Page 334 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Diseases of the Retina
The receptors of the retina (→ A1b) are rods preserved. In retinal detachment from the pig-
(Rs) and three different types of cones (Cs). ment epithelium no deflections are registered
The latter mediate the color sense (red, green, in the ERG. If the retina is completely de-
blue; see below) and are particularly numer- tached, the patient is totally blind.
ous at the site of sharpest vision (fovea centra- Diabetic retinopathy (→ B) is the most com-
Systems lis). The rods mediate black and white vision mon disease of the retina. The cells around the
and particularly predominate in the retinal
thin retinal blood vessels (pericytes) produce
periphery. The light-sensitive outer segments
sorbitol from the increased supply of glucose
(→ p. 290), swell up, and thus narrow the ves-
of the photoreceptors are renewed regularly,
Neuromuscular and Sensory cells are phagocytized. The photoreceptors ened by glycosylation (AGE; → p. 290). This re-
sels. Additionally, the vessel walls are thick-
while the residues of the pigment epithelial
sults in ischemia of the tissues, formation of
transmit their excitation via bipolar cells
(Bps) to the ganglion cells (Gs). Amacrine cells
angiotrophic mediators, increase in vascular
permeability, formation of new vessels, and
(Ams) and horizontal cells (Hcs) form cross-
hemorrhages. This bleeding opacifies the vit-
connections between photoreceptors, bipolar
cells and ganglion cells (→ A1a).
real body, the ischemia destroys the retina
If phagocytosis of the pigment epithelial
and may ultimately lead to blindness.
Night blindness. The visual pigment con-
late and the photoreceptors degenerate (retini-
tis pigmentosa; → A2). Macular degeneration
A and a protein that is different in the rods
10 cells is impaired, metabolic products accumu- sists of 11-cis-retinol, a metabolite of vitamin
that occurs in childhood (Stargardt’s disease)
and the three types of cones (→ C1). In vitamin
is due to a genetic defect of an ATP-binding A deficiency the formation of visual pigment in
transport protein (ABCR) that is normally rods and cones is impaired, resulting in re-
expressed in the outer segment of the photore- duced light perception especially at low light
ceptors. A defect of this transporter can disturb intensity.
the normal turnover of the outer segments. The function of the cones is to provide color
Heterozygote carriers of the genetic defect suf- vision. The pigments of the red, green, and blue
fer from increasing macular degeneration as cones each have different spectral sensitivities.
they grow older. Mutations of the genes for the respective pig-
Electroretinogram (ERG). When light falls ments impair color vision. Partial or complete
on the retina, potential differences can be re- loss of the particular pigment (→ C2) leads to
corded between the cornea and an indifferent weak red color vision or red color blindness
electrode on the ear (→ A3). Sudden exposure (protanomaly or protanopia, respectively),
to light at first generates an a-wave, the sum- green color weakness or blindness (deuter-
mation of potential changes at the receptors. anomaly or deuteranopia), or blue color weak-
It is followed by a b-wave due to potential ness or blindness (tritanomaly or tritanopia).
changes in the bipolar cells and glial cells, and As the genes for the red and green pigments
a c-wave due to potential changes in the pig- are located on the X chromosome, many more
ment epithelium. When the light is turned off, men than women suffer from red or green col-
a d-wave is registered (off-effect), the sum of or blindness.
the potential changes in the photoreceptor If there are no cones, not only is there no
and bipolar cell membranes (reversed poten- color vision, but visual acuity is also greatly re-
tial). duced, because the person can see only with
Occlusion of the central artery causes death much fewer rods in the fovea (rod monochro-
of the amacrine cells, bipolar cells and gan- masia).
glion cells and thus blindness. However, the Color vision can be tested e.g. with tables in
receptors and pigment epithelium survive be- which the numbers can be correctly recog-
cause they are supplied with adequate oxygen nized only by means of the corresponding
324 by the choroid vessels. In the ERG the b-wave cones (→ C3).
is thus absent, but the a-wave and c-wave are
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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