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Abbreviation for autonomic nervous system.
Farlex Partner Medical Dictionary © Farlex 2012


American Nimodipine Study. A clinical trial that assessed nimodipine’s effect on mortality in post-acute stroke patients.
Conclusions Nimodipine has no effect on patients at 48 hours, but may be beneficial at 18 hours.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.


Abbreviation for autonomic nervous system.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

autonomic nervous system



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The parts of the nervous system that control unconscious, involuntary, and visceral body functions. The autonomic nervous system reflexively balances the body's smooth muscle tone, blood pressure, temperature, fluid composition, state of digestion, metabolic activity, and sexual activation. In the central nervous system (CNS), the activities of the autonomic nervous system (ANS) are coordinated in the brainstem (especially in the nucleus of the tractus solitarius) and in the hypothalamus. In the peripheral nervous system (PNS), the ANS comprises the visceral motor axons, the visceral sensory axons, and the enteric nervous system (a neural net within the walls of the gastrointestinal tract). Compared to peripheral somatic axons, the peripheral autonomic axons tend to be small (less than 3 µm in diameter), slowly conducting, and sparsely myelinated. The autonomic motor circuits also differ from somatic motor pathways. Peripheral somatic motor pathways, i.e., the circuitry sending signals to skeletal muscles, are only one axon long; axons of somatic motor neurons in the spinal cord and brainstem synapse directly on the effector cell, a muscle cell. In contrast, peripheral autonomic motor pathways are two axons long. First, an axon (a preganglionic axon) of a visceral motor neuron in the spinal cord or brainstem synapses on a neuron in a peripheral ganglion. Second, the axon (a postganglionic axon) of the ganglion neuron synapses on the effector cell, a smooth muscle cell, a cardiac muscle cell, or a secretory cell.

This autonomic motor circuitry is further subdivided into two parallel subsystems; the sympathetic and the parasympathetic. The subsystems differ in two major ways: 1. In the sympathetic system, the central (preganglionic) neurons are located only in the thoracic and lumbar segments of the spinal cord; in the parasympathetic system, the central neurons are located only in the brainstem and in a short segment of the caudal end of the spinal cord. 2. In the sympathetic system, norepinephrine is the characteristic neurotransmitter of the postganglionic axons; in the parasympathetic system, acetylcholine is the characteristic neurotransmitter of the postganglionic axons. In both the sympathetic and parasympathetic systems, the characteristic neurotransmitter of the preganglionic axons is acetylcholine.Besides their characteristic neurotransmitters, autonomic nerves influence surrounding tissues through the release of other active chemicals including ATP, nitric oxide, and a range of peptides, e.g., substance P and vasoactive intestinal peptide. As a result of their different final transmitters, the effects of the two subsystems differ. Sympathetic stimulation readies an animal for interaction with the outside world and prepares the animal for "fight or flight"; e.g., activation of sympathetic axons increases heart rate and decreases gastrointestinal peristalsis. On the other hand, parasympathetic stimulation relaxes and quiets an animal; e.g., activation of parasympathetic axons decreases heart rate and increases gastrointestinal peristalsis. The accompanying table compares the effects of sympathetic and parasympathetic stimulation on specific tissues. See: parasympathetic nervous system; sympathetic nervous system; illustrationtable


The ANS is distributed throughout the body, and autonomic dysfunction can produce a wide range of symptoms, such as bladder malfunction, blood pressure abnormalities, breathing difficulty, gastrointestinal motility problems, heart arrhythmias, impotence, nasal congestion, sweating disorders, syncope, and visual symptoms. Drugs that act on or mimic autonomic neurotransmitters are commonly used to alleviate these symptoms as well as other conditions, such as glaucoma, heart failure, shock, and thyroid storm. To assess the overall functioning of the ANS, physicians often begin with simple measurements of the reflexive responses of the cardiovascular system; specifically, they measure the changes of blood pressure and heart rate as a person stands from sitting and exercises.

Sympathetic StimulationParasympathetic Stimulation
adipose tissuelipolysis
adrenal cortex
secretion (corticoids)increase
adrenal medulla
secretion (adrenaline)increase
abdominal organsconstriction
skin and mucosaconstriction
wall (detrusor) musclerelaxationcontraction
gall bladder
wall musclerelaxationcontraction
AV node (conduction velocity)increasedecrease
SA node (rate)increasedecrease
atrial muscle (contractility)increasedecrease
ventricular muscle (contractility)increase
wall muscle (tone and motility)decreaseincrease
rectal sphincter musclecontractionrelaxation
secretion (renin)increase
lacrimal gland
secretion (tears)increase
metabolismglycogenolysis, gluconeogenesisglycogen synthesis
bile secretiondecreaseincrease
secretion (airway glands)increase
secretion (mucosal glands)increase
secretion (enzymes and insulin)decreaseincrease
secretion (glucagon)increase
pineal gland
melatonin synthesisstimulation
reproductive tract
blood vesselsconstrictiondilation, erection
salivary gland
skeletal muscleincrease contractility, glycogenolysis
pilomotor musclecontraction
secretion (sweat)increase
wall muscle (tone and motility)decreaseincrease
Medical Dictionary, © 2009 Farlex and Partners

Autonomic nervous system (ANS)

The part of the nervous system that supplies nerve endings in the blood vessels, heart, intestines, glands, and smooth muscles, and governs their involuntary functioning. The autonomic nervous system is responsible for the biochemical changes involved in experiences of anxiety.
Mentioned in: Anxiety
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


Abbreviation for autonomic nervous system.
Medical Dictionary for the Dental Professions © Farlex 2012
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