A nerve cell (also called a neuron) is a specialized cell in your body that carries messages between your brain and the rest of your body. These cells form your nervous system, the communication network that lets you think, move, feel, and react to the world around you.

Nerve cells have a unique shape: a central body with long, branching arms that reach out to connect with other nerve cells. When you decide to pick up a pencil, nerve cells in your brain send electrical signals down through your spinal cord and out to the muscles in your hand, all in a fraction of a second. When you touch something hot, nerve cells in your skin instantly alert your brain, which sends messages back telling your hand to pull away.

Your brain contains roughly 86 billion nerve cells, constantly firing signals to each other. Every thought you have, every memory you form, every movement you make happens because nerve cells are communicating. Some nerve cells are incredibly long: a single nerve cell can stretch from your lower spine all the way down to your toes, nearly three feet long in some adults. Unlike most cells in your body, nerve cells generally can't be replaced once they're damaged, which is why protecting your brain and spinal cord is so important.