New Chapt. 23-phototransduction in retina(1)

New Chapt. 23-phototransduction in retina(1) - CHAPTER 23...

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248 CHAPTER 23 PHOTOTRANSDUCTION IN THE RETINA The vertebrate eye works very much like a photographic camera. An image of an object in visual space is focused by a lens onto a light sensitive surface or retina at the back of the eye. As with a camera, there is a means for controlling the amount of light entering the eye by varying the diameter of an aperture--the pupil at the front of the eye. The general anatomy of the eye may be seen in the drawing below. A ray of light entering the eye first strikes the transparent cornea . Its path then takes it through the clear fluid within the anterior chamber of the eye known as the aqueous humor , then through the pupil of the iris , where it encounters the lens. From the lens it then passes into another clear fluid called the vitreous humor , and finally the ray strikes the retina . Behind the retina is a layer of pigmented epithelium, whose black pigment absorbs light reaching it (i.e., not absorbed by the photoreceptors), assuring that light is not reflected around inside the eye. Fig. 1. The eye and its various parts. Retina The retina is composed of several layers of cells and the processes of these cells. Transduction, where one form of energy (light) is changed into anther form of energy (electrical), occurs in the photoreceptors that actually lie in the innermost layer of the retina (Fig. 2).
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249 Fig. 2. The retina sits on the back of the eye and is composed of several cell types that form a circuit. Light enters the eye through the pupil and lens and is captured by the photoreceptors (rods and cones). Notice that light has to first travel through the axons of ganglion cells, which sit on the surface of the retina, and then through the cell bodies of ganglion cells and bipolar cells before reaching the photoreceptors, that are located on the back of the retina. Surprisingly, a ray of light must penetrate several layers of retinal cells before reaching the photoreceptors. The retina consists both of photoreceptors and a group of nerve cells. Recall that retina forms during embryological development from an out-pocketing of the diencephalon. It is, thus, a part of the central nervous system and its job it is to transformation light first into changes in the membrane potential of its photoreceptors and then into electrical signals for transmission to higher visual centers in the brain. In this section we consider how the photoreceptors capture light, and how the capture of light leads to a change in the membrane potential of the photoreceptor (i.e., how photoreceptors transduce one form of energy into another energy form). In the next chapter we will take up the issue of how the neurons in the retina then process the information conveyed to them by the photoreceptors. A vertebrate photoreceptor: The rod
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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 at Austin.

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New Chapt. 23-phototransduction in retina(1) - CHAPTER 23...

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