that part of the nervous system which represents the motor innervation of smooth muscle, cardiac muscle, and gland cells. It consists of two physiologically and anatomically distinct, mutually antagonistic components: the sympathetic and parasympathetic parts. In both of these parts the pathway of innervation consists of a synaptic sequence of two motor neurons, one of which lies in the spinal cord or brainstem as the preganglionic neuron, the thin but myelinated axon of which (preganglionic or B fiber) emerges with an outgoing spinal or cranial nerve and synapses with one or more of the postganglionic (or, more strictly, ganglionic) neurons composing the autonomic ganglia; the unmyelinated postganglionic fibers in turn innervate the smooth muscle, cardiac muscle, or gland cells. The preganglionic neurons of the sympathetic part lie in the intermediolateral cell column of the thoracic and upper two lumbar segments of the spinal gray matter; those of the parasympathetic part compose the visceral motor (visceral efferent) nuclei of the brainstem as well as the lateral column of the second to fourth sacral segments of the spinal cord. The ganglia of the sympathetic part are the paravertebral ganglia of the sympathetic trunk and the prevertebral or collateral ganglia; those of the parasympathetic part lie either near the organ to be innervated or as intramural ganglia within the organ itself except in the head, where there are four discrete parasympathetic ganglia (ciliary, otic, pterygopalatine, and submandibular). Impulse transmission from preganglionic to postganglionic neuron is mediated by acetylcholine in both the sympathetic and parasympathetic parts; transmission from the postganglionic fiber to the visceral effector tissues is classically said to be by acetylcholine in the parasympathetic part and by noradrenalin in the sympathetic part; recent evidence suggests the existence of further noncholinergic, nonadrenergic classes of postganglionic fibers. Syn: pars autonomica [NA], systema nervosum autonomicum [NA], autonomic part, involuntary nervous system, vegetative nervous system, visceral nervous system.