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.
