This book contains papers which discuss many different stimulus--response systems ranging from those in bacteria and paramecia to those in insects and mammals. Emphasis is placed on the identification and characterization of receptor molecules, physical properties of channel molecules, physical and chemical changes in receptors, channels and other structures after stimuli, and the molecular events in the sequential response, excitation, adaptation and learning phases. Use is made throughout of both traditional and new techniques in electrophysiology, biochemistry, genetics and ultrastructural analysis. By covering a wide range of stimulus--response systems the book invites comparison between systems and generates discussion about common principles underlying information reception, transduction and response.