When the patellar tendon is tapped just below the knee, the tap initiates an action potential in a specialized structure known as a muscle spindle located within the quadriceps . This action potential travels to the L3 and L4 nerve roots of the spinal cord,  via a sensory axon which chemically communicates by releasing glutamate onto a motor nerve . The result of this motor nerve activity is contraction of the quadriceps muscle, leading to extension of the lower leg at the knee (. the lower leg kicks forward). Ultimately, an improper patellar reflex may indicate cerebellar injury.