{"id":4468,"date":"2017-03-29T16:17:24","date_gmt":"2017-03-29T16:17:24","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=4468"},"modified":"2024-04-26T02:20:45","modified_gmt":"2024-04-26T02:20:45","slug":"transduction-of-light","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/transduction-of-light\/","title":{"raw":"Transduction of Light","rendered":"Transduction of Light"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Trace the path of light through the eye to the point of the optic nerve<\/li>\r\n<\/ul>\r\n<\/div>\r\nThe rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, <b>rhodopsin<\/b>, has two main parts Figure 1): an opsin, which is a membrane protein (in the form of a cluster of \u03b1-helices that span the membrane), and retinal\u2014a molecule that absorbs light.\r\n\r\n[caption id=\"attachment_3493\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-3493\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/15000636\/Figure_36_05_04ab-1024x432.jpg\" alt=\" Molecular model A shows the structure of rhodopsin, a trans-membrane protein with seven helices spanning the membrane. A small organic molecule called retinal is tucked inside. B shows the molecular structure of retinal, which has a ring with a hydrocarbon chain attached. A ketone (double bonded oxygen) is at the end of the chain. In cis retinal the chain is kinked. In trans retinal the chain is straight.\" width=\"1024\" height=\"432\" \/> Figure 1.\u00a0(a) Rhodopsin, the photoreceptor in vertebrates, has two parts: the trans-membrane protein opsin, and retinal. When light strikes retinal, it changes shape from (b) a cis to a trans form. The signal is passed to a G-protein called transducin, triggering a series of downstream events.[\/caption]\r\n\r\nWhen light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (<em>cis<\/em>) form of the molecule to its linear (<em>trans<\/em>) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na<sup>+<\/sup> channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus) visual receptors become hyperpolarized and thus driven away from threshold (Figure 2).\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"600\"]<img id=\"9\" class=\"\" src=\"https:\/\/openstax.org\/resources\/c436549273c45bd388eb79e839468e0776ae477d\" alt=\"Illustration A shows the signal transduction pathway for rhodopsin, which is located in internal membranes at the top of rod cells. When light strikes rhodopsin, a G protein called transducing is activated. Transducin has three subunits, alpha, beta and gamma. Upon activation, G D P on the alpha subunit is replaced with G T P. The subunit dissociates, and binds phosphodiesterase. Phosphodiesterase, in turn, converts c G M P to G M P, which closes sodium ion channels. As a result, sodium can no longer enter the cell, and the membrane becomes hyperpolarized. Illustration b shows that the tall, thin rod cell is stacked on top of a bipolar nerve cell. In the dark the membrane is depolarized, and glutamate is released from the rod cell to the axon terminal of the bipolar cell. In the light, no glutamate is released.\" width=\"600\" height=\"602\" data-media-type=\"image\/png\" \/> Figure 2. When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates phosphodiesterase. Phosphodiesterase converts cGMP to GMP, thereby closing sodium channels. As a result, the membrane becomes hyperpolarized. The hyperpolarized membrane does not release glutamate to the bipolar cell.[\/caption]\r\n<h2>Trichromatic Coding<\/h2>\r\n[caption id=\"attachment_3495\" align=\"alignright\" width=\"400\"]<img class=\"wp-image-3495\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/15000805\/Figure_36_05_06.jpg\" alt=\" Graph plots normalized absorbance for rods and S, M and L cones against wavelength. For all four cell types, the trend is an approximately bell-shaped curve with a steeper decrease than increase. For S cones the peak absorbance is 420 nanometers. For rods the peak absorbance is 498 nanometers. For M cones the peak absorbance is 534 nanometers. For L cones the peak absorbance is 564 nanometers.\" width=\"400\" height=\"251\" \/> Figure 3.\u00a0Human rod cells and the different types of cone cells each have an optimal wavelength. However, there is considerable overlap in the wavelengths of light detected.[\/caption]\r\n\r\nThere are three types of cones (with different photopsins), and they differ in the wavelength to which they are most responsive, as shown in Figure 3. Some cones are maximally responsive to short light waves of 420 nm, so they are called S cones (\u201cS\u201d for \u201cshort\u201d); others respond maximally to waves of 530 nm (M cones, for \u201cmedium\u201d); a third group responds maximally to light of longer wavelengths, at 560 nm (L, or \u201clong\u201d cones). With only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has limitations. Primates use a three-cone (trichromatic) system, resulting in full color vision.