light-absorbing compound called retinal (derived from vitamin A) and the protein opsin. When light strikes rhodopsin, retinal and opsin are split in a non-enzymatic reaction and the polarity of the outer segment's plasma membrane changes. Calcium is then released from vesicles on the surface of the outer segment. The calcium causes a transmitter substance to be released from the inner segment at its synapse with the sensor neuron generating an impulse. In the dark, retinal bonds again with opsin to reform rhodopsin.
Cone cells are concentrated in the retina's center and are used for vision in bright light since they give rise to sharp, clear images and can detect color. Each cone cell has an inner segment containing the nucleus and synaptic body and a cone-shaped outer segment, again a specialized, non-motile cilium that contains light-sensitive pigment in the vesicles on its surface. Unlike rod cells, there are three different types of cone cells. Each different type of cone cell has only one of the three different types of cone cell light-sensitive pigments. All three pigments have retinal as their light absorbing compound and are sensitive to the entire spectrum of visible light, yet each pigment's protein component is slightly different giving each type of pigment a maximum absorbency at a different portion of the spectrum. The three types of pigments are blue-absorbing, green-absorbing, and red-absorbing, and , consequently, there are blue-absorbing cone cells, green-absorbing cone cells, and red-absorbing cone cells.
