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Human visual system

9. Februar 2017 - Eye, Ophthalmology

In the Human Visual System we define photoreceptors as that cell or mechanism capable of capturing light. The photoreceptors are located inside the eye and there are two different types: cones and canes.

In the S ystem V isual H uman we define photoreceptors as that cell or mechanism can catch the light . Photoreceptors are located inside the eye and there are two different types: rods and cones .

Distribution of photoreceptors in the eye

Cones form a regular hexagonal tile in the fovea , the highest density of cones is in the descending foveola this density as we move in the peripheral retina. The breadsticks are the fovea following a more disorganized pattern cones. There is an area where there is no photoreceptor, it is the blind spot.

Visual pigments

The sticks contain rhodopsin, which is a protein that exhibits greater sensitivity to wavelengths close to 500nm, ie, to the blue-green light, therefore it is responsible for scotopic vision (low light conditions).

Each cone contains one of three types of opsins: Erythropsin which has greater sensitivity for long wavelengths (red light), chloropsin with greater sensitivity for medium wavelengths (green light) and, finally, cyanopsin with greater sensitivity for The small wavelengths (blue light), so the cones are responsible for the perception of color and give rise to trichromatic vision.

Ultrastructure of synaptic endings of the rods and cones

The information encoded by the photoreceptors is transmitted through their synaptic terminations called pedicles in the case of cones and spherules in the case of rods. Both are full of synaptic vesicles. In the synapses, which is the region of contact between the axómas and the dendrites, there are dense structures called Sinapsis in Cintilla. The cells involved in the processes performed in this area are the bipolar cells, the horizontal cells, the interplexiform cells and the ganglionic cells.

The pedicles form a structure known as a triad in which three processes are found: 2 lateral processes corresponding to horizontal cells and a central process aligned with the synapses in the belt (bipolar cells). In addition there are other types of bipolar cells that have basal contacts with the pedicle. In these synaptic terminations there are approximately 30 Synapses in Belts.

The spherules contain two synapses on a band which form a structure known as a dyad composed of a lateral structure (composed of the axonal terminations of the horizontal cells) and a central element (composed of intervaginant dendrites of bipolar cells for rods). In general, there are no basal contacts in the spherules.

There are also electrical type synapses in the cone-cone and cone-cone retina.

Phototransduction

The phototransduction is the process through which the information captured by the photoreceptor cells is converted into electrical signal and then sends to the brain.

Although the structure of the cones and rods is different, the mechanism of transduction in both is very similar.

Adaptation to shine

The human eye can discriminate a total range of enormous levels (10 ^ 10 levels) but not at the same time. This is where the phenomenon of adaptation to brightness appears that depending on the perceived subjective brightness the eye can discriminate some levels or others.