What is the exact organization of rods and cones in the eye? And does it go horizontal? Vertical? Both?
I've found a few different sources of information, but they're telling me different things like 4 rods to a cone, and such and such. I just can't seem to find the exact answer.
You can think of the retina as a carpet of nerve cells that lines the back of the eye. This "carpet" is made up of several layers of nerve cells, which include photoreceptors (rods and cones), bipolar cells, amacrine cells, horizontal cells, and ganglion cells. Rods and cones contain molecules that react to light, called photopigments. When photons (light particles) react with the photopigments, the photoreceptor cell send an electrical message (called action potential) to the bipolar cells, which in turn send action potentials to the ganglion cells. The ganglion cells have projections (called axons) that extend from the back of the eye, through the optic nerve, to the brain.
The difference between rods and cones is that they have different shapes (rod-like and cone-like) and contain different photopigments. Rods are more sensitive to low-intensity light, but they are not sensitive to color. Cones are color-sensitive but only detect high-intensity light. Overall, there are between 10 and 20 times more rods than cones in the human retina. Cones are concentrated in the center of the retina, especially in a small disc called fovea. Rods are more concentrated in the periphery of the retina. Rods and cones form the first layer of the retina, the one closest to the back of the eye. They sit perpendicularly to the back of the eye. On top of the rods and cones sits the bipolar cell layer, and on top of it sits the ganglion cell layer. The organization of the retina, thus, is "inverted" relative to what you would expect: light coming from the outside enters the eye and then has to go first through the layers of ganglion cells and bipolar cells in order to reach the rods and cones.