The Parasympathetic Nervous System: Definition & Functions

The parasympathetic nervous system is one of the two main parts of the autonomic nervous system. The autonomic nervous system is the part of the nervous system that functions without conscious control, regulating functions like heart rate, blood pressure, breathing, and digestion.

The parasympathetic nervous system (PNS) is responsible for bodily functions that take place when you’re at rest. These activities include digestion, lowering heart rate, reducing muscle tension, removing waste, salivating, and sexual arousal (Biology Dictionary, 2017). This is why the PNS is typically referred to as the “rest and digest” system.

The parasympathetic nervous system is active during everyday activity when nothing particularly stressful is going on. It regulates the automatic bodily functions that are essential to keep us going.

The parasympathetic nervous system is also what brings us back to a state of calm after a period of stress. After danger or a perceived threat has passed, the parasympathetic nervous system allows you to calm down by lowering your heart rate and blood pressure, relaxing muscles, and slowing breathing. It brings your systems back to homeostasis, or balance, and allows your body to relax and recuperate (NCI Dictionary, 2023). Slowing things down allows us to conserve energy so we’re ready for future threats where quick action is needed.

Here are more specifics about what the parasympathetic nervous system does. The parasympathetic nervous system:

• Constricts pupils for better near vision
• Creates tears to keep eyes lubricated
• Increases salivation 
• Slows heart rate, especially after a stressful experience or after exercise
• Constricts airways in the lungs to slow the breathing rate
• Facilitates digestion and stimulates intestinal muscles to move things along
• Stimulates urination by contracting the bladder
• Elicits sexual arousal
• Activates the immune system

From the list above, it’s clear the parasympathetic nervous system is involved in some pretty important life-sustaining functions. If it’s not running properly, it can lead to all kinds of health issues. These include gastrointestinal issues, heart rhythm problems, issues with waste removal, and sexual dysfunction. 

In addition, many of us experience daily stress or even anxiety which can be triggered by things like work pressures, family concerns, discrimination, or even getting stuck in traffic. So the ability to return to a calm state is critical. Imagine if you were unable to come back down from a stressed-out state. Because of its ability to promote a feeling of calm and well-being, the parasympathetic nervous system plays a crucial role in mental health.

Likewise, the ability of the parasympathetic nervous system to create calm is important for physical health. When we’re in a state of tension, stress hormones like cortisol and adrenalin are released which raise your blood pressure, increase your breathing rate, and create muscle tension. Being in this state frequently contributes to many health problems such as heart disease, diabetes, obesity, and depression (APA, 2010). The parasympathetic nervous system also plays another important role in our physical health by regulating the immune system. It’s involved in controlling inflammation and promoting healing.

The second branch of the autonomic nervous system is the sympathetic nervous system (SNS). This is the system that is responsible for the well-known “fight or flight” response. It’s activated when you’re under stress or perceive some kind of danger. Instead of lowering heart rate, breathing, and blood pressure, the sympathetic nervous system increases them. It redirects blood flow away from internal organs and shifts it to large muscles, the heart, and the brain. So it can be thought of as the opposite of the parasympathetic nervous system.  

The sympathetic nervous system affects the entire body through the release of adrenalin which enters the bloodstream to prepare you for action when you feel threatened. By contrast, the parasympathetic nervous system “puts the brake on” and gets your body back to normal function mode. In this way, the two systems work together. The parasympathetic nervous system helps you conserve energy which allows your body to gather strength in times of calm so you’re is prepared for action the next time you encounter a threat.​​

The parasympathetic nervous system is a system of nerves that connects to the spinal cord in two places. The first place of connection is at the brainstem which is where the brain connects to the spinal cord. These nerves are called the parasympathetic nervous system cranial nerves. There are four of these nerves, three that connect to the sensory organs and glands (eyes, salivation glands), and the vagus nerve, which plays the largest role in the parasympathetic nervous system and connects to many abdominal organs. More on the vagus nerve later.

The parasympathetic nervous system also connects to the spinal cord near the sacrum (LeBouef et al., 2022). The connection here controls functions related to organs in that area of the body such as waste removal and sex.

Although the parasympathetic nervous system generally operates outside of our conscious control, there are some things you can do to activate it. In addition to promoting feelings of calm and relaxation, using these techniques can help reduce the risk of chronic diseases like high blood pressure, heart disease, and digestive disorders. Activating the parasympathetic nervous system can also strengthen the immune system and promote healing. Here are some techniques:

