The Eye-Dentistry PDF

The Urinary System  The Eye: Receptor and Neural Function of the Retina Anatomy and Function of the Structural Elements of the Retina Layers of the Retina. Figure 51-1 shows the functional components of the retina, which are arranged in layers from the outside to the insid...

The Urinary System  The Eye: Receptor and Neural Function of the Retina Anatomy and Function of the Structural Elements of the Retina Layers of the Retina. Figure 51-1 shows the functional components of the retina, which are arranged in layers from the outside to the inside as follows: (1) pigmented layer, (2) layer of rods and cones projecting to the pigment, (3) outer nuclear layer containing the cell bodies of the rods and cones, (4) outer plexiform layer, (5) inner nuclear layer, (6) inner plexiform layer, (7) ganglionic layer, (8) layer of optic nerve fibers, and (9) inner limiting membrane. 4 5 After light passes through the lens system of the eye and then through the vitreous humor, it enters the retina from the inside of the eye (see Figure 51-1); that is, it passes first through the ganglion cells and then through the plexiform and nuclear layers before it finally reaches the layer of rods and cones located all the way on the outer edge of the retina. This distance is a thickness of several hundred micrometers; visual acuity is decreased by this passage through such nonhomogeneous tissue. However, in the central foveal region of the retina, the inside layers are pulled aside to decrease this loss of acuity. 6 Foveal Region of the Retina and Its Importance in Acute Vision. The fovea is a minute area in the center of the retina, shown in Figure 51-2, occupying a total area a little more than 1 square millimeter; it is especially capable of acute and detailed vision. The central fovea, only 0.3 millimeter in diameter, is composed almost entirely of cones. These cones have a special structure that aids their detection of detail in the visual image—that is, the foveal cones have especially long and slender bodies, in contradistinction to the much fatter cones located more peripherally in the retina. Also, in the foveal region, the blood vessels, ganglion cells, inner nuclear layer of cells, and plexiform layers are all displaced to one side rather than resting directly on top of the cones, which allows light to pass unimpeded to the cones. 7 8 Rods and Cones. Figure 51-3 is a diagrammatic representation of the essential components of a photoreceptor (either a rod or a cone). As shown in Figure 51-4, the outer segment of the cone is conical in shape. In general, the rods are narrower and longer than the cones, but this is not always the case. In the peripheral portions of the retina, the rods are 2 to 5 micrometers in diameter, whereas the cones are 5 to 8 micrometers in diameter; in the central part of the retina, in the fovea, there are no rods, and the cones are slender and have a diameter of only 1.5 micrometers. 9 10 11 The major functional segments of either a rod or cone are shown in Figure 51-3: (1) the outer segment, (2) the inner segment, (3) the nucleus, and (4) the synaptic body. The light-sensitive photochemical is found in the outer segment. In the case of the rods, this photochemical is rhodopsin; in the cones, it is one of three “color” photochemicals, usually called simply color pigments, that function almost exactly the same as rhodopsin except for differences in spectral sensitivity. In the outer segments of the rods and cones in Figures 51-3 and 51-4, note the large numbers of discs. Each disc is actually an infolded shelf of cell membrane. There are as many as 1000 discs in each rod or cone. 12 Both rhodopsin and the color pigments are conjugated proteins. They are incorporated into the membranes of the discs in the form of transmembrane proteins. The concentrations of these photosensitive pigments in the discs are so great that the pigments themselves constitute about 40 percent of the entire mass of the outer segment. The inner segment of the rod or cone contains the usual cytoplasm with cytoplasmic organelles. Especially important are the mitochondria, which play the important role of providing energy for function of the photoreceptors. The synaptic body is the portion of the rod or cone that connects with subsequent neuronal cells, the horizontal and bipolar cells, which represent the next stages in the vision chain. 13 Pigment Layer of the Retina. The black pigment melanin in the pigment layer prevents light reflection throughout the globe of the eyeball, which is extremely important for clear vision. Without it, light rays would be reflected in all directions within the eyeball and would cause diffuse lighting of the retina rather than the normal contrast between dark and light spots required for formation of precise images. The pigment layer also stores large quantities of vitamin A. This vitamin A is exchanged back and forth through the cell membranes of the outer segments of the rods and cones, which themselves are embedded in the pigment. Vitamin A is an important precursor of the photosensitive chemicals of the rods and cones. 14 Photochemistry of Vision Rhodopsin and Its Decomposition by Light Energy. The outer segment of the rod that projects into the pigment layer of the retina has a concentration of about 40 percent of the light-sensitive pigment called rhodopsin. This substance is a combination of the protein scotopsin and the carotenoid pigment retinal. Furthermore, the retinal is a particular type called 11-cis retinal. This cis form of retinal is important because only this form can bind with scotopsin to synthesize rhodopsin. When light energy is absorbed by rhodopsin, the rhodopsin begins to decompose within a very small fraction of a second, as shown at the top of Figure 51-5. The cause of this rapid decomposition is photoactivation of electrons in the retinal portion of the rhodopsin, which leads to instantaneous change of the cis form of retinal into an all-trans form that has the same chemical structure as the cis form but a different physical structure—it is a straight molecule rather than an angulated molecule. 