Those organ systems of the animal body which mediate overt behavior. Injury to an effector system leads to loss or to subnormal execution of behavior patterns mediated by the system, conditions termed paralysis and paresis, respectively.
Overt behavior consists of either movement or secretion. Movement results from contraction of muscle. Secretion is a function of glands. Neither muscular contraction nor glandular secretion is autonomous but is regulated by an activating mechanism which may be either neural or humoral. In neurally activated systems the effector organ, whether muscle or gland, is supplied by nerve fibers originating from cell bodies situated in the central nervous system or in peripherally located aggregates of nerve cell bodies known as ganglia.
In other effector systems (humeromuscular and humeroglandular) the activating agent is normally a blood-borne chemical substance produced in an organ distant from the effector organ. For example, uterine smooth muscle is uninfluenced by the uterine nerve activity but contracts vigorously when the blood contains pitocin, a chemical substance elaborated by the posterior lobe of the hypophysis.
Finally, some effector systems are hybrid in the sense that both nerves and humors regulate their functions. The smooth muscle of arterioles contracts in response to either nerve stimulation or epinephrine. Secretion of hydrochloric acid by the gastric mucosa is increased by activation of the vagus nerve or by the presence in the blood of histamine. Effector systems with both neural and humoral regulation are never completely paralyzed by denervation but may be deficient in reaction patterns when the quick integrated activation provided by neural regulation is essential. See Nervous system (vertebrate)