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Light is detected by the photoreceptors of the retina, including the rods and cones as well as photosensitive retinal ganglion cells (pRGCs), expressing the photopigment melanopsin. Light information is relayed to the suprachiasmatic nuclei (SCN) via the retinohypothalamic tract (RHT), where it entrains an intracellular molecular clock mechanism, consisting of positive (BMAL1 and CLOCK) and negative (PER and CRY) elements.
