Page 361 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Visual Acuity, Photosensors red curve). In a dark-adapted eye, on the other
hand, the sensitivity of the retina (! B2, blue
Visual acuity is an important measure of eye curve) is completely dependent on the rod dis-
function. Under well-lighted conditions, the tribution (! B1, purple curve). The color-sen-
normal eye should be able to distinguish two sitive cones are therefore used for visual per-
points as separate when the light rays emitted ception in daylight or good lighting (day vision,
by the point objects converge at an angle (α) of photopic vision), while the black and white-
sensitive cones are used to visualize objects in
1 min (1/60 degree) (! A and p. 346). Visual
Central Nervous System and Senses charts with letters or other optotypes (e.g., Landolt sion is associated with a high loss of visual acu-
– 1
acuity is calculated as 1/α (min ), and is 1/1 in
darkness (dim-light vision, night vision, scotop-
subjects with normal vision.
tic vision). The high light sensitivity in night vi-
Visual acuity testing is generally performed using
ity (! p. 354).
rings) of various sizes used to simulate different dis-
Photosensor Function
tances to the test subject. The letters or rings are
Light-absorbing visual pigments and a variety
usually displayed at a distance of 5 m (! A). Visual
of enzymes and transmitters in retinal rods and
acuity is normal (1/1) if the patient recognizes letters
or ring openings seen at an angle of 1 min from a dis-
cones (! C1) mediate the conversion of light
tance of 5 m. Example: It should be possible to iden-
stimuli into electrical stimuli; this is called
tify the direction of the opening of the middle ring
from a distance of 5 m and that of the left ring from a
disks of the retinal rods contain rhodopsin
(! C2), a photosensitive purple-red chromo-
left ring can be localized from the test distance of
protein (visual purple). Rhodopsin consists of
5 m, the visual acuity is 5/8.5 = 0.59.
12 distance of 8.5 m (! A). If only the opening of the photoelectric transduction. The membranous
Photosensors or photoreceptors. The light- the integral membrane protein opsin and the
sensitive sensors of the eye consist of approxi- aldehyde 11-cis-retinal. The latter is bound to a
lysine residue of opsin which is embedded in
mately 6 · 10 rods and 20 times as many cones
6
(! p. 345 E) distributed at variable densities this protein; it is stably kept in place by weak
throughout the retina (! B1). (Certain gan- interactions with two other amino acid resi-
glion cells also contain a light-sensitive pig- dues. Photic stimuli trigger a primary photo-
ment; ! p. 334). The fovea centralis is exclu- chemical reaction in rhodopsin (duration,
– 14
sively filled with cones, and their density 2 · 10 s) in which 11-cis-retinal is converted
rapidly decreases towards the periphery. Rods to all-trans-retinal (! C3). Even without con-
predominate 20–30 degrees away from the tinued photic stimulation, the reaction yields
bathorhodopsin, the intermediates lumirho-
fovea centralis. Approaching the periphery of
the retina, the density of the rods decreases dopsin and metarhodopsin I, and finally meta-
– 3
2
5
continuously from 1.5 ! 10 /mm (maximum) rhodopsin II within roughly 10 s (! D1).
to about one-third this value. No photosensors Metarhodopsin II (MR-II) reacts with a G s-
are present on the optic disk, which is therefore protein (! p. 274) called transducin (G t-pro-
referred to as the blind spot in the visual field. tein), which breaks down into α s and "γ sub-
Clear visualization of an object in daylight units once GDP has been replaced by GTP
requires that the gaze be fixed on it, i.e., that an (! D1). Activated α s-GTP now binds the inhibi-
tory subunit of cGMP phosphodiesterase (I PDE)
image of the object is produced in the fovea
centralis. Sudden motion in the periphery of (! D2). The consequently disinhibited phos-
the visual field triggers a reflex saccade phodiesterase (PDE) then lowers the cytosolic
(! p. 360), which shifts the image of the object concentration of cyclic guanosine mono-
phosphate (cGMP). The activation of a single
into the fovea centralis. Thereby, the retinal
area with the highest visual acuity is selected retinal rhodopsin molecule by a quantum of 6
(! B2, yellow peak), which lies 5 degrees tem- light can induce the hydrolysis of up to 10
poral to the optical axis. Visual acuity cGMP molecules per second. The reaction cas-
decreases rapidly when moving outward from cade therefore has tremendous amplifying
power.
348 the fovea (! B2, yellow field), reflecting the
decreasing density of cone distribution (! B1,
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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