Figure 2.

A more realistic model of a simple circadian system consisting of a set of input (entrainment) pathways, multiple central oscillators, and sets of output pathways. Entraining stimuli include light, mediated through phytochromes (PHY) and cryptochromes (CRY), temperature, and imbibition (not shown). Complexity in input pathways arises from multiple phytochromes and cryptochromes as well as interaction among them and their downstream signaling pathways. Each central oscillator is illustrated as a loop including positive and negative components that yields a self-sustaining oscillation with a period of approximately 24 hr. Coupling between oscillators is suggested by double-headed arrows. Multiple output pathways are drawn as each regulating an overt rhythm with a distinct phase. Some outputs may be driven by individual oscillators whereas others may receive input from more than one oscillator. Additionally, different oscillators may drive separate rhythms with distinct periods.