What is a reflex arc?

A reflex is an involuntary response to a sensory stimulus. The pathway of neurones involved in a reflex is known as a reflex arc. A reflex arc - such as quickly moving your hand away from a hot flame - involves just three neurones. A reflex arc begins with the stimulus - the heat from the hot flame. A receptor - such as the temperature receptors in the skin of the hand create a nerve impulse in a sensory neurone. A sensory neurone then passes the nerve impulse to the spinal cord. An intermediate (or relay) neurone links the sensory neurone to the motor neurone in the spinal cord. A motor neurone carries the nerve impulse from the spinal cord to a muscle in the upper arm. An effector - the muscle in the upper arm that is stimulated to contract produces the effect. Thus, the overall response is the hand pulling away quickly from the hot flame.