\r\n\r\nThe color we perceive is a result of the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation, or about 2 million distinct colors.\r\n<h2>Retinal Processing<\/h2>\r\nVisual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells. A large degree of processing of visual information occurs in the retina itself, before visual information is sent to the brain.\r\n<p id=\"fs-idp51630656\">Photoreceptors in the retina continuously undergo\u00a0<strong><span id=\"term1675\" data-type=\"term\">tonic activity<\/span><\/strong>. That is, they are always slightly active even when not stimulated by light. In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at a baseline; while some stimuli increase firing rate from the baseline, and other stimuli decrease firing rate. In the absence of light, the bipolar neurons that connect rods and cones to ganglion cells are continuously and actively inhibited by the rods and cones. Exposure of the retina to light hyperpolarizes the rods and cones and removes their inhibition of bipolar cells. The now active bipolar cells in turn stimulate the ganglion cells, which send action potentials along their axons (which leave the eye as the optic nerve). Thus, the visual system relies on change in retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain. Sometimes horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells. When a rod or cone stimulates a horizontal cell, the horizontal cell inhibits more distant photoreceptors and bipolar cells, creating lateral inhibition. This inhibition sharpens edges and enhances contrast in the images by making regions receiving light appear lighter and dark surroundings appear darker. Amacrine cells can distribute information from one bipolar cell to many ganglion cells.<\/p>\r\nYou can demonstrate this using an easy demonstration to \u201ctrick\u201d your retina and brain about the colors you are observing in your visual field. Look fixedly at Figure 4\u00a0for about 45 seconds. Then quickly shift your gaze to a sheet of blank white paper or a white wall. You should see an afterimage of the Norwegian flag in its correct colors. At this point, close your eyes for a moment, then reopen them, looking again at the white paper or wall; the afterimage of the flag should continue to appear as red, white, and blue.\r\n\r\n[caption id=\"attachment_3496\" align=\"aligncenter\" width=\"544\"]<img class=\"size-full wp-image-3496\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/15001027\/Figure_36_05_07.jpg\" alt=\"A Norwegian flag is shown in false colors of green, yellow and black (normally, the colors are red, white and blue, like the American flag.\" width=\"544\" height=\"393\" \/> Figure 4.\u00a0View this flag to understand how retinal processing works. Stare at the center of the flag (indicated by the white dot) for 45 seconds, and then quickly look at a white background, noticing how colors appear.[\/caption]\r\n\r\nWhat causes this? According to an explanation called opponent process theory, as you gazed fixedly at the green, black, and yellow flag, your retinal ganglion cells that respond positively to green, black, and yellow increased their firing dramatically. When you shifted your gaze to the neutral white ground, these ganglion cells abruptly decreased their activity and the brain interpreted this abrupt downshift as if the ganglion cells were responding now to their \u201copponent\u201d colors: red, white, and blue, respectively, in the visual field. Once the ganglion cells return to their baseline activity state, the false perception of color will disappear.\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/fc45343d-14f6-49df-b8b9-4f29a5509048\r\n<\/div>\r\n","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Trace the path of light through the eye to the point of the optic nerve<\/li>\n<\/ul>\n<\/div>\n<p>The rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, <b>rhodopsin<\/b>, has two main parts Figure 1): an opsin, which is a membrane protein (in the form of a cluster of \u03b1-helices that span the membrane), and retinal\u2014a molecule that absorbs light.