Parasympathetic Nervous System

Overview

The Parasympathetic Nervous System (PNS) is a division of the Autonomic Nervous System]] (ANS) that exerts subconscious control on a variety of visceral functions. Here we discuss specific features of the PNS; however, general themes are discussed on the autonomic nervous system page.

Physiological Anatomy

Like all autonomic pathways those of the PNS are composed of two-neuron systems composed of Pre- and Post-ganglionic Neurons. The cell bodies of preganglionic neurons reside within the CNS and their axons extend to specialized parasympathetic ganglia outside of the CNS where they synapse on postganglionic neuron cell bodies. The axons of these postganglionic neurons then extend to the target organ.

The cell bodies of parasympathetic preganglionic neurons reside either in the brain stem or in the sacral spinal cord. The axons of these neurons project from these locations along peripheral nerves toward their target organs. The preganglionic fibers synapse on parasympathetic ganglia that are either directly adjacent to or located within the body of the target organs. Thus, parasympathetic preganglionic nerves are quite long, standing in contrast to the short preganglionic fibers of the sympathetic nervous system.

As mentioned above, parasympathetic ganglia lie directly adjacent to or within the walls of target organs. Consequently, postganglionic parasympathetic neurons only travel a short distance between the ganglia and target cells.

The parasympathetic preganglionic neurons that project form the brain stem do so through cranial nerves, specifically CN III, VI, IX, X. Cranial Nerve X, the vagus nerve, is perhaps the most dominant nerve carrying parasympathetic output and accounts for all of the thoracic and abdominal parasympathetic innervation. In fact, nearly 75% of all PNS fibers course through the vagus.
The parasympathetic fibers originating from the sacral levels innervate the descending colon and genitourinary system.

Neurotransmitters and Receptors

The neurotransmitter-receptor combination at all autonomic ganglia is similar and is identical to that of the sympathetic nervous system. Preganglionic neurons secrete acetylcholine which binds the nicotinic acetylcholine receptor on the postganglionic cell body. This interaction is excitatory and thus acts to stimulate the postganglionic cell to fire, transmitting the stimulus through the ganglion.

Nearly all parasympathetic postganglionic neurons secrete acetylcholine. Conveniently, all parasympathetic target organs express the muscarinic receptor. Therefore, the neurotransmitter-receptor combination of the parasympathetic system is much more simple than that of the sympathetic nervous system.

Function

Below we detail the diverse effects that stimulation of parasympathetic fibers exert on target organs. As mentioned, these effects are mediated by muscarinic receptor on target organs. An understanding the distribution of these muscarinic receptors and their physiological effects will allow for an easier appreciation of effects of cholinergic drugs on the body.

Promotes pupillary constriction (i.e. miosis) by stimulating contraction of the circular muscle of the iris. Parasympathetic fibers also stimulate contraction of the ciliary muscle which leads to relaxation of the lens, thus allowing for focusing on near objects.

Lacrimal, salivary, and diverse GI glands are all stimulated by parasympathetic efferents that promote a watery secretion. Thus, the parasympathetic nervous system is responsible for regulating salivary secretion, stomach acid secretion, as well as promoting small intestine secretion, and large intestine secretion. Notably, sweat glands do not have parasympathetic innervation and sweating is completely controlled by the sympathetic system.

Although GI motility is directly controlled by the enteric nervous system, parasympathetic fibers can modulate its activity. In general, parasympathetic outflow promotes peristalisis and relaxation of GI sphincters. These effects are discussed in more detail under Autonomic GI Neural Control.

Parasympathetic innervation of the heart generally leads to reduced contractility, sinus rate, and AV nodal conduction. Together these negative inotropic, chronotropic, and dromotropic effects reduce the heart rate and cardiac output as discussed in detail under autonomic cardiac regulation.

The parasympathetic nervous system has almost no affect on the vasculature. Consequently, the profound effect of the autonomic nervous system on the peripheral vascular resistance is almost completely controlled by modulation of sympathetic fibers.

Stimulation of parasympathetic fibers in bronchioles result in bronchoconstriction and thus increased airflow resistance.

Bladder: Parasympathetic output results in contraction of the detrusor muscle and relaxation of the internal urethral sphincter, thus promoting micturition
Penis: The parasympathetic nervous system is critical for triggering and maintaining an eerection.

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