Scientists believe they are closer to unlocking the secret of the itch. Long a medical mystery, a recent experiment involving mutant mice has pushed researchers closer to learning the truth about what causes the annoying need to scratch.br br The experiment found that mice carried an "itch signal" along a neuron system populated by the Nppb protein. The dedicated neuron system carried the message of "itchiness" up the spine to the brain.br br The neuron carrier system remained discrete from other neuron receptor systems that registered other topical sensations such as heat, cold and pain.