2.79 understand that a coordinated response requires a stimulus, a receptor and an effector

A stimulus is a thing or event that provokes a specific reaction. For example, if you touch a hot object and you react (usually by reflex), the hot object is a stimulus.

Receptors detect stimuli and effectors produce a response.

Receptors are a group of cells that detect an external stimuli, they are situated in the sense organs - the sense organs are the eyes (sight), the nose (smell), the ears (sound), the tongue (taste) and the skin (touch).

Effectors, for example muscle cells and cells found in glands, are cells that coordinate a response to the stimuli. They can react in different ways depending on the stimuli, for example, glands can secrete hormones whereas muscles can contract. They are only found in muscles and glands.

NOTE: receptors and effectors communicate, this is via the hormonal system or nervous system (sometimes it can be both)