The spinal cord is a collection of nerves that travels from the bottom of the brain down your back. There are 31 pairs of nerves that leave the spinal cord and go to your arms, legs, chest and abdomen. These nerves allow your brain to give commands to your muscles and cause movements of your arms and legs. The nerves that control your arms exit from the upper portion of the spinal cord, while the nerves to your legs exit from the lower portion of the spinal cord. The nerves also control the function of your organs including your heart, lungs, bowels, and bladder. For example, signals from the spinal cord control how fast your heart beats and your rate of breathing.