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The Eyes - Study the Visual System Here
1. What is vision? Why is vision important for life on earth?
Vision is the ability of some living beings to perceive, to distinguish and to interpret luminous stimuli.
Vision is important on earth mainly in the terrestrial and in the superficial aquatic habitats because our planet is intensely exposed to sunlight and thus light and colors become distinguishing factors of objects present in the environment, even at distance. This distinction provided new survival strategies for the organisms, new protection mechanisms against external dangers, new ways to find food and to communicate with other individuals, new types of courting and reproduction behaviors, etc. That is, it created new possibilities of interaction with the surrounds and increased capacity to explore new ecological niches.
2. How does photosensitivity in cnidarians, annelids and worms differ from insects, cephalopods and vertebrates?
In the first mentioned group of animals there are photoreceptor cells organized in ocelli or diffusely dispersed in the body. These animals do not form images.
In the animals of the second group the photoreceptor cells are part of more sophisticated structures, the eyes, able to form images and to send them to the nervous system.
3. What are the structures that compose the human vision apparatus?
The organs of the human visual apparatus are the eyes, the optical nerves and the visual areas of the brain (located in the occipital lobes of both hemispheres).
Image Diversity: human eye eye anatomy
4. What are the main structures of the human eye?
The main structures of the human eye are the cornea, the iris, the pupil, the ciliary muscles, the crystalline lens and the retina (the space between the crystalline lens and the retina within the eyeball is filled with vitreous humor).
5. What is the function of the iris and of the pupil?
The iris works like the diaphragm of a photographic camera since it has muscles that contract or relax varying the pupil diameter. When the luminous intensity heightens the parasympathetic nervous system commands the contraction of the pupil; when there is shortage of light the sympathetic nervous system stimulates the dilation of the pupils. These movements depend upon the muscles of the iris.
6. Which is the part of the human visual system where the receptors that sense light, i.e., the photoreceptor cells, are located? How do those cells work?
The photoreceptor cells form the retina, a lamina that covers the internal posterior region of the eyeball. The photosensitive cells of the retina are divided into two types: the cone cells and the rod cells. These cells have pigments that sense specific light wave ranges (frequencies) and trigger action potentials conducted by the optical nerves to the visual area of the brain.
Image Diversity: retina histology cone cells rod cells
7. Since the visual images are projected in an inverted manner on the retina why don't we see things upside down?
Since the crystalline lens is a convex spherical lens it forms inverted images on the retina (every converging lens forms inverted images). The inverted information follows through the optical nerves until the occipital cerebral cortex that contains the visual area of the brain. In the brain the interpretation of the image takes place and the inverted information is reverted.
Image Diversity: vision pathway
8. What type of structure is the crystalline lens? What is its function?
The crystalline is a converging spherical lens. This natural lens has the function to project images of objects onto the retina.
9. What is visual accommodation?
Visual accommodation is the phenomenon of varying the curvature of the crystalline lens to make possible the variation of its refractivity to adjust the images of objects exactly onto the retina. The visual accommodation is accomplished by the action of the ciliary muscles.
The nitid vision depends on the visual accommodation since, if the images are not projected onto the retina but in front or behind it, they will appear blurred. The closer an object is more the ciliary muscles must compress the crystalline lens (increasing its curvature); the more distant an object is more the ciliary muscles must relax.
10. What are the near point and the far point of the vision?
The near point is the closest distance between an object and the eye that makes possible the formed image to be focused, i.e., it is the point in which the ciliary muscles are in their maximum contraction. The far point is the most distant point from the eye in which an object can be placed and its image is still focused, i.e., it is the situation of maximum relaxation of the ciliary muscles. The zone between the near point and the far point is called the accommodation zone.
11. How can the visual deficiencies known as myopia and hypermetropia be optically explained?
Myopia is the visual condition in which the images are formed before (in front of) the retina. Hypermetropia is the visual condition in which the point of image formation is beyond (behind) the retina. Actually myopia is due to an increase in the distance between the retina and the crystalline lens, mainly caused by a slight flattening of the eyeball. In hypermetropia the retina is too close to the crystalline lens due to slight shortening of the eyeball.
In myopia the near point and the far point of vision come closer (the refractivity of the crystalline lens that corresponds to the maximum distension capacity of the ciliary muscles is not enough to provide visual accommodation). In hypermetropia the ciliary muscles are not able to contract more to compensate the inadequate position of the retina, i.e., the near point becomes more distant.
Image Diversity: myopia hypermetropia
12. What are presbyopia and astigmatism?
Presbyopia is the visual impairment in which there is loss of the cililary muscle strength thus reducing the capability of the crystalline lens to adjust images of near objects onto the retina. In presbyopia the near point of vision becomes more distant. The disease generally occurs in old people.
Astigmatism is caused by irregular shape of the refractive structures, mainly the cornea. In astigmatism a single object-point may produce more than one image onto the retina and so the vision becomes distorted.
Image Diversity: presbyopia
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