Autonomic nervous system

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Autonomic nervous system
1503 Connections of the Parasympathetic Nervous System.jpg
Autonomic nervous system innervation
Details
Identifiers
Latin autonomicum systema nervosum
MeSH D001341
TA98 A14.3.00.001
TA2 6600
FMA 9905
Anatomical terminology

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, smooth muscle and glands. [2] The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, its force of contraction, digestion, respiratory rate, pupillary response, urination, and sexual arousal. [3] The fight-or-flight response, also known as the acute stress response, is set into action by the autonomic nervous system. [4]

Contents

The autonomic nervous system is regulated by integrated reflexes through the brainstem to the spinal cord and organs. These functions include control of respiration, cardiac regulation, vasomotor activity, and certain reflex actions such as coughing, sneezing, swallowing and vomiting. Those are then subdivided into other areas and are also linked to autonomic subsystems and the peripheral nervous system. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving autonomic regulatory input from the limbic system. [5]

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 three branches: the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. [6] [7] : 13  [8] [9] The enteric nervous system however is a less recognized part of the autonomic nervous system. [10] The sympathetic nervous system is responsible for setting off the fight-or-flight response. [4] The parasympathetic nervous system is responsible for the body's rest and digestion response. [4] In many cases, both of these systems have "opposite" 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. [5]

There are inhibitory and excitatory synapses between neurons. A third subsystem of neurons has been named as non-noradrenergic, non-cholinergic transmitters because they use other transmitters such as nitric oxide as a neurotransmitter. These functions are integral in autonomic function, in particular in the gut and the lungs. [11]

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.

Structure

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. Gray839.png
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.

The autonomic nervous system has been classically divided into the sympathetic, parasympathetic and enteric nervous systems. [7] : 168  [13] More modern classifications recognize other networks that integral to different organs, such as the intrinsic cardiac nervous system. [14]

The sympathetic division emerges from the spinal cord in the thoracic and lumbar areas, terminating around L2-3. The parasympathetic division has craniosacral "outflow", meaning that the neurons begin at the cranial nerves (specifically the oculomotor nerve, facial nerve, glossopharyngeal nerve and vagus nerve) and sacral (S2-S4) spinal cord. These divisions are distinctive because they require a sequential two-neuron efferent pathway; the preganglionic neuron must first synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first, neuron will have its nerve cell body in the central nervous system and will synapse at the postganglionic, or second, neuron's cell body. The postganglionic neuron will then form junctions within the target organ.

Sympathetic division

The sympathetic nervous system consists of cells with bodies in the lateral grey column from T1 to L2/3. These cell bodies are "GVE" (general visceral efferent) neurons and are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons:

  1. cervical ganglia (3)
  2. thoracic ganglia (12) and rostral lumbar ganglia (2 or 3)
  3. caudal lumbar ganglia and sacral ganglia

These ganglia provide the postganglionic neurons from which innervation of target organs follows. Examples of splanchnic (visceral) nerves are:

These all contain afferent (sensory) nerves as well, known as GVA (general visceral afferent) neurons.

Parasympathetic division

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:

These ganglia provide the postganglionic neurons from which innervations of target organs follows. Examples are:

Enteric nervous system

Development of the enteric nervous system

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]

Intrinsic cardiac nervous system

This section is an excerpt from Intrinsic cardiac nervous system.[ edit ]

The Intrinsic cardiac nervous system (ICNS), also known as the heart's "little brain," is a complex network of neurons and ganglia embedded within the heart tissue that regulates cardiac function independently of the central nervous system. It modulates heart rate, conduction, and cardiac contractility in response to local and external stimuli. [16] [17] It forms part of the autonomic nervous system.

Sensory neurons

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 edit
OrganNerves [19] Spinal column origin [19]
stomach T5, T6, T7, T8, T9, sometimes T10
duodenum T5, T6, T7, T8, T9, sometimes T10
jejunum and ileum T5, T6, T7, T8, T9
spleen T6, T7, T8
gallbladder and liver T6, T7, T8, T9
colon
pancreatic head T8, T9
appendix T10
bladder S2-S4
kidneys and ureters T11, T12

Motor neurons

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.

Function

Function of the autonomic nervous system The Autonomic Nervous System.png
Function of the autonomic nervous system

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]

Target organ/systemParasympatheticSympathetic
Digestive systemIncrease peristalsis and amount of secretion by digestive glandsDecrease activity of digestive system
LiverNo effectCauses glucose to be released to blood
LungsConstricts bronchiolesDilates bronchioles
Urinary bladder and UrethraRelaxes sphincterConstricts sphincter
KidneysNo effectsDecrease urine output
HeartDecreases rateIncrease rate
Blood vesselsNo effect on most blood vesselsConstricts blood vessels in viscera; increase BP
Salivary and lacrimal glandsStimulates; increases production of saliva and tearsInhibits; result in dry mouth and dry eyes
Eye (iris)Stimulates constrictor muscles; constrict pupilsStimulate dilator muscle; dilates pupils
Eye (ciliary muscles)Stimulates to increase bulging of lens for close visionInhibits; decrease bulging of lens; prepares for distant vision
Adrenal medullaNo effectStimulate medulla cells to secrete epinephrine and norepinephrine
Sweat gland of skinNo effectStimulate sudomotor function to produce perspiration

Sympathetic nervous system

Promotes a fight-or-flight response, corresponds with arousal and energy generation, and inhibits digestion

The pattern of innervation of the sweat gland—namely, the postganglionic sympathetic nerve fibers—allows clinicians and researchers to use sudomotor function testing to assess dysfunction of the autonomic nervous systems, through electrochemical skin conductance.

