Morphological and functional diversity of intrinsically photosensitive retinal ganglion cells.

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are a special type of ganglion cell that express the photopigment melanopsin and autonomously respond to light in the mammalian retina. Nearly two decades since their discovery, ipRGCs have been broadly investigated for their basic morphologic features, cell classification, physiologic properties, and functional significance. It is well established that ipRGCs are predominantly responsible for non-image-forming vision, such as circadian rhythm entrainment, pupillary contraction, mood, and sleep regulation. However, there are mounting morphological, physiological, and behavioral pieces of evidence supporting the idea that ipRGCs may contribute to image-forming vision such as color, contrast, and perception. These new compelling pieces of evidence extend our previous thought and provide extensive and profound insights into the melanopsin-mediated light response affecting visual functions. This article summarizes what we currently know about ipRGCs' subtypes, signal transduction, axonal projections, synaptic inputs, and functions with the emphasis on non-image-forming vision and image-forming vision.