Figure 2312

Electron micrographs of portions of the inner and outer segments of cones and rods. a. This electron micrograph shows the junction between the inner and outer segments of the rod cell. The outer segments contain the horizontally flattened discs. The plane of this section passes through the connecting stalk and cilium. A centriole, a cilium and its basal body, and a calyceal process are identified. x32,000. b. Another electron micrograph shows a similar section of a cone cell. The interior of the discs in the outer segment of the cone is continuous with the extracellular space (arrows), x 32,000. (Courtesy of Dr. Toichiro Kuwabara.)

It then takes several days for the disc to reach the tip of the outer segment. In contrast, although visual proteins are constantly produced in retinal cones, the proteins are incorporated into cone discs located anywhere in the outer segment.

Vision is a process by which light striking the retina is converted into electrical impulses that are transmitted to the brain

The impulses produced by light reaching the photoreceptors are conveyed to the brain by an elaborate network of nerves. The conversion of the incident light into nerve impulses is called transduction and involves two basic steps:

• Step 1 is a photochemical reaction that occurs in the outer segment of the rod and cone receptors as absorbed light energy causes conformational changes in the chro-mop bores.

• Step 2 consists of changes in the concentration of internal transmitters within the cytoplasm of the inner segment of the photoreceptors. These changes influence the ionic permeability of the plasma membrane and cause the photoreceptor to become hyperpolarized, thus initiating impulses that are conveyed to the brain.

In rods, absorbed light energy causes conformational changes in retinal, converting it to retinol

The conversion of retinal to retinol results in its release from scotopsin (a reaction called "bleaching"). The cell becomes hyperpolarized as calcium diffuses from the receptor cell and reduces its permeability to sodium. The visual pigment is then reassembled, and calcium is transported back into the cell. The energy for this process is provided by the mitochondria located in the inner segment. Miiller's cells and pigment epithelial cells also participate in the in-terconversion of retinal and retinol and the reactions necessary for the resynthesis of rhodopsin.

During normal functioning of the photoreceptors, the membranous discs of the outer segment are shed and phagocytosed by the pigment epithelial cells (Fig. 23.13). It is estimated that each of these cells is capable of phago-cytosing and disposing of about 7500 discs/day. The discs are constantly turning over, and the production of new discs must equal the rate of disc shedding.

Discs are shed from both rods and cones

In rods, after a period of sleep, a burst of disc shedding occurs as light first enters the eye. The time of disc shedding in cones is more variable. The shedding of discs in cones also enables the receptors to eliminate superfluous membrane. Although not fully understood, the shedding process in cones also alters the size of the discs, so that the conical form is maintained as discs are released from the distal end of the cone.

The outer limiting membrane (layer 3) is formed by a row of zonulae adherentes between Müller s cells

The outer limiting membrane is not a true membrane. It is a row of zonulae adherentes that attaches the apical ends of Miiller's cells (i.e., the end that faces the pigment epithelium) to each other and to the rods and cones (see Fig. 23.10). Because Miiller's cells end at the base of the inner segments of the receptors, they mark the location of this layer. Thus, the supporting processes of Miiller's cells on which the rods and cones rest are pierced by the inner and outer segments of the photoreceptors.

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