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).
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).
The autonomic nervous system. Schematic representation of the autonomic nerves and effector organs based on chemical mediation of nerve impulses. Blue-Grey (blue-grey line), cholinergic; light blue (light blue dotted), adrenergic; dark blue (dark blue dotted line) visceral afferent; solid lines, preganglionic; broken lines, postganglionic. The rectangle at right shows the finer details of the ramifications of adrenergic fibers at any one segment of the spinal cord, the path of the visceral afferent nerves, the cholinergic nature of somatic motor nerves to skeletal muscle, and the presumed cholinergic nature of the vasodilator fibers in the dorsal roots of the spinal nerves. The asterisk (*) indicates that it is not known whether these vasodilator fibers are motor or sensory or where their cell bodies are situated. (Reproduced with permission from Westfall TC, Westfall DP. Neurotransmission: The Autonomic and Somatic Motor Nervous Systems. In: Brunton LL, Chabner BA, Knollmann BC, eds. Goodman & Gilman's: The Pharmacological Basis of Therapeutics, 12e New York, NY: McGraw-Hill; 2011.)
Endocrine glands are a primary effector of homeostatic control systems, which are regulated by the autonomic nervous system through efferent mechanisms that usually ...