The Retinohypothalamic tract (RHT) is a photic input pathway involved in circadian rhythms. The RHT is the input pathway to the suprachiasmatic nucleus (or nuclei), and is important in the entrainment of our 'body clock' to the external environment. The retinal cells involved in the RHT are photoreceptive retinal ganglion cells, which contain melanopsin. The information these retinal cells receive on day-length is interpreted by the SCN, and passed onto the pineal gland, which then secretes the hormone melatonin in response to this exogenous environmental signal.
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