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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.
Anatomy of Human Eye
Gross Anatomy of Retina
Light ray
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
PHOTON
All-trans retinal
Light-Activated Rhodopsin Turns Off a Dark Current Through
cGMP-gated Cation Channel
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.
Rods and Cones Have Different Distributions Across Retina
Only our central foveal vision can detect color
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
Basis of Color Perception Lies In Overlapping But Distinct
Absorption Spectra of Cone Photoreceptors
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.
Rod cells only signal through on-center bipolar cells.
http://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/bipolar-cell-pathways-in-the-vertebrate-retina/
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)
Off-Center Pathway Causes Ganglion Cells to Stop Firing In Response to
Light Center and Dark Surround
Horizontal Cells Provide Lateral Inhibition That Enhances Detection of Contrast
Functional Reasons for Parallel ON and OFF Bipolar Cells
ON bipolar cells are more responsive to dim light than are OFF bipolar cells. Therefore,
loss of ON bipolar cell function causes night blindness.
Possibly because of their different sensitivities to light intensities, the precise
boundaries of light contrast detection are somewhat different.
A computer emulation of “edge detection” using retinal receptive fields.
ON-center and OFF-center stimulation is shown in red and green respectively.