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. 2018 Jan 31;23(2):301. doi: 10.3390/molecules23020301

Figure 1.

Figure 1

Illustration of the SCN-melatonin loop. Solid arrows indicate the neuronal connections and the direction of neuronal projections. Dash arrows indicate the input signals. SCN is the master clock which determines the biological rhythms as well as the melatonin circadian rhythm. Its intrinsic circadian interval is longer than 24 h. The natural photoperiod (photos) serves as an input signal to entrain melatonin circadian rhythm to 24 h; in turn, melatonin functions as a signal of photoperiod to re-entrain the biological rhythm of SCN to 24 h. MRGC: melanopsin-containing retinal ganglion cells; RHT: retino-hypothalamic tract; SCN: suprachasmatic nucleus; PVN: paraventricular nucleus; IMCC: Intermediolateral cell column; SCG: sympathetic cervical ganglion; PG: pineal gland. Mel: melatonin.