The preganglionic neurons of the parasympathetic nervous system leave the central nervous system (CNS) in the third, seventh, ninth, and tenth cranial nerves (gray matter of the brain stem (medial part of the oculomotor nucleus) , Edinger–Westphal nucleus, superior and inferior salivatory nuclei)as well as the second to fourth sacral nerves.
These are cholinergic and release acetylcholine.
Parasympathetic division of the autonomic nervous system (only left half shown).
Four cranial nerves convey preganglionic parasympathetic fibers. The oculomotor, facial, and glossopharyngeal nerves (cranial nerves III, VII, and IX) distribute parasympathetic or fibers to the head. Parasympathetic axons in these nerves synapse with postganglionic neurons in the ciliary, sphenopalatine, submaxillary, and otic ganglia, respectively.
The vagus nerve (cranial nerve X) distributes its autonomic fibers to the thoracic and abdominal viscera via the prevertebral plexuses.
The pelvic nerve (nervus erigentes) distributes parasympathetic fibers to most of the large intestine and to the pelvic viscera and genitals via the hypogastric plexus.
The majority of parasympathetic preganglionic fibers terminate on ganglion cells distributed diffusely or in networks in the walls of the innervated organs. Some preganglionic parasympathetic fibers terminate in parasympathetic ganglia located outside the organs innervated: the ciliary, pterygopalatine, submandibular, otic, and several pelvic ganglia.
Most preganglionic fibers from S2, S3, and S4 run without interruption from their central origin within the spinal cord to either the wall of the viscus they supply or the site where they synapse with terminal ganglion cells associated with the plexuses of Meissner and Auerbach in the wall of the intestinal tract (see Enteric Nervous Systemsection). Because the parasympathetic postganglionic neurons are located close to the tissues they supply, they have relatively short axons. The parasympathetic distribution is confined entirely to visceral structures.
The parasympathetic nervous system has nerves that exit from the cervicosacral spinal cord ; neurons that have parasympathetic activity are also found in some of the cranial nerves
In contrast to the sympathetic nervous system, which uses catecholamines such as norepinephrine and epinephrine, the parasympathetic nervous system uses acetylcholine to exert most of its effects on the body through muscarinic receptors.
Muscarinic receptors: odd-numbered receptors have excitatory function, whereas even-numbered receptors typically have inhibitory function.
- ❍
M 1 , M 3 , and M 5 (odd) receptors : Use the G q pathway described previously. M 3 receptors cause smooth muscle contractions at smooth muscles that aren’t sphincters (because if your sphincters were tight, it would make urinating and defecating difficult!)—an example of this contraction is the detrusor muscle, the smooth muscle of the urinary bladder, promoting urination. The M 3 receptor also increases glandular secretions, important in the parasympathetic-mediated digestion response as well as in bronchial secretions, in which blocking this receptor is helpful to patients with asthma or chronic obstructive pulmonary disease (COPD). Again, the G q pathway causes a calcium surge , leading to smooth muscle contraction through calcium release from the sarcoplasmic reticulum, but the receptors are in locations different from the sympathetic α 1 receptors that also mediate smooth muscle contraction. In general, the odd-numbered muscarinic receptors are excitatory (whereas the even-numbered muscarinic receptors are inhibitory). A way to remember this is it’s odd to be excited about muscarinic receptors. (The M 1 and M 5 receptors are much less clinically important.)
- ❍
M 2 and M 4 (even) receptors: As you may guess from above, M 2 and M 4 receptors are inhibitory and therefore act through the G i pathway. The most important inhibitory muscarinic receptor is the M 2 receptor found on the atria of the heart —the inhibitory actions on the sinus node (the pacemaker of the heart) cause a decreased heart rate as well as decreased contractility of the atria only. The ventricles do not have a high density of these receptors, and ventricular contractility is unaffected.
- ++++++++++++++++
The parasympathetic nervous system mediates the “rest-and-digest” response and promotes gastrointestinal (GI) motility, defecation, and urination
It slows the heart rate, increases intestinal and gland activity (increases blood flow to the intestines, and promotes gland activity, motility(?), and relaxes sphincter muscles promoting urination and defecation.
Evidence is accumulating that the ANS, especially the vagus nerve, also influences immune function and some CNS functions such as seizure discharge.
Short postgangiolic
Anabolic
Craniosacral
https://en.wikipedia.org/wiki/Parasympathetic_nervous_system
