Sympathetic vs. Parasympathetic Nervous System

March 15, 2021

Our nervous system is responsible for receiving sensory information from our environment,  processing it, and generating an appropriate motor response to that information. In other words, without our nervous system, we would not be able to quickly pull our hand away from a burning hot stove if we accidentally touched it or slam on the brakes when we get close to a car in front of us. Our nervous system is vastly complex, and we need it to function properly in order to go about our daily routines. It is broken up into two major parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS is further divided into the sympathetic and parasympathetic nervous systems, aka “fight or flight” or “rest and digest”. The figure below makes the divisions of the nervous system more easy to understand.

In this article, we will explore the differences between the sympathetic and parasympathetic nervous systems and how they contribute to our overall health and wellness.

 

THE SYMPATHETIC NERVOUS SYSTEM

The sympathetic nervous system is activated when we are under stress. When the sympathetic nervous system is activated, so is our “fight or flight” response. This response biologically prepares us to deal with danger or physical threats by releasing large quantities of the hormone epinephrine (better known to most as “adrenaline”) that cause increases in our heart rate, breathing rate, and blood pressure. Occasional activation of the sympathetic nervous system is beneficial in times of acute stress, but consistent activation of the sympathetic nervous system can be detrimental to our overall health and wellbeing. In other words, it is not productive for us to be in “fight or flight” mode all the time. We need to be able to recover from stressful situations and shift our bodies into “rest and digest” mode so that we can relax, recover and respond quickly and adequately. In fact, we should be living in a parasympathetic state most of the time, however, given our modern and Westernized lifestyles that we live today, this is almost never the case. 

Individuals who are stressed out frequently use their sympathetic nervous systems much more than individuals who are not. As a result, those who are under constant stress release larger and more frequent amounts of adrenaline and cortisol into their bodies throughout the day. 

Since adrenaline is responsible for raising our heart rates, breathing rates, and blood pressure, increased adrenaline release can cause negative health outcomes such as rapid heartbeat, high blood pressure (hypertension), and heart palpitations. Releasing too much adrenaline is also responsible for increased anxiety and irritability, poor sleep and digestion, constipation, and a suppressed immune system. Again, this is useful when we are running from a bear or preventing our hand from burning on a hot stove, but not for daily life activities. 

Another hormone that is released when the sympathetic nervous system is activated is called cortisol which is responsible for increased heart rate and blood pressure as well as altered glucose levels. Large quantities of cortisol may result in high blood glucose levels (hyperglycemia) which can eventually lead to weight gain and/or difficulty losing weight, and even type 2 diabetes. Clearly, stress doesn’t just take a toll on our mental health, our physical health suffers greatly as well.

As stated previously, the sympathetic nervous system is activated when we are stressed. Work and school are obviously major reasons as to why we can be stressed, but there are many other factors that can contribute to sympathetic dominance as well. These include:

  • Poor sleep 
  • Toxic relationships
  • Air pollutants
  • Environmental toxins
  • Bacterial/viral infections
  • Processed diets
  • High caffeine intake or stimulant use
  • Endocrine disruptors 
  • Overtraining
  • Low calorie diets
  • Certain medications and more! 

Additionally, if you frequently engage in high intensity, heart-pumping, fast-paced workouts, you can expect your body to be in a constant state of “fight or flight,” especially if you are not prioritizing recovery. 

THE PARASYMPATHETIC NERVOUS SYSTEM

Opposite of the sympathetic nervous system which triggers our “fight or flight” response when we are stressed, the parasympathetic nervous system (PSNS) triggers our “rest and digest” response when we are calm, relaxed, and resting. Basically, the activation of the parasympathetic nervous system undoes the work of the sympathetic nervous system after a stressful situation (Furness, 2009). When the parasympathetic system is activated, it lowers our heart rate and blood pressure while increasing the efficiency of our digestive system (bye bye bloating!). It also encourages the body to repair and rebuild new muscle tissue after workouts and allows for better blood circulation due to the dilation of blood vessels that promote blood flow. 

Activities that trigger the activation of the parasympathetic nervous system include: 

  • Aerobic activities such as walking or yoga
  • Meditation and deep breathing
  • Talk therapy
  • Journaling or drawing
  • Sexual intercourse
  • Getting a massage 
  • Listening to calming music

SUMMARY

The sympathetic nervous system triggers a “fight or flight” response when we are exposed to stressors and makes us easily excitable and anxious. This energy is productive in the short term, but in the long term, it can negatively impact our health. On the other hand, the parasympathetic nervous system triggers our “rest and digest” response when we are calm and resting. All in all, too much of a response from either system is not beneficial. A healthy nervous system requires an appropriate balance between activations of both, the sympathetic and parasympathetic nervous systems.

By Sara De Luca, RD, CDN, CPT and Simone Gmuca, Dietetic Intern

REFERENCES

Furness, J. (2008, November 05). Parasympathetic nervous system. Retrieved February 04, 2021, from https://www.sciencedirect.com/science/article/pii/B9780080450469019902

Sympathetic nervous system. (n.d.). Retrieved February 04, 2021, from https://www.britannica.com/science/sympathetic-nervous-system

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