Explain a reflex arc

A reflex arc is the nerve pathway which follows a reflex action. First a stimulus, like a pin prick, is detected by receptors in the skin. A sensory neurone then sends electrical impulses to a relay neurone, which are located in the spinal cord. These connect sensory neurons to motor neurones. The motor neurone sends electrical impulses to an effector, which causes a response to occur, such as the muscle contracting to pull away from the pin prick.