Any intracellular signal transduction in which the signal is passed on within the cell via a second messenger; a small molecule or ion that can be quickly generated or released from intracellular stores, and can diffuse within the cell. Second-messenger signaling includes production or release of the second messenger, and effectors downstream of the second messenger that further transmit the signal within the cell. [ http://www.ncbi.nlm.nih.gov/pubmed/15221855 https://www.worldcat.org/search?q=bn%3A0815316194 http://en.wikipedia.org/wiki/Second_messenger_system ]