tendon jerk reflex

tendon jerk reflex

rapid reflex contraction of a muscle in response to a sudden stretch, elicited by tapping its tendon, and involving direct (monosynaptic) excitation of alpha motor neurons in the spinal cord by afferent fibres from primary sensory endings in muscle spindles. The best known is the knee jerk: when the patellar tendon is tapped, the quadriceps is caused to contract. Similar rapid monosynaptic reflexes operate, e.g. when the tendon of the biceps is tapped at the elbow or the Achilles tendon at the ankle. Also known as a phasic stretch reflex. See also stretch reflex.