autonomic nervous system(redirected from Sympathetic fibers)
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Related to Sympathetic fibers: Postganglionic sympathetic fibers
Once the symptoms of autonomic dysreflexia are manifest, emergency care is indicated. Efforts are made to lower the blood pressure by placing the patient in a sitting position or elevating the head and upper body to a 45-degree angle. The stimulus must be identified and removed as gently and quickly as possible. If fecal impaction is the cause, the rectum should be coated with an anesthetic ointment prior to attempted removal of the impaction; this prevents increasing the stimulus to autonomic dysreflexia. The physician is notified so that appropriate medical intervention can be initiated. Antihypertensive drugs are a last resort. As soon as the cause is identified and removed, the dysreflexia will disappear. Patients who experience repeated attacks may require surgery to sever the nerves responsible for the exaggerated response to stimulation.
The parts of a system can be referred to as its elements or components; the environment of the system is defined as all of the factors that affect the system and are affected by it. A living system is capable of taking in matter, energy, and information from its environment (input), processing them in some way, and returning matter, energy, and information to its environment as output.
An open system is one in which there is an exchange of matter, energy, and information with the environment; in a closed system there is no such exchange. A living system cannot survive without this exchange, but in order to survive it must maintain pattern and organization in the midst of constant change. Control of self-regulation of an open system is achieved by dynamic interactions among its elements or components. The result of self-regulation is referred to as the steady state; that is, a state of equilibrium. homeostasis is an assemblage of organic regulations that act to maintain steady states of a living organism.
A system can be divided hierarchically into subsystems, which can be further subdivided into sub-subsystems and components. A system and its environment could be considered as a unified whole for purposes of study, or a subsystem could be studied as a system. For example, the collection of glands in the endocrine system can be thought of as a system, each endocrine gland could be viewed as a system, or even specific cells of a single gland could be studied as a system. It is also possible to think of the human body as a living system and the endocrine system as a subsystem. The division of a system into a subsystem and its environment is dependent on the perspective chosen by the person studying a particular phenomenon.
autonomic (visceral motor) division of nervous system[TA]
autonomic nervous system
autonomic nervous system
au·to·no·mic di·vi·sion of ner·vous sys·tem(aw'tō-nom'ik di-vizh'ŭn nĕr'vŭs sis'tĕm)
Synonym(s): divisio autonomica systematis nervosi peripherici [TA] , autonomic nervous system.
autonomic nervous system,
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 Stimulation||Parasympathetic Stimulation|
|skin and mucosa||constriction|
|wall (detrusor) muscle||relaxation||contraction|
|AV node (conduction velocity)||increase||decrease|
|SA node (rate)||increase||decrease|
|atrial muscle (contractility)||increase||decrease|
|ventricular muscle (contractility)||increase|
|wall muscle (tone and motility)||decrease||increase|
|rectal sphincter muscle||contraction||relaxation|
|metabolism||glycogenolysis, gluconeogenesis||glycogen synthesis|
|secretion (airway glands)||increase|
|secretion (mucosal glands)||increase|
|secretion (enzymes and insulin)||decrease||increase|
|blood vessels||constriction||dilation, erection|
|skeletal muscle||increase contractility, glycogenolysis|
|wall muscle (tone and motility)||decrease||increase|
autonomic nervous systemThe part of the nervous system controlling involuntary functions, such as the heart beat, the secretion of glands and the contraction of blood vessels. It is subdivided into the SYMPATHETIC and the PARASYMPATHETIC divisions which are, in general, antagonistic and in balance. The term autonomic derives from the Greek autos , self, and nomos , a law.
autonomic nervous systemthe part of the nervous system that controls the involuntary activities of the body There are two main parts:
- The sympathetic nervous system., in which complexes of SYNAPSES form ganglia alongside the vertebrae, and the preganglionic fibres from the central nervous system are therefore short; the fibres are ADRENERGIC. (b) The parasympathetic nervous system, in which the ganglia are embedded in the wall of the effector so that the preganglionic fibres are long and the postganglionic fibres short; these are CHOLINERGIC.
sympathetic system inhibits peristalsis stimulates contraction in sphincters of bladder and anus inhibits bladder contraction stimulates pacemaker, speeding up heart stimulates arterial constriction inhibits contraction of bronchioles, causing dilation inhibits contraction of iris muscle, causing dilated pupil parasympathetic system stimulates peristalis inhibits contraction in sphincters of bladder and anus stimulates bladder contraction inhibits pacemaker, slowing down heart inhibits arterial constriction, causing dilation stimulates contraction of bronchioles stimulates contraction of iris muscle, causing reduced pupil.
The sympathetic and parasympathetic systems innervate the same end organs, but the effects produced by the two systems generally oppose one another, for example: