The autonomic nervous system is responsible for regulating the body's unconscious actions. The parasympathetic system is responsible for stimulation of "rest-and-digest" or "feed-and-breed"[3] activities that occur when the body is at rest, especially after eating, includingsexual arousal,salivation,lacrimation (tears),urination,digestion, anddefecation. Its action is described as being complementary to that of thesympathetic nervous system, which is responsible for stimulating activities associated with thefight-or-flight response.
Owing to its location, the parasympathetic system is commonly referred to as having "craniosacral outflow", which stands in contrast to the sympathetic nervous system, which is said to have "thoracolumbar outflow".[4]
The parasympathetic nerves areautonomic orvisceral[5][6] branches of theperipheral nervous system (PNS). Parasympathetic nerve supply arises through three primary areas:
Thepelvic splanchnic efferent preganglionic nerve cell bodies reside in thelateral gray horn of thespinal cord at the T12–L1 vertebral levels (the spinal cord terminates at the L1–L2 vertebrae with theconus medullaris), and their axons exit the vertebral column as S2–S4 spinal nerves through thesacral foramina.[7] Their axons continue away from the CNS to synapse at an autonomic ganglion. The parasympatheticganglion where these preganglionic neurons synapse will be close to the organ of innervation. This differs from the sympathetic nervous system, where synapses between pre- and post-ganglionic efferent nerves in general occur at ganglia that are farther away from the target organ.
As in the sympathetic nervous system,efferent parasympathetic nerve signals are carried from the central nervous system to their targets by a system of twoneurons. The first neuron in this pathway is referred to as thepreganglionic orpresynaptic neuron. Its cell body sits in the central nervous system and its axon usually extends to synapse with the dendrites of apostganglionic neuron somewhere else in the body. The axons of presynaptic parasympathetic neurons are usually long, extending from the CNS into a ganglion that is either very close to or embedded in their target organ. As a result, the postsynaptic parasympathetic nerve fibers are very short.[8]: 42
Theoculomotor nerve is responsible for a number of parasympathetic functions related to the eye.[9] The oculomotor PNS fibers originate in theEdinger-Westphal nucleus in the central nervous system and travel through thesuperior orbital fissure to synapse in theciliary ganglion located just behind the orbit (eye).[10] From the ciliary ganglion the postganglionic parasympathetic fibers leave via short ciliary nerve fibers, a continuation of thenasociliary nerve (a branch of ophthalmic division of thetrigeminal nerve (CN V1)). The short ciliary nerves innervate the orbit to control theciliary muscle (responsible foraccommodation) and theiris sphincter muscle, which is responsible formiosis or constriction of the pupil (in response to light or accommodation). There are two motors that are part of the oculomotor nerve known as the somatic motor and visceral motor. The somatic motor is responsible for moving the eye in precise motions and for keeping the eye fixated on an object. The visceral motor helps constrict the pupil.[11]
The parasympathetic aspect of thefacial nerve controls secretion of the sublingual and submandibularsalivary glands, thelacrimal gland, and the glands associated with the nasal cavity. The preganglionic fibers originate within the CNS in the superior salivatory nucleus and leave as theintermediate nerve (which some consider a separate cranial nerve altogether) to connect with the facial nerve just distal (further out) to it surfacing the central nervous system. Just after the facial nervegeniculate ganglion (general sensory ganglion) in thetemporal bone, the facial nerve gives off two separate parasympathetic nerves. The first is thegreater petrosal nerve and the second is thechorda tympani. The greater petrosal nerve travels through the middle ear and eventually combines with the deep petrosal nerve (sympathetic fibers) to form the nerve of thepterygoid canal. The parasympathetic fibers of the nerve of the pterygoid canal synapse at thepterygopalatine ganglion, which is closely associated with the maxillary division of the trigeminal nerve (CN V2). The postganglionic parasympathetic fibers leave the pterygopalatine ganglion in several directions. One division leaves on thezygomatic division of CN V2 and travels on a communicating branch to unite with the lacrimal nerve (branch of the ophthalmic nerve of CN V1) before synapsing at the lacrimal gland. These parasympathetic to the lacrimal gland control tear production.[12]
A separate group of parasympathetic leaving from the pterygopalatine ganglion are the descendingpalatine nerves (CN V2 branch), which include the greater and lesser palatine nerves. The greater palatine parasympathetic synapse on the hard palate and regulate mucous glands located there. The lesser palatine nerve synapses at the soft palate and controls sparse taste receptors and mucous glands. Yet another set of divisions from the pterygopalatine ganglion are the posterior, superior, and inferior lateral nasal nerves; and thenasopalatine nerves (all branches of CN V2, maxillary division of the trigeminal nerve) that bring parasympathetic innervation to glands of the nasalmucosa. The second parasympathetic branch that leaves the facial nerve is the chorda tympani. This nerve carriessecretomotor fibers to thesubmandibular andsublingual glands. The chorda tympani travels through themiddle ear and attaches to thelingual nerve (mandibular division of trigeminal, CN V3). After joining the lingual nerve, the preganglionic fibers synapse at the submandibular ganglion and send postganglionic fibers to the sublingual and submandibular salivary glands.
