What functions of the nervous system enable the body to respond quickly?

The fastest response from the nervous system is from the autonomic system which is involuntary. Heartbeat and reflex movement come from this system which is out of your conscious control. It is centered in the cerebellum and spinal cord. Impulses from your sense organs (a bad smell or a knock to the knee) travel to your spinal cord or cerebellum -- whichever is closest to the stimulus -- and a response (coughing or knee-jerk) travels back to the area to provoke an appropriate reponse. Some voluntary responses can become involuntary responses, such as riding a bicycle. When learning how to balance, the actions are learned over and over until they become automatic. Some involuntary actions can be controlled voluntarily, such as holding your breath or a yogi who can slow his heartrate.