The light-sensitive sensory cells have the same structural design in all vertebrates. Next to the pigmented epithelium lies the outer segment of the photoreceptor cell; it ispartly buried in a pigmented epithelial cell. The outer segment of rod cells is a cylinder (ABC1) containing several hundreds of stacked, disk-shaped membrane pouches of uniform size. The outer segment of cone cells (B2, D) is of a conical shape, and the proximal membrane folds are larger than the distal ones. A thin, eccentric cyto-plasmic bridge, the connecting cilium (ACD3), links the outer segment to the innersegment (AB4). The bridge contains a mod-ified cilium with 9 pairs of microtubules but without the central pair characteristic of other cilia (see p. 285, D5, D6). The connect-ing cilium is relatively long in some species, leaving a distinct space between the two segments (A). In humans, however, it is so short that both segments touch each other without leaving a visible space between them (B). The inner segment contains numerous Golgi stacks, ribosomes, and longitudinally arranged mitochondria. The cell body then tapers to form an axonlike process (AB5) containing neurofilaments and microtubules. The cell nucleus (AB6) lies either at the transition from the inner segment to the axon or within the axon. The cell terminates with an end bulb, the synap-tic terminal (A7). In addition to the usualsynapses, the terminal develops invaginatedsynapses (A8) in which the presynapticmembrane becomes invaginated and sur-rounds the postsynaptic complex on all sides.
The outer segment is the actual receptorpart of the cell where the light is absorbed.The stacked membranes are formed by in-foldings of the plasma membrane (C9) in the proximal part of the outer segment. In the rod cells, they detach from the outer membrane and form isolated disks in the distal part of the segment (C10). The visualpigment of the rod cells,rhodopsin, is boundto the membrane of the disks. The forma-tion of visual pigment in the inner segmentand its migration through the connecting bridge into the outer segment can be fol-lowed using autoradiography by labeling the protein component of rhodopsin with a radioactive amino acid. Once the labeled substance has passed the bridge, it forms a band that migrates to the outer end and then disappears (in rats within 10 days). The migrating band represents a membrane disk that has incorporated labeled rhodop-sin. Thus, new disks are being continuously formed in the rod cells, migrate to the distal end, and are shed there. Fragments of the shed disks have been found in the pig-mented epithelial cells. There is no new for-mation of disks in the outer segments of cone cells (D). The infoldings of the mem-brane are permanent and, in contrast to rod cells, there is no detachment of membrane invaginations from the plasma membrane.
Only rod cells contain rhodopsin. The ab-sorption of light changes the molecular structure of rhodopsin, causing it to break down into its protein and pigment com-ponents. From these components, rhodop-sin is continuously resynthesized in the rods (rhodopsin–retinin cycle). It absorbs light of all wavelengths and, thus, is not in-volved in color vision. Rod cells are light–dark receptors. The three different types ofcone cells each contain a different pigment that absorbs only light of a specific wavelength. Cone cells are color receptors.
There are some animal species in which the retina contains only cones, while it contains only rods in other species (cat, cattle). Ani-mals with a retina containing only rods can-not distinguish between colors. The bull, which is said to react to red, is actually color blind.