Biological Sciences
Human Retina
The human retina is a thin layer of tissue located at the back of the eye that contains photoreceptor cells called rods and cones. These cells convert light into electrical signals that are sent to the brain via the optic nerve, allowing us to see and perceive the world around us. The retina also contains other types of cells that support and nourish the photoreceptors.
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11 Key excerpts on "Human Retina"
- Ruben Adler(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Alfred J. (Chris) Coulombre, to whom these two volumes are dedicated, occupies a site of honor as one of the founding fathers of the cell biology of the retina. Contributing authors have been asked to review the field of their expertise, with emphasis not only on established concepts but also on unanswered ques-tions, emerging issues, and areas of controversy. They have also been asked to describe technical breakthroughs which have had, or might have, significant impact on research in their fields. Correlations between basic retinal cell biology and Human Retinal disease have been made whenever possible. Because of space limitations, some important topics (i.e., glial cells, cell proliferation) are briefly ISSUES AND QUESTIONS IN RETINAL CELL BIOLOGY 3 discussed in this Introduction but have not been alloted individual articles. To orient the reader who may not be familiar with the retina, we also present in the following pages some basic information about retinal structure and development. Moreover, we discuss the advantages of the retina as a model system for cell biology studies, and call attention to problems or questions which have not yet received proper attention. II. The Adult Retina A. Localization within the Eye The retina is the innermost of the three coats that form the wall of the eyeball (Fig. 1). The outer coat is thick and tough and protects the delicate inner struc-FIG. 1. Diagram of a horizontal section of a high vertebrate eye. The cellular organization of a portion of the neural retina (□) is represented in Fig. 2. 4 DEBORA FARBER AND RUBEN ADLER tures. It is opaque in the larger posterior portion, the sclera, and transparent in the smaller anterior section, the cornea. The middle vascular coat, the uvea or choroid, provides the nutrients to the ocular tissues, and its anterior segment, the ciliary body, is the muscular instrument for the process of accommodation car-ried out by the lens.
eBook - PDFBioelectronic Vision: Retina Models, Evaluation Metrics And System Design
Retina Models, Evaluation Metrics, and System Design
- Joao Carlos Martins, Leonel Augusto Sousa(Authors)
- 2009(Publication Date)
- World Scientific(Publisher)
2.2. Referring to the photoreceptors present in the Human Retina: 2.2.1 What are the main characteristics of both types of photoreceptors? 2.2.2 What is their main location based on their relative quantities? 2.2.3 Investigate the process of light transducing at the photoreceptor’s layer of the retina. 2.3. Present the different types of bipolar cells, describing their importance The Human Visual System 55 to vision. 2.4. Present the connections between photoreceptors and bipolar, horizon-tal, and ganglion cells, relating the way that images are transmitted to the brain through the optical nerve. 2.5. Compare the organization of the retinal cells in the fovea against its periphery. Where is the spatial resolution higher? Why? 2.6. Describe the mechanisms, in terms of the retina structure, used to decrease the retina response time. Where are these mechanisms more effec-tive? 2.7. Besides the eye, what are the other main centers of the visual path-way? Describe their organization and enumerate their main characteristics. 2.8. Describe the main advantages and disadvantages of all the centers along the visual pathway with respect to interfacing a visual prosthesis. 2.9. What is the importance of visuotopic organization in the development of a visual prosthesis? Describe the visuotopic organization along the visual pathway, from retina to the visual cortex. This page intentionally left blank This page intentionally left blank
eBook - ePubVisual and Non-Visual Effects of Light
Working Environment and Well-Being
- Agnieszka Wolska, Dariusz Sawicki, Malgorzata Tafil-Klawe, Małgorzata Tafil-Klawe(Authors)
- 2020(Publication Date)
- CRC Press(Publisher)
The lens has just the right curvature for parallel rays of light to pass through each of its parts and be bent exactly enough for all the rays to pass through a single focal point. The more a lens bends the light rays, the greater is its refractive power measured in terms of diopters. In children, this refractive power can be increased from 20 diopters to about 34 diopters, which is an accommodation of about 14 diopters. The elastic lens capsule can change shape (become more or less spherical) in response to the activity of the ciliary muscle, controlled by the autonomic nervous system. This refractive power influences visual acuity, or clarity of vision – in the human eye it is about 25 seconds of arc for discriminating between point sources of light. A person with normal visual acuity looking at two pinpoint light spots 10 m away can barely distinguish the separate spots when they are 1.5 to 2 mm apart.The retina, a part of the eye and a specialized part of the central nervous system, called “the brain’s window to the world”, is the sensory part of the eye [Mayeli 2019]. Its primary job is to convert energy from the sensory stimuli – the photons of light – into an electrical signal, transmitted and analyzed by special regions of the brain in a process called phototransduction. Histologically, the retina consists of three cellular layers, which contain five cell types separated by two synaptic layers. The complex organization suggests the different cells’ participation in various physiological regulatory processes. Phototransduction is carried out in light-sensitive neurons called photoreceptors, located at the rear of the retina. The Human Retina includes two classes of photoreceptors: rods and cones. These make synaptic connections with bipolar cells, which in turn convey information to the retinal ganglion cells, along a “vertical” visual pathway. The axons of the ganglion cells constitute the optic nerve. The signals along this pathway are modulated by inhibitory neurons at two levels: those of the horizontal cells in the outer retina and the amacrine cells in the inner retina [Chapot et al. 2017]. Supporting glial cells (Müller cells) and their cytoplasmatic processes fill the space between photoreceptors and bipolar and ganglion cells. Müller cells are essential for the transmission of light, due to their unique shape, orientation, and refractive index [Franze et al. 2007]. They act as conduits that enable light to reach the photoreceptors with minimal scattering. It seems that every Müller cell is coupled with a partner cone cell. In addition to these cells, microglial cells are present in all layers.
