Retina is the surface area of the eyes of an animal.
In the inner surface of the retina reside the retinal ganglion cell. This is a special type of neurons, which receives visual information through two other neurons: the horizontal cell and the amacrine cells. The defining characteristic of the retinal ganglion cell is that it has a very long axon which extends directly to the brain. There is a liaison between the retinal ganglion cells and the photoreceptor. For the human being, each retina has about one million retinal ganglion cells, while it has one hundred million photoreceptors. But the photoreceptors are not uniformly distributed. In fact, in the center of the retina, each retinal ganglion cell correspond to five or six photoreceptors, whereas in the periphery area, each retinal ganglion cell receives visual information from some thousands photoreceptors. This, in some sense, is reasonable, because the visual information is not uniformly distributed, either. Usually the central area receives the most information of the sights, so a few photoreceptors will be enough.
The role of retinal ganglion cells is that they proceed the visual information. What does this mean? It means that these neurons start to distinguish the motion visual information. More precisely, it is in the inner surface of retina that the horizontal motion is separated from the vertical motion, as well as from other motions, like moving forward, or moving backward. This is very important for all the animals. For example, when there is a predator in front of an animal, if the predator moves a bit, the animal recognizes at once and can prepare for an escape. This process is automatic, that is, seeing a moving creature, then this visual information is sent to the brain, the brain deciding taking a corresponding action after a series of action potentials and chemical reactions. So, viewed this way, we see that the simple actions of the animals is spontaneous, at least with the kick-off of a firing in the outside world.
But this is not exactly the case. In fact, the reality is more complex than what we have thought. For example, a cat can move even without any external stimulus. So in some sense, the brain is a machine automatic. There are fluctuations in the brain, which is the internal stimulus. But which system can have fluctuations? This is an interesting question. We know that, even the microbes are in a scale superior to that of atoms, the latter being the constituents of all the grand molecules, like DNA, proteins, glucides, etc.. But this is not enough to create a living being, which is obvious. Put in another way, just putting together many many DNA, proteins, all sort of things that exist in the body of an animal will not lead to a creation of such animal. That is to say, only the fluctuation in the sense of thermodynamics is not enough. So, we must have forgotten something. Or, perhaps it is just because I haven’t read enough.