The autonomic nervous system (ANS),1 sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates internal organs, and . The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the , its , , , , , and . The autonomic nervous system has historically been branched into three divisions based on which organs and functions are being controlled by each system. This includes the , which regulates the gastrointestinal tract, the and the , which are responsible for, among other things, stimulation of the "rest and digest" and the (or "acute stress response"), respectively. In literature, the acronyms ENS, PSNS and SNS or SANS(where the A stands for autonomic) may be used to abbreviate the aforementioned terms.
Although conflicting reports about its subdivisions exist in the literature, the autonomic nervous system has historically been considered a purely motor system, and has been divided into the aforementioned three branches of ENS, PSNS and SANS.789 The enteric nervous system however is a less recognized part of the autonomic nervous system.10 When it comes to the parasympathetic(calm, digesting, mating) and the sympathetic(acute stress), in many cases, the two of these systems have complementary actions where one system activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitatory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a "quick response mobilizing system" and the parasympathetic is a "more slowly activated dampening system", but even this has exceptions, such as in sexual arousal and orgasm, wherein both play a role.6
Although the ANS is also known as the visceral nervous system and although most of its fibers carry non-somatic information to the CNS, many authors still consider it only connected with the motor side.12 Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control. Overall, the ANS ensures the maintenance of vital functions and allows the body to effectively adapt to cycles of stress and recovery.
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The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands.
visceral nervous system
The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands.
vegetative nervous system
The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands.
Terms the article defines in bold, quoted in context.
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Autonomic nervous system
2,787 words · 28 references · revision 1383561670 · CC BY-SA 4.0
Autonomic nervous system, showing splanchnic nerves in middle, and the vagus nerve as "X" in blue. The heart and organs below in list to right are regarded as viscera.
chromaffin cells of the adrenal medulla (this is the one exception to the two-neuron pathway rule: the synapse is directly efferent onto the target cell bodies)
These ganglia provide the postganglionic neurons from which innervation of target organs follows. Examples of splanchnic (visceral) nerves are:
cervical cardiac nerves and thoracic visceral nerves, which synapse in the sympathetic chain
The parasympathetic nervous system consists of cells with bodies in one of two locations: the brainstem (cranial nerves III, VII, IX, X) or the sacral spinal cord (S2, S3, S4). These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:
Development of the enteric nervous system involves migration of cells from the vagal section of the neural crest, eventually populating the entire gastrointestinal tract.15 Throughout development, tyrosine kinase activity has roles in formation and regulation of enteric ganglia to influence spontaneous, rhythmic, slow waves in the gastrointestinal tract.15
Structure of the enteric nervous system
The enteric nervous system (ENS) is a division of the autonomic nervous system embedded in the gastrointestinal tract walls.15 Having about 200 million neurons, the ENS communicates with the central nervous system while regulating gut function independently.15 The core of this structure consists of two main interconnected neural networks or plexuses: the myenteric plexus (Auerbach's) and the submucosal plexus (Meissner's).15 The myenteric plexus extends the full length of the gut, primarily controlling motility (movement) and secretomotor functions, using nitric oxide to regulate smooth muscle in the ENS.15 The submucosal plexus has a role in secretory regulation by innervating intestinal endocrine cells and blood vessels.15
The visceral sensory system - technically not a part of the autonomic nervous system - is composed of primary neurons located in cranial sensory ganglia: the geniculate, petrosal and nodose ganglia, appended respectively to cranial nerves VII, IX and X. These sensory neurons monitor the levels of carbon dioxide, oxygen and sugar in the blood, arterial pressure and the chemical composition of the stomach and gut content. They also convey the sense of taste and smell, which, unlike most functions of the ANS, is a conscious perception. Blood oxygen and carbon dioxide are in fact directly sensed by the carotid body, a small collection of chemosensors at the bifurcation of the carotid artery, innervated by the petrosal (IXth) ganglion. Primary sensory neurons project (synapse) onto "second order" visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract (nTS), that integrates all visceral information. The nTS also receives input from a nearby chemosensory center, the area postrema, that detects toxins in the blood and the cerebrospinal fluid and is essential for chemically induced vomiting or conditional taste aversion (the memory that ensures that an animal that has been poisoned by a food never touches it again). All this visceral sensory information constantly and unconsciously modulates the activity of the motor neurons of the ANS.
