Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses.
Receptors are often located in the sense organs, such as the ear, eye and skin. (See table to the right)
When a receptor is stimulated, it sends a signal along the nerve cells (called neurones) to the central nervous system. (See diagram to the left.) Usually, the brain - which is part of the nervous system - coordinates a response.
An effector is any part of the body that produces the response. Here are some examples of effectors:
Hormones are chemical messengers produced in glands
and carried by the blood to specific organs in the body.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science