The Ultimate Guide to the Parasympathetic Nervous System

The nervous system is a fascinating and complex network of nerves, synapses, and organs that work together to transmit signals throughout your body. It’s one system in a larger environment, and it’s responsible for everything from movement to unconscious acts like adjusting your vision and maintaining your resting heart rate.

Want to learn more about the parasympathetic nervous system and why it matters for your health? We’ve put together this scientific guide to help you understand how the system works. Plus, you’ll learn what it means for your health and how you can use tools to track metrics and monitor your wellbeing.

What Is the Parasympathetic Nervous System?

Parasympathetic nervous system: A woman relaxing on the couch

Understanding the parasympathetic nervous system (PSNS) can be complicated because it’s part of a much larger network of bodily processes. Essentially, the parasympathetic nervous system, also known as the “rest and digest” system controls bodily functions associated with the body at rest and after digestion. To fully understand this system, you need an overview of the network it works within.

Your body is a complex network of cells and structures that come together to drive movement, create thoughts, and sense the world around you. Within these complex networks, the nervous system plays a key role. The nervous system consists of nerves and neurons that work to transmit signals from one part of the body to another. It’s the process that underlies everything from waving your hand to coughing to standing up.

The nervous system consists of two parts, the peripheral nervous system (PNS) and the central nervous system (CNS). The brain, spinal cord, and nerves make up the central nervous system while neurons and clusters of neurons — known as ganglia — are the key actors in the peripheral nervous system.

Since these systems are so complex, scientists have broken them down even further. The peripheral nervous system is divided into two sections: the autonomic nervous system and the somatic nervous system.

For our discussion of the parasympathetic nervous system, we’ll focus on the functions of the autonomic nervous system, which controls involuntary smooth muscles and glands. The parasympathetic nervous system is one of two subsections of the autonomic nervous system. The autonomic nervous system is responsible for things you can’t voluntarily change at a moment’s notice, including your heart rate and digestive functions.

Now that you have a basic grasp of the entire nervous system, we’ll dive into the functions and actors of the parasympathetic nervous system.

Functions of the Parasympathetic Nervous System

Parasympathetic nervous system: Closeup on an eyeball

The parasympathetic nervous system is one of two divisions of the autonomic nervous system. The other division of the autonomic nervous system is the sympathetic nervous system. That system is responsible for the body’s fight or flight response.

On the other hand, the PSNS controls unconscious movements and signals that occur during a variety of activities. The parasympathetic nervous system is what’s working behind the scenes when you cry, when you digest and process foods, and when you start drooling over a delicious meal. It’s responsible for everything from urination and defecation to sexual arousal.

It works to constrict certain eye muscles to help improve vision at close range, and it makes it easier for you to use the bathroom. It works like this: M3 receptors — which help to slacken smooth muscle — are stimulated to relax the urethra, which makes it easier to urinate. In scientific terms, parasympathetic stimulation signals peristalsis in the ureters — an involuntary constriction that relaxes urination muscles, creating a wave-like effect that moves the contents of your bladder toward the urethra. In essence, the PSNS is responsible for the urge to use the bathroom. Some of the main functions of the parasympathetic nervous system are that it:

  • Stimulates salivation
  • Constricts pupils
  • Contracts the bladder
  • Inhibits the production of adrenaline
  • Decreases heart rate
  • Stimulates digestion
  • Relaxes sphincter muscles
  • Dilates bronchial tubes

The parasympathetic nervous system essentially works behind the scenes to keep things running as smoothly as possible. It’s a system you may not think much about, but it is a key part of most of your daily routines.

Actors Within the Parasympathetic Nervous System

Parasympathetic nervous system: A doctor looks at a brain scan

The parasympathetic nervous system operates mainly within the lungs, heart, bladder, and stomach. It sends signals using parasympathetic nerves — special nerve systems that are rooted in the spinal cord — and cranial nerves, which are rooted in the brain. The system triggers bodily functions when signaled by parasympathetic nerves throughout the body. The main parasympathetic nerves are autonomic or visceral nerves that work with two neurons to carry signals to the target organ.

