spinal cord
A thick nerve bundle in your back that carries signals.
The spinal cord is a thick bundle of nerves running down the center of your backbone, connecting your brain to the rest of your body. Think of it as the body's main information highway: when you decide to wiggle your toes, signals travel from your brain down through your spinal cord to make it happen. When you touch something hot, signals race back up through your spinal cord to your brain so fast that you pull your hand away before you even consciously think about it.
Your spine (the column of bones in your back) protects this crucial cord like armor, since the spinal cord is fragile and extremely important. Damage to the spinal cord can prevent signals from getting through, which is why spinal injuries are so serious and can affect movement and sensation.
The spinal cord is about as thick as your thumb and extends from the base of your brain down to your lower back. Nerves branch off from it at different levels, like exits on a highway, connecting to different parts of your body. The nerves near your neck connect to your arms and hands, while those lower down connect to your legs and feet. Without your spinal cord, your brain couldn't control your body, and your body couldn't tell your brain what it's experiencing.