Eye and Ear
Answers
261.The answer is a.(Kasper, p 162.)LASIK has largely replaced radial,
or refractive, keratotomy. Both methods can be used to decrease the refrac-
tion of light by the cornea. LASIK alters corneal (not lens) anatomy to cre-
ate a new shape that is more flattened in the center and higher at the
periphery of the cornea. This occurs because intraocular pressure will
cause a reshaping of the cornea due to induced weakness produced by the
laser removal of tissue. The purpose of the procedure is to reduce myopia
to eliminate the need for corrective lenses. The reduction in curvature of
the central portion of the cornea results in decreased refractive power of
the cornea. The removal of tissue and the degree of correction required is
estimated by computer simulations.
262.The answer is c.(Newell, pp 322–324. Vaughan, pp 172–173, 187–189.)
Retinal detachment is the result of the accumulation of fluid between the
retina and the RPE. In one type of detachment, rhegmatogenous retinal
detachment, fluid accumulates after a break occurs in the retina. Detachments
without breaks in the retina are called nonrhegmatogenous, or serous, detach-
ments. Vitreous degeneration usually is a prerequisite for retinal detachment
that results in the breaking of the retina. The breakdown of the vitreous pro-
duces traction on the retina, which may already possess an inherent area of
weakness. The site of retinal detachment is the area between the inner and
outer layers of the embryonic optic cup and represents a relatively weak area
of adherence between the retinal and RPE layers.
263.The answer is d. (Junqueira, pp 456–458, 466. Kierszenbaum, pp
244–245.)Visual transduction involves closing of the Na+channel in rod cells
in response to photons of light. Rhodopsin is the visual pigment found in the
outer segments of rod cells and is composed of retinal, a vitamin A derivative,
bound to opsin. When photons strike rhodopsin, 11-cis-retinal is isomerized
to 11-trans-retinal resulting in bleaching, which represents the dissociation
of retinal from opsin. The conformationally-altered opsin then acts on trans-
ducin, a G protein that couples bleaching to cGMP through the action of a
phosphodiesterase enzyme that cleaves cGMP to GMP. Breakdown of cGMP
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