The spinal cord is a column of nervous tissue protected by the spine and running up the back. At intervals sensory and motor neurones emerge.The knee-jerk reflex is a spinal reflex that is used by the body to maintain posture and balance. When the patellar tendon is tapped, it stretches. This causes a reflex arc that causes the extensor muscle at the top of the thigh to contract and inhibits the flexor muscle so that it relaxes. As a result the leg kicks.