The Retina: Principles of Structure and Function - Case Study Example

Retina The human eye, unlike invertebrates is capable of detecting many variations of colors as well as form images of objects that are located at a distance. It is also capable of responding to as few as a single photon of light (Campbell, 1999). The basic parts of the eye, particularly, the iris, lens, pupil and cornea, function to manipulate and focus the light to the retina, which houses the photoreceptor cells (Vander, et. Al, 2001). Cornea is located at the front of the eye and functions by getting light into the eye, acting as a fixed lens; the iris changes size to regulate the amount of light that enters the pupil. The photoreceptors function by receiving information transmitted in the form of light. This information is then transferred to the optic nerves, to the brain through the optic disc, the spot on the lower outside of the retina. Figure 1. Structure of the Eye (Feinberg E.B., 2004) The eye has two major cavities filled with clear water liquid. One, is located between the lens and the cornea, which is filled by the aqueous humor. The other is located behind the lens within the eyeball itself, just touching the retina. This is filled by the jellylike vitreous humour which constitutes the bulk of the eye. These liquids function as liquid lenses. Similar to the cornea and most parts of the eye, their function is to focus the light into the retina as liquid lenses. As said earlier, the retina houses the photoreceptors so that information can then be transferred to the brain for interpretation (Vander, et. Al, 2001). The main question now is why is the retina placed as far inside the eye where information has to be passed from many different parts like cornea, pupil and liquid lenses, and when speed of information transmission is very necessary especially in sensory organs. The last thought exactly answers the structural question of the anatomy of the eye. The retina is structured to layer the inner cavity of the eye to allow more surface area for information to be passed to the many different photoreceptors housed in the retina. The liquid vitreous humor helps disperse this light so that information can pass to the photoreceptors at a single instant instead of the light being directed at a single line and spreading to the many different photoreceptors in the retina. This means that the retina is perfectly structured for speed in processing of information. It is also easier to vascularize this way than if the whole cavity is filled with retinal cells (Feinberg, 2004). What is it with the retina that allows its function? The retina contains two types of photoreceptors named for their shapes: 125 million rod cells and 6 million cone cells, the total of which accounts for about 70% of all body sensory receptors (Campbell, N.A., 1999). Because rods are more sensitive to light and are unable to distinguish between colors, these photoreceptors allow vision only during the night with the limitation of being in black and white. On the other hand, because more light is necessary to stimulate the cones, they are functioning for vision during the day and cannot function at night. It does not have any limitations on colors (Campbell, N.A., et. Al, 1999). It is important to note that the number of the two photoreceptors in the retina is related on the species’ activities. Animals that are nocturnal, for example, have a maximum number of rods and relatively fewer cones compared to those that are active during the day. Some also have very few cones which explain their inability to adapt to daylight and their inability to distinguish between colors (Retina, 2006). The humans have adapted their retinal structure to their increased activity during the day. The fovea, located at the center of the visual field and considered as the most important part of the eye is most concentrated with cones rather than rods (Campbell, N.A., 1999). In addition, this dense concentration is most efficiently packed in hexagonal mosaic which allows greater utilization of the area (Photoreceptors, 2003). Figure 2. The Retina. (Nave, R., n.d.) The thinnest layer of the fovea consisting of only cones are then surrounded by the thickest portion of the entire retina composed of six layers of ganglion cells (Figure 2). This relatively greater number of ganglion cells in the cone-dominant fovea allows for more synaptic interactions compared to the rods-dominant area of the retina. These are necessary so that information can be transferred immediately to the optic nerve (Simple Anatomy of the Retina, 2003). Figure 3 shows the structure of each type of photoreceptor. As said earlier, the photoreceptors are named after their respective shapes: rods are cylindrical or rod-shaped while cones have a triangular tip. Both of which contain a tail ending in synaptic terminals for communication with other cells. Each photoreceptor is composed of stacks of discs containing visual pigments. These visual pigments contain opsin (a protein) to which retinal, a derivative of Vitamin A that absorbs light, is bonded. The configuration of these components change in response to signals, in this case, light, to trigger a signal transduction pathway that would ultimately result to receptor potentials (Cambell, N.A. 1999). The stacking of visual pigment-containing discs is to allow greater surface area for the containment of visual pigments and synaptic terminals and thus, for easier transduction of signals (Photoreceptors, 2003). Figure 3. Upper: Structure of photoreceptors, showing discs containing visual pigments. Lower: Inside each visual pigment (Campbell, N.A. 1999). The whole fovea with all its structures is pigmented by yellow xanthophyll carotenoids, zeaxanthin and lutein (collectively known as macula lutea) (Simple Anatomy of the Retina, 