Nerve Fiber

nerve fiber

[′nərv ‚fī·bər] (neuroscience) The long process of a neuron, usually the axon.

Nerve Fiber

the structural unit along which nerve impulses travel, consisting of the nerve cell outgrowth, or axon, and its surrounding sheath. The axon is also called the axis cylinder.

Nerve fibers are usually 0.5–30 μ thick; some nerve fibers in lower vertebrates and in invertebrates are much thicker (as in the annelid Myxicola, with a nerve fiber thickness of about 1,700 μ). The length of a nerve fiber varies in direct proportion to the size of the animal and can be greater than 1 m. The vertebrate nervous system has medullated, or myelinated, nerve fibers and unmedullated, or unmyelinated, nerve fibers. In both types of fiber, the sheath consists of Schwann’s cells. In unmedullated fibers the Schwann’s cells form the neurilemma, or Schwann’s sheath, which encloses one or more axons. Schwann’s cells in medullated fibers also form the myelin sheath. This consists of a white protein-lipid complex—myelin—which is formed by the repeated convolutions of the Schwann’s cell around the axon. The cytoplasm of the Schwann’s cell is displaced to the periphery of the cell, while the cell membranes “swathe” the axon. The portion of the axon covered in this manner can measure 200 μ to several millimeters in length.

Figure 1. A nerve cell axon

The intervals between adjacent Schwann’s cells are called the nodes of Ranvier (see Figure 1); these are approximately 1 μ long and are free of the myelin sheath. Since the myelin sheath is an insulator, it prevents the current created by a stimulus from jumping to adjacent portions of the axon’s membrane. As a result, the nerve impulse does not spread continuously as it would in an unmyelinated fiber; rather, the impulse in a myelinated fiber passes in stepwise fashion from one node of Ranvier to the next. This method of propagating an impulse is called saltatory conduction. It is a more rapid method than that in unmyelinated fibers.

The rate at which nerve impulses travel along nerve fibers increases in proportion to the thickness of the axons. Several species of animals develop giant nerve fibers to provide for rapid nerve reactions in response to emergency situations. The giant nerve fiber of a squid is approximately 800 μ in diameter; studies using this nerve fiber have revealed several common patterns in the physiology of excitable cell membranes.

