Human Eye

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Light sensitive layer is made of photo-receptors: rods (120 millions) and cones (7 millions) which absorb the light.

Plexiform Layer: nerve cells that process the signals generated by rods and cones and relay them to the optical nerve.

Choroid: carries mayor blood vessels to nourish the retina and absorb the light so that it will not be reflected back (dark pupil!)

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Rods and Cones

Covers an area of 5 cm2. A baseball a mile away gives an image covering one cone.

Cones: for more precise vision, need strong light. help to see colors. Mostly distributed in the center of the retina (fovea).

Rods: for peripheral and night vision. Sensitive to light. Mostly distributed away from fovea.

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Sensitivity

Cones: slow, fine grain, like color film.

Need high level of light (photopic condition, day)

High density, high resolution.

Rods: fast, coarse grain, black & white film

Low level of light (scotopic condition, at night)

No color is obvious.

Adaptation: Changing of retina sensitivity.

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Singal Processing

Trace the signal through the retina:

The retina is a seven-layered structure involved in signal transduction.

Light enters from the GCL side first, and must penetrate all cell types before reaching the rods and cones.

The outer segments of the rods and cones transduce the light and send the signal through the cell bodies of the ONL and out to their axons.

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In the OPL photoreceptor axons contact the dendrites of bipolar cells and horizontal cells. Horizontal cells are interneurons which aid in signal processing

The bipolar cells in the INL process input from photoreceptors and horizontal cells, and transmit the signal to their axons.

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In the IPL, bipolar axons contact ganglion cell dendrites and amacrine cells, another class of interneurons.

The ganglion cells of the GCL send their axons through the OFL to the optic disk to make up the optic nerve. They travel all the way to the lateral geniculate nucleus.

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Fovea

The fovea defines the center of the retina, and is the region of highest visual acuity. The fovea is directed towards whatever object you wish to study most closely - this sentence, at the moment. In the fovea there are almost exclusively cones, and they are at their highest density.