Theglossopharyngeal nerve has parasympathetic fibers that innervate theparotid salivary gland. The preganglionic fibers depart CN IX as thetympanic nerve and continue to the middle ear where they make up a tympanic plexus on the cochlear promontory of the mesotympanum. The tympanic plexus of nerves rejoin and form thelesser petrosal nerve and exit through theforamen ovale to synapse at theotic ganglion. From the otic ganglion postganglionic parasympathetic fibers travel with theauriculotemporal nerve (mandibular branch of trigeminal, CN V3) to the parotid salivary gland.
Thevagus nerve, named after the Latin wordvagus (because the nerve controls such a broad range of target tissues –vagus in Latin literally means "wandering"), contains parasympathetic fibers that originate in thedorsal nucleus of the vagus nerve and thenucleus ambiguus in the CNS. The vagus nerve can be readily identified in the neck both on ultrasound and magnetic resonance imaging. It has several branches. The largest branch is therecurrent laryngeal nerve. From the left vagus nerve the recurrent laryngeal nerve hooks around theaorta to travel back up to the larynx and proximal esophagus while, from the right vagus nerve, the recurrent laryngeal nerve hooks around the rightsubclavian artery to travel back up to the same location as its counterpart. These different paths are a direct result ofembryological development of the circulatory system. Each recurrent laryngeal nerve supplies the larynx, the heart, the trachea and the esophagus.
Another set of nerves that come off the vagus nerves approximately at the level of entering the thorax are thecardiac branches of the vagus nerve. These cardiac branches go on to form cardiac andpulmonary plexuses around the heart and lungs. As the main vagus nerves continue into the thorax they become intimately linked with the esophagus and sympathetic nerves from the sympathetic trunks to form the esophageal plexus. This is very efficient as the major function of the vagus nerve from there on will be control of the gutsmooth muscles andglands. As theesophageal plexus enter the abdomen through theesophageal hiatus anterior and posterior vagus trunks form. The vagus trunks then join with preaortic sympathetic ganglion around the aorta to disperse with the blood vessels and sympathetic nerves throughout the abdomen. The extent of the parasympathetic in the abdomen include the pancreas, kidneys, liver,gall bladder, stomach andgut tube. The vagus contribution of parasympathetic continues down the gut tube until the end of themidgut. The midgut ends two thirds of the way across the transverse colon near thesplenic flexure.[13]
Thepelvic splanchnic nerves, S2–4, work in tandem to innervate the pelvicviscera. Unlike in the cranium, where one parasympathetic is in charge of one particular tissue or region, for the most part the pelvic splanchnics each contribute fibers to pelvic viscera by traveling to one or more plexuses before being dispersed to the target tissue. These plexuses are composed of mixed autonomic nerve fibers (parasympathetic and sympathetic) and include the vesical, prostatic, rectal, uterovaginal, and inferior hypogastric plexuses. The preganglionic neurons in the pathway do not synapse in a ganglion as in the cranium but rather in the walls of the tissues or organs that they innervate. The fiber paths are variable and each individual's autonomic nervous system in the pelvis is unique. The visceral tissues in the pelvis that the parasympathetic nerve pathway controls include those of the urinary bladder, ureters, urinary sphincter, anal sphincter, uterus, prostate, glands, vagina, and penis. Unconsciously, the parasympathetic will cause peristaltic movements of the ureters and intestines, moving urine from the kidneys into the bladder and food down the intestinal tract and, upon necessity, the parasympathetic will assist in excreting urine from the bladder or defecation. Stimulation of the parasympathetic will cause the detrusor muscle (urinary bladder wall) to contract and simultaneously relax the internal sphincter muscle between the bladder and the urethra, allowing the bladder to void. Also, parasympathetic stimulation of the internal anal sphincter will relax this muscle to allow defecation. There are other skeletal muscles involved with these processes but the parasympathetic plays a huge role in continence and bowel retention.
