spinal cord

The spinal cord is the long, soft cable of nervous tissue that runs down the inside of your backbone, from the base of your brain to about the small of your back. Think of it as the body's main information highway: messages from the brain travel down it to reach your muscles and organs, and signals from your skin, joints, and insides travel up it to reach the brain. It is only about as thick as your little finger, yet nearly every command to move and almost every sensation you feel passes through this single bundle.

Bundles of nerve fibers branch off the cord in pairs at each level of the spine, fanning out to a particular region of the body — one pair to the arms, others to the chest, the legs, and so on. The bony rings of the spine wrap around the cord to protect it, the way a hard case shields a delicate wire. Because so much traffic funnels through one channel, an injury high up can cut off the brain's contact with everything below the damaged point, which is why spinal injuries can cause paralysis.

The cord does more than just carry messages — it can also act on its own for fast, automatic responses called reflexes. When you touch something painfully hot, the cord pulls your hand back before the news even reaches your brain, saving the precious fraction of a second it would take to think about it. So the spinal cord is both a relay line between brain and body and a small decision-maker in its own right.