Together, the spinal cord and the brain comprise the central nervous system. The spinal cord, a continuation of the brain stem located at the base of the brain, is protected by the bony vertebral column called the spine. Running approximately two-thirds of the way down the spine and ending at the small of the back, the spinal cord, like the brain, is made up of gray and white matter. The gray matter, the innermost portion of the spinal cord, is shaped like the letter "H" and formed primarily of cell bodies. Cell bodies in the posterior, (backward-facing) legs of the H--called the dorsal horns--are sensory neurons which transmit information from the skin such as pain, pressure, touch, and stretch. Neurons in the anterior (forward-facing) legs--called ventral horns--are motor neurons which signal muscle movement.

The white matter surrounds the gray matter and is made up of nerve...