In cold-blooded vertebrates, deep-brain photoreceptors allow for photoentrainment, the process by which the eyes facilitate setting of the circadian clock. Mammals do not have these receptors; instead, mammalian eyes collect light and send the information back to the SCN through the optic nerve, a pathway called the retinohypothalamic tract (RHT). This is known, in part, because mice with removed eyes cannot reset their clocks.
Oddly enough, about half of all blind people can photoentrain, as can mice that lack functional rods and cones, implying that another receptor capable of processing light exists in the eye. Tracer studies, in which a marker traverses the neuron from one end to the other, indicated more than 20 years ago that a small subset of cells found in the innermost part of the retina innervates the SCN. These retinal ganglion cells (RGCs), which were not known to contain any photopigments of their own, were ...
