Photoreceptor structure

Cone and Rod photoreceptors have same structures as shown in figure below. The Photoreceptors have diameters ranging from 1 to 4 µm, being smaller at fovea, enhancing the acuity there. The inner segment consists of the nucleus and has an axon-like procedure connected to a synaptic terminal. The external segment in the cone cell has its plasma membrane invaginated into several closely packed parallel folds, forming the discs. In rod cells the discs are pinched off the plasma membrane to become entirely intracellular. The disc membrane is compactly packed with visual pigment. In rod cells this is rhodopsin. Each kind of cone cells has its characteristic cone opsin. The outer segment is continually redeveloped from the base, at the same time as its apical tip is phagocytosed by pigment epithelial cells at the rate of 3–4 discs per hour. The Photoreceptors are not capable of mitotic division.

                                     Figure: Photoreceptors: (a) a rod cell; (b) a cone cell.