New Chapt. 24-retinal processing(1)

New Chapt. 24-retinal processing(1) - CHAPTER 24 NEURAL...

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259 CHAPTER 24 NEURAL PROCESSING IN THE RETINA We have seen in a previous lecture that photoreceptors are depolarized in the dark and hyperpolarize in response to light. The changes in membrane potential then have to be passed onto neurons and "processed", so that the visual information in the retina can be coded in trains of action potentials that are conveyed to higher visual centers in the brain. Structure and cell types of the retina The retina is composed of several layers of cells and the processes of these cells. Transduction occurs, as we have already seen, in the photoreceptors that actually lie in the innermost layer of the retina (at the back of the eye). That is, a ray of light must penetrate several layers of retinal cells before reaching the photoreceptors. The arrangement is shown in the figure below. The electrical signals generated by phototransduction in the rods and cones are passed via chemical synapses to other retinal cells. The connections of the photoreceptors are, in principal, quite simple. Photoreceptors (rods and cones) make a synaptic connection with cells called bipolar cells. They are called bipolar cells because they have two processes: one process, or pole, makes a synaptic connection with a rod or cone, from which it receives innervation, while the other process makes a synaptic connection on the third type of cell, the ganglion cell. The ganglion cells sends their axonal process into the optic nerve where they travel to the lateral geniculate body (the visual thalamus). Thus, signals from photoreceptors are passed to bipolar cells and then to ganglion cells. It is these ganglion cells, which, behaving like "typical" neurons, generate action potentials and transmit them over axons into the brain. Fig. 1. Circuitry of the retina showing the major types of cells and how they are synaptically connected.
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260 In addition to the photoreceptors, bipolar cells, and ganglion cells, the retina contains two additional cell types. These cells are arrayed in two additional layers. The first type are horizontal cells, which are found in a layer between the bipolar cells and the photoreceptors. The second type are amacrine cells, which occur in a layer between the ganglion cells and the bipolar cells. In contrast to the first three types of retinal cells, the horizontal and amacrine cells spread their processes laterally for some distance. They seem to be specialized to interconnect adjacent ensembles of photoreceptor-bipolar-ganglion cell combinations. Horizontal cells receive synapses from photoreceptors and make synapses onto bipolar cells. We will see shortly that these horizontal cells are responsible for the creation of lateral interactions in the retina. We will not discuss amacrine cells further.
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This note was uploaded on 09/19/2011 for the course BIO 365R taught by Professor Draper during the Spring '08 term at University of Texas.

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New Chapt. 24-retinal processing(1) - CHAPTER 24 NEURAL...

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