Visual System II:Retinal Processing
Adequate Stimulus: A stimulus of a quality and of sufficientintensity to excite a sensory receptor.
Adequate Stimulus for Visual System: Electromagneticwave with a wavelength between 350 and 750 nm and of sufficient intensity to excite a photoreceptor.
100μm
B
A
Foveacones
Peripheryrodscones
rhodopsin 11-cis retinal
+ opsinall-trans retinal
+ opsin
11-cisretinal
All-transretinal
opsin opsin
When light strikes photoreceptoractivatesG-proteinactivatesc-GMP phosphodiesterasec-CMP hydrolyzed to 5-GMPc-GMP dependent cationchannels closereduced Na+
and C2+ currentshyperpolarization of cellreduces release of glutamate
In dark, cGMP keeps cation(mainly Na+) channels open.The inward Na+ “dark current”keeps the resting membranepotential at about -40 mv. This depolarization keepsvoltage-gated Ca2+ channelsopen, resulting in release (“dripping “)of glutamatefrom the photoreceptor.
bipolar cells
amacrine cells
ganglioncells
horizontalcells
photoreceptors
rods
cones
direct vs. indirect pathways to ganglion cells
visual receptive fields
fovea periphery
Receptive field sizeabout 0.025 mm diam.
Receptive field size about 1.0 mm diam