When you think of the autonomic nervous system, the first point is not to confuse the terms automatic and autonomic. We can breathe automatically courtesy of the somatic nervous system, but the word autonomic is derived from "autonomy," the quality of being independent. In the context of the two great divisions of the nervous system, the autonomic nervous system is largely independent of the somatic system; it consists of a vast auxiliary network of neurons that controls viscera, blood vessels, and glands throughout the body. It is not, however, completely autonomous, because it interacts with the somatic nervous system—it both feeds sensory information from within the body into the somatic systems of the brain and spinal cord (in this case our main concern is the respiratory centers), and is affected by the somatic motor systems in return.
We constantly depend on smooth interactions between the somatic and autonomic nervous systems. You race around the block using your skeletal muscles, which are controlled by the somatic nervous system, but you would not get far unless your autonomic nervous system sped up your heart, stimulated the release of glucose from your liver, and shunted blood from the skin to the skeletal muscles. And if, instead of running around the block, you sit down and read a book after dinner, you flip the pages using your skeletal muscles and depend on the unconscious operation of your autonomic nervous system to digest your meal. Respiration, as it happens, is the foremost function in the body in which signals from internal organs have a constant and continuing effect on somatic function, in this case the rate and depth of breathing, twenty-four hours a day.
If we look at an overview of how the autonomic nervous system operates, controlling autonomic influences from the central nervous system (the brain and spinal cord) are relayed to their visceral targets by two systems of autonomic motor neurons: sympathetic and parasympathetic. The sympathetic nervous system prepares the body for emergencies ("fight or flight") and the parasympathetic nervous system maintains the supportive functions of the internal organs. Between them by definition, these two systems execute the autonomic motor commands from the brain and spinal cord. More of these interactions will be discussed in chapter 10, in which we'll be concerned with Lhe importance of the autonomic nervous system in relaxation.
yo ANATOMY Of HATHA YOGA
Here our concern is limited mainly to breathing, and the first thing to note is that the most important autonomic relationship involving the control of respiration is sensory. This does not mean sensory in regard to something you can feel; it refers to influences from sensory receptors that have an impact on breathing. Specifically, the sensory limb of the autonomic nervous system carries information on oxygen and carbon dioxide levels in the blood and cerebrospinal fluid to the respiratoiy control centers in the brain stem. You would see the important respiratory linkage between the autonomic and somatic systems in operation if you were suddenly rocketed from sea level to the top of Alaska's Mount Denali. You would immediately begin to breathe faster because your somatic respiratory control centers receive autonomic sensory signals that your blood is not getting enough oxygen, not because you make a conscious somatic decision that you had better do something to get more air.
There are also purely autonomic mechanisms that affect breathing in other ways. The most obvious example is familiar to those who suffer from asthma, or from chronic obstructive pulmonary disease (COPD) combined with bronchitis, and that is the difficulty of moving air through constricted airways. It is not very helpful to have healthy skeletal muscles of respira tion if the airways are so constricted that they do not permit the passage o' air. Although this is a complex and multifaceted problem, the autonomic nervous system involvement appears to be straightforward. In quiet time when there is less need for air, the parasympathetic nervous system mildh constricts the smooth muscle that surrounds the airways, especially thi smaller bronchioles, and thereby impedes the flow of air to and from th< alveoli. But in times of emergency or increased physical activity, th sympathetic nervous system opens the airways and allows air to flow mor< easily. Those who have chronic respiratory diseases have an acute awari ness of how difficult it can be to medicate and regulate this system.
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