Innervation
Autonomic nerves travel to organs throughout the body. Most organs receive parasympathetic supply by the vagus nerve and sympathetic supply by splanchnic nerves. The sensory part of the latter reaches the spinal column at certain spinal segments. Pain in any internal organ is perceived as referred pain, more specifically as pain from the dermatome corresponding to the spinal segment.18
Autonomic nervous system's jurisdiction to organs in the human body
Motor neurons of the autonomic nervous system are found in "autonomic ganglia". Those of the parasympathetic branch are located close to the target organ whilst the ganglia of the sympathetic branch are located close to the spinal cord.
The sympathetic ganglia here, are found in two chains: the pre-vertebral and pre-aortic chains. The activity of autonomic ganglionic neurons is modulated by "preganglionic neurons" located in the central nervous system. Preganglionic sympathetic neurons are located in the spinal cord, at the thorax and upper lumbar levels. Preganglionic parasympathetic neurons are found in the medulla oblongata where they form visceral motor nuclei; the dorsal motor nucleus of the vagus nerve; the nucleus ambiguus, the salivatory nuclei, and in the sacral region of the spinal cord.
Sympathetic and parasympathetic divisions typically function in opposition to each other. But this opposition is better termed complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. The sympathetic system is often considered the "fight or flight" system, while the parasympathetic system is often considered the "rest and digest" or "feed and breed" system.
However, many instances of sympathetic and parasympathetic activity cannot be ascribed to "fight" or "rest" situations. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus. Another example is the constant, second-to-second, modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles. In general, these two systems should be seen as permanently modulating vital functions, in a usually antagonistic fashion, to achieve homeostasis. Higher organisms maintain their integrity via homeostasis which relies on negative feedback regulation which, in turn, typically depends on the autonomic nervous system.21 Some typical actions of the sympathetic and parasympathetic nervous systems are listed below.22
History
In 1665, Thomas Willis used the terminology, and in 1900, John Newport Langley used the term, defining the two divisions as the sympathetic and parasympathetic nervous systems.25
Caffeine effects
Caffeine is a bioactive ingredient found in commonly consumed beverages such as coffee, tea, and sodas. Short-term physiological effects of caffeine include increased blood pressure and sympathetic nerve outflow. Habitual consumption of caffeine may inhibit physiological short-term effects. Consumption of caffeinated espresso increases parasympathetic activity in habitual caffeine consumers; however, decaffeinated espresso inhibits parasympathetic activity in habitual caffeine consumers. It is possible that other bioactive ingredients in decaffeinated espresso may also contribute to the inhibition of parasympathetic activity in habitual caffeine consumers.26
Caffeine is capable of increasing work capacity while individuals perform strenuous tasks. In one study, caffeine provoked a greater maximum heart rate while a strenuous task was being performed compared to a placebo. This tendency is likely due to caffeine's ability to increase sympathetic nerve outflow. Furthermore, this study found that recovery after intense exercise was slower when caffeine was consumed prior to exercise. This finding is indicative of caffeine's tendency to inhibit parasympathetic activity in non-habitual consumers. The caffeine-stimulated increase in nerve activity is likely to evoke other physiological effects as the body attempts to maintain homeostasis.27
The effects of caffeine on parasympathetic activity may vary depending on the position of the individual when autonomic responses are measured. One study found that the seated position inhibited autonomic activity after caffeine consumption (75 mg); however, parasympathetic activity increased in the supine position. This finding may explain why some habitual caffeine consumers (75 mg or less) do not experience short-term effects of caffeine if their routine requires many hours in a seated position. The data supporting increased parasympathetic activity in the supine position was derived from an experiment involving participants between the ages of 25 and 30 who were considered healthy and sedentary. Caffeine may influence autonomic activity differently for individuals who are more active or elderly.
The parasympathetic nervous system has been said to promote a "rest and digest" response, promotes calming of the nerves return to regular function, and enhancing digestion. Functions of nerves within the parasympathetic nervous system include:
Dilating blood vessels leading to the GI tract, increasing the blood flow.
Constricting the bronchiolar diameter when the need for oxygen has diminished
Constriction of the pupil and contraction of the ciliary muscles, facilitating accommodation and allowing for closer vision
Stimulating salivary gland secretion, and accelerates peristalsis, mediating digestion of food and, indirectly, the absorption of nutrients
Sexual. Nerves of the peripheral nervous system are involved in the erection of genital tissues via the pelvic splanchnic nerves 2–4. They are also responsible for stimulating sexual arousal.