D. A. SAKHAROV

nerve fiber

fiber

[fi´ber] 1. an elongated threadlike structure.2. dietary fiber.3. nerve f.A f's myelinated fibers of the somatic nervous system having a diameter of 1 to 22 μm and a conduction velocity of 5 to 120 meters per second.accelerating f's (accelerator f's) adrenergic fibers that transmit the impulses that accelerate the heart beat.adrenergic f's nerve fibers of the sympathetic nervous system that liberate norepinephrine" >norepinephrine (and possibly small amounts of epinephrine" >epinephrine) at a synapse when a nerve impulse passes.alpha f's motor and proprioceptive fibers of the A type having conduction velocities of 70 to 120 meters per second and ranging from 13 to 22 micrometers in diameter.arcuate f's any of the bow-shaped fibers in the brain, such as those connecting adjacent gyri in the cerebral cortex, or the external or internal arcuate fibers of the medulla oblongata.association f's nerve fibers that interconnect portions of the cortex" >cerebral cortex within a hemisphere. Short association fibers interconnect neighboring gyri; long fibers interconnect more widely separated gyri and are arranged into bundles or fasciculi.B f's myelinated preganglionic autonomic axons having a fiber diameter less than 3 μm and a conduction velocity of 3 to 15 meters per second.beta f's touch and temperature fibers of the A type having conduction velocities of 30 to 70 meters per second and ranging from 8 to 13 micrometers in diameter.C f's postganglionic unmyelinated fibers of the autonomic nervous system; also, the unmyelinated fibers at the dorsal roots and at free nerve endings having a diameter of 0.3 to 1.3 μm and a conduction velocity of 0.6 to 2.3 meters per second.cholinergic f's nerve fibers such as the parasympathetic fibers that liberate acetylcholine" >acetylcholine at a synapse when a nerve impulse passes.collagen f's (collagenous f's) the soft, flexible, white fibers that are the most characteristic constituent of all types of connective tissue, consisting of the protein collagen, and composed of bundles of fibrils that are in turn made up of smaller units (microfibrils) that show a characteristic crossbanding with a major periodicity of 65 nm.Corti's f's pillar cells.crude fiber the fiber that remains after food is digested with alkali and acid, which destroys all soluble and some insoluble fiber. It is mainly lignin and cellulose.depressor f's afferent nerve fibers that when stimulated reflexly cause diminished vasomotor tone and thus decreased arterial pressure.dietary fiber that portion of ingested foodstuffs that cannot be broken down by intestinal enzymes and juices and, therefore, passes through the small intestine and colon undigested. It is composed of cellulose (which is the “skeleton” of plants), hemicellulose, gums, lignin, pectin, and other carbohydrates indigestible by humans. Dietary fiber is not to be confused with crude fiber, which is the term used in the USDA Handbook and other tables listing the composition of foods. Crude fiber is mainly lignin and cellulose and is the residue remaining after a food has been subjected to a standardized treatment with dilute acid and alkali. Crude fiber measurements usually underestimate actual total dietary fiber by at least 50 per cent.
Vegetables, cereals, and fruits are the main sources of dietary fiber. Although bran is advertised as an excellent source of fiber, it is not unique nor is it as nutritious as fruits and vegetables and some other whole unprocessed cereals. The typical diet in Western countries contains 10 to 30 grams of dietary fiber.
The primary effects of dietary fiber are to increase the bulk of the stool and make it softer by taking up water as it passes through the colon, and to absorb organic wastes and toxins and carry them out of the intestinal tract. The increase in stool bulk hastens the passage of feces and may reduce the length of time the intestinal wall is exposed to toxic substances.Benefits of a High Fiber Diet. Dietary fiber is helpful in the treatment and prevention of uncomplicated constipation. Unlike strong laxatives, it presents no problems when taken on a long-term basis. Metamucil, a medicinal fecal softener, is made from seed husks and is often prescribed for persons having problems with normal bowel activity. Hemorrhoids are aggravated by straining on defecation, and so there is some basis for recommending a high fiber diet for persons who have this condition.
The symptoms of diverticular disease, which is an outpouching of the wall of the colon with subsequent inflammation, are relieved by a high fiber diet. There is evidence to support the theory that the more rapid passage of softer stools through the colon decreases the pressure exerted against its walls and thereby prevents formation of diverticula.
The symptoms of irritable bowel syndrome often can be mitigated by fiber. The bulk of fiber keeps the colon mildly distended, thus preventing the development of pockets of high pressure that cause spasm. However, inflammatory bowel disease in which there is a narrowing of the bowel, as in some cases of crohn's disease, can be worsened by more roughage in the intestinal tract.
Fiber does have the capacity to unite with intestinal bile salts and dietary cholesterol, preventing their absorption from the gut and hastening their elimination via the intestinal tract. Because of these properties, fiber has been advocated as a preventive measure against the formation of gallstones and the production of atherosclerotic plaques in the blood vessels.
In diabetes mellitus, fiber, when eaten with other foods, somewhat reduces the rise in blood glucose that occurs after eating. Fiber slows the rate of carbohydrate breakdown and absorption from the intestinal tract. The American Cancer Society suggests a diet rich in fiber as a way to lower the incidence of certain kinds of cancer, particularly colorectal cancer.
Some people may have difficulties with a high fiber diet. It can produce abdominal pain, bloating, flatus, and diarrhea. These side effects can be controlled if the fiber is introduced to the diet in small amounts and with an increase in fluid intake. Excessive amounts of fiber can also impair absorption of essential minerals.elastic f's yellowish fibers of elastic quality traversing the intercellular substance of connective tissue.gamma f's fibers that conduct touch and pressure impulses and innervate the intrafusal fibers of the muscle spindle; they conduct at velocities of 15 to 40 meters per second and range from 3 to 7 μm in diameter.gray f's unmyelinated fibers found largely in the sympathetic nerves.insoluble fiber that not soluble in water, composed mainly of lignin, cellulose, and hemicelluloses and primarily found in the bran layers of cereal grains. Its actions include increasing fecal bulk and decreasing free radicals in the gastrointestinal tract.intrafusal f's modified muscle fibers which, surrounded by fluid and enclosed in a connective tissue envelope, compose the muscle spindle.light f's muscle fibers poor in sarcoplasm and more transparent than dark fibers.Mahaim f's short direct connections between the lower atrioventricular node or bundle of His and the ventricular septum, resulting in preexcitation of the ventricular septum and a delta wave. Only right sided connections have been described.medullated f's (medullated nerve f's) myelinated fibers.motor f's nerve fibers transmitting motor impulses to a muscle fiber.muscle fiber any of the cells of skeletal or cardiac muscle" >muscle tissue. Skeletal muscle fibers are cylindrical multinucleate cells containing contracting myofibrils" >myofibrils, across which run transverse striations. Cardiac muscle fibers have one or sometimes two nuclei, contain myofibrils, and are separated from one another by an intercalated disk; although striated, cardiac muscle fibers branch to form an interlacing network.muscle f's, fast twitch paler-colored muscle fibers of larger diameter than slow twitch fibers, and having less sarcoplasm and more prominent cross-striping; used for forceful and rapid contractions over short periods of time.muscle f's, slow twitch small dark muscle fibers rich in mitochondria, myoglobin, and sarcoplasm and with only faint cross-striping; designed for slow but repetitive contractions over long periods of time.myelinated f's grayish white nerve fibers encased in a sheath" >myelin sheath; see myelin" >myelin.nerve fiber a slender process of a neuron, especially the prolonged axon that conducts nerve impulses away from the cell; classified as either myelinated fibers or unmyelinated fibers according to whether they have or do not have a sheath" >myelin sheath.nonmedullated f's unmyelinated fibers.osteogenetic f's (osteogenic f's) precollagenous fibers formed by osteoclasts and becoming the fibrous component of bone matrix.postganglionic f's nerve fibers passing to involuntary muscle and gland cells, the cell bodies of which lie in the autonomic ganglia.preganglionic f's nerve fibers passing to the autonomic ganglia, the cell bodies of which lie in the brain or spinal cord.pressor f's afferent nerve fibers that when stimulated reflexly cause or increase vasomotor tone and thus increase arterial pressure.projection f's bundles of axons that connect the cerebral cortex with the subcortical centers, brain stem, and spinal cord.Purkinje f's modified cardiac fibers in the subendocardial tissue that constitute the terminal ramifications of the conducting system of the heart. The term is sometimes used loosely to denote the entire system of conducting fibers.radicular f's fibers in the roots of the spinal nerves.ragged red f's muscle fibers characterized by large collections of structurally abnormal mitochondria below the sarcolemmal surface and within the fiber itself that stain red; seen in myopathy" >mitochondrial myopathy and certain other myopathic disorders.reticular f's immature connective tissue fibers, staining with silver, forming the reticular framework of lymphoid and myeloid tissue, and occurring in interstitial tissue of glandular organs, the papillary layer of the skin, and elsewhere.Sharpey's f's 1. collagenous fibers that pass from the periosteum and are embedded in the outer circumferential and interstitial lamellae of bone.2. terminal portions of principal fibers that insert into the cementum of a tooth.soluble fiber that with an affinity for water, either dissolving or swelling to form a gel; it includes gums, pectins, mucilages, and some hemicelluloses, and is primarily found in fruits, vegetables, oats, barley, legumes, and seaweed. It acts to decrease the rate of stomach emptying and increase transit time through the intestine, and also binds bile acids, increasing their excretion. Soluble fiber appears to specifically lower levels of low-density lipoproteincholesterol.somatic f's (somatic nerve f's) nerve fibers, afferent or efferent, that stimulate or activate skeletal muscle and somatic tissues.spindle f's the microtubules radiating from the centrioles during mitosis and forming a spindle-shaped configuration.unmyelinated f's nerve fibers that lack a sheath" >myelin sheath; see myelin" >myelin.visceral f's (visceral nerve f's) nerve fibers, afferent or efferent, that stimulate or activate smooth muscle and glandular tissues.

nerve fi·ber

the axon of a nerve cell, ensheathed by oligodendroglial cells in brain and spinal cord, and by Schwann cells in peripheral nerves.

nerve fiber

n. A threadlike process of a neuron, especially the prolonged axon that conducts nerve impulses.

nerve fi·ber

(nĕrv fībĕr) Nerve cell axon, ensheathed by oligodendroglial cells in brain and spinal cord.

nerve fiber

nerve fiber

nerve′ fi`ber

nerve fiber

nerve fiber

Nerve Fiber

nerve fiber

Nerve Fiber

nerve fiber

fiber

nerve fi·ber

nerve fiber

nerve fi·ber

nerve fiber

Synonyms for nerve fiber

noun a threadlike extension of a nerve cell

Synonyms

Related Words