Parasympathetic nervous system

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:[ citation needed ]

Enteric nervous system

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:

Neurotransmitters

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 Autonomic nervous system.jpg
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
Sistema Nervioso Autonomo.svg

At the effector organs, sympathetic ganglionic neurons release noradrenaline (norepinephrine), along with other cotransmitters such as ATP, to act on adrenergic receptors, with the exception of the sweat glands and the adrenal medulla:

A full table is at Table of neurotransmitter actions in the ANS.

Autonomic nervous system and the immune system

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]

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. It is important to note that 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. [28]

See also

References

  1. ↑ Lawrence, Eleanor. "ANS". Henderson's Dictionary of Biological Terms (10th ed.). p. 1. ISBN   0-470-21446-5.
  2. ↑ "autonomic nervous system" at Dorland's Medical Dictionary
  3. ↑ Schmidt, A; Thews, G (1989). "Autonomic Nervous System". In Janig, W (ed.). Human Physiology (2 ed.). New York, NY: Springer-Verlag. pp. 333–370.
  4. 1 2 3 Chu, Brianna; Marwaha, Komal; Sanvictores, Terrence; Awosika, Ayoola O.; Ayers, Derek (2024). "Physiology, Stress Reaction". StatPearls Publishing, US National Library of Medicine. PMID   31082164 . Retrieved 2024-12-02.
  5. 1 2 Allostatic load notebook: Parasympathetic Function Archived 2012-08-19 at the Wayback Machine – 1999, MacArthur research network, UCSF
  6. ↑ Langley, J.N. (1921). The Autonomic Nervous System Part 1. Cambridge: W. Heffer.
  7. 1 2 Jänig, Wilfrid (2008). Integrative action of the autonomic nervous system : neurobiology of homeostasis (Digitally printed version. ed.). Cambridge: Cambridge University Press. ISBN   978052106754-6.
  8. ↑ Furness, John (9 October 2007). "Enteric nervous system". Scholarpedia. 2 (10): 4064. Bibcode:2007SchpJ...2.4064F. doi: 10.4249/scholarpedia.4064 .
  9. ↑ Willis, 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.
  10. ↑ Pocock, Gillian (2006). Human Physiology (3rd ed.). Oxford University Press. pp. 63–64. ISBN   978-0-19-856878-0.
  11. ↑ Belvisi, 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. PMID   1350993.
  12. ↑ Costanzo, Linda S. (2007). Physiology . Hagerstwon, MD: Lippincott Williams & Wilkins. p.  37. ISBN   978-0-7817-7311-9.
  13. ↑ Langley, J. N. (1921). The autonomic nervous system: Part 1. W. Heffer. p. 10.
  14. ↑ Wake, Emily; Brack, Kieran (August 2016). "Characterization of the intrinsic cardiac nervous system". Autonomic Neuroscience. 199: 3–16. doi:10.1016/j.autneu.2016.08.006. PMID   27568996.
  15. 1 2 3 4 5 6 7 Sharkey KA, Mawe GM (April 2023). "The enteric nervous system". Physiological Reviews. 103 (2): 1487–1564. doi:10.1152/physrev.00018.2022. PMC   9970663 . PMID   36521049.
  16. ↑ Fedele, Laura; Brand, Thomas (2020-11-24). "The Intrinsic Cardiac Nervous System and Its Role in Cardiac Pacemaking and Conduction". Journal of Cardiovascular Development and Disease. 7 (4): 54. doi: 10.3390/jcdd7040054 . ISSN   2308-3425. PMC   7712215 . PMID   33255284.
  17. ↑ ARMOUR, J. ANDREW (February 2007). "The little brain on the heart" (PDF). CLEVELAND CLINIC JOURNAL OF MEDICINE.
  18. ↑ Essential Clinical Anatomy. K. L. Moore and A. M. Agur. Lippincott, 2 edition (2002). Page 199
  19. 1 2 Unless 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.
  20. ↑ Neil A. Campbell, Jane B. Reece: Biologie. Spektrum-Verlag Heidelberg-Berlin 2003, ISBN   3-8274-1352-4
  21. ↑ Goldstein, 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.
  22. ↑ Pranav Kumar. (2013). Life Sciences : Fundamentals and practice. Mina, Usha. (3rd ed.). New Delhi: Pathfinder Academy. ISBN   9788190642774. OCLC   857764171.
  23. ↑ Hadhazy, Adam (February 12, 2010). "Think Twice: How the Gut's "Second Brain" Influences Mood and Well-Being". Scientific American. Archived from the original on December 31, 2017.
  24. ↑ Zhu 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. PMID   35766834.
  25. ↑ Johnson, 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
  26. ↑ Zimmerman-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. PMID   25850440. S2CID   23539284.
  27. ↑ Bunsawat, Kanokwan; White, Daniel W; Kappus, Rebecca M; Baynard, Tracy (2015). "Caffeine delays autonomic recovery following acute exercise". European Journal of Preventive Cardiology. 22 (11): 1473–1479. doi: 10.1177/2047487314554867 . PMID   25297344. S2CID   30678381.
  28. ↑ Monda, M.; Viggiano, An.; Vicidomini, C.; Viggiano, Al.; Iannaccone, T.; Tafuri, D.; De Luca, B. (2009). "Espresso coffee increases parasympathetic activity in young, healthy people". Nutritional Neuroscience. 12 (1): 43–48. doi:10.1179/147683009X388841. PMID   19178791. S2CID   37022826.
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