Distribution
Due to their different tasks, the cones and rods in the eye are also distributed differently in their density. The cones serve for sharp vision with color differentiation during the day. They are therefore most common in the center of the retina (yellow spot – macula lutea) and are the only receptors present in the central fovea (fovea centralis) (no rods).
The visual fossa is the place of sharpest vision and is specialized in daylight. The rods have their maximum density parafoveal, i.e. around the central fovea. In the periphery, the density of the photoreceptors decreases rapidly, whereas in the more distant parts, there are almost only rods left.
Size
Cones and rods share the building plan to a certain degree, but then vary.In general, rods are slightly longer than cones. Rod photoreceptors have an average length of about 50 μm and a diameter of about 3 μm, at the most densely packed locations, i.e. for rods the parafoveal region. Cone photoreceptors are somewhat shorter than the rods and have a diameter of 2 μm in the fovea centralis, the so-called visual pit, in the region with the highest density.
Quantity
The human eye has an overwhelming number of photoreceptors. One eye alone has about 120 million rod receptors for scotopic vision (in darkness), while for daytime vision there are about 6 million cone receptors. Both receptors transmit their signals convergently to about one million ganglion cells, whereby the axons (cell extensions) of these ganglion cells form a bundle that makes up the optic nerve (nervus opticus) and pulls it into the brain, where the signals can be processed centrally.
Comparison of rods and cones
As already described, rods and cones have slight differences in structure, which is not serious. More important is their different function. Rods and cones are much more sensitive to light and can therefore also detect low light incidence, but only distinguish between light and dark.
They are also slightly thicker than cones and are transmitted convergently, so their resolving power is lower. Cones, on the other hand, require more light incidence, but can enable color vision through their three subforms. Due to their smaller diameter and the less strongly converging transmission, up to 1:1 transmission in the fovea centralis, they have an excellent resolution, which is however only usable during daytime.
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