How does the knee-jerk reflex use stretch receptors and the spinal cord?

The knee-jerk reflex is a classic example of an involuntary reflex that relies on mechanoreception to maintain muscle function and posture.

When a sharp tap is applied to the patellar tendon, it causes a sudden, passive stretch of the quadriceps muscle. This mechanical stimulus is detected by muscle spindles, which are specialized proprioceptors arranged parallel to the muscle fibers. The stretch increases the frequency of nerve impulses sent from these receptors along sensory (afferent) nerves toward the spinal cord.123

Within the spinal cord, these sensory impulses directly activate motor (efferent) nerves that lead back to the same quadriceps muscle. This creates a rapid, automatic circuit that triggers a compensatory contraction of the muscle, resulting in the characteristic kicking motion. Because this circuit is contained entirely within the spinal cord, the knee-jerk reflex is considered a purely spinal response that does not require input from the brain to occur. Testing this reflex is a vital clinical tool for assessing the integrity of the spinal cord's motor mechanisms.321