- autonomic nervous system
- the part of the peripheral nervous system responsible for the control of involuntary muscles (e.g. the heart, bladder, bowels) and hence those bodily functions that are not consciously directed, including regular beating of the heart, intestinal movements, sweating, salivation, etc. The autonomic system is subdivided into sympathetic and parasympathetic nervous systems. Sympathetic nerves lead from the middle section of the spinal cord and parasympathetic nerves from the brain and lower spinal cord. The heart, smooth muscles, and most glands receive fibres of both kinds: the interplay of sympathetic and parasympathetic reflex activity (the actions are often antagonistic) governs their working. For example, the parasympathetic system is responsible for slowing the heart rate and constricting the pupillary muscles; the sympathetic system increases the heart rate and dilates the pupil. Sympathetic nerve endings liberate noradrenaline as a neurotransmitter; parasympathetic nerve endings release acetylcholine.
Medical dictionary. 2011.