Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence of something like ipRGCs was first suspected in 1927 when rodless, coneless mice still responded to a light stimulus through pupil constriction; this implied that rods and cones are not the only light-sensitive neurons in the retina. Yet research on these cells did not advance until the 1980s. Recent research has shown that these retinal ganglion cells, unlike other retinal ganglion cells, are intrinsically photosensitive due to the presence of melanopsin, a light-sensitive protein. Therefore, they constitute a third class of photoreceptors, in addition to rod and cone cells.
An ipRGC, shown here as a complied image of the retina from proximal inner nuclear layer to the ganglion cell layer with fluorescent labeling of melanopsin
Mammalian eye
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Mammals normally have a pair of eyes. Although mammalian vision is not so excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families possessing a trichromatic color perception.
Human eye seen from the front in its orbit surrounded by the eyelid and eyelashes, showing the anterior segment with the iris (light brown in this individual), pupil, and sclera visible
Eyelids and eyelashes are a unique feature of mammalian eyes, and primarily serve to protect the eyes. However, the lids also play a role in nonverbal communication, and the lashes in human culture and sexuality.