autonomic nervous system (ANS)

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Figure 1: Efferent nerve pathways from the brainstem and spinal cord. Shown on the right: somatic, to skeletal muscles. Shown on the left: autonomic. B brain stem, C cervical, T thoracic, L lumbar, S sacral segments of the spinal cord. (Red shaded regions are those with no autonomic outflow.)
Figure 2: The autonomic nervous system. Actions on the heart and on smooth muscle. Sympathetic actions on the left and parasympathetic actions on the right. Solid arrows: stimulation (contraction or secretion); broken arrows: inhibition.

autonomic nervous system (ANS)

the system of nerves that regulates body functions which have no direct voluntary control. Consists of motor (efferent) nerves that supply the heart, secretory glands and smooth muscle (e.g. in blood vessels, intestinal tract and airways). The two divisions of the ANS are the sympathetic nervous system and the parasympathetic nervous system. In some organs and tissues these have opposing effects, respectively promoting an increase or a decrease in activity, e.g. sympathetic nerves stimulate the heart but quieten the gut, parasympathetic vice versa. Other organs or tissues are supplied by only one of the divisions, e.g. most blood vessels only by sympathetic and digestive secretory glands only by parasympathetic. Preganglionic fibres originate within the central nervous system (CNS) and relay at ganglia outside the CNS; thence postganglionic fibres run to the relevant tissue or organ. visceral afferents carry information from the various sites which are subject to autonomic reflex control. Figure 1, Figure 2.