The human nervous system carries stimuli from sensory receptors to the brain by two main parts: the central nervous system and the peripheral nervous system. The brain and spinal cord make up the central nervous system. The spinal cord represents the major nerve track in all vertebrates.

In humans, it extends from the base of the brain to the middle of the back, about 18 inches long. It's composed of 31 paired spinal nerves that transmit information about internal and external conditions to the brain and also distribute impulses that mediate reflex actions from the brain to other parts of the body. Essentially, all thoughts and actions depend on the proper functioning of the nervous system. Any disturbance in the nervous system can affect the entire body.

Spinal cord injury, SCI, refers to any injury of the nerves within the spinal canal. This can result in the loss of mobility and feeling in the body. The effect depends on the level and type of the injury. The type of injury can be broken into two classifications: complete and incomplete. Complete injury results in 100 percent loss of function below the level of the injury site. Incomplete injury results in only some loss of function below the level of injury. The level is determined by the location along the spine at which the injury occurs. The closer the injury is to the brain, the greater the loss of sensation and movement. A paraplegic is someone who is unable to move or feel his or her lower body. A quadriplegic is someone who has lost feeling and movement in their upper and lower body. Traumas, including car accidents, shootings, and sports related accidents, frequently cause spinal damage. Diseases, such as polio, and birth defects, such as spinabifida, can also result in spinal damage. Over 400,000 people in the United States suffer with SCI and the majority, 82 percent, are men between the ages of 16 and 30.