when the patellar tendon is stimulated, it leads to a brief stretch of the agonist quadriceps muscle, as well as activating the muscle spindles. The resultant sensory information is sent to the tray matter in the spinal cord. At the same time that the α-motoneuron is excited, leading to contraction of the quadriceps it also inhibits the antagonist hamstring muscle. As there is no load, and thus no muscle tension, the Golgi tendon organs are inactive, and together with spindles lead the quadriceps to relax.
See Action, Agonist muscle, Antagonist muscle, Autonomic nervous system, Denervation, Exteroceptive pain reflexes, Golgi tendon organ, Gray matter, Intentionality, Motor equivalence, Reflex arc, Reflexology