ANATOMY

Detailed explanation-1: -In physiology, a stimulus is a detectable change in the physical or chemical structure of an organism’s internal or external environment. The ability of an organism or organ to detect external stimuli, so that an appropriate reaction can be made, is called sensitivity (excitability).

Detailed explanation-2: -nerves and nervous system …in the environment is the stimulus; the reaction of the organism to it is the response.

Detailed explanation-3: -External stimuli are factors outside of the body that are taken in by receptors and will lead to a response. Examples of external stimuli include temperature changes, vision, hearing, tasting, smelling, and even sunlight for plants.