The parasympathetic nervous system (PSNS) is one of the three divisions of the
autonomic nervous system
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies internal organs, smooth muscle and glands. The autonomic nervous system is a control system t ...
, the others being the
sympathetic nervous system and the
enteric nervous system. The
enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.
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" activities that occur when the body is at rest, especially after eating, including
sexual arousal,
salivation,
lacrimation (tears),
urination,
digestion, and
defecation
Defecation (or defaecation) follows digestion, and is a necessary process by which organisms eliminate a solid, semisolid, or liquid waste material known as feces from the digestive tract via the anus. The act has a variety of names ranging f ...
. Its action is described as being complementary to that of the
sympathetic nervous system, which is responsible for stimulating activities associated with the
fight-or-flight response
The fight-or-flight or the fight-flight-or-freeze response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It was first des ...
.
Nerve fibres of the parasympathetic nervous system arise from the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
. Specific nerves include several
cranial nerves, specifically the
oculomotor nerve,
facial nerve,
glossopharyngeal nerve
The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior (closer to the nose) to the vagus nerve. ...
, and
vagus nerve
The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...
. Three
spinal nerves in the
sacrum (S2–4), commonly referred to as the
pelvic splanchnic nerves
Pelvic splanchnic nerves or nervi erigentes are splanchnic nerves that arise from sacral spinal nerves S2, S3, S4 to provide parasympathetic innervation to the organs of the pelvic cavity.
Structure
The pelvic splanchnic nerves arise from th ...
, also act as parasympathetic nerves.
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".
Structure
The parasympathetic nerves are
autonomic or ''visceral'' branches of the
peripheral nervous system
The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brai ...
(PNS). Parasympathetic nerve supply arises through three primary areas:
# Certain
cranial nerve
Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...
s in the cranium, namely the preganglionic parasympathetic nerves (CN III, CN VII, CN IX and CN X) usually arise from specific nuclei in the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
(CNS) and synapse at one of four parasympathetic
ganglia:
ciliary,
pterygopalatine,
otic, or
submandibular. From these four ganglia the parasympathetic nerves complete their journey to target tissues via
trigeminal branches (
ophthalmic nerve,
maxillary nerve
In neuroanatomy, the maxillary nerve (V) is one of the three branches or divisions of the trigeminal nerve, the fifth (CN V) cranial nerve. It comprises the principal functions of sensation from the maxilla, nasal cavity, sinuses, the pala ...
,
mandibular nerve
In neuroanatomy, the mandibular nerve (V) is the largest of the three divisions of the trigeminal nerve, the fifth cranial nerve (CN V). Unlike the other divisions of the trigeminal nerve ( ophthalmic nerve, maxillary nerve) which contain only ...
).
# The
vagus nerve
The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...
does not participate in these cranial
ganglia as most of its parasympathetic fibers are destined for a broad array of ganglia on or near thoracic viscera (
esophagus
The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
,
trachea
The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from t ...
,
heart
The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
,
lungs
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either si ...
) and abdominal viscera (
stomach,
pancreas
The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an ...
,
liver
The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
,
kidneys,
small intestine, and about half of the
large intestine). The vagus innervation ends at the junction between the midgut and hindgut, just before the
splenic flexure of the
transverse colon.
# The
pelvic splanchnic efferent preganglionic nerve cell bodies reside in the
lateral gray horn of the
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
at the T12–L1 vertebral levels (the spinal cord terminates at the L1–L2 vertebrae with the
conus medullaris
''Conus'' is a genus of predatory sea snails, or cone snails, marine gastropod mollusks in the family Conidae.Bouchet, P.; Gofas, S. (2015). Conus Linnaeus, 1758. In: MolluscaBase (2015). Accessed through: World Register of Marine Species at ...
), and their axons exit the vertebral column as S2–S4 spinal nerves through the
sacral foramina. Their axons continue away from the CNS to synapse at an autonomic ganglion. The parasympathetic
ganglion
A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympathe ...
