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