Parasympathetic Nervous System: What to Know

Medically Reviewed by Jabeen Begum, MD on October 13, 2022
5 min read

You’ve probably heard the phrase “fight or flight” to describe the body's response to an extreme, acute stressor. Your sympathetic nervous system automatically kicks your body into high gear when you need to defend yourself or run from a real or perceived threat.

Have you ever wondered how the body calms down after an experience like this? The opposite of the sympathetic nervous system is the parasympathetic nervous system. The parasympathetic nervous system is responsible for the body’s relaxation response, and it also plays a large part in regulating digestion, heart rate, and breathing.

The parasympathetic nervous system, or the PNS, is part of the body’s autonomic nervous system, the ANS. This is a division of the body’s peripheral nervous system as opposed to the central nervous system, which is mostly housed in the spinal cord. The ANS controls systems and processes in the body that aren’t under our conscious control. For example, it produces the sensation of panic when we’re in danger and the feeling of hunger when we need food. It also regulates how many of our internal organs respond to certain conditions. 

The PNS comprises a network of small neurons and large nerves that run throughout the body and regulate breathing, digestion, sexual function, heart rate, and more. If the parasympathetic nervous system didn’t exist, the sympathetic nervous system would take over during times of stress — and your body would not have a way of turning off its dramatic response.

The parasympathetic nervous system is responsible for various functions of internal organs, such as their capacity for relaxation, constriction, and mucus production. While the sympathetic nervous system uses a form of adrenaline called norepinephrine to activate receptors in different organs, the peripheral system uses acetylcholine as its main neurotransmitter. 

Acetylcholine stimulates muscarinic receptor sites around the body and is used to contract muscles in the digestive system, help the bladder urinate, and return breathing to normal after a period of stress. Consider the list below as a sample of the many responsibilities of the parasympathetic nervous system:

Eyes. The PNS uses acetylcholine to stimulate muscarine receptors in the eye and contract the pupil. This constriction affects our vision and helps us see close-up objects clearly.

Heart. The PNS decreases heart rate when the body is under stress. It also affects circulation due to its ability to constrict blood vessels.

Lungs. Whether your lungs are irritated due to bronchitis or an asthma episode, the PNS takes over to regulate mucus secretions and manage the width of airways. Sometimes this constriction is helpful — but in the case of an asthma attack, too much constriction can be fatal.

Digestion. Through its vast network of nerves, the PNS aids in digestion as it helps the bowel muscles contract and push food through the small and large intestines.

There are 12 nerves called “cranial nerves” attached to the brain. These nerves are involved in processing senses like smell, taste, and touch. They also assist in facial movement and signaling between your brain and your head, neck, and the upper part of your chest and arms. Four of these cranial nerves belong to the parasympathetic nervous system. They constrict the pupils, produce saliva, and perform other small but important sensory activities. 

The tenth cranial nerve, called the vagus nerve, is a large nerve that includes almost 75% of the parasympathetic system. It is connected to several organs, including parts of the digestive tract and the heart, and branches into smaller nerves throughout the body. 

The term “brain-gut connection” refers to the large role the vagus nerve plays in conditions such as post-traumatic stress disorder, depression, and inflammatory bowel disease. These conditions are thought to involve both the gut and the brain. Many researchers devote their careers to studying how to improve the lives of patients with these conditions by stimulating the vagus nerve to modulate the parasympathetic nervous system.

A few telltale signs can suggest that your parasympathetic nervous system isn’t functioning correctly. You might notice parasympathetic nervous system effects in the following physical and mental areas:

  • Anxiety disorders, panic attacks, or PTSD
  • Irritable bowel syndrome or an inflammatory disorder that affects the digestive tract
  • Asthma and increased mucus in the nose
  • Heart rate issues like palpitations or an irregular heartbeat
  • Sexual problems (usually erectile dysfunction)

Some specific conditions can impact your parasympathetic nervous system.

Nerve or brain trauma. Getting injured or experiencing physical nerve damage can affect the PNS. If you’ve hurt your spinal cord or sustained a blow to the head, you may see a neurologist to discuss potential damage to your nervous system. Be sure to seek emergency care as soon as possible after a back or head injury.

Autonomic neuropathy. This condition can be the result of bacterial or viral infections, but it’s most often caused by type 2 diabetes. Symptoms include low blood pressure when standing, low blood sugar without shakiness and other common signs, exercise issues, and digestion problems such as bloating, constipation, diarrhea, and heartburn. 

Multiple system atrophy. This is a brain condition that has neurological symptoms similar to Parkinson’s disease. You may first notice movement problems and then your parasympathetic nervous system, or “automatic” bodily systems, will start malfunctioning.

It’s important to see your doctor as soon as possible if you have trouble regulating your heartbeat, body temperature, or blood pressure. Not experiencing “danger” signals from your body means that you could be very ill without knowing it.

Keeping the parasympathetic nervous system healthy is a multifaceted approach that combines stress reduction with beneficial lifestyle habits. Think about integrating the following lifestyle changes and precautions into your daily life to keep your peripheral nervous system functioning as it should:

  • Eat well and get enough high-quality sleep.
  • Don’t use substances like alcohol to self-medicate anxiety.
  • Decrease lifestyle stress.
  • Don’t engage in dangerous activities that could cause accidents and nerve damage.
  • Get screened for diabetes regularly to prevent late-stage nerve damage.

Because the PNS is involved in regulating and controlling so many bodily systems, it’s important to visit your doctor if you notice any symptoms that signal nervous system trouble. If you have a medical condition or disease, ask your primary care doctor for additional information and resources about how this fascinating and complex system may be involved in your medical condition or disease.