Figure 1A is a schematic representation of the salient structural features of the vertebrate rod cell. As indicated, the cell has two distinct anatomical regions, the inner and outer segments. The inner segment contains the subcellular organelles required for the usual metabolic processes of the cell and terminates in a synaptic foot. The outer segment is specialized for phototransduction and is densely packed with a highly ordered Stack of flattened membrane saccules generally referred to as discs, the individual membranes of which contain the integral membrane protein rhodopsin in high concentration. The rims of the discs have an exceedingly small radius of curvature and are specialized structures with a composition distinctly different from the planar regions of the membrane. The adjacent membranes of a disc lie in close apposition, resulting in a very attenuated intradiscal spacing of only about 30 Å. The highly regular spacing between adjacent discs (approximately 150 Å) and between the disc rims and the plasma membrane of the outer segment is apparently achieved by filamentous structures that bridge the interdisc and disc-plasma membrane spaces. This cytoskeletal network is presumably responsible for the observed structural rigidity of the outer segment.
KeywordsOuter Segment Light Response Rhodopsin Molecule Visual Transduction Plasma Membrane Ionic Channel
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