Retinal cone cells

Figure 30-10 (A) Schematic drawing of a synapse. (B) Electron micrograph showing the synaptic junctions in the basal part (pedicle) of a retinal cone cell of a monkey.403 Each pedicle contains synaptic contacts with 12 triads, each made up of processes from a bipolar cell center that carries the principal output signal and processes from two horizontal cells that also synapse with other cones. A ribbon structure within the pedicle is characteristic of these synapses. Note the numerous synaptic vesicles in the pedicle, some arranged around the ribbon, the synaptic clefts, and the characteristic thickening of the membranes surrounding the cleft (below the ribbons). Micrograph courtesy of John Dowling.

The retinol that is delivered to the retinas of the eyes in this manner is accumulated by rod and cone cells. In the rods (which are the better characterized of the two cell types), retinol is oxidized by a specific retinol dehydrogenase to become 2iW-trans retinal and then converted to 11-eis retinal by reti-... 

Arikawa, K., Molday, L. L., Molday, R. S. and Williams, D. S. Localization of peripherine/rds in the disk membranes of cone and rod photoreceptors relationship to disk membrane morphogenesis and retinal degeneration. /. Cell Biol. 116 659-667,1992. 

In colour vision there are three specific types of cone cell corresponding to red, green and blue receptors. The chromophore is the same for all three colours, being 11-cis-retinal bound to a protein which is structurally similar to opsin. Colour selectivity is achieved by positioning specific amino acid side chains along the chromophore so as to perturb the absorption spectrum of the chromophore. 

A (carotene) Retinoic acid and retinol act as growth regulators, especially in epithelium Retinal is important in rod and cone cells for vision... 

Studies on teleost fish and mammals have demonstrated the existence of non-rod, non-cone ocular photoreceptors. In the case of VA opsin in the roach, electrophysiological evidence suggests that one function of this photosensory photopigment is to modulate the activity of retinal horizontal cells. To what end remains unclear, but this fits with the general role of horizontal cells in the teleost... 

Rod and cone cells are the light sensitive receptor cells in the retina of the human eye. About three million rod cells are responsible for our vision in dim light, whereas the hundred million cone cells are responsible for our vision in the bright light and for the perception of bright colours. In the rod cells, ll-cw-retinal is converted to rhodopsin. 

Gtl/ Gt2 Photons (rhodopsin and color opsins in retinal rod and cone cells) t cGMP phosphodiesterase - cGMP (phototransduction)... 

Cyclic GMP has a second mode of action in the vertebrate eye it causes ion-specific channels to open in the retinal rod and cone cells. We return to this role of cGMP in the discussion of vision in Section 12.7. 

The membranes of the rod discs are -60% protein and 40% lipid (Table 8-3). About 80% of the protein is rhodopsin (visual purple), a lipoprotein that is insoluble in water but soluble in detergent solutions. Digito-nin is widely used to disperse rhodopsin molecules because it causes no change in optical properties. In addition to rhodopsin, in the outer segment discs of frog retinal rods, there are -65 molecules of phospholipid and smaller amounts of other materials for each molecule of rhodopsin (Table 8-3). The cone cells have a similar architecture but have a different shape and contain different light receptors. The receptors in the cones are present in deep indentations of the plasma membrane rather than in discs within the cytoplasm. 

Some details about cone cells and invertebrate vision. The biochemistry of retinal cones is less well known but is similar to that of rod cells. Cone pigments are present in the plasma membrane rather than in isolated discs (Fig. 23-40C). Different a, P, and y subunits of transducin are formed in rods and cones.522 Many differences are seen among various invertebrate visual systems. Inositol triphosphate (IP3) and Ca2+ often serve as signals of photoexcitation. G proteins also play prominent roles.522... 

An important cause of blindness is retinitis pigmentosa, an inherited disease affecting about one in 3000 persons. Symptoms include progressive night blindness, degeneration of the rod cells, and gradual loss of cone cells and of nerve function in the retina. 

The Visual Pigments Are Found in Rod and Cone Cells Rhodopsin Consists of 11-cri-Retinal Bound to the... 

The light-sensitive protein complex in the rods, rho-dopsin, consists of 1 l-c/s-retinal bound as a Schiff s base to a protein, opsin. Rhodopsin is an integral constituent of membranes that form a stack of disks at one end of the cell. Cone cells, which are responsible for the perception of color, contain similar complexes in infoldings of the plasma membrane. 

Another major concern for toxicity of vigabatrin is sever persistent visual field constriction associated with retinal cone system dysfunction [63, 82], The effect did not appear to be reversible upon discontinuation of the drug. Vigabatrin also causes GABA to accumulate in retinal glial cells in rats, suggesting a mechanism for the toxic effect [83]. 

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