Of fight and flight

Excerpt

Development of the sympathetic nervous system is a critically important component of evolution and natural selection. Faced with a predator, early animals, including humans, could flee or perish. Survival required rapid delivery of oxygen to exercising muscles. The sympathetic nervous system is a key component of this process, working through cardiac and peripheral mechanisms.1 At low levels of exercise, cardiac output is increased primarily by withdrawal of parasympathetic tone, resulting in a rise in heart rate.2 This mechanism of parasympathetic withdrawal, while adequate with low levels of exercise, cannot meet the demands of strenuous exercise. As exercising muscles rapidly deplete their supply of oxygen, anaerobic metabolism replaces aerobic metabolism. Anaerobic metabolism generates lactate and other anaerobic metabolic products, which activate metabolic receptors in skeletal muscle. These metabolic receptors provide a strong stimulus for vasodilation, to augment blood flow and oxygen delivery to exercising skeletal muscle.3 As cardiac output begins to fail to fill the dilated peripheral vasculature, blood pressure falls and carotid and cardiopulmonary baroreceptors are stimulated. The sympathetic nervous system is activated.4 Such sympathetic activation in stress situations, first described as “fight-or-flight response” by Cannon nearly a century ago,5 was subsequently applied by Scholander nearly 50 years later as the “master switch of life”.6 …