What is the parasympathetic nervous system?

The parasympathetic nervous system (PNS) is a major subdivision of the autonomic nervous system, which controls the function of the body's organs, blood vessels, and smooth muscles. While most actions of the parasympathetic nervous system are automatic and involuntary, some, like breathing, work in concert with the conscious mind. Largely considered the control system when external conditions are calm and normal, the PNS promotes a slower heartbeat, slower respiratory rate, increased perspiration and salivation, smaller pupils, improved waste elimination, 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 an "eat and rest" response. In a complex homeostatic process, the sympathetic and parasympathetic systems act in opposition but in concert, much like the accelerator and brakes on a car, to keep vital body functions in balance.

All parasympathetic nervous systems consist of spinal and cranial segments. Near the coccyx 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 segments of the PNS consist of sensory components, which carry information to the brain, and motor components, which provide appropriate feedback to end organs. Sensory cells control blood pressure, oxygen and carbon dioxide levels, blood sugar concentrations, and the contents of the stomach and intestine, while motor neurons, clustered in small ganglia near target organs, modulate the body's responses to information gathered by sensory cells.

Acetylcholine is the main chemical messenger released at the neuronal junctions of the parasympathetic nervous system. Muscarinic receptors, named for their sensitivity to muscarin derived from Amanita muscaria mushrooms, are the main final receptors of the PNS. Acetylcholine molecules activate muscarinic receptors on the plasma membranes of nerve cells by binding to intracellular proteins. Once acetylcholine binds to proteins, a cascade of events leads to the end-organ response. Scientists have discovered five subtypes of muscarinic receptors, each with a different gene.

Dysautonomia refers to autonomic nervous system dysfunction where the sympathetic or parasympathetic nervous system exerts a disproportionate amount of influence on the body. Viral infections, toxic exposures, trauma, and heredity have been implicated as causative factors for the condition. Symptoms include aches and pains, fainting, fatigue, anxiety attacks, rapid heart rate, and low blood pressure. Examination of patients with dysautonomia by clinicians generally yields few, if any, objective physical or laboratory findings. There is no widely accepted treatment approach for dysautonomia, and therapeutic attempts are primarily directed at alleviating symptoms, not curing the dysfunction.

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