The chain of nerve cells that runs from the brain through the spinal cord out to the muscle is called the motor pathway. Normal muscle function requires intact connections all along this motor pathway. Damage at any point reduces the brain's ability to control the muscle's movements. This reduced efficiency causes weakness, also called paresis. Complete loss of communication prevents any willed movement at all. This lack of control is called paralysis.

The types of paralysis are classified by region:

• Monoplegia, affecting only one limb
• Diplegia, affecting the same body region on both sides of the body (both arms, for example, or both sides of the face)
• Hemiplegia, affecting one side of the body
• Paraplegia, affecting both legs and the trunk
• Quadriplegia, affecting all four limbs and the trunk.

Paraplegia occurs after injury to the lower spinal cord, and quadriplegia occurs after damage to the upper spinal cord at the level of the shoulders or higher (the nerves controlling the arms leave the spine at that level). Both Paraplegia and Quadriplegia are devastating conditions that affect hundreds of accident victims each year. Many will never reach full recovery and will require lifelong medical attention.