The knee jerk reflex, or patellar reflex, is a deep tendon reflex seen as a sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon. Tapping the patellar ligament stimulates the muscle spindles in the quadriceps. Impulses travel from the muscle spindles to the spinal cord. In the spinal cord, synapses occur with motor neurons and interneurons. The motor (efferent) neurons send activating impulses to the quadriceps causing the muscles to contract and extend the knee. The interneuron (relay neuron) forms a connection between the other neurons and interneurons. Interneurons are neither motor nor sensory. Interneurons transmit impulses that inhibit the antagonistic muscles (hamstrings). An abnormality of the reaction suggests that there may be damage to the central nervous system.
