LECTURE 17: VISIONORGANIZATION AND FUNCTION OF RETINA

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LECTURE 17 VISION--ORGANIZATION AND FUNCTION OF
RETINA
REQUIRED READING Kandel text, Chapter 26
The retina is part of the central nervous system,
consisting of light-sensing neurons (rods and
cones), interneurons (bipolar, horizontal, and
amacrine cells), and retinal ganglion cells that
provide the sole output to higher brain
centers. Retinal neurons and circuits are
designed to provide information on spatial and
temporal visual novelty, and accomplish this by
filtering out absolute light intensity
(brightness)and amplifying spatial
differences in light intensity (contrast) and
temporal differences in light
intensity. Color vision is accomplished by
parallel processing of a light region by cones
possessing distinct photoreceptors with
different absorption spectra.
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Anatomy of Human Eye
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Gross Anatomy of Retina
Light ray
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Rhodopsins GPCRs With Tethered Light-Activated
Ligand
Rhodopsin consists of a GPCR called opsin, on
which is linked a vitamin A derivative called
retinal. When retinal is an all-trans isomer, it
activates opsin, which signals to the
heterotrimeric G protein transducin.
11-cis retinal All-trans retinal
PHOTON
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Light-Activated Rhodopsin Turns Off a Dark
Current Through cGMP-gated Cation Channel
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Photoreceptor Cells and Bipolar Cells Do Not Fire
Action Potentials
Voltage in photoreceptor cells ranges between -40
mV (when dark current is on) down to -70 mV
(when current is shut off). The cGMP-gated
cation channels conduct calcium as well as
sodium, thereby mediating neurotransmitter
glutamate release. Light-induced
hyperpolarization stops glutamate
release. Target bipolar cells also have
cGMP-gated channels, and depolarization of
bipolar cells also promotes glutamate release.
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Rods and Cones Have Different Distributions
Across Retina
Only our central foveal vision can detect color
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Cones Each Express One of Three Distinct
Photoreceptors Rods All Express a Common
Photoreceptor Not Found in Cones
Rods can detect as little as ONE PHOTON of
light, while cones are less sensitive
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Basis of Color Perception Lies In Overlapping But
Distinct Absorption Spectra of Cone Photoreceptors
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Receptive Field Reflects Vertical and Horizontal
Neural Projections On-Center and Off-Center
Bipolar Cells
Any cone signals vertically to both off-center
and on-center bipolar cell.
Off-Center Bipolar Has elevated resting
potential Hyperpolarizes when cone is
stimulated Has ionotropic GluRs
On-Center Bipolar Depolarizes when cone is
stimulated Has metabotropic GluRs that
hyperpolarize cell (in some cases, by same
mechanism as rhodopsin)
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Off-Center Pathway Causes Ganglion Cells to Stop
Firing In Response to Light
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Horizontal Cells Provide Lateral Inhibition That
Enhances Detection of Contrast
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