The parasympathetic nervous system (PNS) is a chief subdivision of the autonomic nervous system, which controls the function of body organs, blood vessels, and smooth muscles. Whereas most of the actions of the parasympathetic nervous system are automatic and involuntary, some, such as breathing, work in concert with the conscious mind. Largely considered to be the controlling system when external conditions are calm and normal, the PNS promotes a slower heart beat, a slower respiratory rate, increased perspiration and salivation, smaller pupils, enhanced waste disposal, and sexual arousal. Unlike the other subdivision of the autonomic nervous system, the sympathetic nervous system, which mediates the "fight or flight" response, the PNS functions when conditions do not require immediate action in a "digest and rest" response. In a complex homeostatic process, the sympathetic and parasympathetic systems act in opposing but concerted ways, much like the accelerator and brakes of an automobile, to keep vital body functions balanced.
All parasympathetic nervous systems consist of spinal and cranial segments. Near the tailbone or sacrum, the PNS originates from the second, third, and fourth sacral nerves, which innervate the pelvic organs. In the brain, the parasympathetic system arises from four of the cranial nerves: the oculomotor nerve, the facial nerve, the glossopharyngeal nerve, and the vagus nerve. All PNS segments consist of sensory components, which carry information to the brain, and motor components, which deliver appropriate feedback to the end organs. Sensory cells monitor blood pressure, oxygen and carbon dioxide levels, blood sugar concentrations, and stomach and bowel contents, while motor neurons, grouped into small ganglia close to the target organs, modulate the body's responses to the information collected by the sensory cells.
Acetylcholine is the principal chemical messenger released in the neuronal junctions of the parasympathetic nervous system. Muscarinic receptors, so named because of their sensitivity to muscarine derived from Amanita muscaria mushrooms, are the chief end-receptors of the PNS. The acetylcholine molecules activate the muscarinic receptors in the plasma membranes of nerve cells by connecting to intracellular proteins. Once acetylcholine binds to the proteins, a cascade of events leads to the end-organ response. Scientists have discovered five subtypes of muscarinic receptors, each having a distinct gene.
Dysautonomia refers to dysfunction of the autonomic nervous system where either the sympathetic or the parasympathetic nervous system exerts a disproportionate amount of influence on the body. Viral infections, toxic exposures, trauma, and heredity have all been implicated as causative factors for the condition. Symptoms include aches and pains, fainting spells, fatigue, anxiety attacks, rapid heart rate, and low blood pressure. Examination of dysautonomia patients by physicians typically yields few, if any, objective physical or laboratory findings. There is no widely accepted treatment approach for dysautonomia, and therapeutic attempts are largely directed at mitigating the symptoms, not curing the dysfunction.