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The parasympathetic nervous system(PSNS) is part of theautonomic nervous system, using long nerve fibers to automatically regulate bodily functions through quick responses.
Also called the “rest and digest” system, it conserves energy and promotes relaxation of the body after undergoing stress.
The Autonomic Nervous System Has Two Main Divisions: Sympathetic and Parasympathetic The parasympathetic nervous system (PSNS) leads to decreased arousal in areas such as the eyes, glands that produce saliva, nerves in the stomach, blood vessels, and nerves that go to the bladder.
Parasympathetic vs. sympathetic nervous systemThe PSNS works in opposition to thesympathetic nervous system(SNS), which activates the “fight and flight” response which stimulates involuntary bodily processes during times of stress.The parasympathetic branch stimulates digestion and the urinary system when relaxed, whereas the SNS slows them down as these processes are not required during heightened stress.Theneuronsof the SNS also haveshorter pathwaysthan those of the parasympathetic nervous system.This shorter distance allows for a quicker signal transmission; sometimes the responses happen before a person is consciously aware of them.Together, the SNS and PNS work tomaintain homeostasisin the body (the balance of internal physiological mechanisms).
Parasympathetic vs. sympathetic nervous system
The PSNS works in opposition to thesympathetic nervous system(SNS), which activates the “fight and flight” response which stimulates involuntary bodily processes during times of stress.
The parasympathetic branch stimulates digestion and the urinary system when relaxed, whereas the SNS slows them down as these processes are not required during heightened stress.
Theneuronsof the SNS also haveshorter pathwaysthan those of the parasympathetic nervous system.
This shorter distance allows for a quicker signal transmission; sometimes the responses happen before a person is consciously aware of them.
Together, the SNS and PNS work tomaintain homeostasisin the body (the balance of internal physiological mechanisms).
Functions
The PSNS helps to recover the body after thefight or flight responsekicks in, so the main functions will be theopposite of this stress response.
Key functions of the PSNS include:
Without proper PSNS function, the body would struggle tomaintain homeostasis, potentially remaining in a constant state of heightened arousal.
The PSNS’s ability to quickly restore balance after stress helps protect against the adverse effects of chronic stress on the body’s systems.
Nerves of the PSNS
Nerve fibers of the PSNS arise within the central nervous system.The primary nerves involved arecranial nerves. Below are some of the main cranial nerves in the PSNS:
The PNS is made up ofspinal nervetypesthat project from the sacral region of the spinal cord (known as S2, S3, and S4).
The sacrum nerves send signals to regions of the bladder, reproductive organs, and the colon. The axons of these nerves are typically quite long in length andextend into ganglia(a group of neuron cell bodies) in the rest of the body.
The ganglia are usually located close to the target organs, allowing the PSNS to send and receive signals throughout the body quickly.
Alike to the sympathetic branch, the PSNS consists ofpreganglionic and postganglionic neurons.
The preganglionic nerves are located in the brain stem or sacral levels of the spinal cord and then project out to the parasympathetic ganglia, usually located near the organs.
The postganglionic neurons will then complete the journey to the organs and activate the specific organs.
Neurotransmitters of the Parasympathetic nervous system
The parasympathetic nervous system primarily usesacetylcholine as its mainneurotransmitter(chemical messenger), released at both preganglionic and postganglionic nerve endings.
When nerve impulses trigger its release, acetylcholine travels to receptor sites on target organs, where it can eitherstimulate or inhibit specific responses.
In the cardiovascular system, acetylcholine slows heart rate and reduces the force of heart muscle contractions.
Additionally, acetylcholine helpsslow breathing and stimulates various gland secretionsthroughout the body.
Dysregulated Parasympathetic Nervous System
Overactivation
When the parasympathetic nervous system becomes overactive, it can lead to anexcess of acetylcholine in the body.
This surplus can cause several uncomfortable symptoms including muscle weakness, cramps, excessive tearing of the eyes, blurred vision, increased salivation, and diarrhea.
In severe cases, overactivation may lead to paralysis.
Underactivation
An underactive parasympathetic system often results from nerve damage and prevents the body from properly “calming down” after stress.
People with an underactive PSNS may experienceconsistently elevated blood pressure and heart rate, even without obvious triggers.
This chronic state of arousal occurs because the body cannot effectively counteract sympathetic nervous system activation.
Signs of PSNS Damage and Dysfunction
Damage to the parasympathetic nervous system can occur due to various causes, including diabetes, aging, Parkinson’s disease, or chronic fatigue syndrome. Common signs of PSNS damage include:
Medical evaluation is recommended for anyone experiencing these symptoms, as treatment options vary depending on the underlying cause.
Activating Your Parasympathetic Nervous System
There are some ways in which you can help activate your parasympathetic nervous system, promoting relaxation and recovery from stress.
Deep breathing exercises, particularly those emphasizing long exhales, effectively trigger the PSNS.
The “box breathing” technique—inhaling for four counts, holding for four, exhaling for four, and holding for four—is especially effective and can be done anywhere.
Physical activitiesthat combine gentle movement with breathing, such as yoga, tai chi, or gentle stretching, can stimulate parasympathetic activity.
Progressive muscle relaxation, where you systematically tense and relax different muscle groups, also helps activate the PSNS.
Lumen. (n.d.).Functions of the Autonomic Nervous System. Retrieved May 5, 2021 from https://courses.lumenlearning.com/boundless-ap/chapter/functions-of-the-autonomic-nervous-system/
Nall, R. (2020, April 23). Your Parasympathetic Nervous System Explained. Healthline. https://www.healthline.com/health/parasympathetic-nervous-system
Tindle, J., & Tadi, P. (2020). Neuroanatomy, Parasympathetic Nervous System.StatPearls[Internet].
Saul McLeod, PhD
BSc (Hons) Psychology, MRes, PhD, University of Manchester
Saul McLeod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.
Olivia Guy-Evans, MSc
BSc (Hons) Psychology, MSc Psychology of Education
Olivia Guy-Evans is a writer and associate editor for Simply Psychology. She has previously worked in healthcare and educational sectors.
