A reflex arc is the pathway that a nerve reflex, such as the knee jerk reflex, follows.
A tap on the knee stimulates sensory receptors, generating a nerve signal.
The signal travels along a nerve to the spinal cord.
In the spinal cord, the signal is transmitted from the sensory nerve to a motor nerve.
The motor nerve sends the signal back to a muscle in the thigh.
The muscle contracts, causing the lower leg to jerk upward. The entire reflex occurs without involving the brain.