16 17 Because the three-dimensional orientation of the reactive sites of the all-trans retinal no longer fits with the orientation of the reactive sites on the protein scotopsin, the all-trans retinal begins to pull away from the scotopsin. The immediate product is bathorhodopsin, which is a partially split combination of the all-trans retinal and scotopsin. Bathorhodopsin is extremely unstable and decays in nanoseconds to lumirhodopsin. This product then decays in microseconds to metarhodopsin I, then in about a millisecond to metarhodopsin II, and finally, much more slowly (in seconds), into the completely split products scotopsin and all-trans retinal. It is the metarhodopsin II, also called activated rhodopsin, that excites electrical changes in the rods, and the rods then transmit the visual image into the central nervous system in the form of optic nerve action potential. 18 Re-Formation of Rhodopsin. The first stage in re-formation of rhodopsin, as shown in Figure 51-5, is to reconvert the all-trans retinal into 11-cis retinal. This process requires metabolic energy and is catalyzed by the enzyme retinal isomerase. Once the 11-cis retinal is formed, it automatically recombines with the scotopsin to re-form rhodopsin, which then remains stable until its decomposition is again triggered by absorption of light energy. 19 Role of Vitamin A for Formation of Rhodopsin. Note in Figure 51-5 that there is a second chemical route by which all-trans retinal can be converted into 11-cis retinal. This second route is by conversion of the all-trans retinal first into all-trans retinol, which is one form of vitamin A. Then the all-trans retinol is converted into 11-cis retinol under the influence of the enzyme isomerase. Finally, the 11-cis retinol is converted into 11-cis retinal, which combines with scotopsin to form new rhodopsin. Vitamin A is present both in the cytoplasm of the rods and in the pigment layer of the retina. Therefore, vitamin A is normally always available to form new retinal when needed. Conversely, when there is excess retinal in the retina, it is converted back into vitamin A, thus reducing the amount of light- sensitive pigment in the retina. 20 Excitation of the Rod When Rhodopsin Is Activated by Light The Rod Receptor Potential is Hyperpolarizing, Not Depolarizing. When the rod is exposed to light, the resulting receptor potential is different from the receptor potentials in almost all other sensory receptors because excitation of the rod causes increased negativity of the intrarod membrane potential, which is a state of hyperpolarization. This is exactly opposite to the decreased negativity (the process of “depolarization”) that occurs in almost all other sensory receptors. How does activation of rhodopsin cause hyperpolarization? The answer is that when rhodopsin decomposes, it decreases the rod membrane conductance for sodium ions in the outer segment of the rod. This causes hyperpolarization of the entire rod membrane in the following way. 22 Figure 51-6 shows movement of sodium and potassium ions in a complete electrical circuit through the inner and outer segments of the rod. The inner segment continually pumps sodium from inside the rod to the outside, and potassium ions are pumped to the inside of the cell. Potassium ions leak out of the cell through nongated potassium channels that are confined to the inner segment of the rod. As in other cells, this sodium-potassium pump creates a negative potential on the inside of the entire cell. However, the outer segment of the rod, where the photoreceptor discs are located, is entirely different; here, the rod membrane, in the dark state, is leaky to sodium ions that flow through cyclic guanosine monophosphate (cGMP)-gated channels. In the dark state, cGMP levels are high, permitting positively charged sodium ions to continually leak back to the inside of the rod and thereby neutralize much of the negativity on the inside of the entire cell. Thus, under normal dark conditions, when the rod is not excited, there is reduced electronegativity inside the membrane of the rod, measuring about −40 millivolts rather than the usual −70 to −80 millivolts found in most sensory receptors. 23 24 When the rhodopsin in the outer segment of the rod is exposed to light, it is activated and begins to decompose. The cGMP-gated sodium channels are then closed, and the outer segment membrane conductance of sodium to the interior of the rod is reduced by a three-step process (Figure 51-7): (1) light is absorbed by the rhodopsin, causing photoactivation of the electrons in the retinal portion, as previously described; (2) the activated rhodopsin stimulates a G protein called transducin, which then activates cGMP phosphodiesterase, an enzyme that catalyzes the breakdown of cGMP to 5′- cGMP; and (3) the reduction in cGMP closes the cGMP-gated sodium channels and reduces the inward sodium current. Sodium ions continue to be pumped outward through the membrane of the inner segment. Thus, more sodium ions now leave the rod than leak back in. Because they are positive ions, their loss from inside the rod creates increased negativity inside the membrane, and the greater the amount of light energy striking the rod, the greater the electronegativity becomes—that is, the greater is the degree of hyperpolarization. At maximum light intensity, the membrane potential approaches −70 to −80 millivolts. 25 26 Transmission of Most Signals Occurs in the Retinal Neurons by Electrotonic Conduction, Not by Action Potentials. The only retinal neurons that always transmit visual signals by means of action potentials are the ganglion cells, and they send their signals all the way to the brain through the optic nerve. Electrotonic conduction means direct flow of electric current, not action potentials, in the neuronal cytoplasm and nerve axons from the point of excitation all the way to the output synapses. Even in the rods and cones, conduction from their outer segments, where the visual signals are generated, to the synaptic bodies is by electrotonic conduction. 27 That is, when hyperpolarization occurs in response to light in the outer segment of a rod or a cone, almost the same degree of hyperpolarization is conducted by direct electric current flow in the cytoplasm all the way to the synaptic body, and no action potential is required. Then, when the transmitter from a rod or cone stimulates a bipolar cell or horizontal cell, once again the signal is transmitted from the input to the output by direct electric current flow, not by action potentials. The importance of electrotonic conduction is that it allows graded conduction of signal strength. Thus, for the rods and cones, the strength of the hyperpolarizing output signal is directly related to the intensity of illumination; the signal is not all or none, as would be the case for each action potential. 28