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 two
neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...
s. The first neuron in this pathway is referred to as the
preganglionic or
presynaptic neuron. Its cell body sits in the central nervous system and its axon usually extends to synapse with the dendrites of a
postganglionic 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.
Cranial nerves
The
oculomotor nerve is responsible for a number of parasympathetic functions related to the eye. The oculomotor PNS fibers originate in the
Edinger-Westphal nucleus in the central nervous system and travel through the
superior orbital fissure
The superior orbital fissure is a foramen or cleft of the skull between the lesser and greater wings of the sphenoid bone. It gives passage to multiple structures, including the oculomotor nerve, trochlear nerve, ophthalmic nerve, abducens ne ...
to synapse in the
ciliary ganglion located just behind the orbit (eye). From the ciliary ganglion the postganglionic parasympathetic fibers leave via short ciliary nerve fibers, a continuation of the
nasociliary nerve (a branch of ophthalmic division of the
trigeminal nerve
In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and che ...
(CN V
1)). The short ciliary nerves innervate the orbit to control the
ciliary muscle (responsible for
accommodation) and the
iris sphincter muscle, which is responsible for
miosis 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.
The parasympathetic aspect of the
facial nerve controls secretion of the sublingual and submandibular
salivary glands, the
lacrimal gland, and the glands associated with the nasal cavity. The preganglionic fibers originate within the CNS in the superior salivatory nucleus and leave as the
intermediate nerve
The intermediate nerve, nervus intermedius, nerve of Wrisberg or Glossopalatine nerve, is the part of the facial nerve (cranial nerve VII) located between the motor component of the facial nerve and the vestibulocochlear nerve (cranial nerve VIII ...
(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 nerve
geniculate ganglion
The geniculate ganglion (from Latin ''genu'', for "knee") is a collection of pseudounipolar sensory neurons of the facial nerve located in the facial canal of the head. It receives fibers from the facial nerve. It sends fibers that supply the ...
(general sensory ganglion) in the
temporal bone, the facial nerve gives off two separate parasympathetic nerves. The first is the
greater petrosal nerve and the second is the
chorda 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 the
pterygoid canal. The parasympathetic fibers of the nerve of the pterygoid canal synapse at the
pterygopalatine ganglion
The pterygopalatine ganglion (aka Meckel's ganglion, nasal ganglion, or sphenopalatine ganglion) is a parasympathetic ganglion found in the pterygopalatine fossa. It is largely innervated by the greater petrosal nerve (a branch of the facial ner ...
, which is closely associated with the maxillary division of the trigeminal nerve (CN V
2). The postganglionic parasympathetic fibers leave the pterygopalatine ganglion in several directions. One division leaves on the
zygomatic division of CN V
2 and travels on a communicating branch to unite with the lacrimal nerve (branch of the ophthalmic nerve of CN V
1) before synapsing at the lacrimal gland. These parasympathetic to the lacrimal gland control tear production.
A separate group of parasympathetic leaving from the pterygopalatine ganglion are the descending
palatine nerves (CN V
2 branch), which include the greater and lesser palatine nerves. The greater palatine parasympathetic synapse on the hard palate and regulate mucus glands located there. The lesser palatine nerve synapses at the soft palate and controls sparse taste receptors and mucus glands. Yet another set of divisions from the pterygopalatine ganglion are the posterior, superior, and inferior lateral nasal nerves; and the
nasopalatine nerves (all branches of CN V
2, maxillary division of the trigeminal nerve) that bring parasympathetic innervation to glands of the nasal
mucosa. The second parasympathetic branch that leaves the facial nerve is the chorda tympani. This nerve carries
secretomotor fibers to the
submandibular and
sublingual
Sublingual (abbreviated SL), from the Latin for "under the tongue", refers to the pharmacological route of administration by which substances diffuse into the blood through tissues under the tongue.
The sublingual glands receive their primary ...
glands. The chorda tympani travels through the
middle ear and attaches to the
lingual nerve (mandibular division of trigeminal, CN V
3). After joining the lingual nerve, the preganglionic fibers synapse at the submandibular ganglion and send postganglionic fibers to the sublingual and submandibular salivary glands.
