Parasympathetic Tone: Function, Task & Diseases

Parasympathetic tone is a measure of the state of excitation of the parasympathetic autonomic nervous system as a counterpart to the sympathetic nervous system. A high parasympathetic tone has a calming effect on the internal organs, enables regeneration and serves to build up reserves. The body is returned to normal mode from the sympathetically controlled exceptional situation, in which the body is prepared for peak performance and for flight or attack.

What is parasympathetic tone?

Parasympathetic tone is a measure of the state of arousal of the parasympathetic autonomic nervous system as a counterpart to the sympathetic nervous system. The vegetative or autonomic nervous system, through which most bodily and organ functions are controlled independently of the will, consists of the sympathetic, parasympathetic, and enteric nervous systems (the nervous system for the digestive tract, which is also autonomic but certainly responds to signals from the sympathetic and parasympathetic nervous systems). The parasympathetic and sympathetic nervous systems can be understood as antagonists, but they also work together synergistically at the same time in sudden stressful situations by putting the body into alert mode in the best possible way. While a high sympathetic tone is associated with an increased concentration of stress hormones and sets the body up for peak physical performance and flight or fight, the parasympathetic tone brings the body back from the exceptional situation by blocking the stress hormones. It initiates a metabolic program that serves to regenerate, build up and relieve stress. There is a sophisticated interplay between sympathetic and parasympathetic tone for the diverse control of the internal organs, metabolism and hormone balance. Direct measurement of parasympathetic tone is not possible because the parasympathetic system is mainly inhibitory to the concentration of stress hormones. Some indication of parasympathetic tone is given by the concentration of acetylcholine, a neurotransmitter that has inhibitory effects on the stress hormone cortisol. It is thus possible to infer the respective parasympathetic tone by measuring cortisol concentration and by heart rate variability.

Function and task

Parasympathetic tone provides an indication of the momentary activity of the parasympathetic autonomic nervous system, which must always be viewed in the context of the function and tasks of the sympathetic nervous system. On the one hand, the parasympathetic nervous system acts as an antagonist of the sympathetic nervous system; on the other hand, the parasympathetic nervous system can also work synergistically, i.e. complementarily, with the sympathetic system. This happens above all in sudden stress situations, in which the metabolism programs the body with a multitude of individual reactions for the greatest possible efficiency while at the same time reducing the risk of bleeding in the event of injury. The metabolism is prepared in the short term for flight or attack. The change in bodily functions caused by a high stress level can be harmful in some cases, such as an increase in blood pressure. Therefore, the increase in parasympathetic tone after the end of an acute stress situation serves the purpose of returning the circulation and metabolism to normal mode as soon as possible. This gives the body and muscle tissue an opportunity to regenerate and build up reserves. Apart from the global metabolic change, the parasympathetic tone influences the function of some internal organs such as the heart, bronchi, digestive tract, gall bladder, liver, pancreas, ureter and others. In the heart, an increase in parasympathetic tone causes a slowing of the heart rate, a slowing of the conduction of excitation from the sinus node in the right atrium to the AV node and within the AV node, so that the signal to contract the two chambers is delayed a little. In the bronchi, increased parasympathetic tone causes dilation of the blood vessels (vasodilation), so that there is greater blood flow and therefore greater secretion of mucus and constriction of the bronchi. In the digestive tract, the parasympathetic nervous system influences its own autonomic nervous system, the enteric nervous system, causing it to be more active. In addition to sexual arousal, men need a certain parasympathetic tone for an erection to occur.The smooth ring muscles that clasp the corpus cavernosum arteries slacken under the parasympathetic influence, allowing blood to rush into the corpus cavernosum and cause an erection.

Diseases and ailments

A change in parasympathetic tone interacting with sympathetic tone is subject to many influences and can be sensitively disturbed by disease, drugs, or toxins. The most common complaints resulting from a disturbance in the balance between the two antagonists are attributed to changes in lifestyle compared to ancient societies. In particular, people who frequently experience stressful conditions that cannot be relieved by increased physical activity suffer from permanently elevated concentrations of stress hormones. The parasympathetic tone can be too low under such conditions, so that the metabolism is dominated by the sympathetic tone and corresponding complaints such as primary hypertension, sleep disorders, restlessness or similar symptoms can develop. The digestive tract can also show functional disturbances due to a too low parasympathetic tone because of missing or too weak impulses from the parasympathetic system. A few decades ago, the diagnosis of vegetative dystonia was often made for nonspecific complaints that were due to an apparent dysfunction of the autonomic nervous system. Today, this term is controversial because it is often “only” a disturbed balance between sympathetic and parasympathetic tone. Primary nerve disorders leading to similar symptoms are very rare. However, sensitive disorders can be triggered by neurotoxins, which are also synthesized in nature in the form of venomous spiders, snakes, cube jellyfish and other animals and used to catch prey or defend against attack. If parasympathetic tone disorders are diagnosed, drugs are available to stimulate the parasympathetic system or inhibit activity.