The retina

The retina

The retina (from Latin rete = net) lines the eyeball 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 arranged in several layers, which are essential for the visual capacity of the human eye. 1, 3

Rods, cones and nerve cells

The exterior side of the retina that is facing away from the light consists of the so-called retinal pigment epithelium (RPE), which supplies and protects the sensory cells (photoreceptors), the rods and cones. 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 light impulses into electrical signals and transmit them to the downstream horizontal and bipolar cells. This amplifies contrast, enables directional movement detection, following lines and much more. By way of several intermediary relays, the information reaches the so-called ganglion cells, whose axons jointly form the optic nerve. This channels the signals to the visual cortex via the lateral geniculate nucleus. The visual cortex is the part of the brain that is responsible for our vision. It further processes the signals and assembles them 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 for as long as possible. The RETINA IMPLANT Alpha AMS has been developed to substitute the basic function of these decayed cells right at the anatomically correct location: the conversion of incident light signals and the activation of bipolar cells.

Papilla, macula and fovea

The macula and the papilla are two special structures in the retina. The papilla, where the optic nerve exits the eye, is the only point of the retina that contains no photoreceptors. The human eye is therefore “blind” in this place, which is why the corresponding part of our visual field is also referred to as the blind spot. The macula - also known as the yellow spot - is located in the centre of the retina. In its centre is the so-called fovea, a tiny area of the retina that exclusively contains cones, but no rods. It is the place of our sharpest vision, which we use to focus on everything we look at. Our macula picks up all the fine details and important information from our surroundings, such as recognising faces, reading, watching television, or checking our watch. The remaining portion of the retina, which covers more than 95% of its total area, among other purposes serves to detect movements and allow spatial orientation. This does not require the complete focus of an object.1, 2, 3, 4

 

How does vision work?

When we look at an object, the rays of light reflected by this object first meet the cornea. Its strongly outward-curved (convex) shape concentrates the incident light. Light thus focussed penetrates the iris, which functions similar to an aperture in a camera: If the surroundings are bright, it constricts and the pupil, the circular opening in its centre, becomes smaller. If it is dark, the pupil dilates further to allow more light to enter the eye. The lens is located behind the pupil. It also is convex on its front and rear side. The lens therefore again concentrates the incident light, which causes the object under consideration to be precisely reflected onto the macula, the place of our sharpest vision in the posterior area of the retina. Since the lens is elastic, it can change its shape with the help of the ciliary muscle to extend or contract. Based on this adaptation, it is able to refract the incident light by different degrees, which allows us to focus on objects that are close by as well as far away. 1, 2 ,3

 

Microsccades

When focussing on an object only a very small area of it will actually appear sharp. To supply the brain with more information from the surroundings of the object in focus, the human eye applies a “trick”: very fast micro-movements of the eye, so-called saccades, focus on single aspects of the surrounding for fractions of a second, and collect further information for the brain to complete the picture. Scientists have recently discovered that we utilise this mechanism predominantly when focussing on one point in particular. The eyes will then deviate for fractions of a degree from the point on which we fix our gaze. These microsaccades are practically not recognisable. They enable us to react to our surroundings despite focussing on one particular aspect.5

 


Sources:
1 Lang GK et al. Augenheilkunde. 5. Auflage. Thieme, Stuttgart 2014
2 Grehn F. Augenheilkunde. 30. Auflage. Springer, Heidelberg 2008
3 Pschyrembel Klinisches Wörterbuch. 266. Auflage. De Gruyter, Berlin 2014
4 Onlineinformation der PRO RETINA Deutschland e.V.: http://www.pro-retina.de/netzhauterkrankungen/makula-degeneration/altersabhaengige-makuladegeneration/krankheitsbild/die-makula. Letzter Aufruf: Januar 2017
5 Chen C.-Y. et al.: Neuronal Response Gain Enhancement prior to Microsaccades. 2015, Current Biology 25, 2065–2074