results in the closing of the gated Na+channels, a reduction in permeability to
Na+,and hyperpolarization of the cell membrane. The signal spreads to the
inner segment and through gap junctions to nearby photoreceptor cells. In
the presence of cGMP, the Na+channel remains open; in its absence, the chan-
nel closes and the cell hyperpolarizes. Therefore, the rods and cones differ
from other receptors in that hyperpolarization of the cell membranes occurs
rather than the depolarization that occurs in other neural systems. Closing the
channel slows down the release of the visual transmitter.
264.The answer is b.(Junqueira, pp 462–467.)The retinal pigment
epithelium (RPE) is a single layer of cells that phagocytose old components
of photoreceptor cells. It is derived from the outer layer of the optic cup
and is continuous from the ora serrata retinae to the optic nerve. Microvilli
are prominent on the apical surfaces of the RPE and play an important role
in the maintenance of the blood-retinal barrier. In addition, the RPE syn-
thesizes melanin and stores vitamin A for the photoreceptor cells. Rod and
cone perikarya and amacrine cells are found in the photosensitive retina
derived from the inner layer of the optic cup.
265.The answer is b.(Kumar, pp 1437–1440. Kasper, pp 2163–2164. Newell,
pp 507–510. Vaughan, pp 191–194. Kierszenbaum, pp 526–527.)Retinopathy is
one of the major complications of diabetes mellitus. In diabetic retinopathy
pathologic changes usually begin with thickening of the basement membrane
of small retinal vessels. The abnormal vessels develop microaneurysms, which
leak and hemorrhage with resultant ischemia of the retinal tissue. New vessels
proliferate in response to ischemia and production of angiogenic factors. Loss
of phagocytic capacity of the RPE occurs in retinal dystrophy, but is not a char-
acteristic of diabetic retinopathy. Retinopathy also occurs with prematurity
when retinal vascularization is disturbed.
266.The answer is c.(Junqueira, pp 456–463.)The structure labeled B is
the sinus venosus sclerae (canal of Schlemm), which carries aqueous
humor to the scleral veins and the systemic vasculature. Blockage of the
canal of Schlemm, the trabecular meshwork, or the scleral veins results in
glaucoma. The overall figure shows the region of the iridocorneal angle and
other associated structures in the eye. This region is extremely important in
the production and outflow of the aqueous humor and in the distribution
of zonule fibers to the lens. The iris (C)contains both sphincter (D)and
402 Anatomy, Histology, and Cell Biology