A study published in 2016, suggests that all sacral autonomic output may be sympathetic; indicating that the rectum, bladder and reproductive organs may only be innervated by the sympathetic nervous system. This suggestion is based on detailed analysis of 15 phenotypic and ontogenetic factors differentiating sympathetic from parasympathetic neurons in the mouse. Assuming that the reported findings most likely applies to other mammals as well, this perspective suggests a simplified, bipartite architecture of the autonomic nervous system, in which the parasympathetic nervous system receives input from cranial nerves exclusively and the sympathetic nervous system from thoracic to sacral spinal nerves.[14]
Autonomic nervous system's jurisdiction to organs in thehuman bodyedit
The afferent fibers of the autonomic nervous system, which transmit sensory information from the internal organs of the body back to the central nervous system, are not divided into parasympathetic and sympathetic fibers as the efferent fibers are.[8]: 34–35 Instead, autonomic sensory information is conducted bygeneral visceral afferent fibers.
General visceral afferent sensations are mostly unconscious visceral motor reflex sensations from hollow organs and glands that are transmitted to the CNS. While the unconsciousreflex arcs normally are undetectable, in certain instances they may sendpain sensations to the CNS masked asreferred pain. If theperitoneal cavity becomes inflamed or if the bowel is suddenly distended, the body will interpret the afferent pain stimulus assomatic in origin. This pain is usually non-localized. The pain is also usually referred todermatomes that are at the same spinal nerve level as the visceral afferentsynapse.
Heart rate is largely controlled by the heart's internal pacemaker activity. Considering a healthy heart, the main pacemaker is a collection of cells on the border of the atria and vena cava called the sinoatrial node. Heart cells have the ability to generate electrical activity independent of external stimulation. As a result, the cells of the node spontaneously generate electrical activity that is subsequently conducted throughout the heart, resulting in a regular heart rate.
In absence of any external stimuli, sinoatrial pacing contributes to maintain the heart rate in the range of 60-100 beats per minute (bpm).[16] At the same time, the two branches of the autonomic nervous system act in a complementary way increasing or slowing the heart rate. In this context, the vagus nerve acts on sinoatrial node slowing its conduction thus actively modulating vagal tone accordingly. This modulation is mediated by the neurotransmitter acetylcholine and downstream changes to ionic currents and calcium of heart cells.[17]
The vagus nerve plays a crucial role in heart rate regulation by modulating the response of sinoatrial node; vagal tone can be quantified by investigating heart rate modulation induced by vagal tone changes. As a general consideration, increased vagal tone (and thus vagal action) is associated with a diminished and more variable heart rate.[18][19] The main mechanism by which the parasympathetic nervous system acts on vascular and cardiac control is the so-calledrespiratory sinus arrhythmia (RSA). RSA is described as the physiological and rhythmical fluctuation of heart rate at the respiration frequency, characterized by heart rate increase during inspiration and decrease during expiration.
Another role that the parasympathetic nervous system plays is in sexual activity. In males, thecavernous nerves from theprostatic plexus stimulate smooth muscles in the fibrous trabeculae of the coiledhelicine arteries of penis to relax and allow blood to fill the twocorpora cavernosa and thecorpus spongiosum of the penis, making it rigid to prepare for sexual activity. Upon emission of ejaculate, the sympathetics participate and causeperistalsis of theductus deferens and closure of the internalurethral sphincter to prevent semen from entering the bladder. At the same time, parasympathetics cause peristalsis of the urethral muscle, and thepudendal nerve causes contraction of the bulbospongiosus (skeletal muscle is not via PN), to forcibly emit the semen. During remission the penis becomes flaccid again. In the female, there is erectile tissue analogous to the male yet less substantial that plays a large role in sexual stimulation. The PN cause release of secretions in the female that decrease friction. Also in the female, the parasympathetics innervate thefallopian tubes, which helps peristaltic contractions and movement of theoocyte to the uterus for implantation. The secretions from the female genital tract aid in sperm migration. The PN (and SN to a lesser extent) play a significant role in reproduction.[8]
The parasympathetic nervous system uses chieflyacetylcholine (ACh) as itsneurotransmitter, althoughpeptides (such ascholecystokinin) can be used.[20][21] The ACh acts on two types of receptors, themuscarinic andnicotiniccholinergic receptors. Most transmissions occur in two stages: When stimulated, thepreganglionic neuron releases ACh at theganglion, which acts on nicotinic receptors ofpostganglionic neurons. The postganglionic neuron then releases ACh to stimulate the muscarinic receptors of the target organ. Niconitic receptors transmit outgoing signals from the presynaptic to the postsynaptic cells within the sympathetic and parasympathetic nervous system, and are the receptors used in thesomatic nervous system for signalling muscular contraction in theneuromuscular junction. The muscarinic receptors are mainly present in the parasympathetic nervous system but also appear in the sweat glands of the sympathetic nervous system.
