Nerve

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

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.

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 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.

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 first diagnose disorders of the nerves by a physical examination, including the testing of reflexes, walking, muscle weakness, directed movement, proprioception, and the sense of touch. During the exam, the doctor asks questions about the patients' symptoms and medical history. The initial exam can be followed with tests such as nerve conduction study and electromyography (EMG).

See also

• Connective tissue in the peripheral nervous system
• Dermatome (anatomy)
• Nerve agents
• Nerve block
• Nerve fiber
• Nerve injury
• Nervous system
• Neuronal oscillations
• Neuropathy
• Peripheral nerve injury
• Peripheral nerve injury classification
• Synapse

Additional images

Peripheral nerve, cross section

Nerves of head and neck: the cranial nerves

I-IV: olfactory - optic - oculomotor - trochlear

V: trigeminal: trigeminal ganglion
V1: ophthalmic: lacrimal - frontal (supratrochlear, supraorbital) - nasociliary (long root of ciliary, long ciliary, infratrochlear, posterior ethmoidal, anterior ethmoidal) - ciliary ganglion (short ciliary)
V2: maxillary: middle meningeal - in the pterygopalatine fossa (zygomatic, zygomaticotemporal, zygomaticofacial, sphenopalatine, posterior superior alveolar)
in the infraorbital canal/infraorbital nerve (middle superior alveolar, anterior superior alveolar)
on the face (inferior palpebral, external nasal, superior labial, infraorbital plexus) - pterygopalatine ganglion (deep petrosal, nerve of pterygoid canal)
branches of distribution (palatine, nasopalatine, pharyngeal)
V3: mandibular: nervus spinosus - medial pterygoid - anterior (masseteric, deep temporal, buccal, lateral pterygoid)
posterior (auriculotemporal, lingual, inferior alveolar, mylohyoid, mental) - otic ganglion - submandibular ganglion

VI: abducens

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

IX: glossopharyngeal: fasciculus solitarius - nucleus ambiguus - ganglia (superior, petrous) - tympanic - carotid sinus

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: accessory XII: hypoglossal

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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