What is Nerve?

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

Spinal nerves
	ventral root - dorsal root - dorsal root ganglion - cauda equina - gray ramus communicans - white ramus communicans suboccipital posterior divisions: cervical (greater occipital, third occipital) - thoracic - lumbar - sacral - coccygeal anterior divisions: cervical plexus - brachial plexus - thoracic nerves: (intercostal - intercostobrachial - subcostal) - lumbosacral plexus

•  • [ e] 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) • Alderman's nerve • in the neck (pharyngeal branch, superior laryngeal ext and int, recurrent laryngeal) • in the thorax (pulmonary branches, esophageal plexus) • in the abdomen (gastric plexuses, celiac plexus, gastric plexus)
XI-XII	accessory • hypoglossal

•  • [ e] Nerves of head and neck: the cervical plexus
superficial	lesser occipital • greater auricular • transverse cervical • supraclavicular
deep	ansa cervicalis • phrenic

•  • [ e] 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

•  • [ e] Nerves - autonomic nervous system (sympathetic nervous system/ganglion and parasympathetic nervous system/ganglion)
Head/cranial	Ciliary ganglion (Short ciliary nerves) - Pterygopalatine ganglion (Nerve of pterygoid canal) - Submandibular ganglion - Otic ganglion
Neck/cervical	cervical ganglia: (Superior, Middle, Inferior) - Stellate ganglion Cavernous plexus - Internal carotid
Chest/thorax	paravertebral: Thoracic ganglia Cardiac plexus - Thoracic aortic plexus Thoracic splanchnic nerves - cardiac nerves (Superior, Middle, Inferior)
Abdomen/Lumbar	paravertebral: Lumbar ganglia Celiac plexus - Celiac ganglia (Hepatic, Splenic, Pancreatic) - Aorticorenal (Abdominal aortic plexus, Renal/Suprarenal) - Superior mesenteric (Gastric) Inferior mesenteric (Spermatic, Ovarian) hypogastric: Superior hypogastric - Inferior hypogastric (Vesical, Prostatic, Uterovaginal) enteric nervous system: Meissner's plexus • Auerbach's plexus Lumbar splanchnic nerves
Pelvis/sacral	paravertebral: Sacral ganglia Ganglion impar Pelvic splanchnic nerves - Sacral splanchnic nerves
All	Sympathetic trunks - Ganglia of sympathetic trunk - Prevertebral ganglia - Rami communicans (White, Gray)

•  • [ e] Nerves of lower limbs and lower torso: the lumbosacral plexus
lumbar plexus	iliohypogastric - ilioinguinal - genitofemoral (femoral branch/lumboinguinal, genital branch) - lateral cutaneous of thigh (patellar) - obturator (anterior, cutaneous, posterior, accessory) - femoral (anterior cutaneous branches, saphenous)
sacral/coccygeal plexus	to quadratus femoris - to obturator internus - to the piriformis - superior gluteal - inferior gluteal - posterior cutaneous of thigh (inferior cluneal, perineal branches) sciatic: tibial (medial sural cutaneous, sural, medial calcaneal, medial plantar, lateral plantar) - common fibular (lateral sural cutaneous, deep fibular, superficial fibular, medial dorsal cutaneous, intermediate dorsal cutaneous)
pudendal plexus	perforating cutaneous - pudendal (dorsal of the penis/clitoris, inferior anal, perineal and posterior scrotal/labial) - anococcygeal
cutaneous innervation of the lower limbs