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. Author manuscript; available in PMC: 2023 Jun 1.
Published in final edited form as: Semin Cell Dev Biol. 2021 May 14;126:45–55. doi: 10.1016/j.semcdb.2021.04.026

Figure 2.

Figure 2.

Organization of the circadian system at the conceptual, cellular, network, and systems levels. A. Conceptual three-part model of the circadian system. B. Cellular timekeeping involves transcriptional-translational feedback loops that can be modified by inter- and intracellular signaling. Arrows indicate activation and bars indicate repression. C. The SCN network comprises multiple neuronal subtypes organized into shell and core compartments. Schematic illustrates the location of neuronal subtypes mentioned in text. AVP: arginine vasopressin, VIP: vasoactive intestinal polypeptide, GRP: gastrin releasing peptide, CB: calbindin, CR: calretinin. D. The SCN sends time of day cues to other clock tissues in the brain and body.