Sight

Mechanism of the sight in the eye

The light passes initially by the Cornée. It crosses then the iris, the Cristallin, then the aqueous humor. At each stage, it can be modified qualitatively and/or quantitatively (ex: sunglasses, opacification of the crystalline lens). It reaches then the Rétine. At this stage, the light, made up of electromagnetic waves, is converted into electric impulses by the components of the retina the photoreceivers (cones approximately 10 million, sticks approximately 120 million) and the Neuron S, then transmitted to the central nervous system by the optical nerve. The two optical nerves (right and left) intersect on the level of the optical chiasma and project towards thalamus on the level of the side geniculate bodies. From those, information is relayed towards the visual surfaces of the cortex.

The retinal photoreceivers cones or Bâtonnet S are connected via bipolar neurons to the ganglionic cells whose axons constitute the optical nerve. This cone or stick contains a chemical pigment which is modified by the light, this modification produces electricity in the neuron (stimulation). The cell then cancels chemically the modification of the pigment in order to give again its formula to him first (stop of the stimulation of the neuron).

Several sticks are often connected to the same neuron, it is enough that only one stick is illuminated so that the neuron is stimulated. This type of neuron is then very sensitive to the quantity of Lumière (its power). The cones are often connected to only one neuron and they contain a pigment sensitive only to a “Couleur” (space restricted and contiguous wavelengths). There are three types of pigments for the cones, respectively sensitive to the red (big wavelength), green (average wavelength), and blue (short wavelength). There is a fourth, which is a rather rare genetic exception.

The eye is thus sensitive only to three colors. It is the treatment and the recombination of these three stimulations, carried out in the brain, which will give the feeling of the other colors. The absence of one or several types of cones in the eye makes insensitive with the types of corresponding wavelengths. This was had a presentiment of by the doctor John Dalton, it gave him its name: The Daltonism. Beyond (Infra-red) and in-on this side these wavelengths (Ultraviolet) we do not see.

Each cone or stick is activated by the light, it passes then in an insensitive state during a certain time, and becomes again activable. These various times are due to the photochemical reactions between luminous energy and the various pigments. The duration when the cone (or stick) is not sensitive any more to a change of the light is time that it needs to reconstitute its pigment. As long as the concentration of pigment in the cell did not reach a certain threshold, the neuron continues to be stimulated. It is part of the explanation of the phenomenon of retinal Persistance, one “sees” luminous traces whereas the light stopped.

Amétropies and Pathologies

The eyes can be affected by Amétropies, like the Myopie, the Hypermétropie, the Astigmatisme which can deteriorate the clearness of the perceived image. The Presbytie highlights the ageing of the Cristallin, the Nystagmus when has accompanies a weak vision to him. Other pathologies are also the Strabisme, the Daltonisme and the cataract. After the birth, one can also see appearing allergies due to certain substances or food.

The vitreous humor allows in the light of moving in the eye to the retina, then with the optical nerve.

See too

Internal bonds

External bonds

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