<\/p>\n<div id=\"attachment_3493\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3493\" class=\"size-large wp-image-3493\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/15000636\/Figure_36_05_04ab-1024x432.jpg\" alt=\"Molecular model A shows the structure of rhodopsin, a trans-membrane protein with seven helices spanning the membrane. A small organic molecule called retinal is tucked inside. B shows the molecular structure of retinal, which has a ring with a hydrocarbon chain attached. A ketone (double bonded oxygen) is at the end of the chain. In cis retinal the chain is kinked. In trans retinal the chain is straight.\" width=\"1024\" height=\"432\" \/><\/p>\n<p id=\"caption-attachment-3493\" class=\"wp-caption-text\">Figure 1.\u00a0(a) Rhodopsin, the photoreceptor in vertebrates, has two parts: the trans-membrane protein opsin, and retinal. When light strikes retinal, it changes shape from (b) a cis to a trans form. The signal is passed to a G-protein called transducin, triggering a series of downstream events.<\/p>\n<\/div>\n<p>When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (<em>cis<\/em>) form of the molecule to its linear (<em>trans<\/em>) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na<sup>+<\/sup> channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus) visual receptors become hyperpolarized and thus driven away from threshold (Figure 2).<\/p>\n<div style=\"width: 610px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"9\" class=\"\" src=\"https:\/\/openstax.org\/resources\/c436549273c45bd388eb79e839468e0776ae477d\" alt=\"Illustration A shows the signal transduction pathway for rhodopsin, which is located in internal membranes at the top of rod cells. When light strikes rhodopsin, a G protein called transducing is activated. Transducin has three subunits, alpha, beta and gamma. Upon activation, G D P on the alpha subunit is replaced with G T P. The subunit dissociates, and binds phosphodiesterase. Phosphodiesterase, in turn, converts c G M P to G M P, which closes sodium ion channels. As a result, sodium can no longer enter the cell, and the membrane becomes hyperpolarized. Illustration b shows that the tall, thin rod cell is stacked on top of a bipolar nerve cell. In the dark the membrane is depolarized, and glutamate is released from the rod cell to the axon terminal of the bipolar cell. In the light, no glutamate is released.\" width=\"600\" height=\"602\" data-media-type=\"image\/png\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 2. When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates phosphodiesterase. Phosphodiesterase converts cGMP to GMP, thereby closing sodium channels. As a result, the membrane becomes hyperpolarized. The hyperpolarized membrane does not release glutamate to the bipolar cell.<\/p>\n<\/div>\n<h2>Trichromatic Coding<\/h2>\n<div id=\"attachment_3495\" style=\"width: 410px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3495\" class=\"wp-image-3495\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/15000805\/Figure_36_05_06.jpg\" alt=\"Graph plots normalized absorbance for rods and S, M and L cones against wavelength. For all four cell types, the trend is an approximately bell-shaped curve with a steeper decrease than increase. For S cones the peak absorbance is 420 nanometers. For rods the peak absorbance is 498 nanometers. For M cones the peak absorbance is 534 nanometers. For L cones the peak absorbance is 564 nanometers.\" width=\"400\" height=\"251\" \/><\/p>\n<p id=\"caption-attachment-3495\" class=\"wp-caption-text\">Figure 3.\u00a0Human rod cells and the different types of cone cells each have an optimal wavelength. However, there is considerable overlap in the wavelengths of light detected.<\/p>\n<\/div>\n<p>There are three types of cones (with different photopsins), and they differ in the wavelength to which they are most responsive, as shown in Figure 3. Some cones are maximally responsive to short light waves of 420 nm, so they are called S cones (\u201cS\u201d for \u201cshort\u201d); others respond maximally to waves of 530 nm (M cones, for \u201cmedium\u201d); a third group responds maximally to light of longer wavelengths, at 560 nm (L, or \u201clong\u201d cones). With only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has limitations. Primates use a three-cone (trichromatic) system, resulting in full color vision.<\/p>\n<p>The color we perceive is a result of the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation, or about 2 million distinct colors.<\/p>\n<h2>Retinal Processing<\/h2>\n<p>Visual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells. A large degree of processing of visual information occurs in the retina itself, before visual information is sent to the brain.