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SENSORY SYSTEMS

An individual is diagnosed with retinitis pigmentosa. Which of the following is the most likely cause of this disease?

a. Degeneration of area 17 of the cerebral cortex

b. Degeneration of bipolar cells of the retina

c. Degeneration of amacrine cells

d. Degeneration of retinal ganglion cells

e. Degeneration of photoreceptors

The answer is E. (Ropper, pp 211, 950. Purves, p 265.) In one form of retinitis pigmentosa, there is a genetic defect with respect to rhodopsin. The result of this defect is the production of defective opsin. As a consequence, rod cells are affected, leading to a reduced response to light. Consequently, there is degeneration of photoreceptors which die by apoptosis. At a later time, cone cells also appear to degenerate. Other components of the retina (such as retina/ganglion cells) and CNS neurons (such as those located in area 17) are not directly affected and vision is not totally lost.

A 21-year-old man, who previously had normal vision, suddenly becomes blind. A detailed examination and analysis indicates that he was infected by a virus that selectively attacked and destroyed the same retinal cells that are attacked in felines as determined from experimental studies. In such studies, the cells attacked by the virus are ones that produce action potentials in response to changes in position of selective objects presented to the visual field of a cat. Which of the following retinal cells are destroyed by this virus?

a. Amacrine cells

b. Rods

c. Cones

d. Ganglion cells

e. Horizontal cells

The answer is D. (Kandel, pp 512-521. Siegel and Sapru, pp 275-281.) The only cell in the retina that is capable of producing an action potential is the ganglion cell. As indicated earlier, the ganglion cell gives rise to optic nerve fibers, which terminate as optic tract fibers in the lateral geniculate nucleus. As a result of action potentials generated in the ganglion cells, volleys of impulses are transmitted over these fibers, resulting in the appropriate responses in the neurons of the lateral geniculate nucleus. Therefore, the patient became blind of damage to the ganglion cells whose axons form the optic nerve and optic tract, thus depriving the lateral geniculate nucleus and visual cortex from receiving visual inputs.

A patient is diagnosed by the same optometrist as being farsighted. To correct this person’s defect, which of the following lenses should the ...

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