Sight is the most important sense that human ever have. By sight we perceive aprox. 80% of environment.

Retina is sensible to electromagnetic radiations with wavelenght between 400 - 700 nm. Visual reception implies the transformation of light electromagnetic energy in nervous influx (electric signal).

This action spends in rod and cones cells.

In their structure are photosensible macromolecules called "retinian purpura". There are many types of "retinian purpura" : rods contains a single visual pigment - rodopsine; cones contains three different kind of pigments, called iodopsines. Structurally, a visual pigment has two components : a nonproteic group - retinen and an other proteic group - opsine. The retinen is an A vitamin aldehyde. The opsine differ from a pigment to another. The rod cells uses the same opsine - scotopsine.

Retinen exists in two isomere forms : cis, which combines easilly with opsines, and formming the characteristic visual pigment, and trans form which unbinds from opsine, producing pigment decomposing. Passing from cis form to trans form is done in presence of light. The cis form regeneration done in darkness with an enzime help.

The photoreception mechanism is following :

Cone cells assumes colour eyesight and rod cells the black and white perception. The visual receptors sensitivity is very big. However, rod cells are more sensitive than cone cells.

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