n.1. an organ, such as a muscle or a gland, that responds to neural stimulation by producing a particular physical response or initiating a specific physiological event. 2. a motor nerve ending that triggers activity in tissue cells, such as causing a muscle to contract or a gland to secrete. 3. in motor control, the part of the body that interacts with the environment during an action (e.g., the hand during a reaching movement).