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. Author manuscript; available in PMC: 2015 Jan 1.
Published in final edited form as: Front Neuroendocrinol. 2013 Nov 25;35(1):111–139. doi: 10.1016/j.yfrne.2013.11.003

Figure 1. Representation of circadian timing system.

Figure 1

The circadian clock has been represented as having three components: input pathways, a central oscillator (or pacemaker), and output pathways. Input pathways such as photic signals from the retina, or temperature, can influence the master oscillator in the suprachiasmatic nucleui (SCN) of the hypothalamus which produces the endogenous biological rhythm that synchronize the rest of the body. Output pathways to target sites entail both neural connections and diffusible signals, and these regulate clock-controlled biological processes. Additional pathways (shown as dotted lines) include multiple interlocking positive or negative feedback from clock controlled activities. One prominent feedback mechanism are the systemically secreted hormones which can then influence the circadian system at all levels, including input pathways, the central oscillator, and output pathways. Adapted from Kriegsfeld et al. (2002).