The spinal cord is the major bundle of nerves that carries nerve impulses to and from the brain to the rest of the body. The brain and the spinal cord constitute the central nervous system. Motor and sensory nerves outside the central nervous system constitute the peripheral nervous system. Another diffuse system of nerves that controls involuntary functions, such as blood pressure and temperature regulation, are called the sympathetic and parasympathetic nervous systems.
The spinal cord is about 18 inches long and extends from the base of the brain, down the middle of the back, to about the waist. The nerves that lie within the spinal cord are upper motor neurons (UMNs), and their function is to carry the messages back and forth from the brain to the spinal nerves along the spinal tract. The spinal nerves that branch out from the spinal cord to the other parts of the body are called lower motor neurons (LMNs).