parasympathetic nervous system

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Parasympathetic nervous system

A 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 System


the 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.


parasympathetic nervous system

[¦par·ə‚sim·pə′thed·ik ′nər·vəs ‚sis·təm]
The craniosacral portion of the autonomic nervous system, consisting of preganglionic nerve fibers in certain sacral and cranial nerves, outlying ganglia, and postganglionic fibers.
References in periodicals archive ?
However, this does not imply that there is no specific physiological effect of acupuncture treatment as we found an acupuncture-specific increased parasympathetic tone accompanied by an increase of pain-related QoL.
The burst pacing with higher AF inducibility than premature beat indicated that in addition to the parasympathetic tone, the rapid focal firing within LSPV was another condition for AF inducement.
These physiologic changes were paralleled by feelings of well-being and mental brightness, which might be attributed to the enhancement of neural sympathetic activity, as revealed by the rise of the NA but not Ad plasma levels, together with the increase of parasympathetic tone, as revealed by the neurophysiologic, neurochemical, and clinical parameters.
"One hypothesis is that omega-3 fatty acids have some effects on the autonomic nervous system--in particular, by increasing parasympathetic tone. In generally healthy individuals without CVD, it is sometimes thought that an increase in autonomic tone may play a role in the development of AF," he said in an interview.
Some physiologic measures of parasympathetic tone, like heart beat-to-beat variability, go down.
It has been shown that HRV depends on the relationship of the sympathetic and parasympathetic tone. The smaller the deviation in our R-intervals (the lower the heart rate variability), the more increase in sympathetic tone.
The pupil has an exclusively autonomic innervation, pupil diameter reflecting the balance between sympathetic and parasympathetic tone, thus constituting a sensitive test system of central and peripheral autonomic activity.
By reducing allostatic load and restoring normal homeostasis, yogic relaxation (Shavasana) may be able to help correct the imbalance of autonomic nervous system by enhancing parasympathetic tone and by reducing sympathetic overactivity.
The important point here is that although they are similar, decreased parasympathetic tone is not the same as excessive sympathetic activity.
If the body's clock malfunctions, this affects the circadian rhythmicity in sympathetic and parasympathetic tone and in the cortisol awakening response, which in turn alter sympathetic reactivity to stress and the HPA-axis response.
Five time-domain indices were measured: the standard deviation of all normal-to-normal RR intervals in the entire recording (SDNN); the mean of the standard deviations of the normal RR intervals in all 5-min segments in the entire 24-h recording (SDNN index); and the mean of the standard deviation of the averaged normal-to-normal RR intervals in all 5-min segments of the 24-h recording (SDANN index) represent the autonomic function; the root mean square of successive differences normal RR intervals and the percent of the differences between the adjacent normal-to-normal intervals >50 ms (pNN50), both of them are considered as a maker of parasympathetic tone.

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