Parasympathetic nervous system

Source: Wikipedia, the free encyclopedia.
"PSNS" redirects here. For the naval shipyard in Washington, see Puget Sound Naval Shipyard.
Parasympathetic nervous system
Autonomic nervous system innervation, showing the parasympathetic (craniosacral) systems in blue.
Details
Identifiers
Latinpars parasympathica divisionis autonomici systematis
Acronym(s)PSNS
MeSHD010275
TA98A14.3.02.001
TA26661
FMA9907
Anatomical terminology

The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system.[1][2] The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.[3]

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"

salivation, lacrimation (tears), urination, digestion, and defecation. 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
.

Nerve fibres of the parasympathetic nervous system arise from the central nervous system. Specific nerves include several cranial nerves, specifically the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve. Three spinal nerves in the sacrum (S2–4), commonly referred to as the pelvic splanchnic nerves, 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".[5]

Structure

The parasympathetic nerves are autonomic or visceral [6][7] branches of the peripheral nervous system (PNS). Parasympathetic nerve supply arises through three primary areas:

  1. Certain
    trigeminal branches (ophthalmic nerve, maxillary nerve, mandibular nerve
    ).
  2. The
    splenic flexure of the transverse colon
    .
  3. The
    sacral foramina.[8] Their axons continue away from the CNS to synapse at an autonomic ganglion. The parasympathetic ganglion
    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,

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.[9]
: 42 

Cranial nerves

The

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.[12]

The parasympathetic aspect of the

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 (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 (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, 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 the zygomatic 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.[13]

A separate group of parasympathetic leaving from the pterygopalatine ganglion are the descending

mucosa. The second parasympathetic branch that leaves the facial nerve is the chorda tympani. This nerve carries secretomotor fibers to the submandibular and sublingual glands. The chorda tympani travels through the middle ear and attaches to the lingual 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.

The

auriculotemporal
nerve (mandibular branch of trigeminal, CN V3) to the parotid salivary gland.

Vagus nerve

The

embryological development
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

splenic flexure.[14]

Pelvic splanchnic nerves

The

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.[15]

Autonomic nervous system's jurisdiction to organs in the human body edit
Organ Nerves[16]
Spinal column origin[16]
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
  • proximal colon
    )
  • distal colon
    )
pancreatic head
 T8, T9
appendix T10
bladder S2-S4
kidneys and ureters T11, T12

Function

Further information: Parasympathetic rebound

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.

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 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 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).[17] 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.[18]

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.[19][20] 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

fallopian tubes, which helps peristaltic contractions and movement of the oocyte 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.[9]

Receptors

The parasympathetic nervous system uses chiefly

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:

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.[24]

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, salivation, lacrimation, urination, digestion and defecation).

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.[25]

The parasympathetic nervous system decreases respiration and heart rate and increases digestion. Stimulation of the parasympathetic nervous system results in:

  • Constriction of pupils
  • Decreased heart rate and blood pressure
  • Constriction of bronchial muscles
  • Increase in digestion
  • Increased production of saliva and mucus
  • Increase in urine secretion[26]

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.[27]

References

  1. ISBN 9781152710191
    .
  2. ISBN 978-0-19-856878-0
    .
  3. ^ "14.1B: Divisions of the Autonomic Nervous System". Medicine LibreTexts. 2018-07-21. Retrieved 2021-11-14.
  4. PMID 17786266
    .
  5. ^ "- YouTube". www.youtube.com. Archived from the original on 2021-11-17. Retrieved 2021-05-17.
  6. ^ "visceral nerve fibers - definition of visceral nerve fibers in the Medical dictionary – by the Free Online Medical Dictionary, Thesaurus and Encyclopedia". Medical-dictionary.thefreedictionary.com. Retrieved 2012-07-06.
  7. ^ "Visceral nerve – RightDiagnosis.com". Wrongdiagnosis.com. 2012-02-01. Retrieved 2012-07-06.
  8. ^ "The Vertebral Column and Spinal Cord". www.emery.edu. 1997-08-21. Retrieved 2013-03-21.
  9. ^
    ISBN 978-0-7817-6274-8
    .
  10. ^ Rico Garófalo, Xavier (21 June 2019). "Sistema nervioso simpático y parasimpático: qué son, diferencias y funciones". MedSalud. Retrieved 14 September 2019.
  11. ^ Castillero Mimenza, Oscar (2016). "Sistema nervioso parasimpático: funciones y recorrido". Psicología y Mente.
  12. ^ Joyce, Christopher; Le, Patrick H.; Peterson, Diana C. (2023). "Neuroanatomy, Cranial Nerve 3 (Oculomotor)". StatPearls. StatPearls Publishing. Retrieved 28 December 2023.
  13. PMC 3652637
    .
  14. ^ Netter. Atlas of Human Anatomy, Fourth Ed. Saunders Elsevier. 2003.
  15. PMID 27856909
    .
  16. ^
    ISBN 978-0-7817-5940-3
    .
  17. S2CID 44378765
    .
  18. PMID 24834378
    .
  19. doi:10.1111/j.1532-7795.2011.00762.x
    .
  20. S2CID 5934042
    .
  21. PMID 7491953
    .
  22. S2CID 28329900
    .
  23. PMID 22001099
    .
  24. ^ Colquhoun, David. "Nicotinic acetylcholine receptors" (PDF). www.ucl.ac.uk/. University College London. Retrieved 4 March 2015.
  25. OCLC 1076268769.{{cite book}}: CS1 maint: location missing publisher (link
    )
  26. ^ "Parasympathetic Nervous System - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2023-03-07.
  27. ISBN 978-1-4377-1679-5
Retrieved from "https://en.wikipedia.org/w/index.php?title=Parasympathetic_nervous_system&oldid=1211249433"