Page 367 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Retinal Processing of Visual Stimuli AP frequency of ON ganglion cells (! B1).
Stimulation of the periphery, on the other
Light stimuli hyperpolarize the sensor poten- hand, leads to a decrease in AP frequency, but
tial of photosensors (! A, left) from ca. – 40 mV excitation occurs when the light source is
to ca. – 70 mV (maximum) due to a decrease in switched off (! B2). This type of RF is referred
conductance of the membrane of the outer to as an ON field (central field ON). The RF of
+
sensor segment to Na and Ca 2+ (! p. 348ff.). OFF ganglion cells exhibits the reverse re-
sponse and is referred to as an OFF field (cen-
The potential rises and falls much more
Central Nervous System and Senses sor potential is proportional to the logarithm (! p. 344). They invert the impulses from pho-
sharply in the cones than in the rods. As in
tral field OFF). Horizontal cells are responsible
for the functional organization of the RFs
other sensory cells, the magnitude of the sen-
of stimulus intensity divided by threshold-
tosensors in the periphery of the RF and trans-
intensity (Fechner’s law). Hyperpolarization
mit them to the sensors of the center. The op-
decreases glutamate release from the receptor.
posing central and peripheral responses lead
When this signal is relayed within the retina, a
to a stimulus contrast. At a light–dark inter-
distinction is made between “direct” signal
face, for example, the dark side appears darker
and the light side brighter. If the entire RF is ex-
flow for photopic vision and “lateral” signal
flow for scotopic vision (see below). Action
posed to light, the impulses from the center
glion cells (! A, right), but stimulus-depend-
Simultaneous contrast. A solid gray circle appears
ent amplitude changes of the potentials occur
darker in light surroundings than in dark surround-
in the other retinal neurons (! A, center).
ings (! C, left). When a subject focuses on a black-
12 potentials (APs) can only be generated in gan- usually predominate.
These are conducted electrotonically across
and-white grid (! C, right), the white grid lines ap-
the short spaces in the retina (! p. 48ff.). pear to be darker at the cross-sections, black grid
lines appear lighter because of reduced contrast in
Direct signal flow from cones to bipolar cells is con- these areas. This effect can be attributed to a variable
ducted via ON or OFF bipolar cells. Photostimulation sum of stimuli within the RFs (! C, center).
leads to depolarization of ON bipolar cells (signal in-
version) and activation of their respective ON gan- During dark adaptation, the center of the RFs
glion cells (! A). OFF bipolar cells, on the other hand, increases in size at the expense of the pe-
are hyperpolarized by photostimulation, which has riphery, which ultimately disappears. This
an inhibitory effect on their OFF ganglion cells. leads to an increase in spatial summation
”Lateral” signal flow can occur via the following (! p. 353 C3), but to a simultaneous decrease
pathway: rod ! rod–bipolar cell ! rod–amacrine in stimulus contrast and thus to a lower visual
cell ! ON or OFF bipolar cell ! ON or OFF ganglion acuity (! p. 349 B2).
cell. Both rod–bipolar cells and rod–amacrine cells Color opponency. Red and green light (or
are depolarized in response to light. Rod–amacrine
cells inhibit OFF bipolar cells via a chemical synapse blue and yellow light) have opposing effects in
and stimulate ON bipolar cells via an electrical syn- the RFs of ! ganglion cells (! p. 358) and more
apse (! p. 50). centrally located cells of the optic tract
(! p. 357 E). These effects are explained by
A light stimulus triggers the firing of an AP in Hering’s opponent colors theory and ensure
ON ganglion cells (! A, right). The AP contrast (increase color saturation; ! p. 356)
frequency increases with the sensor potential in color vision. When a subject focuses on a
amplitude. The APs of ON ganglion cells can be color test pattern (! p. 359 C) for about 30 min
measured using microelectrodes. This data can and then shifts the gaze to a neutral back-
be used to identify the retinal region in which ground, the complementary colors will be
the stimulatory and inhibitory effects on AP seen (color successive contrast).
frequency originate. This region is called the RFs of higher centers of the optic tract (V1,
receptive field (RF) of the ganglion cell. Retinal V2; ! p. 358) can also be identified, but their
ganglion cells have concentric RFs comprising characteristics change. Shape (striate or angu-
a central zone and a ringlike peripheral zone lar), length, axial direction and direction of
354 distinguishable during light adaptation (! B). movement of the photic stimuli play impor-
Photic stimulation of the center increases the
tant roles.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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