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The Parasympathetic System . Function. The parasympathetic nervous system (PSNS) is one of three main divisions of the autonomic nervous system (ANS), the other two being the sympathetic and enteric systems .
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Function The parasympatheticnervoussystem(PSNS) isone of threemaindivisions of the autonomicnervoussystem (ANS), the othertwobeing the sympathetic and entericsystems. The ANS isresponsible for regulation of internalorgans and glands, whichoccursunconsciously. To be specific, the parasympatheticsystemisresponsible for stimulation of "rest-and-digest" or "feed and breed” activitiesthatoccurwhen the body isatrest, especiallyaftereating, including sexualarousal, salivation, lacrimation (tears), urination, digestion and defecation. Itsactionisdescribedasbeingcomplementary to that of one of the othermainbranches of the ANS, the sympatheticnervoussystem, whichisresponsible for stimulatingactivitiesassociated with the fight-or-flightresponse.
Relation to sympatheticnervoussystem Sympathetic and parasympatheticdivisionstypicallyfunction in opposition to eachother. Thisnaturaloppositionisbetterunderstoodascomplementary in nature ratherthanantagonistic. The sympatheticdivisiontypicallyfunctions in actionsrequiringquickresponses. The parasympatheticdivisionfunctions with actionsthat do notrequire immediate reaction. A usefulmnemonic to summarize the functions of the parasympatheticnervoussystemis SLUDD (salivation, lacrimation, urination, digestion and defecation). The parasympatheticnervoussystemmayalso be knownasParasympatheticdivision.
Pathways Asistrue in the sympatheticnervoussystem, efferentparasympatheticnervesignals are carried from the centralnervoussystem to their targets by a system of twoneurons. The first neuronin thispathwayisreferred to as the preganglionic or presynapticneuron. Itscell body sits in the centralnervoussystem and itsaxonusuallyextends to a ganglionsomewhere else in the body whereitsynapses with the dendrites of the secondneuron in the chain. Thissecondneuronisreferred to as the postganglionic or postsynapticneuron. The axons of presynapticparasympatheticneurons are usually long: Theyextend from the CNS into a ganglionthatiseitherveryclose to or embedded in their target organ. As a result, the postsynapticparasympatheticnervefibers are very short.
Cranialnerves CN 0 – Terminal CN I – Olfactory CN II – Optic CN III – Oculomotor CN IV – Trochlear CN V – Trigeminal CN VI – Abducens CN VII – Facial CN VIII – Vestibulocochlear CN IX – Glossopharyngeal CN X – Vagus CN XI – Accessory CN XII – Hypoglossal
Pathways In the cranium, preganglionic PSN (CN III, CN VII, and CN IX) usuallyarise from specific nuclei in the Central Nervous System (CNS) and synapseatone of fourparasympatheticganglia: ciliary, pterygopalatine, otic, or submandibular. From thesefourganglia the PSN complete theirjourney to target tissues via CN V (trigeminal) branches (ophthalmicnerve CN V1, maxillarynerve CN V2, mandibularnerve CN V3).
Pathways The vagusnerve (CN X) doesnotparticipate in thesecranialgangliaasmost of its PSN fibers are destined for a broad array of ganglia on or nearthoracic viscera (esophagus, trachea, heart, lungs) and abdominal viscera (stomach, pancreas, liver, kidneys). Ittravelsall the way down to the midgut/hindgutjunction, whichoccurs just before the splenicflexure of the transverse colon.
Pathways The pelvicsplanchnicefferentpreganglionicnervecellbodiesreside in the lateralgrayhorn of the spinalcordat the T12-L1 vertebrallevels (the spinalcordterminatesat the L1-L2 vertebrae with the conusmedullaris), and theiraxons exit the vertebralcolumnas S2-S4 spinalnervesthrough the sacralforamina. Theiraxons continue away from the CNS to synapseat an autonomicganglion. The PSN ganglionwherethesepreganglionicneuronssynapsewill be close to the organ of innervation. Thisdiffers from the sympatheticnervoussystem, wheresynapsesbetweenpre- and post-ganglionicefferentnerves in general occuratgangliathat are fartheraway from the target organ.
Types of muscarinicreceptor The fivemaintypes of muscarinicreceptors: The M1 muscarinicreceptors(CHRM1) are located in the neuralsystem. The M2 muscarinicreceptors(CHRM2) are located in the heart, and act to bring the heart back to normalafter the actions of the sympatheticnervoussystem: slowing down the heart rate, reducingcontractileforces of the atrialcardiacmuscle, and reducingconductionvelocity of the sinoatrialnode (SA node) and atrioventricularnode (AV node). Note, theyhave a minimaleffect on the contractileforces of the ventricularmuscle due to sparse innervation of the ventricles from the parasympatheticnervoussystem.
Types of muscarinicreceptor The M3 muscarinicreceptors(CHRM3) are located endothelialcells of bloodvessels, aswellas the lungscausingbronchoconstriction. bloodvesselsisvasodilation, asacetylcholinecausesendothelialcells to produce nitricoxide, whichdiffuses to smoothmuscle and results in vasodilation. smoothmuscles of the gastrointestinaltract (GIT), which help in increasingintestinalmotility and dilatingsphincters. in manyglandsthat help to stimulatesecretion in salivaryglands and otherglands of the body on the detrusormuscle and urothelium of the bladder, causingcontraction. The M4 muscarinicreceptors: Postganglioniccholinergicnerves, possible CNS effects The M5 muscarinicreceptors: Possibleeffects on the CNS
