Pinched nerves occur 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, numbness, weakness, or paralysis. Patients may feel these symptoms in areas far from the actual site of damage, a phenomenon called referred pain. Referred pain occurs because when a nerve is damaged, signaling is defective from all parts of the area which the nerve receives input, not just the site of the damage. 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. This initial exam can be followed with tests such as nerve conduction study and electromyography (EMG).

See also

• Nervous system
• Dermatomic area

Additional images

• 

  Peripheral nerve, cross section

ar:عصب