From HomoExcelsior

1. A muscle, a gland, or an organ capable of responding to a stimulus, especially a nerve impulse. 2. A nerve ending that carries impulses to a muscle, a gland, or an organ and activates muscle contraction or glandular secretion. 3. A small molecule that when bound to the allosteric site of an enzyme causes either a decrease or an increase in the activity of the enzyme. 4. A device used to produce a desired change in an object in response to input.

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