LECTURE 17: VISION--ORGANIZATION AND FUNCTION OF RETINA

REQUIRED READING: Kandel text, Chapter 26

The retina is part of the central central nervous system, consisting of light-sensing neurons (rods and conesrods and cones), interneurons (bipolarbipolar, horizontal,horizontal, and amacrineamacrine cells), and retinal ganglion cellsretinal 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

11-cis retinal

All-trans retinal

PHOTON

Rhodopsins: GPCRs With Tethered Light-Activated Ligand

Rhodopsin consists of a GPCR called opsinopsin, on which is linked avitamin A derivative called retinalretinal.

When retinal is an all-trans isomerall-trans isomer, it activates opsin, which signals tothe heterotrimeric G protein transducintransducin.

Light-Activated Rhodopsin Turns Off a Dark Current ThroughcGMP-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 colorOnly our central foveal vision can detect color

Cones Each Express One of Three Distinct PhotoreceptorsRods All Express a Common Photoreceptor Not Found in Cones

Rods can detect as littleas ONE PHOTON of light,

while cones are lesssensitive

Basis of Color Perception Lies In Overlapping But DistinctAbsorption 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 BipolarHas elevated resting potential

Hyperpolarizes when coneis stimulated

Has ionotropic GluRsionotropic GluRs

On-Center BipolarDepolarizes when cone

is stimulatedHas metabotropic GluRsmetabotropic GluRsthat hyperpolarize cell

(in some cases, bysame 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 preciseboundaries of light contrast detection are somewhat different.

A computer emulation of “edge detection” using retinal receptive fields.A computer emulation of “edge detection” using retinal receptive fields.ON-centerON-center and and OFF-centerOFF-center stimulation is shown in stimulation is shown in redred and and greengreen respectively. respectively.