top of page

The Autonomic Nervous System - General Overview

Updated: Sep 18, 2020

Hey Readers! Terms such as "Autonomic imbalance," parasympathetic state," "and sympathetic overdrive" are used in the health and wellness fields quite a lot these days, but do you really know what these concepts refer to? For this week’s blog post, I will take a semi-deep dive into the Autonomic Nervous System (ANS).

Obviously a discussion on the nervous system can get incredibly complex. So, for the purposes of this post, I will keep the information to the basics as best I can. In this post, I will review what the ANS is, the two divisions of the ANS and what they do, and ANS dysfunction/imbalance. Stay tuned for part 2 of this series, where I will discuss how yoga and breathing/meditation practices help to re-balance the ANS.

The Autonomic Nervous System (ANS) - What Is It?

The human nervous system consists of the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system, which includes all of the nerves outside of the brain and spinal cord. The peripheral nervous system is further broken down into the somatic nervous system and the autonomic nervous system. The somatic nervous system innervates (i.e. has neural connections with) skeletal muscles, controlling movement therein. The autonomic nervous system (ANS) innervates your digestive organs, reproductive and waste removal systems, cardiac muscle (i.e. in the heart), smooth muscle (e.g. in the lower esophagus, blood vessel walls, etc.), and glands. You consciously use your somatic nervous system all the time to move your body through space. In contrast, your ANS operates mostly on a subconscious, or involuntary, level. In response to changing conditions (within the body or outside of it), the ANS shunts blood to “needy” areas, speeding or slowing the heart rate, adjusting blood pressure and body temperature, and increasing or decreasing stomach secretions.

The ANS is further divided into the sympathetic nervous system (SNS) and parasympathetic nervous system (PNS). The SNS and PNS generally innervate the same organs/tissue but cause essentially opposite effects. In a "healthy" body (meaning no physical, mental, cognitive or emotional disease), this dual innervation helps to maintain homeostasis in the body since each Autonomic division counterbalances the other one. Let’s discuss the SNS and PNS in more detail.

What Does the Sympathetic Nervous System Do?

The sympathetic nervous system (SNS) is often referred to as the “fight, flight, or freeze” system. In general, the SNS is mobilized, or activated, during extreme situations – for example, during exercise, excitement, or emergencies. SNS activity is evident when we are excited, frightened, or in pain. The function of the SNS is to stimulate an appropriate response to some threat, whether that threat is life-threatening (e.g. you are being chased by a hungry lion) or not (e.g. you are exercising or dealing with a stressful situation at work). When the SNS is mobilized to respond, blood is shunted to active skeletal muscles and the vigorously working heart – this increases muscle tone and heart rate. The bronchioles in the lungs (i.e. the little tubes that transport air in/out of your lungs) dilate, increasing breathing rate. Your pupils dilate so that you can see better amidst the danger or stressor. The liver releases more glucose into the blood to accommodate the increased energy needs in the body. Your adrenal glands release cortisol, which helps to regulate blood pressure and metabolism of fats, carbohydrates, and proteins. At the same time, nonessential activities, such as digestion and elimination, are halted. When the SNS is activated, it typically brings about feelings of alertness, high energy, and heightened perception.

Activation of the SNS is super important when we are dealing with legit stress (e.g. a car accident or dangerous situation). However, chronic activation of the SNS is NOT good for the human body. In our busy “go go go” culture, we tend to spend a lot of our time in a heightened sympathetic state, as we are “stressed out” and insanely busy much of the time. We often overschedule ourselves and our children, and we are constantly bombarded with excitatory stimuli (e.g. children, alarm clocks, social media, sports, tutors, music lessons, financial stress, etc...), leaving little time during the day for simple, quiet down-time. Stress is associated with a broad range of health and social problems and has a negative effect on pretty much every bodily function (but that is a topic for a future blog post!).

What does the Parasympathetic Nervous System Do?

The PNS is often referred to as the “rest, digest, and recovery” system because it is primarily responsible for bodily maintenance and day-to-day functioning of your internal organs. I liken the PNS to getting maintenance done on your car. When you take time to get the oil changed, tires rotated, and the air filters replaced in your car, your vehicle simply runs much better. If you neglect to provide basic maintenance for your car, you will likely not get as many miles out of it, and it will likely have other issues that require time and money to fix. Your body is very similar to this. Your PNS essentially provides basic maintenance to your body, so your body can get the maximum amount of "mileage" possible. It does so by directing energy to vital activities that keep the body running smoothly and efficiently. For example, PNS stimulation increases the mobility of the digestive tract, promoting digestion. PNS activity also increases smooth muscle contraction in the urinary and excretory systems, leading to elimination of urine and feces. Blood pressure, heart rate, and respiratory rate fall to low-normal levels when the PNS is activated. The pupils constrict in the eyes to protect the retinas from excessive light. When the PNS is activated, it typically brings about feelings of calm, ease, and satisfaction.