The
glossopharyngeal
The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior (closer to the nose) to the vagus nerve. B ...
nerve has parasympathetic fibers that innervate the
parotid salivary gland. The preganglionic fibers depart CN IX as the
tympanic 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 the
lesser petrosal nerve and exit through the
foramen ovale to synapse at the
otic ganglion. From the otic ganglion postganglionic parasympathetic fibers travel with the
auriculotemporal nerve (mandibular branch of trigeminal, CN V
3) to the parotid salivary gland.
Vagus nerve
The
vagus nerve, named after the Latin word ''vagus'' (because the nerve controls such a broad range of target tissues – ''vagus'' in Latin literally means "wandering"), has parasympathetic functions that originate in the
dorsal nucleus of the vagus nerve and the
nucleus ambiguus
The nucleus ambiguus ("ambiguous nucleus" in English) is a group of large motor neurons, situated deep in the medullary reticular formation named by Jacob Clarke. The nucleus ambiguus contains the cell bodies of neurons that innervate the muscle ...
in the CNS. The vagus nerve is an unusual cranial parasympathetic in that it doesn't join the trigeminal nerve in order to get to its target tissues. Another peculiarity is that the vagus has an
autonomic ganglion
An autonomic ganglion is a cluster of nerve cell bodies (a ganglion) in the autonomic nervous system. The two types are the sympathetic ganglion and the parasympathetic ganglion
Parasympathetic ganglia are the autonomic ganglia of the parasympa ...
associated with it at approximately the level of C1 vertebra. The vagus gives no parasympathetic to the cranium. The vagus nerve is hard to track definitively due to its ubiquitous nature in the
thorax
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the c ...
and
abdomen
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...
so the major contributions will be discussed. Several parasympathetic nerves come off the vagus nerve as it enters the thorax. One nerve is the
recurrent laryngeal nerve
The recurrent laryngeal nerve (RLN) is a branch of the vagus nerve ( cranial nerve X) that supplies all the intrinsic muscles of the larynx, with the exception of the cricothyroid muscles. There are two recurrent laryngeal nerves, right and ...
, which becomes the inferior laryngeal nerve. From the left vagus nerve the recurrent laryngeal nerve hooks around the
aorta to travel back up to the larynx and proximal esophagus while, from the right vagus nerve, the recurrent laryngeal nerve hooks around the right
subclavian artery
In human anatomy, the subclavian arteries are paired major arteries of the upper thorax, below the clavicle. They receive blood from the aortic arch. The left subclavian artery supplies blood to the left arm and the right subclavian artery suppl ...
to travel back up to the same location as its counterpart. These different paths are a direct result of
embryological development
Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...
of the circulatory system. Each recurrent laryngeal nerve supplies the trachea and the esophagus with parasympathetic secretomotor innervation for glands associated with them (and other fibers that are not PN).
Another nerve that comes off the vagus nerves approximately at the level of entering the thorax are the
cardiac nerves. These cardiac nerves go on to form cardiac and
pulmonary 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 gut
smooth muscles and
glands
In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream ( endocrine gland) or into cavities inside the body or its outer surface ( exocrine gland).
Structure
...
. As the
esophageal plexus
The esophageal plexus (oesophageal plexus in British-English) is formed by nerve fibers from two sources, branches of the vagus nerve, and visceral branches of the sympathetic trunk. The esophageal plexus and the cardiac plexus contain the same ty ...
enter the abdomen through the
esophageal 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 and
gut tube. The vagus contribution of parasympathetic continues down the gut tube until the end of the
midgut. The midgut ends two thirds of the way across the transverse colon near the
splenic flexure.
Pelvic splanchnic nerves
The
pelvic splanchnic nerves, S2–4, work in tandem to innervate the pelvic viscera. 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.
Function
Sensation
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.
