What’s the parasympathetic nervous system?

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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 of the actions of the parasympathetic nervous system are automatic and involuntary, some, such as breathing, work in concert with the conscious mind. Widely regarded as the control system when external conditions are calm and normal, the PNS promotes a slower heart rate, slower breathing rate, increased sweating and salivation, smaller pupils, increased 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 work in opposing but concerted ways, much like the accelerator and brakes of an automobile, to keep the body’s vital 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 derives 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 provide appropriate feedback to the end organs. Sensory cells monitor blood pressure, oxygen and carbon dioxide levels, blood sugar concentrations, and stomach and intestinal contents, while motor neurons, grouped in small ganglia near target organs, modulate the body’s responses to information collected by sensory cells.

Acetylcholine is the primary chemical messenger released at the neuronal junctions of the parasympathetic nervous system. Muscarinic receptors, so called because of their sensitivity to muscarine derived from the Amanita muscaria mushrooms, are the major terminal receptors of the PNS. Acetylcholine molecules activate muscarinic receptors in the plasma membranes of nerve cells by attaching 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 distinct gene.

Dysautonomia refers to autonomic nervous system dysfunction in which the sympathetic or 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 in the condition. Symptoms include aches and pains, fainting, fatigue, anxiety attacks, rapid heart rate, and low blood pressure. Examination of patients with dysautonomia by physicians typically yields few, if any, objective physical or laboratory findings. There is no widely accepted treatment approach for dysautonomia, and treatment efforts are largely directed at alleviating symptoms, not curing the dysfunction.

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