Autonomic nervous system
Autonomic nervous system | |
---|---|
Details | |
Identifiers | |
Latin | autonomici systematis nervosi |
MeSH | D001341 |
TA98 | A14.3.00.001 |
TA2 | 6600 |
FMA | 9905 |
Anatomical terminology |
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the
The autonomic nervous system is regulated by integrated
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
There are
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.[10] Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control.
Structure
The autonomic nervous system has been classically divided into the
The autonomic nervous system is unique in that it requires 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 begin at the "outflow" and will synapse at the postganglionic, or second, neuron's cell body. The postganglionic neuron will then synapse at the target organ.[citation needed]
Sympathetic division
The sympathetic nervous system consists of cells with bodies in the
- paravertebral ganglia(3) of the sympathetic chain (these run on either side of the vertebral bodies)
- cervical ganglia (3)
- thoracic ganglia (12) and rostral lumbar ganglia (2 or 3)
- caudal lumbar ganglia and sacral ganglia
- prevertebral ganglia (celiac ganglion, aorticorenal ganglion, superior mesenteric ganglion, inferior mesenteric ganglion)
- 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
- thoracic splanchnic nerves (greater, lesser, least), which synapse in the prevertebral ganglia
- lumbar splanchnic nerves, which synapse in the prevertebral ganglia
- sacral splanchnic nerves, which synapse in the inferior hypogastric plexus
These all contain afferent (sensory) nerves as well, known as
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:
- cranial nerve III), geniculate (cranial nerve VII),
- pterygopalatine (cranial nerve VIIand IX),
- ottic in inner ear space (cranial nerve IX)
- tympanic nerve of VII with C9, C10, C5 (cranial nerves VII, XI, X, V) in promontory plexus in middle ear space
- trigeminal ganglion specially sensory (only mastication motor) is common with other ones
- in or near the wall of an organ innervated by the vagus (sacral nervesplexus (S2, S3, S4)
these ganglia provide the postganglionic neurons from which innervations of target organs follows. Examples are:
- the postganglionic parasympathetic splanchnic (visceral) nerves
- the vagus nerve, which passes through the thorax and abdominal regions innervating, among other organs, the heart, lungs, liver and stomach
Enteric Nervous System
Development of the Enteric Nervous System:
The intricate process of enteric nervous system (ENS) development begins with the migration of cells from the vagal section of the neural crest. These cells embark on a journey from the cranial region to populate the entire gastrointestinal tract. Concurrently, the sacral section of the neural crest provides an additional layer of complexity by contributing input to the hindgut ganglia. Throughout this developmental journey, numerous receptors exhibiting tyrosine kinase activity, such as Ret and Kit, play indispensable roles. Ret, for instance, plays a critical role in the formation of enteric ganglia derived from cells known as vagal neural crest. In mice, targeted disruption of the RET gene results in renal agenesis and the absence of enteric ganglia, while in humans, mutations in the RET gene are associated with megacolon. Similarly, Kit, another receptor with tyrosine kinase activity, is implicated in Cajal interstitial cell formation, influencing the spontaneous, rhythmic, electrical excitatory activity known as slow waves in the gastrointestinal tract. Understanding the molecular intricacies of these receptors provides crucial insights into the delicate orchestration of ENS development.[11]
Structure of the Enteric Nervous System:
The structural complexity of the enteric nervous system (ENS) is a fascinating aspect of its functional significance. Originally perceived as postganglionic parasympathetic neurons, the ENS earned recognition for its autonomy in the early 1900s. Boasting approximately 100 million neurons, a quantity comparable to the spinal cord, the ENS is often described as a "brain of its own." This description is rooted in the ENS's ability to communicate independently with the central nervous system through parasympathetic and sympathetic neurons. At the core of this intricate structure are the myenteric plexus (Auerbach's) and the submucous plexus (Meissner's), two main plexuses formed by the grouping of nerve-cell bodies into tiny ganglia connected by bundles of nerve processes. The myenteric plexus extends the full length of the gut, situated between the circular and longitudinal muscle layers. Beyond its primary motor and secretomotor functions, the myenteric plexus exhibits projections to submucosal ganglia and enteric ganglia in the pancreas and gallbladder, showcasing the interconnectivity within the ENS. Additionally, the myenteric plexus plays a unique role in innervating motor end plates with the inhibitory neurotransmitter nitric oxide in the striated-muscle segment of the esophagus, a feature exclusive to this organ. Meanwhile, the submucous plexus, most developed in the small intestine, occupies a crucial position in secretory regulation. Positioned in the submucosa between the circular muscle layer and the muscularis mucosa, the submucous plexus's neurons innervate intestinal endocrine cells, submucosal blood arteries, and the muscularis mucosa, emphasizing its multifaceted role in gastrointestinal function. Furthermore, ganglionated plexuses in the pancreatic, cystic duct, common bile duct, and gallbladder, resembling submucous plexuses, contribute to the overall complexity of the ENS structure. In this intricate landscape, glial cells emerge as key players, outnumbering enteric neurons and covering the majority of the surface of enteric neuronal-cell