10 - Autonomic nervous system

Abstract

Publisher Summary The autonomic nervous system consists of two major divisions, the sympathetic and the parasympathetic. The preganglionic sympathetic fibers and all the parasympathetic fibers are cholinergic, while most postganglionic sympathetic fibers except those innervating sweat glands and vasodilator fibers in muscle and skin are adrenergic. Non-adrenergic, non-cholinergic fibers in which a peptide is the putative transmitter also exist. Important physiological functions of the autonomic nervous system are control of heart rate and blood pressure that are regulated through the baroreflexes whose afferent limbs arise from the carotid sinus and aortic arch. The preganglionic fibres in the sympathetic nervous system are cholinergic. Vasomotor tone determines the peripheral resistance and is controlled almost entirely by the sympathetic nervous system. Adrenergic fibers are chiefly responsible for maintaining constrictor tone in muscle resistance beds and vasodilatation results from inhibition of the tonic vasoconstrictor fiber activity. The eccrine sweat glands, which occur over most of the body, are innervated by postganglionic sympathetic nerve fibers in which acetylcholine are neurotransmitter. Autonomic disorders occur in many peripheral neuropathies. The impairment of sweating on the extremities is very common as a result of the degeneration of the sympathetic efferent fibers in the peripheral nerve trunks. When small diameter myelinated and unmyelinated fibres in afferent and efferent nerves are involved by the disease processes, for example, diabetes and amyloid disease, there is impairment of baroreflexes and postural hypotension occurs. Autonomic disturbances occur in other peripheral neuropathies such as those due to porphyria, carcinoma, chronic renal failure, and vincristine.