Which is the part of the human visual system where the receptors that sense light, i.e., the photoreceptor cells, are located? How do those cells work?

The photoreceptor cells create the retina, a lamina that covers the internal posterior region of the eyeball. The photosensitive cells of the retina are separated into two types: the cone cells and the rod cells. These cells have pigments that sense exact light wave ranges (frequencies) and trigger action potentials conducted by the optical nerves to the visual area of the brain.

Image Diversity: retina histology cone cells rod cells