In order to activate our PNS, we must find time during the day to reduce stress so that our SNS can "turn off." The problem in our modern society is that we rarely give ourselves quiet times of rest during the day. For example, think of the last time you had to wait in line for something (maybe at Starbucks or the grocery store). Did you simply wait quietly, just breathing, or did you get on social media or flip through radio stations, essentially keeping yourself busy while you waited? I will be honest - I am definitely guilty of keeping my mind occupied while I am waiting for something. Often, when I am waiting in line for something or waiting for my next client, I get on my phone and scroll through social media or play a game (like Solitaire or Dr. Mario). What if, instead of bombarding our sensory system with so many stimuli during the day, we used those short periods of waiting to our advantage by simply breathing quietly? These moments of quiet are so important to your entire body because they give your PNS a chance to perform its maintenance functions, allowing your whole body to feel, move, and breath better.

ANS Dysfunction/Imbalance

Normally, both ANS divisions are partially active, creating balance in the body and mind. In “typical” individuals, either the SNS or the PNS takes over depending on the situation, and then the balance between the two systems is restored when the situation resolves. However, in some individuals, the balance is not restored, causing imbalance, or dysfunction, in the ANS. ANS dysfunction can have far-reaching effects and even threaten life itself.

Physiological and psychological stress can disrupt the balance in your ANS. Repeated, or continuous, exposure to episodes of stress can alter ANS balance, leading to reduced parasympathetic activity and shifting to sympathetic predominance. If SNS predominance is allowed to continue over a period of time without intervention, it can lead to a wide range of health issues (physically and mentally). Dysfunction in the ANS has been documented in individuals with: autism, anxiety, depression, post-traumatic stress disorder (PTSD), emotional regulation issues, and panic disorder. Additionally, prolonged ANS imbalance has also been linked to: hypertension (i.e. high blood pressure), heart issues, weight gain, digestive issues, immunodeficiency, headaches, trouble sleeping, reductions in joint mobility, and chronic pain. Some of these issues are discussed below.

The ANS plays a pivotal role in controlling emotional regulation (i.e. the process of monitoring and adjusting emotional responses to environmental stimuli). In a healthy individual, emotional responses are modulated by the PNS, which down-regulates the activity of the SNS. This down-regulation from the PNS results in less impulsive reactivity to emotionally charged situations, giving the individual time to respond to a situation maturely, gracefully, and with compassion. If a person has an overactive SNS, he/she will likely react to situations very impulsively, often causing the person to feel regret, shame, and embarrassment when the situation is over. Think of the last time you felt stressed or overwhelmed (e.g. getting your kids ready for school, dealing with your finances, etc.). Imagine how you felt in your body in that time of stress - probably tense, tight, hot, short of breath, irritated, and possibly more. Now imagine a novel stressor presenting itself to you (e.g. your pet vomits everywhere, or you realize you forgot to account for a big bill in this month's paycheck, etc.). How calm and regulated is your response? Probably not very calm at all. In fact, you probably "blew up" and lost control because your emotional reaction was being guided by your SNS and not your PNS. If your SNS is chronically activated, you will likely respond to stressful situations impulsively and with very little logical analysis of the situation.

Autonomic imbalance with sympathetic overactivity can lead to hypertension. Hypertension (i.e. increased blood pressure) can result from an overactive SNS response promoted by chronic high levels of stress. Hypertension is a serious issue because it increases the workload of the heart, and this can cause heart disease. Hypertension also increases the “wear-and-tear” on artery walls, potentially causing an aneurysm. An aneurysm is an abnormal bulge, or ballooning of the wall of a blood vessel. Aneurysms can rupture which can lead to permanent complications, or even death.

Sympathetic overactivity can also lead to increased levels of cortisol in the blood – and OMG, there are a TON of negative consequences to having chronically elevated cortisol. After a stressful situation, cortisol levels in your blood should lower back to baseline levels. However, for many of us who live in a "sympathetic dominant state," our cortisol levels remain elevated because our body still "thinks" we need to be in "fight, flight, or freeze" mode. Chronically elevated cortisol can lead to anxiety and depression, headaches, heart disease, memory and concentration issues, digestion problems, immune system deficits, trouble sleeping, and weight gain.

Overactivation of the SNS can also affect the mobility of your joints (see blog post about joint mobility for more info). If you have not read that blog post, I will summarize briefly here (but I do recommend you use the link above to go back and read that blog post). Your SNS can increase the tone, or tension, in your muscles at rest. This is because the SNS is our arousal system, our "on-switch," and the job of the SNS is to prepare your muscles to either run, fight, or freeze during stressful situations. The problem with chronic SNS activation is that your muscles can become shortened over time and the persistent contraction can lead to trigger points within the muscle tissue. The muscle shortening and development of trigger points can reduce the range of motion (ROM) in various joints in your body. And this reduced ROM can ultimately lead to areas of pain and the potential for injury (again, see joint mobility blog for more info).

