Parasympathetic nervous 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. 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. In contrast to the sympathetic division, which predominates during acute stress responses, parasympathetic activity promotes energy conservation, metabolic storage (for example by enhancing glycogen synthesis), and the maintenance of internal homeostasis over longer time scales.

Beyond its role in visceral motor control, parasympathetic activity—particularly via the vagus nerve—forms part of an "inflammatory reflex" in which efferent vagal fibers, acting through cholinergic signaling, dampen the release of pro-inflammatory cytokines and thereby modulate innate immune responses. Non-invasive indices such as heart rate variability (HRV), especially measures that reflect vagal modulation of the sinoatrial node, are widely used as surrogate markers of cardiac parasympathetic tone in research and clinical settings.

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.

Because of 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 (PNS). Parasympathetic nerve supply arises through three primary areas:

1. Certain cranial nerves 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 (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, mandibular nerve).
2. The vagus nerve (CN X) 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, trachea, heart, lungs) and abdominal viscera (stomach, pancreas, liver, 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.
3. The pelvic splanchnic efferent preganglionic nerve cell bodies reside in the intermediolateral cell column (also termed the sacral parasympathetic nucleus) of the sacral spinal cord segments S2–S4, and their axons exit the vertebral column in the anterior rami of the corresponding spinal nerves before forming the pelvic splanchnic nerves. Their axons continue away from the CNS to synapse at autonomic ganglia that are usually in, or very close to, the wall of 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 neurons. 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. In contrast to the relatively discrete end-organs of the somatic nervous system, many parasympathetic postganglionic fibers form varicosities along their length and release neurotransmitter diffusely, influencing a wider field of smooth muscle and glandular tissue.

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 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 (CN V₁)). 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 (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.

A separate group of parasympathetic leaving from the pterygopalatine ganglion are the descending palatine 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 the nasopalatine nerves (all branches of CN V₂, 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 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 glossopharyngeal 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 V₃) 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"), contains parasympathetic fibers that originate in the dorsal nucleus of the vagus nerve and the nucleus 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 the recurrent 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 to travel back up to the same location as its counterpart. These different paths are a direct result of embryological 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 the cardiac branches of the vagus nerve. These cardiac branches 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. As the esophageal plexus 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. Centrally, vagal preganglionic neurons receive convergent input from the nucleus tractus solitarii, hypothalamus, and limbic structures such as the amygdala, integrating visceral sensory information with emotional and cognitive influences on autonomic output.

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. This reinterpretation has been controversial, and subsequent anatomical and physiological analyses have argued that the sacral autonomic outflow fulfills classical criteria for a parasympathetic division and should continue to be classified as such, in line with Langley's original tripartite scheme.