The ankle jerk reflex, also known as the Achilles reflex, is an involuntary response triggered by tapping or striking the Achilles tendon, located at the back of the ankle. The reflex causes the calf muscles to contract, resulting in a brief and automatic upward movement of the foot. It is mediated by the stimulation of sensory nerve fibers in the muscle, which send signals to the spinal cord and trigger a reflex arc involving the motor neurons that control muscle contraction. The ankle jerk reflex is commonly used in clinical examinations to assess the integrity of the nervous system, specifically the reflex arc involving the nerves and muscles associated with the ankle joint.