There are special nerves that sense when something irritates the skin. Those nerves send signals through a nerve to the spinal cord. The signals travel up the spinal cord through the medulla and send a little signal to the cerebellum that tells your muscles where the itch is and how to coordinate the movement that will allow you to scratch it, if and when you decide to scratch it. In the millisecond that passes before you scratch, the signal goes to the periaqueductal gray (PAG) shown in the small blue circle just above the medulla. The PAG plays a critical role in motivated behavior and in behavioral responses to threatening stimuli.