Instead, autonomic sensory information is conducted by
general 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 unconscious
reflex arcs normally are undetectable, in certain instances they may send
pain
Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...
sensations to the CNS masked as
referred pain. If the
peritoneal cavity becomes inflamed or if the bowel is suddenly distended, the body will interpret the afferent pain stimulus as
somatic in origin. This pain is usually non-localized. The pain is also usually referred to
dermatomes that are at the same spinal nerve level as the visceral afferent
synapse.
Vascular effects
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 exhibit automaticity which is 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). 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.
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. The main mechanism by which the parasympathetic nervous system acts on vascular and cardiac control is the so-called
respiratory 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.
Sexual activity
Another role that the parasympathetic nervous system plays is in sexual activity. In males, the
cavernous nerves from the
prostatic plexus stimulate smooth muscles in the fibrous trabeculae of the coiled
helicine arteries of penis to relax and allow blood to fill the two
corpora cavernosa and the
corpus spongiosum of the penis, making it rigid to prepare for sexual activity. Upon emission of ejaculate, the sympathetics participate and cause
peristalsis
Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, whi ...
of the
ductus deferens
The vas deferens or ductus deferens is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled tube w ...
and closure of the internal
urethral sphincter to prevent semen from entering the bladder. At the same time, parasympathetics cause peristalsis of the urethral muscle, and the
pudendal 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 the
fallopian tubes
The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the ovaries. The fallopian tubes are part of the female reproductive system. In ot ...
, which helps peristaltic contractions and movement of the
oocyte
An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...
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.
Receptors
The parasympathetic nervous system uses chiefly
acetylcholine
Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...
(ACh) as its
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell.
Neu ...
, although
peptides (such as
cholecystokinin) can be used.
The ACh acts on two types of receptors, the
muscarinic and
nicotinic cholinergic receptors. Most transmissions occur in two stages: When stimulated, the
preganglionic neuron releases ACh at the ganglion, which acts on nicotinic receptors of
postganglionic neurons. The postganglionic neuron then releases ACh to stimulate the muscarinic receptors of the target organ.
Types of muscarinic receptors
The five main types of muscarinic receptors:
* The
M1 muscarinic receptors () are located in the neural system.
* The
M2 muscarinic receptors () are located in the heart, and act to bring the heart back to normal after the actions of the sympathetic nervous system: slowing down the
heart rate, reducing contractile forces of the
atrial cardiac muscle, and reducing conduction velocity of the
sinoatrial node and
atrioventricular 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.
* The
M3 muscarinic receptors
The muscarinic acetylcholine receptor, also known as cholinergic/acetylcholine receptor M3, or the muscarinic 3, is a muscarinic acetylcholine receptor encoded by the human gene CHRM3.
The M3 muscarinic receptors are located at many places in t ...
() are located at many places in the body, such as the endothelial cells of blood vessels, as well as the lungs causing
bronchoconstriction. The net effect of innervated M3 receptors on blood vessels is
vasodilation, as acetylcholine causes endothelial cells to produce
nitric oxide, which diffuses to smooth muscle and results in vasodilation. They are also in the smooth muscles of the
gastrointestinal tract, which help in increasing intestinal motility and dilating sphincters. The M3 receptors are also located in many glands that help to stimulate secretion in
salivary glands and other glands of the body. They are also located on the detrusor muscle and urothelium of the bladder, causing contraction.
* The
M4 muscarinic receptors: Postganglionic cholinergic nerves, possible CNS effects
* The
M5 muscarinic receptors: Possible effects on the CNS
Types of nicotinic receptors
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.
Relationship to sympathetic nervous system
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,
saliva
Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can ...
tion,
lacrimation,
urination,
digestion and
defecation
Defecation (or defaecation) follows digestion, and is a necessary process by which organisms eliminate a solid, semisolid, or liquid waste material known as feces from the digestive tract via the anus. The act has a variety of names ranging f ...
).
Clinical significance
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 of
glycogen, 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.
History
The terminology ‘Parasympathetic nervous system’ was introduced by
John 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.
References
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