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The central nervous system includes the brain and the spinal cord; the remainder belongs to the peripheral nervous system. The peripheral nervous system consists of two components: the somatic nervous system, which predominantly controls skeletal muscle (e.g., the “outer world”), and the autonomic nervous system, which predominantly controls smooth muscle, glands, and cardiac muscle (e.g., the “inner world”).1 The autonomic nervous system mainly includes sympathetic division (the so-called “fight-or-flight” response) and parasympathetic division (the “rest-and-digest” response).
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This chapter provides a brief overview of the autonomic nervous system for the rehabilitation physician. This system plays a crucial role in adjusting the physiological state of the body to maintain circulatory homeostasis, or the stability of the “inner world.”2 The autonomic nervous system not only carries out the command of the brain, but also functions as a reflex circuit by using the sensory feedback of the organs to precisely adapt its output.3 The brain sets the balance of sympathetic and parasympathetic division based on the biological, physiological, or psychological situation, causing changes in the emphasis of the autonomic nervous system output (i.e., sympathetic dominant or parasympathetic dominant).3 If the balance is disturbed, either by behavior or by disease of the organ, this may lead to pathological changes that could affect the functioning of the whole individual.3 In general, excessive sympathetic activation is associated with specific medical conditions, such as hypertension, heart failure, myocardial infarction, sleep apnea, metabolic syndrome, kidney disease, polycystic ovary syndrome, gestational hypertension, and preeclampsia. In some situations, sympathetic inhibition and/or parasympathetic activation may be the underlying mechanisms, for example, neurally mediated syncope, postprandial hypotension, and multiple system atrophy (Fig. 7–1).
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Endocrine glands are a primary effector of homeostatic control systems, which are regulated by the autonomic nervous system through efferent mechanisms that usually consist ...