Rod and cone photoreceptors have similar structures (Figure 1). Photoreceptors have diameters ranging from 1 to 4 μm, being smaller at the fovea, enhancing visual acuity there. The inner segment contains the nucleus and has an axon-like process connected to a synaptic terminal. The outer segment in the cone cell has its plasma membrane Figure 1 Photoreceptors: (a) a rod cell; (b) a cone cell. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429212840/ec7c9128-9b44-4171-97f1-ac3f1921b079/content/fig1_G3.tif" xmlns:xlink="http://www.w3.org/1999/xlink"/> 141 invaginated into numerous closely packed parallel folds, forming discs. In rod cells the discs are pinched off the plasma membrane to become completely intracellular. The disc membrane is densely packed with visual pigment. In rod cells this is rhodopsin. Each type of cone cells has its characteristic cone opsin. The outer segment is continually regenerated from the base, whilst its apical tip is phagocytosed by pigment epithelial cells at the rate of 3-4 discs per hour. Photoreceptors are incapable of mitotic division.