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 non-axon glial cells that ensheath the axons in myelin.
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 axons are often arranged together, forming mixed nerves. For example, the median nerve controls motor and sensory function in the hand.
Billions of long nerve cells, called neurons,make up the bodies's nervous system. Neurons receive and transmit chemical-electrical messages to and from the brain. Each neuron is long and thin. One end receives messages and the other transmits the message to the next neuron.The messages "jump" across a gap from one neuron cell to another.
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 forms an unbroken tube which extends from the surface of the spinal cord to the level at which the axon synapses with its muscle fibres or ends in sensory receptors. 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 central nervous system 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 major nerves have descriptive names. Inside the central nervous system, distinguishable bundles of axons are termed tracts rather than nerves.
The signals that nerves carry, sometimes called nerve impulses, are also known as action potentials. These are rapidly (up to 120 m/s) travelling electrical waves, which typically begin in the cell body of a neuron and propagate down the axon to its tip or "terminus." The signals cross over from the terminus of the axon to the adjacent neurotransmitter receptor through a gap called the synapse. Motor neurons innervate or activate muscles groups.
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. 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, 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, signalling is defective from all parts of the area from 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).
|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 cranial nerves|
|I-IV||olfactory • optic • oculomotor (superior branch, inferior branch) • trochlear|
|V: trigeminal||trigeminal ganglion • ophthalmic • maxillary • mandibular|
|VI: abducens||no significant branches|
|VII: facial||nervus intermedius • geniculate • inside facial canal (greater petrosal, nerve to the stapedius, chorda tympani) • at exit from stylomastoid foramen (posterior auricular, digastric • stylohyoid) • on face (temporal, zygomatic, buccal, mandibular, cervical)|
|VIII: vestibulocochlear||cochlear (striae medullares, lateral lemniscus) • vestibular (Scarpa's ganglion)|
|IX: glossopharyngeal||nucleus ambiguus • ganglia (superior, petrous) • tympanic (tympanic plexus, lesser petrosal) • carotid sinus • pharyngeal branches • pharyngeal plexus|
|X: vagus||ganglia (jugular, nodose) • in the jugular fossa (meningeal branch, auricular branch) • in the neck (pharyngeal branch, superior laryngeal ext and int, recurrent laryngeal, superior cervical cardiac) • in the thorax (inferior cardiac, pulmonary branches, anterior vagal trunk, posterior vagal trunk) • in the abdomen (celiac branches - renal branches - hepatic branches of anterior vagal trunk - anterior gastric branches of anterior vagal trunk - posterior gastric branches of posterior vagal trunk)|
|XI-XII||accessory • hypoglossal|
|Nerves of head and neck: the cervical plexus|
|superficial||Lesser occipital • Greater auricular • Transverse cervical • Supraclavicular|
|deep||Ansa cervicalis • Phrenic|
|Nerves of upper limbs (primarily): the brachial plexus|
|Supraclavicular||dorsal scapular • suprascapular • to the subclavius • long thoracic|
|Infraclavicular: lateral cord||musculocutaneous (lateral cutaneous of forearm) • lateral pectoral|
lateral head of median (anterior interosseous, palmar, common palmar digital, proper palmar digital)
|Infraclavicular: medial cord||medial pectoral • medial cutaneous of forearm • medial cutaneous of arm
ulnar (muscular branches, dorsal branch, palmar branch, superficial branch, deep branch)
medial head of median
|Infraclavicular: posterior cord||subscapular (upper, lower) • thoracodorsal
axillary (superior lateral cutaneous of arm)
radial (muscular, inferior lateral cutaneous of arm, posterior cutaneous of arm, posterior cutaneous of forearm, superficial branch, deep branch, 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 (Short ciliary nerves) - Pterygopalatine ganglion (Nerve of pterygoid canal) - Submandibular ganglion - Otic ganglion|
|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: Thoracic splanchnic nerves
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|
|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)|
|sacral plexus (L4-S4)||muscular: superior gluteal/inferior gluteal - to quadratus femoris - to obturator internus - to the piriformis
sciatic: tibial (medial sural cutaneous, sural, medial calcaneal, medial plantar, lateral plantar)
sciatic: common fibular (lateral sural cutaneous, deep fibular, superficial fibular, medial dorsal cutaneous, intermediate dorsal cutaneous)
cutaneous: posterior cutaneous of thigh (inferior cluneal, perineal branches) - perforating cutaneous
|coccygeal plexus (S4-Co)||pudendal: inferior anal - perineal (deep, posterior scrotal ♂/labial ♀) - dorsal of the penis ♂/clitoris ♀|
|cutaneous innervation of the lower limbs|