There are two kinds of photosensitive sensing systems in the retina of human beings and most vertebrates. One is called rod system or night vision system, which is a photosensitive transducing system composed of rod cells, bipolar cells and ganglion cells. This is because rod cells are highly sensitive to light and can feel the stimulation of light at night to produce vision. The other is composed of cone cells, secondary cells and ganglion cells, and is called the optic vertebra system or daytime light perception system. This is because cone cells are less sensitive to light and can only cause vision under the strong light of daytime, but cone cells can distinguish colors and see the details and contours of the surface of objects clearly, with strong spatial resolution.
The main basis for proving the existence of these two relatively independent photosensitive sensing systems is:
(1) The distribution of rod cells and cone cells in human retina is extremely uneven. The closer to the retina, the more rod cells and the less cone cells. The closer to the center of retina, the fewer rod cells and the more cone cells. In the fovea of macula, there are only cone cells, ignoring rod cells. Corresponding to the above-mentioned cell distribution, the characteristics of human vision are that the visual acuity and color vision collected by medical education network are concave in the light and poor in the dark; On the contrary, the peripheral part of the retina can feel the stimulation of weak light, but it is colorless and has poor clarity.
(2) When two kinds of photosensitive cells, bipolar cells and cells form information transmission paths, their contact modes are the same. Convergence is common in rod-like systems, that is, multiple rod-like cells are connected to the same bipolar cell, and multiple bipolar cells are connected to the same ganglion cell. The convergence of intercellular connections in pyramidal system is much smaller. It can even be seen in the fovea that a pyramidal cell is only connected with a bipolar cell, and bipolar cells are only connected with a ganglion cell. This low convergence or non-convergence "single line connection" makes the cone system have high resolution.
(3) According to the characteristics of animal species, some animals that only move in the daytime, such as reptiles and chickens, have only cone cells in their retinas, but ignore rod cells; Other animals that only move at night, such as owls, have only rod cells but no cone cells in their retinas.
(4) Rod cells contain only one kind of visual pigment, namely rhodopsin, while cone cells contain three kinds of visual pigments with different absorption spectrum characteristics, which is consistent with the fact that rod system has no color perception and cone system has color perception.