Stimulus (physiology)

In physiology, a stimulus is a detectable change in the internal or external environment. When a stimulus is applied to a sensory receptor, it elicits or influences a reflex via stimulus transduction. A stimulus is often the first component of a homeostatic control system. When a sensory nerve and a motor nerve communicate with each other, it is called a nerve stimulus.

In psychology, a stimulus is part of the stimulus-response relationship of behavioural learning theory.