PHYSIOLOGY OF THE CIRCADIAN SYSTEM IN ANIMALS AND HUMANS

Virtually all organisms have developed an internal timing system capab le of reacting to and anticipating environmental stimuli with a progra m of appropriately timed metabolic, physiologic, and behavioral events . The predominant biological rhythms coincide with the geophysical cyc le of day and night-the circadian rhythms. The suprachiasmatic nuclei comprise the primary pacemaker in mammals, exhibiting the properties f undamental to a rhythm-generating structure. This article summarizes r ecent research that has elucidated mechanisms of signal transduction w ithin the circadian system. The roles of various neurochemicals and ho rmones in transmitting the circadian timing signal are described. Prop erties of the circadian system, including photic and nonphotic entrain ment, phase response curves, masking, and the intrinsic variability in the system are discussed.