Transmission of stimuli in the retina
In the retina mainly 4 different cell types are responsible for the transmission of the light stimulus. The signal is transmitted not only vertically (from the outer retinal layers to the inner retinal layers), but also horizontally. Horizontal transmission is performed by horizontal and amacrine cells, vertical transmission by bipolar cells.
The cells influence each other and thus change the original signal initiated by the cones and rods. The ganglion cells are located in the innermost nerve cell layer of the retina. The cell extensions of the ganglia then move to the blind spot where they bundle to the optic nerve and leave the eye to enter the brain.
At the blind spot (one in each eye), i.e. the beginning of the optic nerve, there are understandably no cones and rods and no visual perception takes place. By the way, you can easily find your own blind spots:By the way: Not only light can cause signals to be generated in the cones and rods. A blow to the eye or strong rubbing triggers a corresponding electrical impulse similar to light. Anyone who has rubbed their eyes before will certainly have noticed the bright patterns that you think you see.
Visual path and conduction to the brain
After the nerve processes of the ganglion cells have bundled to form the optic nerve, they move together through a hole in the back wall of the eye socket (optic canalis). Behind it, the two optic nerves meet in the optic chiasm. Some of the nerves cross over (the fibers of the medial half of the retina) to the other side, while others do not change sides (the fibers of the lateral half of the retina).
This ensures that the visual impressions of a complete half of the face are switched to the other side of the brain.Before the fibers in the Corpus geniculatum laterale, a part of the thalamus, are switched to another nerve cell, some optic nerve fibers branch off to deeper reflex centers in the brainstem. Examination of the ocular reflex function may therefore be very helpful in locating the damaged area on the way from the eye to the brain. Behind the Corpus geniculatum laterale, nerve cords continue to the primary visual cortex, which is collectively known as visual radiation.
This is where the visual impulses are consciously perceived for the first time. However, there is no interpretation or classification yet. The primary visual cortex is ordered retinotopically.
This means that a specific area in the visual cortex corresponds to a specific location on the retina. The site of sharpest vision (fovea centralis) is represented on approximately 4/5 of the primary visual cortex. Fibers from the primary visual cortex mainly pull into the secondary visual cortex, which is arranged like a horseshoe around the primary visual cortex.
Here, finally, the interpretation of the perceived takes place. The received information is compared with information from other areas of the brain. From the secondary visual cortex, nerve fibers travel to practically all brain regions.
And so, little by little, an overall impression of what has been seen is created, in which a lot of additional information such as distance, movement and, above all, the classification of what kind of object is involved, is now incorporated. Around the secondary visual cortex there are further visual cortex fields, which are no longer ordered retinotopically and take over very specific functions. For example, there are areas that connect what is visually perceived with speech, that prepare and calculate the corresponding reactions of the body (e.g. “catch the ball!”) or that store what is seen as a memory. More information on this topic can also be found here: Visual path
