| 单词 | parasympathetic nervous system | |||
| 释义 | parasympathetic nervous systemparasympathetic nervous systempar·a·sym·pa·thet·ic nervous system(păr′ə-sĭm′pə-thĕt′ĭk)parasympathetic nervous system
parasympathetic nervous systemparasympathetic nervous system:see nervous systemnervous system,network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. Virtually all members of the animal kingdom have at least a rudimentary nervous system. ..... Click the link for more information. . Parasympathetic nervous systemA portion of the autonomic system. It consists of two neuron chains, but differs from the sympathetic nervous system in that the first neuron has a long axon and synapses with the second neuron near or in the organ innervated. In general, its action is in opposition to that of the sympathetic nervous system, which is the other part of the autonomic system. It cannot be said that one system, the sympathetic, always has a excitatory role and the other, the parasympathetic, an inhibitory role; the situation depends on the organ in question. However, it may be said that the sympathetic system, by altering the level at which various organs function, enables the body to rise to emergency demands encountered in flight, combat, pursuit, and pain. The parasympathetic system appears to be in control during such pleasant periods as digestion and rest. The alkaloid pilocarpine excites parasympathetic activity while atropine inhibits it. See Autonomic nervous system, Sympathetic nervous system Parasympathetic Nervous Systemthe part of the autonomic nervous system whose ganglia are located very close to or in the innervated organs. The centers of the parasympathetic nervous system are found in the mesencephalon and medulla oblongata and in the sacral part of the spinal cord. The fibers of the parasympathetic nervous system proceed to the viscera as part of the third (oculomotor), seventh (facial), ninth (glossopharyngeal), and especially the tenth (vagus) cranial nerves and as part of the pelvic nerve. In many cases, the parasympathetic and sympathetic nervous systems are directly opposed to one another in their effect on organs. Whereas sympathetic stimulation will act to accelerate and intensify heart contractions, raise the blood pressure, and dilate the pupils, parasympathetic stimulation will act to slow and weaken the heartbeat, lower arterial pressure, and contract the pupils. Efferent neurons of the parasympathetic nervous system are typically situated in the innervated organ proper and not in the separate trunk, as is the case in the sympathetic nervous system. The mediator substance formed in the nerve endings of the parasympathetic nervous system is acetylcholine; the predominant mediator substance in the sympathetic nervous system is norepinephrine. O. M. BENIUMOV parasympathetic nervous system[¦par·ə‚sim·pə′thed·ik ′nər·vəs ‚sis·təm]parasympathetic nervous systemparasympathetic nervous systemThe predominant secretion of the nerve endings of the parasympathetic nervous system is acetylcholine, which acts on organs of the body to either excite or inhibit certain activities. For example, stimulation of the parasympathetic system causes constriction of the pupil of the eye and contraction of the ciliary muscle; increase of the glandular secretion of enzymes, as in the case of the pancreas; increased peristalsis; and a slowed heart rate. It often happens that excitation of the sympathetic nervous system results in an effect opposite that of the parasympathetic system; however, most organs are under the almost exclusive control of either one or the otherof the two nervous systems that compose the autonomic nervous system. parasympathetic nervous systempar·a·sym·pa·thet·ic ner·vous sys·tem(par'ă-sim'pă-thet'ik nĕr'vŭs sis'tĕm)Compare: sympathetic nervous system parasympathetic nervous systemOne of the two divisions of the AUTONOMIC NERVOUS SYSTEM. The parasympathetic system leaves the central nervous system in the 3rd, 7th, 9th and 10th CRANIAL NERVES and from the 2nd to the 4th SACRAL segments of the spinal cord. Parasympathetic action constricts the pupils of the eyes, promotes salivation and tearing, slows the heart, constricts the BRONCHI, increases the activity of the intestines, contracts the bladder wall and relaxes the SPHINCTERS and promotes erection of the penis. See also SYMPATHETIC NERVOUS SYSTEM.Parasympathetic nervous systempar·a·sym·pa·thet·ic ner·vous sys·tem(par'ă-sim'pă-thet'ik nĕr'vŭs sis'tĕm)parasympathetic nervous system
Synonyms for parasympathetic nervous system
|
|||
| 随便看 |
|
英语词典包含2567994条英英释义在线翻译词条，基本涵盖了全部常用单词的英英翻译及用法，是英语学习的有利工具。