DEFINITION:  a reflex action is a rapid authomatic response to a stimulus which is not under the voluntary control of the brain it is describe as an involuntary action.

NB:  the nervous part way taken by nerve impulses in a reflex action is called a reflex arc.

A STIMULUS

A stimulus is any change in the internal or external environment of an organism.

HOW THE BODY REACT WHEN A HAND TOUCHES AN OBJECT

• When a hand touches a hot object heat receptors in the skin are stimulated
• The receptor convert the heat stimulus into electrical impulses which are fired along the sensory neurone to the spinal cord through the dorsal root
• In the arey matter of the spinal cord the impulses pass across the synase to the relay neurone. This is assisted by acethycoline
• From the relay neurone, the impulses pass across another synapse assisted by acetycoline to the motor neurone. The motor neurone carries the impulses through the ventral root the bicep muscles of the arm
• This causes the biceps muscle to contract and bends the arm there by removing it from the hot object.