Chronic pain, such as in Fibromyalgia or arthritis, can lead to overactivation of the SNS, and underactivation of the PNS, because the pain makes your nervous system "think" you are in danger. Thus, those with chronic pain disorders are at a higher risk for ANS dysfunction and all of the issues discussed in this blog post. Stay tuned for a future blog series that will discuss yoga's effect on chronic pain (Hint: yoga helps to reduce chronic pain).

Yoga and meditation help to rebalance activity in the ANS, by decreasing SNS activity and increasing PNS activity, and by increasing the ability to shift in/out of SNS/PNS states flexibly. This creates an environment for healthy cells to thrive in our body (Part 2 of this blog post will take a deeper look at how yoga and meditation can help restore balance to our ANS, so stay tuned).


The ANS is responsible for mediating your responses to various situations that you encounter during the day. Our bodies perform best when we have a balance between both divisions of the ANS – the SNS and the PNS. We truly require both of these systems to maintain optimal performance in our body and mind. It is equally important to be able to activate a sympathetic response or parasympathetic response, based on your current environment. Obviously if you are being chased by a grizzly bear, you NEED your SNS to mobilize so you can try to escape the danger. Also, you obviously need down-time to allow your PNS to take care of your body and perform "maintenance" functions. The problem is that many of us have an ANS imbalance, typically with SNS overactivity, due to our stressful and crazy, busy schedules. This imbalance can lead to a large number of health issues in both your body and mind. It is super important to have at least a basic knowledge of these two systems in your body so that you can take care of yourself in the best possible way. Part 2 of this series will explore what yoga and meditation practices actually do to the ANS, based on current scientific research.

As always, the information presented in this blog post is derived from my own study of neuroscience and human movement. If you have specific questions about your nervous system, please consult with your physician. If you are interested in private yoga sessions with me, Jackie, you can book services on my website ("Book Online" from the menu at the top of the page), or you can email me at for more information about my services. Also, please subscribe to my website so you can receive my weekly newsletters (scroll to the bottom of the page where you can submit your email address). This will help keep you "in-the-know" about my latest blog releases and other helpful yoga and wellness information. Special thanks to my amazing husband for doing the artwork in the two sketches in this post. Thanks for reading!

~Namaste, Jackie Allen, M.S., M.Ed., CCC-SLP, RYT-200, RCYT


Fran, J. et al. (2020). Yoga in school sports improves functioning of autonomic nervous system in young adults: A non-randomized controlled pilot study. PLOS One. 1 – 17.

Kim, S.H. et al. (2013). Mind-body Practices for Posttraumatic Stress Disorder. Journal of Investigative Medicine. 61(5): 827 – 834.

Kuppusamy, M. et al. (2020). Effects of yoga breathing practice on heart rate variability in healthy adolescents: a randomized controlled trial. Integrative Medicine Research. 9: 28 – 32.

Marieb, E.N. (2004). Human Anatomy & Physiology, 6th Edition. Pearson.

Miller, J. (2014). The Roll Model: A Step-by-step Guide to Erase Pain, Improve Mobility, and Live Better in Your Body. Victory Belt Publishing, Inc.

Nagendra, H. et al. (2015). Cognitive Behavior Evaluation Based on Physiological Parameters among Young Healthy Subjects with Yoga as Intervention. Computational and Mathematical Methods in Medicine. 2015: 1 – 13.

Papp, M.E., et al. (2013). Increased heart rate variability but no effect on blood pressure from 8 weeks of hatha yoga – a pilot study. BMC Research Notes. 6(59): 1 – 9.

Telles, S., et al. (2014). Blood Pressure and Heart Variability during Yoga-Based Alternate Nostril Breathing Practice and Breath Awareness. Medical Science Monitor Basic Research. 20: 184 – 193.

Telles, S., et al. (2016). Heart rate variability in chronic low back pain patients randomized to yoga or standard care. BioMed Central Complementary and Alternative Medicine. 16: 1 – 7.

Tyagi, A. and Cohen, M. (2016). Yoga and heart rate variability: A comprehensive review of the literature. International Journal of Yoga. 9(2): 97 – 113.

Vinay, A.V., et al. (2016). Impact of short-term practice of yoga on heart rate variability. International Journal of Yoga. 9(1): 62 – 66.

Wadden, K.P. et al. (2018). Yoga Practioners Uniquely Activate the Superior Parietal Lobule and Supramarginal Gyrus During Emotional Regulation. Frontiers in Integrative Neuroscience. 12(60): 1 – 11.

62 views0 comments

Recent Posts

See All


bottom of page