This blog post of an exclusive excerpt from my new book, The Physiology of Yoga.

The autonomic nervous system is the division of the peripheral nervous system that regulates involuntary processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. This system has three divisions: the sympathetic nervous system (our fight-or-flight response); the parasympathetic nervous system (our rest-and-digest response); and the enteric nervous system (our second brain).

The sympathetic nervous system (SNS) works with the endocrine system to trigger the fight-or-flight response, a term coined by Cannon (1915) to describe an animal’s immediate response to danger. This is often referred to as the stress response, and while the word stress tends to have a negative connotation, a degree of stress is vital for our everyday functioning and survival. Without this system, it would be even harder to get out of bed in the morning, let alone run to catch a bus, or step out of the way of a runner on the sidewalk. The SNS is composed of many pathways that innervate nearly every living tissue in the body and is triggered, via the amygdalae, whenever we experience a situation that our brain perceives as being threatening. Danger, pain, upsetting feelings, and low blood sugar all activate the SNS. Muscular contractions are sympathetic in origin and therefore even a large component of our yoga asana practice is linked with the SNS.

The SNS primarily regulates blood vessels. An increase in sympathetic signals leads to vasodilation (widening) of the coronary vessels (vessels that supply the cardiac muscle of the heart) and the vessels that supply the skeletal muscles and external genitalia. All other vessels in the body will vasoconstrict (narrow). Sympathetic activation increases our heart rate, increases the contractile force of the heart, raises our blood pressure, decreases motility of the large intestine, and causes pupillary dilation and perspiration. The SNS also directly stimulates the adrenal glands to produce the hormone and neurotransmitter epinephrine (adrenaline). All these actions prepare the body for immediate physical action. The SNS is constantly active even in nonstressful situations; for example, it is active during the normal respiratory cycle when sympathetic activation during inspiration dilates the airways, allowing for an appropriate inflow of air. The SNS also works to regulate your blood pressure every time you stand up.

The endocrine system is also heavily involved in the stress response. A diverse collection of neurons from multiple brain regions including the brain stem and amygdalae innervate specific neurons in the hypothalamus, which synthesizes and secretes a hormone called corticotropin-releasing factor. Corticotropin-releasing factor targets the neighboring pituitary gland, often referred to as the master gland, which, in turn, releases a substance called adrenocorticotropic hormone. Adrenocorticotropic hormone consequently acts on the adrenal gland and triggers the release of cortisol. This release causes the acceleration of heart and lung action, constriction of blood vessels in many parts of the body, metabolism of fat and glucose for muscular action, dilation of the blood vessels supplying our major muscle groups, relaxation of the bladder, inhibition of erection, loss of hearing, loss of peripheral vision, and shaking. The systems that are not required during the stress response become temporarily suppressed, including the immune, digestive, and reproductive systems.

The actions of the endocrine system here are normally tightly regulated to ensure that the body can respond quickly to stressful events and return to a normal state just as rapidly. In the brain, cortisol participates in a negative feedback loop, meaning that it targets the hypothalamus to control its own production. This full cycle is known as the hypothalamic-pituitary-adrenal (HPA) axis.

Barlow (2002) suggested that a freeze response can occur in some threatening situations. It is understood that freezing can be activated at intermediate levels of threat, when fleeing or aggressive responses are likely to be ineffective. In the context of predatory attack, some animals will freeze or play dead; this includes motor and vocal inhibition with an abrupt initiation and cessation (Schmidt et al. 2008). The freezing response is again initiated in the amygdalae (Applegate et al. 1983) and both the SNS and the parasympathetic nervous systems become activated (Iwata, Chida, and LeDoux 1987). The physiological parameters will vary, depending on which system is dominant at a certain point in time. Freezing is therefore not a passive state but can be thought of as a parasympathetic brake on the motor system—or attentive immobility.

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Reference:

Cannon, W. 1915. Bodily Changes in Pain, Hunger, Fear and Rage: An Account of Recent Researches Into the Function of Emotional Excitement. New York: Appleton and Company.