• Slow breathing techniques – A study showed that consciously slowing your breath rate to 10 breaths per minute lowers heart rate and blood pressure and promotes relaxation. It was also found to reduce stress and improve mood (Zaccaro et al., 2018). 
• Massage – It’s no secret that a massage softens your muscles and makes you feel relaxed. That release of muscle tension can activate the parasympathetic nervous system, in a sense telling your nervous system that you’re no longer on high alert. Massage also stimulates the vagus nerve which is the main nerve of the parasympathetic nervous system. 
• Aromatherapy – Certain essential oils such as lavender, chamomile, and ylang-ylang, have been shown to promote relaxation and activate the parasympathetic nervous system (Lehrner et al., 2005). You can use a diffuser, add a few drops to a bath or massage oil, or simply inhale the scent directly. 
• Yoga – Studies show that yoga can activate the parasympathetic nervous system and reduce the activity of the sympathetic nervous system which relieves anxiety and improves mood (Streeter et al., 2012). It accomplishes this in two ways. First, simply directing your mind to focus on your body in the present moment turns your attention away from worries about the past or future. Also, some specific poses and techniques help bring blood pressure down and induce calm, most involving taking your head below the heart. We’ll get into that in more detail later.
• Meditation & Mindfulness – As with yoga, meditation also increases parasympathetic activity and reduces sympathetic activity (Tang et al., 2009). It also does this by turning your mind toward the present moment by watching your thoughts and feelings without judgment. We often use a tool such as watching the breath to give the mind something to focus on. This way we’re less distracted by intrusive thoughts or worries. When distracting thoughts happen (which they will), you gently and without judgment direct your mind back to the breath. 

​
Other methods for activating the parasympathetic nervous system include going for a walk in nature, connecting with friends, laughing, visualizing a peaceful setting, or my favorite, petting your dog.​​

What is the Vagus Nerve?
The vagus nerve is the main contributor to the parasympathetic nervous system and is one of the largest nerves in the body. The name “vagus” means wandering in Latin, which describes the long and winding path this nerve takes through our body. It extends from the brain stem through the neck, the thorax (upper back), and down to the abdomen, connecting to major organs of the body (Breit et al., 2017). 

Because of its size and connection to so many organs, the vagus nerve plays a large role in the parasympathetic nervous system and oversees a large array of crucial bodily functions. It’s responsible for digestion, heart rate, breath rate, blood pressure, and reflexes like coughing, sneezing, and swallowing (Breit et al., 2017).

Vagus Nerve and the Gut-Brain Connection
The vagus nerve acts in a bi-directional manner. Not only does it send information to digestive organs about what to do, but it also sends information about what’s going on in those organs back to the brain. This is known as the “gut-brain” connection. This bi-directional communication suggests that the gut is a source of information to the brain similar to our sense organs. Thus, the brain-gut connection is thought to play a role not only in digestive issues but also in mood and anxiety. There is also evidence that gut bacteria influence our experience of anxiety and depression (Carabotti et al., 2015). So consuming things like probiotics that are thought to contribute to gut health may be helpful for the treatment of not only digestive disorders but also depression (Breit et al., 2017).

Vagal Tone
How well the vagal nerve is working is known as “vagal tone”. This refers to the activity level of the vagus nerve. Higher vagal tone is associated with the ability to efficiently regulate stress responses. The best measurement of vagal tone is heart rate variability (HRV) which is how much the time between your heartbeats varies. Having high HRV indicates strong vagal tone and that the parasympathetic nervous system is able to return you to a more relaxed state quickly (Singh et al., 2018). Vagal tone can be improved with many of the same techniques used to activate the parasympathetic nervous system. 

Parasympathetic Nervous System Example: Vasovagal Response
I recently had a personal experience related to the vagus nerve, although I didn’t know it at the time. After having some stitches removed from my hand, I suddenly felt like I was going to pass out. In a slightly panicky tone, my doctor said “It’s a vasovagal response. You have to lie down!”. (Huh?) After lying down for a few minutes, it passed. When I got home, a quick Google search told me that a vasovagal response is when stimulation of the vagus nerve causes your heart rate and blood pressure to drop suddenly. It’s usually triggered by something stressful—in my case, pain. Perhaps my vagus nerve overreacted a bit.​​

During a stressful or dangerous situation, the sympathetic nervous system increases your breathing rate to get more oxygen circulating to the skeletal muscles to prepare you for action. But after the stress is over, the parasympathetic nervous system constricts the airways of the lungs to slow breathing back down to a normal rate. This returns you to a feeling of calm. 

Often a situation triggers anxiety or fear even though we’re not actually in danger, such as with public speaking. Since the breath is partly under voluntary control, this is where slow deep breathing can help. This technique gets more oxygen to the brain and tells your body that everything is okay, thus helping to promote the activity of the parasympathetic nervous system.​​

For people that have experienced trauma, the nervous system can lose the ability to regulate the balance between the sympathetic nervous system and the parasympathetic nervous system. For some, the sympathetic nervous system, the fight or flight part of the nervous system, can be in hyperarousal mode almost constantly. Essentially, the sympathetic nervous system is stuck in the “on” position. This can lead to symptoms associated with PTSD like hypervigilance. For others, the parasympathetic nervous system may be overactive resulting in depression or feeling like they’re completely shut down (Schauer & Elbert, 2010). Both ends of the spectrum take a toll, both physically and emotionally. ​

Most of us know that pupils dilate and contract according to the amount of light available. But emotional states can also trigger pupil dilation and contraction. 
​
Have you looked into someone’s eyes when they’re under a lot of stress? Sometimes it’s subtle, but their pupils become larger. This is because the sympathetic nervous system signals the dilator muscle in the iris (the colored part of the eye) to contract which pulls the iris outward, causing the pupil to become bigger. The reason it’s beneficial in times of threat is that allowing more light to enter gives the eye more sensitivity and the ability to see faint stimuli which are needed when you’re on the lookout for danger (Mathôt, 2017). 

