The knee-jerk reflex, also known as the patellar reflex, is an automatic response that occurs when the knee is tapped just below the kneecap. This tap stretches the quadriceps muscle, triggering sensory neurons that send a signal to the spinal cord. The spinal cord then sends a signal back to the quadriceps, causing the muscle to contract and the leg to kick 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 helps maintain posture and balance, and it is often tested by doctors to assess the health of the nervous system.