The parasympathetic nervous system (PSNS) is one of two major divisions of the larger autonomic system in your body. Its function is to keep the basic functions of your body working as they should.

Your nervous system is a wild and wonderful network of nerves that act in different key functions to keep your body moving, responding, sensing, and more.

The parasympathetic and sympathetic portions of the autonomic system are two halves of the same whole. This article will examine the parasympathetic nervous system, what it controls, and how it works with the sympathetic nervous system.

Parasympathetic nervous system definition

Doctors often call the parasympathetic nervous system the “rest and digest” side, while the sympathetic is the “fight or flight.”

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Your PSNS starts in your brain and extends out via long fibers that connect with special neurons near the organ they intend to act on. Once PSNS signals hit these neurons, they have a short distance to travel to their respective organs.

Examples of the areas the PSNS acts on include:

  • eyes
  • lacrimal glands that produce tears
  • parotid glands that also produce saliva
  • salivary glands that produce saliva
  • nerves in the stomach and trunk
  • nerves that go to the bladder
  • nerves and blood vessels responsible for the male erection

The PSNS is kind of a “business as usual” system that keeps the basic functions of your body working as they should.

There are several special receptors for the PSNS in your heart called muscarinic receptors. These receptors inhibit sympathetic nervous system action. This means they’re responsible for helping you maintain your resting heart rate. For most people, the resting heart rate is between 60 and 100 beats per minute.

On the other hand, the sympathetic nervous system (SNS) increases heart rate. A faster heart rate (usually) pumps more oxygen-rich blood to the brain and lungs. This can give you the energy to run from an attacker or heighten your senses in another scary situation.

According to an article in the journal Circulation from the American Heart Association, a person’s resting heart rate can be one indicator of how well a person’s PSNS, specifically the vagus nerve, is working. This is usually only the case when a person doesn’t take medications that affect heart rate, like beta-blockers, or have medical conditions affecting the heart.

For example, heart failure reduces the response of the parasympathetic nervous system. The results can be an increased heart rate, which is the body’s way of trying to improve the amount of blood it pumps through the body.

The cranial nerves are paired nerves that are responsible for many movements and sensations that take place in your body’s head and neck. The nerves all start in the brain. There are 12 cranial nerves labeled using Roman numerals from I to XII, with the first set of nerves located at the brain’s front.

Major cranial nerves

  • III. Oculomotor nerve. This nerve helps to constrict the pupil, which makes it appear smaller.
  • VII. Facial nerve. This nerve controls secretions of saliva and mucus in the mouth and nose, respectively.
  • IX. Glossopharyngeal nerve. These nerves go to the parotid salivary glands that provide extra saliva to the tongue and beyond.
  • X. Vagus nerve. An estimated 75 percent of all parasympathetic nerve fibers in the body come from this nerve. This nerve has branches in many key organs, including the stomach, kidneys, liver, pancreas, gallbladder, bladder, anal sphincter, vagina, and penis.

Other cranial nerves

The remaining nerves have either motor function (help something move) or sensory function (sense pain, pressure, or temperature). Some of these nerves are both motor and sensory. Many of these are parasympathetic nerves.

For the most part, if you know the actions of the PSNS, you can consider the sympathetic nervous system to have opposite reactions. However, there are times when the systems are opposites, but instead complement each other.

Here are some key differences in the two:

LocationKey areas affected include the lungs, heart, bladder, and stomach.Key areas affected include the lungs, heart, smooth muscle, and exocrine and endocrine glands, like the sweat glands and saliva.
ActionsConstricts pupils; causes salivation; slows down the heart rate; tightens the bronchi in the lungs; enacts digestion; releases bile; makes the bladder contract Dilates pupils; keeps you from salivating; speeds up the heart; widens the bronchi; inhibits digestion; keeps the bladder from contracting
SpeedSlower than the sympathetic division Faster than the PSNS

An easy acronym to remember how and where the PSNS works is SLUDD. This stands for:

  • Salivation: As part of its rest-and-digest function, the PSNS stimulates production of saliva, which contains enzymes to help your food digest.
  • Lacrimation: Lacrimation is a fancy word for making tears. Tears keep your eyes lubricated, preserving their delicate tissues.
  • Urination: The PSNS contracts the bladder, which squeezes it so urine can come out.
  • Digestion: The PSNS stimulates the release of saliva to promote digestion. It also enacts peristalsis, or the movement of the stomach and intestines, to digest food as well as release bile for the body to digest fats.
  • Defecation: The PSNS constricts the sphincters in the intestine and moves digested food material down the digestive tract so a person can have a bowel movement.

Keeping these things in mind, you can see why doctors may also call the parasympathetic system the “feed and breed” system.

Your PSNS is a vital part of your body’s key functions. When it doesn’t work properly, you can face several bodily dysfunctions that affect your health. If you think you may be having trouble with one of your body’s parasympathetic nervous system functions, talk with your doctor to find out how you can get help.