TheM2 muscarinic receptors (CHRM2) are located in the heart, and act to bring the heart back to normal after the actions of the sympathetic nervous system: slowing down theheart rate, reducing contractile forces of theatrial cardiac muscle, and reducing conduction velocity of thesinoatrial node andatrioventricular node. They have a minimal effect on the contractile forces of the ventricular muscle due to sparse innervation of the ventricles from the parasympathetic nervous system.
TheM3 muscarinic receptors (CHRM3) are located at many places in the body, such as the endothelial cells of blood vessels, as well as the lungs causingbronchoconstriction. The net effect of innervated M3 receptors on blood vessels isvasodilation, as acetylcholine causes endothelial cells to producenitric oxide, which diffuses to smooth muscle and results in vasodilation. They are also in the smooth muscles of thegastrointestinal tract, which help in increasing intestinal motility and dilating sphincters. The M3 receptors are also located in many glands that help to stimulate secretion insalivary glands and other glands of the body. They are also located on the detrusor muscle and urothelium of the bladder, causing contraction.[22]
In vertebrates, nicotinic receptors are broadly classified into two subtypes based on their primary sites of expression: muscle-type nicotinic receptors (N1) primarily for somatic motor neurons; and neuronal-type nicotinic receptors (N2) primarily for autonomic nervous system.[23]
Sympathetic and parasympathetic divisions typically function in opposition to each other. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. A mnemonic to summarize the functions of the parasympathetic nervous system is SSLUDD (sexual arousal,salivation,lacrimation,urination,digestion anddefecation).
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The functions promoted by activity in the parasympathetic nervous system are associated with our day-to-day living. The parasympathetic nervous system promotes digestion and the synthesis ofglycogen, and allows for normal function and behavior.
Parasympathetic action helps in digestion and absorption of food by increasing the activity of the intestinal musculature, increasing gastric secretion, and relaxing the pyloric sphincter. It is called the “rest and digest” division of the ANS.[24]
The parasympathetic nervous system decreases respiration and heart rate and increases digestion. Stimulation of the parasympathetic nervous system results in:
The terminology ‘Parasympathetic nervous system’ was introduced byJohn Newport Langley in 1921. He was the first person who put forward the concept of PSNS as the second division of the autonomic nervous system.[25]
^Nunan D, Sandercock GR, Brodie DA (November 2010). "A quantitative systematic review of normal values for short-term heart rate variability in healthy adults".Pacing and Clinical Electrophysiology.33 (11):1407–17.doi:10.1111/j.1540-8159.2010.02841.x.PMID20663071.S2CID44378765.
^Diamond LM, Fagundes CP, Butterworth MR (2011). "Attachment Style, Vagal Tone, and Empathy During Mother-Adolescent Interactions".Journal of Research on Adolescence.22 (1):165–184.doi:10.1111/j.1532-7795.2011.00762.x.
^Grossman P, Wilhelm FH, Spoerle M (August 2004). "Respiratory sinus arrhythmia, cardiac vagal control, and daily activity".American Journal of Physiology. Heart and Circulatory Physiology.287 (2): H728–34.doi:10.1152/ajpheart.00825.2003.PMID14751862.S2CID5934042.
^Takai, Noriyasu; Shida, Toru; Uchihashi, Kenji; Ueda, Yutaka; Yoshida, Yo (April 1998). "Cholecystokinin as Neurotransmitter and Neuromodulator in Parasympathetic Secretion in the Rat Submandibular Gland".Annals of the New York Academy of Sciences.842 (1):199–203.Bibcode:1998NYASA.842..199T.doi:10.1111/j.1749-6632.1998.tb09649.x.PMID9599311.
^Moro, C; Uchiyama, J; Chess-Williams, R (December 2011). "Urothelial/lamina propria spontaneous activity and the role of M3 muscarinic receptors in mediating rate responses to stretch and carbachol".Urology.78 (6): 1442.e9–15.doi:10.1016/j.urology.2011.08.039.PMID22001099.
^Colquhoun, David; Ogden, David C.; Mathie, Alistair (December 1987). "Nicotinic acetylcholine receptors of nerve and muscle: Functional aspects".Trends in Pharmacological Sciences.8 (12):465–472.doi:10.1016/0165-6147(87)90040-X.
^Barrett, Kim E.; Barman, Susan M.; Yuan, Jason; Brooks, Heddwen L. (2019).Ganong's Review of Medical Physiology (26th ed.). McGraw Hill Professional.ISBN978-1-260-12241-1.OCLC1076268769.[page needed]
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