bodies with laminar extensions. Resembling the astrocytes of the central nervous system, enteric glial cells respond to cytokines by expressing MHC class II antigens and generating interleukins. This underlines their pivotal role in modulating inflammatory responses in the intestine, adding another layer of sophistication to the functional dynamics of the ENS. The varied morphological shapes of enteric neurons further contribute to the structural diversity of the ENS, with neurons capable of exhibiting up to eight different morphologies. These neurons are primarily categorized into type I and type II, where type II neurons are multipolar with numerous long, smooth processes, and type I neurons feature numerous club-shaped processes along with a single long, slender process. The rich structural diversity of enteric neurons highlights the complexity and adaptability of the ENS in orchestrating a wide array of gastrointestinal functions, reflecting its status as a dynamic and sophisticated component of the nervous system.[12]
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
Innervation
Autonomic nerves travel to organs throughout the body. Most organs receive parasympathetic supply by the
Organ | Nerves[14] | Spinal column origin[14]
|
---|---|---|
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
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.[16] Some typical actions of the sympathetic and parasympathetic nervous systems are listed below.[17]
Target organ/system | Parasympathetic | Sympathetic |
---|---|---|
Digestive system | Increase peristalsis and amount of secretion by digestive glands | Decrease activity of digestive system |
Liver | No effect | Causes glucose to be released to blood |
Lungs | Constricts bronchioles | Dilates bronchioles |
Urinary bladder/ Urethra | Relaxes sphincter | Constricts sphincter |
Kidneys | No effects | Decrease urine output |
Heart | Decreases rate | Increase rate |
Blood vessels | No effect on most blood vessels | Constricts blood vessels in viscera; increase BP |
Salivary and Lacrimal glands | Stimulates; increases production of saliva and tears | Inhibits; result in dry mouth and dry eyes |
Eye (iris) | Stimulates constrictor muscles; constrict pupils | Stimulate dilator muscle; dilates pupils |
Eye (ciliary muscles) | Stimulates to increase bulging of lens for close vision | Inhibits; decrease bulging of lens; prepares for distant vision |
Adrenal Medulla | No effect | Stimulate medulla cells to secrete epinephrine and norepinephrine |
Sweat gland of skin | No effect | Stimulate sudomotor function to produce perspiration |
Sympathetic nervous system
Promotes a fight-or-flight response, corresponds with arousal and energy generation, and inhibits digestion
- Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction
- Blood flow to skeletal muscles and the lungs is enhanced (by as much as 1200% in the case of skeletal muscles)
- Dilates epinephrine, which allows for greater alveolaroxygen exchange
- Increases myocytes), thereby providing a mechanism for enhanced blood flow to skeletal muscles
- Dilates pupils and relaxes the ciliary muscleto the lens, allowing more light to enter the eye and enhances far vision
- Provides coronary vessels of the heart
- Constricts all the intestinal sphinctersand the urinary sphincter
- Inhibits peristalsis
- Stimulates orgasm
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]
- Dilating blood vessels leading to the GI tract, increasing the blood flow.
- Constricting the bronchiolar diameter when the need for oxygen has diminished
- Dedicated cardiac branches of the myocardium)
- Constriction of the pupil and contraction of the accommodationand 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.
Enteric nervous system
The enteric nervous system is the intrinsic nervous system of the
- Sensing chemical and mechanical changes in the gut
- Regulating secretions in the gut
- Controlling peristalsis and some other movements
Neurotransmitters
At the effector organs, sympathetic ganglionic neurons release
- 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 nicotinic receptors. Stimulation of the adrenal medulla releases adrenaline(epinephrine) into the bloodstream, which acts on adrenoceptors, thereby indirectly mediating or mimicking sympathetic activity.
A full table is found 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.[19]
History
The specialised system of the autonomic nervous system was recognised by Galen.[citation needed]
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.[20]
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.[21]
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.[22]
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.[23]
See also
- Dysautonomia
- Feeling
- International Society for Autonomic Neuroscience
- Polyvagal Theory
- Medullary ischemic reflex
References
- ^ "autonomic nervous system" at Dorland's Medical Dictionary
- ^ Schmidt, A; Thews, G (1989). "Autonomic Nervous System". In Janig, W (ed.). Human Physiology (2 ed.). New York, NY: Springer-Verlag. pp. 333–370.
- ^ UCSF
- ^ Langley, J.N. (1921). The Autonomic Nervous System Part 1. Cambridge: W. Heffer.
- ISBN 978052106754-6.
- .
- ISBN 0323022251.
- ISBN 978-0-19-856878-0.
- PMID 1350993.
- ISBN 978-0-7817-7311-9.
- ISSN 0028-4793.
- ISSN 0028-4793.
- ^ Essential Clinical Anatomy. K.L. Moore & A.M. Agur. Lippincott, 2 ed. 2002. Page 199
- ^ ISBN 978-0-7817-5940-3.
- ISBN 3-8274-1352-4
- ISBN 9780824704087. Archived from the original(PDF) on 2018-12-06. Retrieved 2018-12-05.
- OCLC 857764171.
- ^ 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.
- PMID 35766834.
- ISBN 978-1-4377-1679-5
- S2CID 23539284.
- S2CID 30678381.
- S2CID 37022826.
External links
- Autonomic nervous system article in Scholarpedia, by Ian Gibbins and Bill Blessing
- Division of Nervous System Archived 2021-03-05 at the Wayback Machine