Begin Retinal Physiology Section

Dr. T. W. Kraft9/24/08

RETINAL PHYSIOLOGY

SOME basic rules

1) Photoreceptors hyperpolarize in response to the absorption of photons of light.

2) Depolarization leads to an increase in neurotransmitter release

3) Ganglion cells produce action potentials

on G cells spike at light onset

off G cells spike when the light goes off

4) Photoreceptors, bipolar cells, horizontal cells convey information through graded electrical potentials that are proportional to the stimulus they receive.

I. On and Off Pathways

Signals about increasing and decreasing light intensity are conveyed by separate ON and OFF pathways.

Cones have synapses with both ON and OFF bipolar cells

ON = depolarization of bipolar cells to light

OFF = hyperpolarization of bipolar cells to light

A. OFF pathway

1. Dark

(a) Cone is depolarized/ Glutamate release is high

(b) Glutamate binds to receptors on OFF bipolar cells, opens ion channel

(d) OFF ganglion cell is depolarized – spike rate increased

2. Light

a) Cone is hyperpolarized/ Glutamate release is reduced

b) OFF bipolar cell ion channels close

c) OFF bipolar cell is hyperpolarized

d) OFF ganglion cell is hyperpolarized (action potential frequency declines)

B. ON pathway

1. Dark

a) Cone is depolarized/ Glutamate release is high

b) Glutamic acid binding at receptors of ON bipolar cell activates G protein

c) G protein mediates closing of ON bipolar cell ion channels

d) ON bipolar cell is hyperpolarized

e) ON ganglion cell is hyperpolarized – spike rate reduced

2. Light

a) Cone is hyperpolarized/ Glutamate release is reduced

b) G protein is deactivated in absence of glutamic acid binding

c) ON bipolar cell ion channels open

d) ON bipolar cell is depolarized

e) ON ganglion cell is depolarized (spike, action potential freq. Increases)

II. Rod Signals

Rod (on) bipolar cell: rod signals go to a single type of bipolar cell. The rod bipolar cells do not make synaptic contact with ganglion cells directly. Instead, rod bipolar cells go to AII amacrine cells whose output is onto the terminal end of cone bipolar cells.

To leave the retina, rod signals must ride “piggyback” on the cone pathways.
This result is the culmination of several layers of convergence in the rods’ signaling pathway. The flow of information is rod → rod bipolar cell → AII amacrine cell → cone bipolar cell → ganglion cell

III. Receptive Field YOU NEED TO UNDERSTAND/DEFINE RECEPTIVE FIELD AND RETINAL TILING

The receptive field of a retinal neuron is the region of visual field (or area of retinal space illuminated) within which a stimulus will affect the neurons behavior.

Photoreceptor RF vs. ganglion cell RF

Photoreceptors -> Bipolar cells -> ganglion cells have increasing receptive field size due to signal convergence.

Difference due to convergence in signaling pathways

A. Tiling (Pg. 654) = an array of discrete elements that covers a surface without gaps or overlap (like your bathroom floor tiles). The retina will have many tilings, for example a rod photoreceptor tiling, a blue cone photoreceptor tiling, a rod bipolar cell tiling, an AII amacrince cell tiling, a directionally sensitive ganglion cell tiling; a blue-ON, yellow-OFF bistratified ganglion cell tiling. Each of the cell types or circuit types will cover the entire retina with “tiles” or basic elements of a certain size with a specific amount of overlap with the adjacent basic element.

One student description:

“Because the retina can not sketch in detail all of the visual input at one time, the visual system responds by detailed sketching of only a small part at a time. This is done at the fovea. The tiles, or retinal circuits are very small in/near the fovea and larger away from the fovea, but, the retinal image is sampled over the entire retina. This is done by arranging the vertical pathway neurons in a tile-like fashion (tiling). This allows for a continuous, singular image of the visual world. “

The highest density of tiles is seen in the regular tiling by hexagons of the cone inner segments of the fovea. The Muller cell cytoplasm serves as grout between the tiles. The tiling size (density) varies across the retina because of the changing density and size of the tiles—the rods and cones, but it is complete. The same is true of other nerve cell types. For example there are tilings of midget and diffuse bipolar cells, as well as blue cone bipolar and rod bipolar cells. The same is true of amacrine and ganglion cells as well. Every type of physiological or anatomical subtype tiles the entire retina.

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