2003). These pigments are essential to prevent the destruction of the fovea, and thus, blindness. The macula lutea then, together with the lens acts as a short wavelength filter to protect ultraviolet irradiation damage (Rodieck, 1973). Behind the retina is a thin, pigmented inner layer called choroid. It may be surprising to know that it is this part that receives the greatest blood flow in the eye. This function, however, is very significant especially in the maintenance of the photoreceptors (Henkind, et. Al., 1979). Anatomically, the photoreceptors are housed in the outermost layer of the retina, closest to the choroid. Figure 4. Pathway of information processing in the retina (Campbell, N.A., 1999) The processing of information begins in the retina. Figure 4 provides an illustration on the pathway of this information processing. In the retina, the order of information processing starts with the photoreceptors, which then synapse with the neurons to the ganglion cells. Amarcine cells and Horizontal cells are placed between the photoreceptors and ganglion cells to assist in the integration and spreading of visual signals from one photoreceptor cell to another or from one horizontal/amarcine cell to another, before being sent to the brain. The ganglion cells then convey the resulting sensations with the production of action potentials, to the brain, through the optic nerve. The illustration does not show, however the difference in connections between the rods and cones. While the rods are connected to nerve fibers collectively and while these fibers could be activated by even a single rod, the cones located in the fovea have individual nerve fibers attached (Hecht, 1987). Retinitis Pigmentosa Retinitis Pigmentosa (RP) is an inherited disorder characterized by abnormalities in the functioning of the photoreceptors or the retinal pigment epithelium (RPE) (Retinitis Pigmentosa, 2006). The most common form of RP is that involving the degeneration of the rod photoreceptors. The degeneration of the retina usually starts at the peripheral retina where the rod photoreceptors are concentrated. At this point, the patient becomes “partially” night-blind (Hamel, 2003). The early stages of RP vary. Night blindness can be present in the first years of the disease; it can appear during the second decade and to some, night blindness can manifest even later (Hamel, 2003). Peripheral visual defects can then be observed during the earlier stages but these defects can be limited during the day since the cone photoreceptors are not yet affected. Because the initial symptoms do not apply during the day and because visual acuity remain normal (there is no signs of bone spicule-shaped pigment deposits; there is normal color vision), these symptoms are usually dismissed and are difficult to diagnose. However, even at this early stage, electroretinogram (ERG) test can already show decreased amplitude of the b wave, which is a significant sign of RP. Figure 5. Pigment deposits on the eye of a patient with retinitis pigmentosa (Simple Anatomy of the Retina, 2003) Night blindness continues to worsen and manifestations of this degeneration such as difficulty in driving during the night or walking in the dark become more obvious. Soon, as the peripheral vision continues to deteriorate even in the day light, the vision of the patient can be reduced to tunnel vision. At this stage the presence of bone spicule-shaped pigment deposits and further degeneration of the retina become more prominent during examinations (see Figure 5). There would be narrowing of the blood vessels of the retina and paling of the optic disc (Hamel, 2003). Still, the fovea is spared of the degeneration. As said earlier, the fovea is cone-dominant and this disease usually affects the rod photoreceptors. Black pigment and thinned blood vessels in the optic nerve characterizes Retinitis pigmentosa (Simple Anatomy of the Retina, 2003). Total loss of the peripheral vision happens at the end stage of the disease: pigment deposits become widespread; blood vessels become very thin and the optic disc becomes very pale. There is also degeneration of the chorea and the area near the fovea. There are times when even the central visual field becomes affected. At such point, there is total loss of vision. Works Cited Campbell, N.A., et. Al. (1999). Biology: Photoreceptors. USA: Addison Wesley Longman, Inc. Feinberg, E.B. (2004). Eye. In Medline Plus. Retrieved 29 Dec 2006 from http://www.nlm.nih.gov/medlineplus/ency/imagepages/1094.htm. Hamel, D.C. (2003). Retinitis Pigmentosa. In Orphanet Encyclopedia. Retrieved 29 Dec 2006 from http://www.orpha.net/data/patho/GB/uk-RetinitisPigmentosa.pdf. Hecht, E. (1987) Optics. USA: Addison Wesley. Henkind , P., Hansen, R.I. and Szalay, J. (1979) Ocular circulation. In "Physiology of the human eye and visual system" (Ed. Records, R.E.) pp 98-155. Harper & Row, new York. Nave, R. (n.d.) The Retina. Retrieved 29 Dec 2006 from http://hyperphysics.phy-astr.gsu.edu/hbase/vision/retina.html. Photoreceptors. (2003). In Webvision. Retrieved 29 December 2006 from http://webvision.med.utah.edu/photo1.html. Retina. (2006). In Wikipedia, The Free Encyclopedia. Retrieved, 30 Dec 2006, from http://en.wikipedia.org/w/index.php?title=Retina&oldid=96612530 Retinitis pigmentosa. (2006). In Wikipedia, The Free Encyclopedia. Retrieved 29 Dec 29, 2006, from http://en.wikipedia.org/w/index.php?title=Retinitis_pigmentosa&oldid=95923619 Rodieck, R. W. (1973) The vertebrate retina: principles of structure and function. W. H. Freeman and Company, San Francisco. Simple Anatomy of the Retina. (2003). In Webvision. Retrieved 29 Dec 2006 from http://webvision.med.utah.edu/sretina.html. Vander, A.J., et. Al. (2001). Human Physiology: The Mechanisms of Body Function. New York: McGraw-Hill Higher Education. Read More