The knee-jerk reflex, also known as the patellar reflex, is an involuntary response that occurs when the knee is tapped just below the kneecap. This tap stretches the quadriceps muscle in the thigh, triggering a signal that travels through the spinal cord and back to the muscle, causing it to contract. This results in the leg kicking forward. This reflex is a simple example of a neural pathway called a reflex arc, which involves sensory neurons, interneurons, and motor neurons. The knee-jerk reflex is often tested by doctors to assess the function of the nervous system and can indicate potential issues if the response is abnormal.