The view is the most used of the five senses and is one of the main means we use to collect information from our environment. More than 75% of the information we receive about the world around us consists of visual information.
- 1 Anatomy of the eye
- 2 The cornea
- 3 The retina
- 4 The perception of color
The eye is the organ of vision. Humans have 2 eyes located under the forehead and eyebrows on the front of the face, within the area of the eye (orbit) of the skull. An eye is located on each side of the bridge of the nose. Only about one sixth of the eyeball can be seen. The rest of the eye is protected by bones and surrounding tissues, including muscles and fat.
The human eye is a highly specialized sensory organ. It contains numerous accessory structures that modify visual stimuli before they are captured by visual receptor cells.
The eyes are often compared to a camera. They are responsible for capturing the light and then transforming it into an "image". Both also have lenses to focus the incoming light. Just as a camera focuses the light on the film to create an image, the eye focuses the light on a specialized layer of cells, called the retina, to produce an image.
Cross section of the human eye.
The eye is designed to focus the visual image on the retina with the least possible distortion. It allows the visualization of near or distant objects, the focus can be adjusted by means of the action of the ciliary musculature that changes the shape of the lens. The intensity of the light that reaches the retina is controlled by the musculature of the iris, which changes the size of the pupillary orifice.
When the light that strikes the retina must cross most of the layers that form it before reaching the receptor cells (photoreceptors) which are located in the outermost part. Beyond the retina is the choroid layer, which contains pigment cells that eliminate retrograde reflexes and absorb any photon that passes away from the photoreceptor layers.
The cornea is the transparent layer in the most anterior and central part of the eye. It is located just in front of the iris, which is the colored part of the eye. The main objective of the cornea is help focus the light when it enters. If one wears contact lenses, the contact lens rests on the cornea.
The retina is the third inner layer of the eye and is formed by a light-sensitive tissue. The optics of the eye creates an image of the visual world in the retina (through the cornea), which fulfills the same function as the film in a camera.
In the retina we find five types of cells: photoreceptors, bipolar cells, ganglion cells, amacrine cells and horizontal cells. All are arranged in different layers: three layers of cell bodies separated by two layers of neural processes.
The macula is located in the central part of the retina and has the highest concentration of cones. It is the area of the retina that is responsible for providing a clear central vision.
The choroid is a layer of tissue that lies between the retina and the sclera. It is mainly composed of blood vessels. The choroid helps nourish the retina.
The light activates the receptor cells of the photoreceptor layer, but of all retinal cells, the only ones that emit action potentials are ganglion cells and some amacrine cells. The rest of the cells generate gradual local potentials. The axons of the ganglion cells form the optic nerve, through which the retinal information is transmitted to the CNS.
Cross section of the retina.
The retina is oriented in such a way within the eye that the light has to cross all its layers before reaching the photoreceptors and be absorbed.
Due to the configuration and morphology of the retina, some problems arise, one is that the axon beam of ganglion cells, when leaving the area of the eyeball, creates a kind of space without receptors in the retina. This area without receivers is called blind point. But our visual system takes advantage of the information captured by the receivers around the blind spot to "fill" the gaps in the retinal images created by it.
Demonstration of the visual blind spot. Get about 45 cm from the screen. Close the right eye and with the left eye look at the star. Move your head slightly (forward and backward) and there will come a time when the black circle will disappear from your field of vision, it will be when the circle projects over your blind spot in the left eye.
light incident on the retina, as it must cross all layers before reaching the photoreceptors, may suffer some distortion. This problem, however, is minimized with the existence of the fovea, which is a central area of the retina of about 0.33 mm in diameter specialized in acute and detailed vision. The fovea is also the place of the retina where we find the highest density of receptors and is the area responsible for providing a detailed view. This is why we move our eyes continuously, because the details of the visual scene that interest us at all times are projected on the fovea.
Section of the retina in which the fovea is observed.
In the human retina we find two types of photoreceptors, which are the basis of two different systems of neural processing of light energy:
- Cones: neurons that favor vision in good light conditions. It implies a much more detailed neural processing. The cones have a differential sensitivity at different wavelengths, which makes color vision possible.
- Canes: neurons that favor vision in low light conditions. They are located mainly in the peripheral retina.
In the photoreceptors we find three functionally different parts: an external segment connected to an internal segment that contains the cell nucleus and the synaptic terminal through which contacts with other retinal cells are established.
The structural differences between the two types of photoreceptors correlate with functional differences between the two systems. So, canes are much more sensitive to light than cones and, therefore, in night light conditions only the canes contribute to the vision. In contrast, in daylight conditions the cones contribute to most of the vision. In general, however, the retina contains more rods than cones.
We could say that we have a scotopic retina that uses the canes and that is basic for night vision and a photopic retina that uses cones and that is basic for a daytime vision.
The retinal is synthesized from the Vitamin A. The deficiency of this vitamin can lead to what is known as night blindness, which begins with the degeneration of the external segment of the photoreceptors. If left untreated, it ends with total loss of vision. Variations between pigments make different receptors sensitive to different wavelengths.
Color vision gives us a sharper and more detailed vision. Our visual system uses two systems, the trichromatic and the opposition of colors, for the coding of information related to colors.
The human retina and that of some primates contain three types of cones, each of these with a different opsin in the external segment. Each opsin has a preference for the absorption of a certain wavelength:
The colors we perceive are basically determined for the relative activation of the blue, green and red colors. Thomas Young demonstrated in 1802 that all colors can be created with the right mix of the first three and proposed that in the retina there had to be some system that had a relationship.
Alterations in color perception
As for dysfunctions that may occur in color vision, the truth is that there are few cases of people with a specific color blindness (achromatopsia); but there are many cases of people with dysfunctions in this type of perception. Most of these alterations are congenital and have already been well characterized, such as the protanopia, deuteranopia and tritanopia.
Protanopia and deuteranopia involve genes on the X chromosome (This makes men more predisposed than women). People with protanopia and deuteranopia confuse green and red. In the first case, its red cones are supposed to contain opsin, green cones (they use green and blue) and in the second case it would turn upside down (they use red and blue) The genes that code opsin for green and for the red they are located adjacent to each other on the X chromosome, while the gene that encodes the pigment of the blue cones is found in another non-sexual chromosome.
The tritanopia It is produced by a gene that causes difficulties in the perception of short wave colors. Therefore, they see the world with shades of green and red preferably (for example, the sky sees it as a bright green color).
Some alterations that affect the retina, such as the glaucoma or retinitis pigmentosa, also cause alterations in color perception. It seems that cones are much more vulnerable to alterations than rods; and within the cones, the ones that most easily suffer are the preferred short wavelengths (cones for blue).Related tests
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