When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response.
But sometimes, a very quick response is needed, one that does not need the involvement of the brain. This is a reflex action.
Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The diagrams below show each stage of the reflex arc. The yellow and green shape top rightof each diagram is a cross-section through the spine, as it would appear if you looked down through the head of the man whose shadow you see.
The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science ]