Signal transduction: the system that cells use to create cellular responses to the signals. Cells that respond to the stimulus: sensory cells. These cells secrete ligands, the molecules that bind directly to receptors on target cells to produce biological responses. Autocrine signalling: targets receptors on the same cell that related them. Paracrine signalling: targets cells near the signalling cell, diffuses through ec uid. Endocrine signalling: uses the circulatory system to target cells long distances away. Direct contact: using gap junctions or something similar to communicate. Ligands activate intracellular messaging, other chemicals called relay molecules pass along info. Ligands must nd their speci c receptor that they are looking for. Once they bond, the receptor changes shape, which alters its shape. Antagonist: inhibitory molecule, blocks receptor from the ligand. Receptors are located on the surface of the cell. Transmembrane receptors bind to the molecules that are too big to pass through.