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.