What reflex is checked by tapping on the patellar tendon?

Tapping on the patellar tendon is a classic clinical test used to evaluate the stretch reflex, specifically targeting the quadriceps muscle. When the patellar tendon is tapped, it stretches the tendon and consequently the muscle fibers of the quadriceps. This stretch is detected by specialized receptors called muscle spindles, which then activate sensory neurons.

These sensory neurons transmit the signal to the spinal cord, where they synapse with alpha motor neurons that innervate the quadriceps muscle. Upon activation, the motor neurons cause contraction of the quadriceps muscle, which results in a noticeable kick of the leg. This rapid response to the muscle stretch exemplifies the characteristics of a stretch reflex, emphasizing its role in maintaining posture and reacting to sudden changes.

While this reflex is indeed monosynaptic, as it involves a single synapse between the sensory neuron and the motor neuron, the more accurate and specific term related to the action of tapping on the patellar tendon is the stretch reflex. Other reflexes mentioned, such as the palpebral reflex (related to blinking) and withdrawal reflex (which involves moving away from a painful stimulus), do not pertain to this specific action.