In physiology, a stimulus (pl. stimuli) is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. 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 neuron and a motor neuron communicate with each other, it is called a nerve stimulus.