Why does a spinal cord injury cause paralysis?
The spine is the column of bones that protects the spinal cord. It starts at your head and goes to your lower back. Muscles and ligaments connect the vertebrae. Discs are soft and ride between the vertebrae. You have seven cervical vertebrae, 12 thoracic vertebrae, five sacral vertebrae, and three to five coccygeal vertebrae. The spinal cord, a bundle of nerves, runs from the base of your brain through the spine and carries signals to and from other parts of your body to the brain.
When your brain cannot send a signal to another part of your body, you are paralyzed. If you injure the cervical part of the spinal cord—the upper part of it—you find yourself paralyzed from the neck down. This results in quadriplegia. If you suffer an injury between the waist to just below the cervical vertebrae, you could become a paraplegic—your torso and legs may become paralyzed.
The injury to the spinal cord does not allow the signals to your brain to travel to the part of the body that can no longer move. If you injure the cervical part of the spine and you think you want to grab a soda can, your arm and hand never receive the signal and don't move.