In contrast, the parasympathetic nervous system is responsible for another set of muscles in the iris that constricts it to make the pupils smaller. This constriction is present during everyday activities when things are calm or after a stressful situation has ended. This explains why pupils are generally smaller when you’re relaxed. The advantage of smaller pupils is it allows objects to appear sharper, just like a camera does when you use a smaller aperture. This is helpful for normal daily activities that require focus and near vision such as reading, cooking, or watching TikTok.

Salivation is another example of how the sympathetic nervous system and the parasympathetic nervous system work together. When we’re in situations of danger or fear, energy needs to be directed to parts of the body required to respond to threats (eyes, muscles, and the heart). So the sympathetic nervous system directs salivation to slow down. You may notice a dry mouth when you’re nervous and probably aren’t thinking about having a snack.

But after the perceived threat is gone, the flow of saliva is turned back up. The parasympathetic nervous system sends a message via acetylcholine that stimulates the salivary glands. This is a good time to replenish energy in case of future threats.

​Want to learn more? Check out these articles:​

• APA. (2010). APA survey raises concern about health impact of stress on children and families. American Psychological Association. Retrieved April 23, 2023.
• Biological Dictionary Editors (2017, April 29). Parasympathetic nervous system – definition and function. Biology Dictionary. Retrieved April 22, 2023.
• Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2017). Vagus Nerve as Modulator of the Brain–Gut Axis in Psychiatric and Inflammatory Disorders. Frontiers in Psychiatry, 9. 
• Carabotti, M., Scirocco, A., Maselli, M. A., & Severi, C. (2015). The gut-brain axis: Interactions between enteric microbiota, central and enteric nervous systems. Annals of Gastroenterology : Quarterly Publication of the Hellenic Society of Gastroenterology, 28(2), 203-209. 
• Duan, H., Cai, X., Luan, Y., Yang, S., Yang, J., Dong, H., Zeng, H., & Shao, L. (2021). Regulation of the Autonomic Nervous System on Intestine. Frontiers in Physiology, 12. 
• LeBouef, T., Yaker, Z., & Whited, L. (2022, May 8). Physiology, autonomic nervous system – statpearls – NCBI bookshelf. National Library of Medicine. Retrieved April 22, 2023.
• Lehrner, J., Marwinski, G., Lehr, S., Johren, P., & Deecke, L. (2005). Ambient odors of orange and lavender reduce anxiety and improve mood in a dental office. Physiology & Behavior, 86(1-2), 92–95. 
• Mathôt, S. (2017). Pupillometry: Psychology, Physiology, and Function. Journal of Cognition, 1(1). 
• NCI Dictionary. (2023). NCI Dictionary of Cancer terms. National Cancer Institute. Retrieved April 21, 2023.
• Singh, N., Moneghetti, K. J., Christle, J. W., Hadley, D., Plews, D., & Froelicher, V. (2018). Heart Rate Variability: An Old Metric with New Meaning in the Era of using mHealth Technologies for Health and Exercise Training Guidance. Part One: Physiology and Methods. Arrhythmia & Electrophysiology Review, 7(3), 193-198. 
• Schauer, M., & Elbert, T. (2010). Dissociation following traumatic stress. Zeitschrift Für Psychologie / Journal of Psychology, 218(2), 109–127. 
• Streeter, C. C., Gerbarg, P. L., Saper, R. B., Ciraulo, D. A., & Brown, R. P. (2012). Effects of yoga on the autonomic nervous system, gamma-aminobutyric-acid, and allostasis in epilepsy, depression, and post-traumatic stress disorder. Medical Hypotheses, 78(5), 571–579. 
• Tindle, J., & Tadi, P. (2022, October 31). Neuroanatomy, parasympathetic nervous system. National Center for Biotechnology Information. Retrieved April 23, 2023.
• Tang, Y.-Y., Ma, Y., Fan, Y., Feng, H., Wang, J., Feng, S., Lu, Q., Hu, B., Lin, Y., Li, J., Zhang, Y., Wang, Y., Zhou, L., & Fan, M. (2009). Central and autonomic nervous system interaction is altered by short-term meditation. Proceedings of the National Academy of Sciences, 106(22), 8865–8870. 
• Zaccaro, A., Piarulli, A., Laurino, M., Garbella, E., Menicucci, D., Neri, B., & Gemignani, A. (2018). How Breath-Control Can Change Your Life: A Systematic Review on Psycho-Physiological Correlates of Slow Breathing. Frontiers in Human Neuroscience, 12. ​​