Stimulus and response

The body keeps sensing changes around it and inside it, and responding to them. Organs and systems have to work together to do this — they are co-ordinated.

A change that the body can detect is a stimulus. It is detected by a receptor, and the action that follows is the response, carried out by an effector — a muscle or a gland.

Stimulus → Receptor → Co-ordinator → Effector → Response

An example

A ball flies towards you (stimulus); your eyes see it (receptor); the brain decides; your arm muscles (effector) move to catch it (response).