Exercise and the autonomic nervous system
The autonomic nervous system (ANA) is a part of our peripheral nervous system that controls our involuntary physiological responses. There are two components of the ANS – the sympathetic and parasympathetic nervous systems. The sympathetic system is involved in our “fight or flight” response and gets us ready for action, whereas the parasympathetic system is involved in resting and digesting. We need both systems in balance to keep us healthy and functioning properly.
The longest nerve of the ANS is known as the vagus nerve and is our 10th cranial nerve https://pubmed.ncbi.nlm.nih.gov/27010234/). It is called a mixed nerved because of its bidirectional involvement between regions of the brain and components of the body such as the heart, lungs, and gut. As an integral component of the ANS, the vagus nerve serves to regulate bodily functions that are not consciously controlled such as the beat of the heart, breathing, and digestive processes.
At rest, there is an equilibrium between the sympathetic and parasympathetic nervous systems. When we experience a stressful event, however, the sympathetic nervous system becomes activated and the system is thrown out of balance. Normally, the stress ends, balance is restored, and all is well again. However, in situations of constant stress where the balance is disrupted for an extended period of time, certain stress-related gastrointestinal diseases (such as functional dyspepsia, irritable bowel syndrome, Crohn’s disease, or ulcerative colitis) may ensue.
When it comes to these stress-related disorders, a major goal for physicians (and patients!) is the restoration of the balance between the parasympathetic and sympathetic nervous system and regaining normal functioning of the vagus nerve. Physical activity may just be one of the ways to do this!
Heart rate variability (HRV) or the variation in the time between heart beats is a marker of ANS and vagal nerve functioning. Having a high HRV means that you have a heart that is “responsive to demands”, a function that is “believed to bestow a survival advantage” (https://pubmed.ncbi.nlm.nih.gov/20548976/). HRV decreases as we age and a decreased HRV is associated with poorer cardiovascular health and outcomes (Routledge et al., 2010).
One study investigated the differences in HRV between endurance trained athletes and sedentary subjects (https://pubmed.ncbi.nlm.nih.gov/1423437). Compared to the sedentary individuals, athletes displayed a significantly elevated HRV – a marker of improved vagal nerve tone. More recently, a group of scientists in France investigated whether 14 weeks of high-intensity interval training 4 days a week in older men could lead to improvements in HRV (https://pubmed.ncbi.nlm.nih.gov/15834767/). 11 healthy, active older men (average age of 73.5 +/- 4.2 years) participated in this study. The training protocol entailed 45 minutes of stationary cycling sessions that consisted of 9 repeated and consecutive 5-minute bouts of cycling, 4 minutes at 65% of maximum heart rate and 1 minute of 85% of maximum heart rate. Similar to the cross-sectional study, exercise training enhanced measures of HRV. This study indicates that long-term exercise can be used as a way to combat the age-related impairments seen in autonomic nervous system functioning.
Excitingly, we at the Suzuki Lab are currently conducting studies to look at both the relationship between cardiopulmonary fitness and HRV as well as the long-term effects of exercise on HRV in previously low fit and middle-aged individuals. To date, the research indicates that exercise helps support a healthy autonomic nervous system, providing balance between the sympathetic and parasympathetic components of ANS. These effects may serve to decrease the risk of stress-related disorders. Future research is needed to identify the best type, duration, and intensity of exercise that will produce optimal improvements in HRV.
References:
Bonaz, B., Sinniger, V., & Pellissier, S. (2016). Vagal tone: effects on sensitivity, motility, and inflammation. Neurogastroenterology & Motility, 28(4), 455-462.
Dixon, E. M., Kamath, M. V., McCartney, N., & Fallen, E. L. (1992). Neural regulation of heart rate variability in endurance athletes and sedentary controls. Cardiovascular research, 26(7), 713-719.
Pichot, V., Roche, F., Denis, C., Garet, M., Duverney, D., Costes, F., & Barthélémy, J. C. (2005). Interval training in elderly men increases both heart rate variability and baroreflex activity. Clinical Autonomic Research, 15(2), 107-115.
Routledge, F. S., Campbell, T. S., McFetridge-Durdle, J. A., & Bacon, S. L. (2010). Improvements in heart rate variability with exercise therapy. Canadian Journal of Cardiology, 26(6), 303-312.
