The Spinal Cord
The spinal cord as part of the nervous system is the largest nerve in the body. Specifically, nerves are cordlike structures which are made up of nerve fibers. These nerve fibers are responsible for the communication systems of the body which include sensory, motor and autonomic functions.
The spinal cord is surrounded by protective bone segments called the vertebral column, or the spinal column. It is made up of seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae and five sacral vertebrae.
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). These spinal nerves exit and enter at each vertebral level and communicate with specific areas of the body. The sensory portions of the LMN carry messages about sensation from the skin and other body parts and organs to the brain. The motor portions of the LMN send messages from the brain to the various body parts to initiate actions such as muscle movement.