The organ of vision has a complex structure that allows the performance of the visual act. This consists of four distinct stages:
- Formation of an image on the retina through the optical system (cornea, aqueous humor, lens and vitreous humor).
- Generation of a neural impulse.
- Transmission of neural impulses through the optic nerve.
- Interpretation of the neural impulses, in the cerebral cortex.
The retina may therefore be considered a specialized part of the central nervous system, able to capture light through specialized receptors, called rods and cones, and to generate a neural impulse which, by way of the neurons of the optic nerve, transmits the message to specialized areas of the brain for decoding and interpretation.
The optic nerve system begins to develop in the fetus at around the fourth week of life and ends 10 weeks after birth. It is estimated that the visual acuity of newborns is about 20/400.
Visual acuity is lost with age due to a variety of causes, such as changes in lens opacity, which leads to cataracts; structural changes in the ciliary muscles that prevent proper alignment of the crystalline lens and cause presbyopia; or alterations of the retina and of neurons, that lose their ability to respond properly to light stimuli.
During the embryonic period as well as in adulthood and old age, preserving neuronal structure that enables the transmission of nerve impulses is key to obtaining and maintaining the highest level of visual acuity. Within these structures we have to stress the importance of nerve cell membranes, which are made up of phospholipids and polyunsaturated fatty acids and are vital to the transmission of nerve impulses.
Essential fatty acids, including long chain polyunsaturated (EPA and DHA), are structural components of all tissues and are indispensable for cell membrane synthesis. The brain, retina and other neural tissues are rich in these fatty acids. Thus, DHA makes up 60% of polyunsaturated fatty acids in the retina and 40% in the brain. Membrane phospholipid DHA is present in the photoreceptors of the retina and of synaptic nerve endings.
The polyunsaturated fatty acids in the photoreceptors of the retina increase its capacity for processing light stimuli, so people who have a deficiency of these fatty acids require greater light stimulation to elicit the same level of photoelectric response as those with adequate levels of EPA and DHA.
Other components crucial to maintaining visual acuity are the pigments lutein and zeaxanthin. They are found in high concentrations in the macula, or central part of the retina, which is where the greatest number of photoreceptor cells, cones and rods, is located. Their main function is to protect these cells from the aggression of oxygen free radicals that are formed by exposure to light. Observational studies have noted that a diet high in lutein and zeaxanthin is related to a reduced risk both of cataracts and age-related macular degeneration.
Vitamins also play an important part in the preservation of the nerve cells involved in vision. Thus, vitamins E and B2 have a protective antioxidative effect and vitamin B12 is essential for nerve tissue repair when damaged. Vitamin B12 and folic acid are essential to avoid high blood levels of homocysteine, which is a neurotoxin.
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