Nerve
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Image:Nerves of the left upper extremity.gif
- "Nerves" redirects here. For the musical group, see The Nerves. For other uses, see Nerve (disambiguation).
A nerve is an enclosed, cable-like bundle of nerve fibers or axons, which includes the glia 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.
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 fibers 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 nerves carry, sometimes called nerve impulses, are also known as action potentials: rapidly 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 phenonmenon called referred pain. Referred pain occurs because when a nerve is damaged, signalling 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
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