The human body has some reflexes that, in a way, allow the spinal cord to briefly control a limb in lieu of the brain. It takes slightly longer for a message to reach the brain than it does for a message to reach the spinal cord; in case of an extreme emergency, time is of the essence, so basically your central nervous system buys you a tiny bit of time by stopping the message at your spine rather than your brain.
For example, if you reach out and touch a hot pan on a stove, once the pain signal has traveled up the nerves from your hand to your spine, your central nervous system automatically causes you to recoil.
This is why your doctor tests your reflexes by tapping you on the knee with a little mallet thingy--if something about the reflex is abnormal, that could be a sign of damage to the central nervous system.