Signaling between cells provides for long-range integration of metabolism, and usually results in a response that is slower than is seen with signals that originate within the cell. Communication between cells can be mediated, for example, by surface-to-surface contact and, in some tissues, by for- mation of gap junctions, allowing direct communication between the cytoplasms of adjacent cells. However, for energy metabolism, the most important route of communication is chemical signaling between cells by bloodborne hormones or by neurotransmitters.