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: thesympathetic nervous system, theparasympathetic nervous system, and theenteric 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 activateddampening system", but even this has exceptions, such as insexual arousal andorgasm, wherein both play a role.[5]
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 thesomatic 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.
Autonomic nervous system, showingsplanchnic 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 sympathetic division emerges from thespinal cord in thethoracic andlumbar areas, terminating around L2-3. The parasympathetic division has craniosacral "outflow", meaning that the neurons begin at thecranial nerves (specifically theoculomotor nerve,facial nerve,glossopharyngeal nerve andvagus nerve) andsacral (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.
The sympathetic nervous system consists of cells with bodies in thelateral 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:
paravertebral ganglia (3) of the sympathetic chain (these run on either side of the vertebral bodies)
chromaffin cells of theadrenal 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 ofsplanchnic (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: thebrainstem (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 theneural 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]
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: themyenteric plexus (Auerbach's) and thesubmucosal plexus (Meissner's).[15] The myenteric plexus extends the full length of the gut, primarily controllingmotility (movement) andsecretomotor functions, usingnitric 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: thegeniculate,petrosal andnodose ganglia, appended respectively to cranial nerves VII, IX and X. These sensory neurons monitor the levels ofcarbon 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 thenucleus 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.
Autonomic nerves travel to organs throughout the body. Most organs receive parasympathetic supply by thevagus nerve and sympathetic supply bysplanchnic nerves. The sensory part of the latter reaches thespinal column at certainspinal segments. Pain in any internal organ is perceived asreferred pain, more specifically as pain from thedermatome corresponding to the spinal segment.[18]
Autonomic nervous system's jurisdiction to organs in thehuman bodyedit
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, thesalivatory 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 achievehomeostasis.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 andparasympathetic nervous systems are listed below.[22]
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 and 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
Stimulatesudomotor function to produce perspiration
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
Constriction of the pupil and contraction of theciliary muscles, facilitatingaccommodation and allowing for closer vision
Stimulatingsalivary gland secretion, and acceleratesperistalsis, 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 thepelvic splanchnic nerves 2–4. They are also responsible for stimulating sexual arousal.
The enteric nervous system is the intrinsic nervous system of thegastrointestinal 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 theadrenal medulla, there is no postsynaptic neuron. Instead, the presynaptic neuron releases acetylcholine to act onnicotinic receptors. Stimulation of the adrenal medulla releasesadrenaline (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]
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 is abioactive ingredient found in commonly consumed beverages such as coffee, tea, and sodas. Short-term physiological effects of caffeine include increasedblood 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 maximumheart rate while a strenuous task was being performed compared to aplacebo. 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 maintainhomeostasis.[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]
^abJänig, Wilfrid (2008).Integrative action of the autonomic nervous system : neurobiology of homeostasis (Digitally printed version. ed.). Cambridge: Cambridge University Press.ISBN978052106754-6.
^Willis, William D. (2004). "The Autonomic Nervous System and its central control". In Berne, Robert M. (ed.).Physiology (5. ed.). St. Louis, Mo.: Mosby.ISBN0323022251.
^Pocock, Gillian (2006).Human Physiology (3rd ed.). Oxford University Press. pp. 63–64.ISBN978-0-19-856878-0.
^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.PMID1350993.
^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.ISBN9780824704087. Archived fromthe original(PDF) on 2018-12-06. Retrieved2018-12-05.
^Pranav Kumar. (2013).Life Sciences : Fundamentals and practice. Mina, Usha. (3rd ed.). New Delhi: Pathfinder Academy.ISBN9788190642774.OCLC857764171.
^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.ISBN9780470650714.PMID35766834.
