Circadian Photoreception in Humans and Mice

Light reception by eye and signal transduction to SCN. Rods and cones, photoreceptor cells located in the inner retina, mediate the perception of light. There are other light-sensitive cells located in the inner nuclear layer (INL) and ganglion cell layer (GCL) that are involved in signal transduction to the SCN via the retinohypothalamic tract (RHT). The cryptochromes CRY1 and CRY2 are expressed in the INL and GCL, and melanopsin is expressed in a small fraction of ganglion cells. In this representation, we show only the axons that transmit light signals from inner retina and outer retina to the SCN. The ONL and INL are partially redundant for signaling to the SCN. Similarly, CRY1, CRY2, and melanopsin (and possibly other unknown photopigments expressed in ganglion cells) may be functionally redundant in the inner retina. The visual pathway, initiated from the outer retina, is omitted for simplicity.