Nerve
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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
Additional images
 Peripheral nerve, cross section |
Nerves: spinal nerves | |
|---|---|
| Cervical (8) | anterior (Cervical plexus, Brachial plexus) - posterior (Posterior branches of cervical nerves, Suboccipital, Greater occipital, Third occipital) |
| Thoracic (12) | anterior (Intercostal, Intercostobrachial - T2, Thoraco-abdominal nerves - T7-T11, Subcostal - T12) - posterior (Posterior branches of thoracic nerves) |
| Lumbar (5) | anterior (Lumbar plexus, Lumbosacral trunk) - posterior (Posterior branches of the lumbar nerves, Superior cluneal L1-L3) |
| Sacral (5) | anterior (Sacral plexus) - posterior (Posterior branches of sacral nerves, Medial cluneal nerves) |
| Coccygeal (1) | anterior (Coccygeal plexus) - posterior (Posterior branch of coccygeal nerve) |
Nerves of head and neck: the cervical plexus (C1-C4) | |
|---|---|
| superficial | C2-C3: Lesser occipital • Greater auricular • Transverse cervical C3-C4: Supraclavicular |
| deep | C1-C3: Ansa cervicalis (superior root, inferior root) C3-C5: Phrenic |
Nerves of upper limbs (primarily): the brachial plexus (C5-T1) | |
|---|---|
| Supraclavicular | root (dorsal scapular, long thoracic) - upper trunk (suprascapular, to the subclavius) |
| Infraclavicular: lateral cord | lateral pectoral
musculocutaneous (lateral cutaneous of forearm) median/lateral root: anterior interosseous - palmar - recurrent - common palmar digital (proper palmar digital) |
| Infraclavicular: medial cord | medial pectoral
cutaneous: medial cutaneous of forearm • medial cutaneous of arm ulnar: muscular - palmar - dorsal (dorsal digital nerves) - superficial (common palmar digital, proper palmar digital) - deep median/medial root: see above |
| Infraclavicular: posterior cord | subscapular (upper, lower) • thoracodorsal
axillary (superior lateral cutaneous of arm) radial: muscular - cutaneous (posterior of arm, inferior lateral of arm, posterior of forearm) - superficial (dorsal digital nerves) - deep (posterior interosseous) |
| Other | cutaneous innervation of the upper limbs |
Nerves - autonomic nervous system (sympathetic nervous system/ganglion/trunks and parasympathetic nervous system/ganglion) | |
|---|---|
| Head/cranial | Ciliary ganglion: roots (Sensory, Parasympathetic, Sympathetic) - Short ciliary
Pterygopalatine ganglion: deep petrosal - nerve of pterygoid canal |
| Neck/cervical | paravertebral ganglia: Cervical ganglia (Superior, Middle, Inferior) - Stellate ganglion prevertebral plexus: Cavernous plexus - Internal carotid |
| Chest/thorax | paravertebral ganglia: Thoracic ganglia
prevertebral plexus: Cardiac plexus - Esophageal plexus - Pulmonary plexus - Thoracic aortic plexus splanchnic nerves: cardiopulmonary - thoracic cardiac nerves: Superior - Middle - Inferior |
| Abdomen/Lumbar | paravertebral ganglia: Lumbar ganglia
prevertebral ganglia: Celiac ganglia (Aorticorenal) - Superior mesenteric ganglion - Inferior mesenteric ganglion prevertebral plexus: Celiac plexus - (Hepatic, Splenic, Pancreatic) - aorticorenal (Abdominal aortic plexus, Renal/Suprarenal) - Superior mesenteric (Gastric) - Inferior mesenteric (Spermatic, Ovarian) - Superior hypogastric (hypogastric nerve, Superior rectal) - Inferior hypogastric (Vesical, Prostatic/Cavernous nerves of penis, Uterovaginal, Middle rectal) splanchnic nerves: Lumbar splanchnic nerves enteric nervous system: Meissner's plexus • Auerbach's plexus |
| Pelvis/sacral | paravertebral ganglia: Sacral ganglia - Ganglion impar splanchnic nerves: Pelvic splanchnic nerves - Sacral splanchnic nerves |
| All | Rami communicans (White, Gray) - Preganglionic fibers - Postganglionic fibers |
Nerves of lower limbs and lower torso: the lumbosacral plexus (L1-Co) | |||||
|---|---|---|---|---|---|
| lumbar plexus (L1-L4) | iliohypogastric: lateral cutaneous branch - anterior cutaneous branch
ilioinguinal: anterior scrotal ♂/labial ♀ genitofemoral: femoral branch/lumboinguinal - genital branch lateral cutaneous of thigh: patellar obturator: anterior (cutaneous) - posterior - accessory femoral: anterior cutaneous branches - saphenous (infrapatellar, medial crural cutaneous) | ||||
| sacral plexus (L4-S4) |
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| coccygeal plexus (S4-Co) | pudendal: inferior anal - perineal (deep, posterior scrotal ♂/labial ♀) - dorsal of the penis ♂/clitoris ♀ anococcygeal | ||||
| cutaneous innervation of the lower limbs | |||||
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Acknowledgement and Attribution Regarding Sources of Content
Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .
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