There probably are no purely autonomic or somatic centers of integration; extensive overlap occurs; somatic responses always are accompanied by visceral responses, and vice versa. Auto-nomic reflexes can be elicited at the level of the spinal cord and are manifested by sweating, blood pressure alterations, vasomotor responses to temperature changes, and reflex emptying of the urinary bladder, rectum, and seminal vesicles. The hypothalamus and the STN are principal loci of integration of ANS functions, including regulation of body temperature, water balance, carbohydrate and fat metabolism, blood pressure, emotions, sleep, respiration, and reproduction. Signals are received through ascending spinobulbar pathways, the limbic system, neostriatum, cortex, and to a lesser extent other higher brain centers.
The CNS can produce a wide range of patterned autonomic and somatic responses. Highly integrated patterns of response generally are organized at the hypothalamic level and involve autonomic, endocrine, and behavioral components. More limited patterned responses are organized at other levels of basal forebrain, brainstem, and spinal cord.
DIVISIONS OF THE PERIPHERAL AUTONOMIC SYSTEM: EFFERENT NERVES
On the efferent side, the ANS consists of two large divisions: (1) the sympathetic or thoracolumbar, and, (2) the parasympathetic or craniosacral (see Figure 6-1).
The neurotransmitter of all preganglionic autonomic fibers, all postganglionic parasympathetic fibers, and a few postganglionic sympathetic fibers is acetylcholine (ACh). Adrenergic fibers comprise the majority of the postganglionic sympathetic fibers; here, the transmitter is norepinephrine (NE, noradrenaline). The terms cholinergic and adrenergic are used to describe neurons that release ACh or NE, respectively.
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