Physiological Basis Of Circadian Organization

The study of the anatomical and physiological organization of circadian systems of insects has a long and productive history. The heuristic model generally used in these studies is illustrated in Fig. 2. There are four essential elements: (1) a pacemaker or oscillator that generates the primary timing signal, (2) photoreceptors for entrainment, and two coupling pathways, (3) one that mediates the flow of entrainment information from the photoreceptor to the pacemaker and (4) another that couples the pacemaker to the effector mechanisms that it controls. The model identifies several basic questions. Can the anatomical location of the circadian clock be identified? What are the pathways and mechanisms by which inputs to the pacemaking system regulate its phase and period? Finally, what are the neural and endocrine signals

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