The Retinohypothalamic tract (RHT) is a photic input pathway involved in circadian rhythms. The RHT is an input pathway from the mammalian retina to the suprachiasmatic nuclei[1] (SCN) in the brain. The SCN receive and interpret information on environmental light, dark and daylength, important in the entrainment of the 'body clock'.
The retinal cells involved in the RHT are intrinsically photosensitive ganglion cells (ipRGCs) which contain the photopigment melanopsin. These cells project directly to the SCN. On the basis of information provided by the ipRGCs, the SCN can coordinate periferal 'clocks' and direct the pineal gland to secrete the hormone melatonin.