Stimulus

Receptors are cells that detect a stimulus — a change in the environment. A coordination centre receives and processes information from the receptors; examples include the brain, spinal cord and pancreas. Effectors are muscles or glands that bring about a response which helps restore optimum conditions.

A change that can be detected is a stimulus—for example, light reaching the eye or heat affecting the skin.

A stimulus is a detectable change in the internal or external environment. A sensory receptor is a specialised cell or group of cells that detects a particular type of stimulus. Examples include light receptors in the retina and receptors in the skin that detect pressure, touch or temperature.

Light intensity may alter, the skin may encounter pressure or heat, and the chemical composition of food may be detected. Such a detectable change is a stimulus. Detecting and responding to stimuli helps an organism avoid harm and adjust its activities to changing conditions.