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(1) In anatomy and physiology, a muscle (or group of muscles) that acts jointly and functions homogeneously with another muscle (or group of muscles) in the same movement.
Examples of synergists in mammals (including humans) are the external intercostal and intercartilagtnous muscles and the muscles of the diaphragm, which, by contracting simultaneously, ensure the act of inhalation. Muscles that act together in performing a particular movement may be antagonists in the execution of other movements. The phenomenon of synergism is ensured by the regulatory activity of the central nervous system.
(2) In pharmacology, one of several medicinal agents used jointly whose combined effect on the body exceeds the effect exerted by each separately. Direct synergists act on the same reactive systems of the body, such as the simultaneous influence of two adrenomimetic substances. Indirect synergists exert the same end effect but differ in their mechanisms of action. For example, adrenaline and atropine act along different pathways to produce dilatation of the pupils; when taken together, the overall effect of these substances on the pupil exceeds the effect of each separately.