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FIGURE 5 Cellular anatomy of the retina. Photoreceptors, bipolar cells, and ganglion cells form the "straight-through" retinal pathway with synaptic relays for processing visual information in the outer and inner plexiform layers. Horizontal and amacrine cells provide lateral interactions within outer and inner plexiform layers, respectively. Interplexiform cells (not shown) feed back information from the inner to the outer plexiform layer.

which provide feedback from inner to outer plexiform layers.

The visual pathway not only must transmit information about the intensity of light present in the environment and its spectral properties but must also convey information about the location of light sources. The anatomic arrangement of the visual system must be such that spatial information about the visual field can be encoded and transmitted to higher cortical centers for processing. Thus, retinal circuitry is highly ordered along vertical and horizontal planes. In the vertical plane, cell bodies and synapse are arranged in distinct layers. Superimposed on this laminar structure are horizontally repeating modular arrangements of cells that form functional columns or channels (not to be confused with ion channels in membranes)—for example, a color-specific channel consisting of a cone or small group of cones and their associated bipolar and ganglion cells.

ON and OFF Channels of Visual Information

Two types of functional channels established by retinal circuitry are designated as ON and OFF channels (Fig. 6). An OFF channel is formed by a column of cells that are all depolarized when the light is decreased. All photoreceptors are depolarized in the dark and therefore release more glutamate when the light is OFF. Many bipolar cells exhibit OFF responses, as well, because they are depolarized by the increased levels of glutamate released by photoreceptors in the dark. Other bipolar cells have the opposite (ON) response and are hyperpo-larized when light levels decrease. The reason for this

FIGURE 6 Synaptic interactions of ON and OFF channels in the retina. (A) When light levels are decreased, cone photoreceptors are depolarized (blue) and release more glutamate, which interacts with excitatory glutamate receptors ( + ) on OFF bipolar cells (OFF BCs) and inhibitory (metabotropic) glutamate receptors (—) on ON bipolar cells (ON BCs). OFF and ON bipolar cells in turn excite OFF and ON ganglion cells (GCs), respectively. (B) When light levels are increased, the cone photoreceptors hyperpolarize (gray) and release less glutamate, resulting in less excitation of the OFF BCs and less inhibition of the ON BCs. This signal is passed on to the GCs. Thus, OFF cells collectively form an OFF channel whose component cells are active when the light is off, and ON cells form an ON channel whose components are active when the light is on.

cones in dark cones in light cones in dark

cones in light

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