During our day-to-day lives we detect changes in the environment and react appropriately. An external stimulus is detected by one or more neurons, which sends the sensory information to the central nervous system, where it is processed. If a motor response is initiated, it usually involves a series of action potentials which produce a muscle contraction and a movement of one or more parts of the body. A simple reflex is perhaps the easiest of this type of stimulus-response reaction. A loud sound or something flying at your eye makes you blink, while a tap on the tendon under the knee cap produces the knee-jerk (or myotactic) reflex.