Signaling cells secrete ligands that bind to target cells and initiate a chain of events within the target cell. The four categories of signaling in multicellular organisms are paracrine signaling, endocrine signaling, autocrine signaling, and direct signaling across gap junctions.

Endocrine signals are carried long distances through the bloodstream by hormones, and autocrine signals are received by the same cell that sent the signal or other nearby cells of the same kind. Internal receptors are found in the cell cytoplasm.

endocrine signal — long-distance signal that is delivered by ligands (hormones) traveling through an organism's circulatory system from the signaling cell to the target cell. internal receptor — (also, intracellular receptor) receptor protein that is located in the cytosol of a cell and binds to ligands that pass through the plasma membrane. paracrine signal — signal between nearby cells that is delivered by ligands traveling in the liquid medium in the space between the cells. autocrine signal — signal that is sent and received by the same or similar nearby cells.