The billions of cells whose job it is to keep your body informed of conditions outside and inside it are called nerve cells, or neurons. Neurons transmit messages throughout the body by passing signals, or impulses, from one to the other.
Sensory neurons are nerve cells which carry impulses from the eyes, ears, nose, mouth, and skin to the brain and spinal chord. They allow us to see and hear, smell and taste, and tell us whether we are hot or cold, and let us feel pleasure or pain.
Motor neurons are nerve cells which carry these impulses from the brain to the muscles in the body, telling them to sit, stand, run, or talk.
Still other neurons work automatically, without signals from the brain. These neurons keep your heart beating, your blood flowing, your lungs breathing, and your stomach digesting food, all automatically, with no command from your brain.
Some impulses travel through your body at a speed of more than 300 feet per second, while others go as slowly as 1.5 feet per second!