Cranial nerves are also among the key actors in the PSNS. The body has a total of 12 cranial nerves that are rooted in the nuclei of the brain and spread out to vital organs. Because of this structure, the PSNS is often said to have a craniosacral flow — a flow of information from the brain to other organs.

The cranial nerves carry signals from the brain to ganglia — a structure that consists of cell bodies including neurons. These ganglia comprise four parasympathetic synapses including the ciliary ganglion, which control pupil dilation, and the pterygopalatine ganglion, which produce tears and mucus. The other two synapses include the otic ganglion and the submandibular ganglion, which control salivation. 

These signals are transmitted through nerve fibers known as preganglionic fibers. After the ganglion processes the message, the PSNS sends the singnal to the relevant organ, using fibers known as postganglionic fibers. The nerve fibers also contain neurons that are involved in the transmission process and known respectively as preganglionic neurons and postganglionic neurons.

Think of the parasympathetic ganglia like train stations — a train arrives with a passenger holding a briefcase of information and departs to deliver that briefcase to another location. You can also think of it as a post office. A letter or message comes through the postal depot and is then transmitted out to its destination.  

In the PSNS, there are 4 main nerve types that perform most actions.

  • Oculomotor nerve: This cranial nerve is located in the eye and is responsible for constricting the pupil. This makes the pupil smaller, bringing in less light. It also contracts the ciliary muscle — an eye muscle responsible for vision — helping to improve vision at close distances.
  • Facial nerve: The facial nerve is responsible for controlling the salivary glands and glands in the mouth and nose. This parasympathetic nerve also controls the lacrimal glands, which produce tears.
  • Glossopharyngeal nerve: This nerve delivers signals to the salivary glands known as parotids glands.
  • Spinal nerves: Spinal nerves are made of desiccated pathways from the spine to the heart that help manage overall heart control.
  • Vagus nerve: The vagus nerve originates in vagal nuclei that are responsible for an incredible amount of body functions. According to StatPearls, the vagus nerve acts on “the viscera of the abdomen and thorax” and is especially important because it “carries 75% of all parasympathetic fibers”. The nerve delivers messages to a host of organs from the bladder and stomach in the gastrointestinal tract to the kidneys, liver, and sexual organs.

Why the PSNS Matters

Parasympathetic nervous system: A man holds a heart-shaped cutout up to his chest

The parasympathetic system is incredibly important when it comes to normal bodily functions. If something within the system breaks down, you may develop issues including digestive problems and heart problems. The latter is particularly important since the PSNS plays an important role in heart health that we haven’t covered yet.

The PSNS includes special heart receptors known as muscarinic receptors. The function of these receptors is to prevent stimulation from the other autonomic system — the sympathetic nervous system, which triggers the fight or flight response in stressful situations. Research shows a correlation between sympathetic nervous system activity and negative outcomes in patients with heart failure.

On the other hand, parasympathetic nervous system activation may help to protect heart health. That’s because these systems work in parallel. While the sympathetic nervous system can trigger increased heart rate and fight or flight responses, the PSNS tends to relax and constrict body responses.

Additional research shows the parasympathetic nervous system may help protect heart health by regulating blood pressure and heart rate. This system sends signals for vasodilation — or widening — of the blood vessels, helping to lower high blood pressure.

The PSNS may also help regulate the resting heart rate. A normal resting heart rate is between 60 and 100 beats per minute. If you’re not sure what your resting heart rate is, you can easily calculate it at home.

Use a health tracker like Biostrap’s wristband to get accurate readings of your resting heart rate. Alternatively, you can take your heart rate by placing two fingers on your wrist and counting the number of beats for one minute. If your resting heart rate is abnormal, it may be a sign that something within the parasympathetic nervous system isn’t working properly.

The PSNS and Your Health: How to Monitor Metrics

The parasympathetic nervous system is a key component of overall health. It’s responsible for unconscious movements that are vital to digestion, vision, and heart health. Monitoring your health can help you identify underlying issues that may affect your PSNS and other important health networks.

The Biostrap App and wearables are a great way to track and monitor your health. From monitoring your resting heart rate and oxygen saturation levels to tracking your sleep habits, the tool offers unrivaled insights into your health.

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