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Our Parasympathetic Nervous System

Updated: Jan 4


Boulder, Colorado

December 3, 2020


I heard of the parasympathetic nervous system (PSNS) when I learned about the relaxation benefits of meditative breathing. I wish I could say by now I am good at that type of breathing. I am when I concentrate on it but it still doesn't come naturally. Like most of us, I tend to "shallow" breathe, only filling the top part of my lungs. If you really focus on filling your lungs and trying to take the same amount of time to breathe in as you do to exhale, you will find you can't help but start to relax.


The following article from "Healthline" provides a good description of the PSNS and how if differs from the sympathetic nervous system (SNS).


Our nervous system is a network of nerves that act in different key functions to keep our body performing functions such as moving, responding, and sensing.

In the simplest terms, the parasympathetic and sympathetic portions of the autonomic system are two halves of the same whole.

Doctors often call the PSNS the “rest and digest” side while the sympathetic is the “fight or flight.”

PSNS Function

Our PSNS starts in our 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 our body working as they should.


Parasympathetic nervous system and our heart

There are a number of special receptors for the PSNS in your heart called muscarinic receptors. These receptors inhibit sympathetic nervous system (SNS) 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 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.

Parasympathetic cranial nerves

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.

Sympathetic and parasympathetic nervous system

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:


Examples of parasympathetic responses

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 PSNS the “feed and breed” system.


The takeaway

Our PSNS is a vital part of your body’s key functions. When it doesn’t work properly, we can face a number of bodily dysfunctions that affect our health.

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