Why does the knee-jerk reflex still function in a person with a severed spinal cord?
The knee jerk response involves a neural circuit which passes through the spinal cord, but which doesn’t involve higher neural processing from the brain. When the patellar tendon is struck by the doctor’s little hammer, it triggers stretch sensors that trigger firing of sensory neurons which carry a report of the stimulus to the spinal cord. There, the neurons synapse with short, interneurons which fire in response to the incoming message from the knee. The interneurons, contained entirely within the spinal cord, then synapse with motor neurons which carry a message outbound to the quadriceps muscle of the thigh. It contracts, resulting in the classic knee jerk in response to the original bang on the knee. Since this process does not involve any processing by the brain, it can occur even when the spinal cord is severed above the point where the reflex is generated. By the way…ever notice the time lag between the whack on the knee and the jerk of the muscle? That is how long it takes
