Why would scratching one part of the body cause another part to itch?

It is called Referred Pain. Physicians and scientists have known about referred pain since the late 1880s. Despite an increasing amount of literature on the subject, the mechanism of referred pain is unknown, although there are several hypotheses. Referred pain is pain perceived at a location other than the site of the painful stimulus/ origin. It’s the result of a network of interconnecting sensory nerves. This network supplies many different tissues. When there is an injury at one place in the network, this pain can be interpreted in the brain to radiate nerves. And can give pain elsewhere in the related areas of the network. Several neuro physiological theories have been suggested: • Convergence-projection theory • Convergence-facilitation theory • Axon-reflex theory • Hyperexcitabillity theory • Thalamic convergence theory You can check the references here. 1. http://en.wikipedia.org/wiki/Referred_pain 2. http://www.physio-pedia.com/Referred_Pain 3. http://howmed.net/physiology/pain/