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Lecture 6

PSYB51H3 Lecture Notes - Lecture 6: Amacrine Cell, Opsin, Rhodopsin

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Matthias Niemeier

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Photoreceptor Density across the Retina
o Highest density of cones is in the fovea
Visual Pigments
o Visual pigments are molecules with two parts
o Chromophores (captures photons)
o Opsin (absorbs light of a specific wavelength)
o 2 types of visual pigments
o Rhodopsin
o 3 subtypes for cones
S-cones (5-10%)
M-cones (~30%)
L-cones (twice as many as M-cones) (~60%)
o Process of photoactivation
1. Hyperpolarization
2. Graded potentials
o Most other neurons
1. Depolarization
2. Action potentials
Whistling in the Dark: Dark and Light Adaptation
o 4 mechanisms important for dark and light adaptation
o Pupil dilation
o Photoreceptors
o Photopigments replacement
o Adaptive circuitry
o 5th mechanism that doesn’t adapt is the most important one
o Neural circuitry of the retina
Filters out contrast information and ‘ignores’ absolute luminance levels
o Pupil dilation
o 16-fold improvement in sensitivity
o Photoreceptors
o Receptor adaptation works at different rates/to different extend for rods and
o Photopigment replacement
o Abundance of photopigments
o Bleaching of photopigments
o Adaptive mechanisms in retina circuitry
o e.g. rods connect to ganglion cells through specialized bipolar and amacrine cells
but during daylight these special amacrines are inhibited
o Neural circuitry of the retina accounts for why we are not bothered by variations in
overall light levels
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