While settling into the groove of one’s vinyasa sequence it feels as if our skeletal-muscular system is taking all the important stretches for us. Yet, the stretch we don’t quite feel may actually have the most profound effect on the course of our practice. Unlike our skeletal musculature, the smooth muscle of our bronchioles contains what are known as “stretch receptors.” When our respiratory rate decreases to roughly 6 cycles per minute (Pramanik et al.), slow wave signals stimulate these receptors, which in turn triggers the propagation of inhibitory postsynaptic potentials along sympathetic nerve fibers. The net effect is parasympathetic dominance, or in other words, relaxation of blood pressure and heart rate (Jerath et al).
An experiment devised by researchers at the University of Delaware looked at the link between bronchoconstriction and the coupling of stretch receptor (SAR) endings. To test whether coupling augments smooth muscle tone in the presence of deep breathing, researchers measured SAR spike time as a function of airway resistance. They found that both the rate and depth of breath contributed to an increase in reliable SAR spike time and a decrease in bronchial resistance. Next, the scientists wondered if the converse principle would hold true, that is, that an increase in bronchial tone causes an increase in SAR coupling. To test this prediction, test groups were given an M3 receptor agonist to increase bronchial tone prior to breath work. Investigators found that M3 agents exert their effect without influencing SAR spike time. Taken together, these findings suggest that deep breathing specifically decreases stretch receptor coupling events and therefore also decreases one’s bronchial tone (Chen et al.).
Pramanik T, Sharma HO, Mishra S, Mishra A, Prajapati R, Singh S. Immediate effect of slow pace bhastrika pranayama on blood pressure and heart rate. J Altern Complement Med, 2009 Mar; 15(3):293-5.
Jerath R, Edry JW, Barnes VA, Jerath V. Physiology of long pranayamic breathing: neural respiratory elements may provide a mechanism that explains how slow deep breathing shifts the autonomic nervous system. Med Hypotheses, 2006; 67(3):566-71.
Chen Y, Marchenko V, and Rogers RF. Pulmonary stretch receptor spike time precision increases with lung inflation amplitude and airway smooth muscle tension. J Neurophysiol, 2011 May; 105(5): 2590–2600.