The enteric nervous system is the intrinsic nervous system of the gastrointestinal system. It has been described as the "second brain of the human body".23 Its functions include:
Sensing chemical and mechanical changes in the gut
A flow diagram showing the process of stimulation of adrenal medulla that makes it release adrenaline, that further acts on adrenoreceptors, indirectly mediating or mimicking sympathetic activity
Acetylcholine is the preganglionic neurotransmitter for both divisions of the ANS, as well as the postganglionic neurotransmitter of parasympathetic neurons. Nerves that release acetylcholine are said to be cholinergic. In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter to stimulate muscarinic receptors.
At the adrenal medulla, there is no postsynaptic neuron. Instead, the presynaptic neuron releases acetylcholine to act on nicotinic receptors. Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream, which acts on adrenoceptors, thereby indirectly mediating or mimicking sympathetic activity.
Recent studies indicate that ANS activation is critical for regulating the local and systemic immune-inflammatory responses and may influence acute stroke outcomes. Therapeutic approaches modulating the activation of the ANS or the immune-inflammatory response could promote neurologic recovery after stroke.24
7Jänig, Wilfrid (2008). Integrative action of the autonomic nervous system : neurobiology of homeostasis (Digitally printed version. ed.). Cambridge: Cambridge University Press. ISBN 978052106754-6.
9Willis, William D. (2004). "The Autonomic Nervous System and its central control". In Berne, Robert M. (ed.). Physiology (5. ed.). St. Louis, Mo.: Mosby. ISBN 0323022251.
10Pocock, Gillian (2006). Human Physiology (3rd ed.). Oxford University Press. pp. 63–64. ISBN 978-0-19-856878-0.
11Belvisi, Maria G.; David Stretton, C.; Yacoub, Magdi; Barnes, Peter J. (1992). "Nitric oxide is the endogenous neurotransmitter of bronchodilator nerves in humans". European Journal of Pharmacology. 210 (2): 221–2. doi:10.1016/0014-2999(92)90676-U. PMID1350993.
12Costanzo, Linda S. (2007). Physiology. Hagerstwon, MD: Lippincott Williams & Wilkins. p. 37. ISBN 978-0-7817-7311-9.
18Essential Clinical Anatomy. K. L. Moore and A. M. Agur. Lippincott, 2 edition (2002). Page 199
19Unless specified otherwise in the boxes, the source is: Moore, Keith L.; Agur, A. M. R. (2002). Essential Clinical Anatomy (2nd ed.). Lippincott Williams & Wilkins. p. 199. ISBN 978-0-7817-5940-3.
21Goldstein, David (2016). Principles of Autonomic Medicine (PDF) (free online version ed.). Bethesda, Maryland: National Institute of Neurological Disorders and Stroke, National Institutes of Health. ISBN 9780824704087. Archived from the original (PDF) on 2018-12-06. Retrieved 2018-12-05.
22Pranav Kumar. (2013). Life Sciences : Fundamentals and practice. Mina, Usha. (3rd ed.). New Delhi: Pathfinder Academy. ISBN 9788190642774. OCLC857764171.
24Zhu L, Huang L, Le A, Wang TJ, Zhang J, Chen X, Wang J, Wang J, Jiang C (June 2022). "Interactions between the Autonomic Nervous System and the Immune System after Stroke". Compr Physiol. 2022 (3): 3665–3704. doi:10.1002/cphy.c210047. ISBN 9780470650714. PMID35766834.{{cite journal}}: CS1 maint: periodical has ISBN (link)
25Johnson, Joel O. (2013), "Autonomic Nervous System Physiology", Pharmacology and Physiology for Anesthesia, Elsevier, pp. 208–217, doi:10.1016/b978-1-4377-1679-5.00012-0, ISBN 978-1-4377-1679-5
26Zimmerman-Viehoff, Frank; Thayer, Julian; Koenig, Julian; Herrmann, Christian; Weber, Cora S.; Deter, Hans-Christian (May 1, 2016). "Short-term effects of espresso coffee on heart rate variability and blood pressure in habitual and non-habitual coffee consumers- a randomized crossover study". Nutritional Neuroscience. 19 (4): 169–175. doi:10.1179/1476830515Y.0000000018. PMID25850440. S2CID23539284.
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