The retina lines the eyeball (a) from the inside. The only openings are the areas of the lens and the outlet of the optic nerve (papilla) at the rear of the eyeball. The retina comprises various types of highly specialised sensory and nerve cells (b-h) arranged in several layers, which are essential for the visual capacity of the human eye.
The exterior side of the retina that is facing away from the light consists of the so-called retinal pigment epithelium (RPE) (h), which supplies and protects the sensory cells (photoreceptors), the rods (f) and cones (g). While the cones, which are primarily located in the centre of the retina, allow us to perceive colours in daylight, the rods located mainly in the periphery enable us to see in twilight and dark conditions. Our ability to distinguish colours decreases in low light conditions, until we can only discern shades of grey in complete darkness.
Both photoreceptor types, rods and cones, convert the incident Iight patterns into electrical signals and transmit them to the downstream horizontal (e) and bipolar cells (d). This amplifies contrast, enables directional movement detection, following lines and much more. By way of several intermediary relays, the information finally reaches the so-called ganglion cells (b), whose axons jointly form the optic nerve. The optic nerve transmits the information pre-processed by the retina to the visual cortex in the brain, where the signals are further processed and assembled into an image.
In people suffering from retinitis pigmentosa the photoreceptor cells gradually decay, beginning with the peripheral rods and followed by the central cones.
The therapy of transcorneal electrical stimulation (TES) aims to decelerate this degeneration as much as possible through electrical stimulation of the retina.