<\/p>\n<p id=\"fs-idp51630656\">Photoreceptors in the retina continuously undergo\u00a0<strong><span id=\"term1675\" data-type=\"term\">tonic activity<\/span><\/strong>. That is, they are always slightly active even when not stimulated by light. In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at a baseline; while some stimuli increase firing rate from the baseline, and other stimuli decrease firing rate. In the absence of light, the bipolar neurons that connect rods and cones to ganglion cells are continuously and actively inhibited by the rods and cones. Exposure of the retina to light hyperpolarizes the rods and cones and removes their inhibition of bipolar cells. The now active bipolar cells in turn stimulate the ganglion cells, which send action potentials along their axons (which leave the eye as the optic nerve). Thus, the visual system relies on change in retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain. Sometimes horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells. When a rod or cone stimulates a horizontal cell, the horizontal cell inhibits more distant photoreceptors and bipolar cells, creating lateral inhibition. This inhibition sharpens edges and enhances contrast in the images by making regions receiving light appear lighter and dark surroundings appear darker. Amacrine cells can distribute information from one bipolar cell to many ganglion cells.<\/p>\n<p>You can demonstrate this using an easy demonstration to \u201ctrick\u201d your retina and brain about the colors you are observing in your visual field. Look fixedly at Figure 4\u00a0for about 45 seconds. Then quickly shift your gaze to a sheet of blank white paper or a white wall. You should see an afterimage of the Norwegian flag in its correct colors. At this point, close your eyes for a moment, then reopen them, looking again at the white paper or wall; the afterimage of the flag should continue to appear as red, white, and blue.<\/p>\n<div id=\"attachment_3496\" style=\"width: 554px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3496\" class=\"size-full wp-image-3496\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/15001027\/Figure_36_05_07.jpg\" alt=\"A Norwegian flag is shown in false colors of green, yellow and black (normally, the colors are red, white and blue, like the American flag.\" width=\"544\" height=\"393\" \/><\/p>\n<p id=\"caption-attachment-3496\" class=\"wp-caption-text\">Figure 4.\u00a0View this flag to understand how retinal processing works. Stare at the center of the flag (indicated by the white dot) for 45 seconds, and then quickly look at a white background, noticing how colors appear.<\/p>\n<\/div>\n<p>What causes this? According to an explanation called opponent process theory, as you gazed fixedly at the green, black, and yellow flag, your retinal ganglion cells that respond positively to green, black, and yellow increased their firing dramatically. When you shifted your gaze to the neutral white ground, these ganglion cells abruptly decreased their activity and the brain interpreted this abrupt downshift as if the ganglion cells were responding now to their \u201copponent\u201d colors: red, white, and blue, respectively, in the visual field. Once the ganglion cells return to their baseline activity state, the false perception of color will disappear.<\/p>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_fc45343d-14f6-49df-b8b9-4f29a5509048\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/fc45343d-14f6-49df-b8b9-4f29a5509048?iframe_resize_id=assessment_practice_id_fc45343d-14f6-49df-b8b9-4f29a5509048\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-4468\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":20,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"}]","CANDELA_OUTCOMES_GUID":"41bb77de-b447-40da-a860-bb9524753729, 645c7e13-895b-4740-a6c6-86053ec8f7c1","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-4468","chapter","type-chapter","status-publish","hentry"],"part":3798,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/4468","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":9,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/4468\/revisions"}],"predecessor-version":[{"id":8597,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/4468\/revisions\/8597"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/3798"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/4468\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=4468"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=4468"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=4468"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=4468"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}