Describe the reflex arc.

A stimulus (for example a pin) stimulates the receptor which causes a nerve impulse in the sensory neuron. The impulse travels to a synapse in the spinal cord, where chemicals diffuse across the gap in the synapse and stimulate an impulse in the relay neurone. This travels to another synapse and an impulse in the motor neurone, which travels to the effector, causing a response (for example moving hand)