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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.
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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 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 body'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, called a synapse, from one neuron 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 septae 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.
Within the endoneurium, the individual nerve fibers are surrounded by a low protein liquid called endoneurial fluid. The endoneurium has properties analogous to the "blood-brain barrier" in that it excludes many types of chemical molecules from crossing into the endoneurial fluid from the blood stream. Because of this endoneurial fluid is considered a "privileged space" within the body - similar to the status of cerebro-spinal fluid in the central nervous system. During the development of nerve edema from nerve irritation or injury, the amount of endoneurial fluid can increase at the site of irritation. This effect can be seen by Magnetic resonance Neurography imaging. This is why MR neurography can show nerve irritation.
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) traveling 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.
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. 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).
See also
Additional images
 Peripheral nerve, cross section |
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Wikipedia content modification information:
- This page was last modified on 19 August 2008, at 04:05.
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