eBook - PDF- Hugh Davson(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
T w o other types of nerve cell are present in the retina, namely the horizontal and amacrine cells, with their bodies in the inner nuclear layer; the ramifications of their dendritic and axonal processes contribute to the outer and inner plexiform layers respectively; their largely horizontal organizations permits them to mediate connections between receptors, bipolars and ganglion cells. Besides the nerve cells there are numerous neuroglial cells, e.g. those giving rise to the radial fibres of Muller, which act as supporting and insulating structures. THE RODS AND CONES The photosensitive cells are, in the primate and most vertebrate retinae, of two kinds, called rods and cones, the rods being usually much thinner than the cones but both being built on the same general plan as illustrated in Figure 5.3. The light-sensitive pigment is contained in the outer segment, o, which rests on the pigment epithelium; the other end is called the synaptic body, s, and it is through this that the effects of light on the receptor are transmitted to the bipolar or horizontal cell. In the rods this is called the rod spherule, and in the cone the cone pedicle. The human rod is some 2 μ thick and 60 μ long; the cones vary greatly in size and 168 PHYSIOLOGY OF THE EYE Fig. 5.1 Cross-section through the adult Human Retina in the periphery of the central area, showing details of stratification at a moderate magnification. The left-hand figure shows the structures as they appear when stained with a non-selective method (e.g. haematoxylin and eosin). The right-hand figure reproduces schematically the same view as it would appear from a study with an analytical method such as Golgi's or Ehrlich's, using many sections to get all details.
eBook - PDF- George Spilich(Author)
- 2023(Publication Date)
- Wiley(Publisher)
5.2 The Retina 117 At each stage of the visual processing stream, raw sensory data is further refined until it forms a meaningful mental image. You might compare your visual system to a computer, in which a bit is an elementary unit of a signal. Your visual system is a marvelous device that takes in as many as ten million bits of visual data per second at the level of the retina. Along with the useful bits of data that will form our visual perception is a lot of background noise, or random fluctuations that don’t actually contribute to the picture. At each processing stage in the visual stream, the noise is reduced, and the information-laden signal is enhanced. We start with raw physical energy in the form of light, and after many steps in various neural structures, the process culminates in an appreciation of something as visually nuanced as Leonardo da Vinci’s Mona Lisa. We will begin our discussion by looking at the retina, that very important cellular layer at the back of the eye. 5.1 Before You Go On Section Summary The physical characteristics of visible light waves are translated into brightness and color by our perceptual system. Light enters the eye, and the image of the world is refracted by the cornea and lens onto the retina. When the focal plane does not fall correctly on the retina, visual difficulties such as myopia and hyperopia occur. Visual acuity and contrast sensitivity are two measures of vision. As light enters the eye, it is first processed at the retina, then in the subcortical centers, and finally at the visual cortex. Comprehension Check 1. Visible light, infrared light, microwaves, X-rays, and gamma waves . a. differ mainly in their intensity b. are all part of the electromagnetic spectrum c. differ in that the first two are light and the last three are radiation d. are all at different frequencies of the visible spectrum 2. The fibrous structure that protects the eye from physical damage is the .
eBook - PDFInvertebrate Photoreceptors
A Comparative Analysis
- Jerome J. Wolken(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
V. THE VERTEBRATE RETINAL PHOTORECEPTORS One might wonder why, in the midst of our discussion of invertebrate photoreceptors, I have turned to the vertebrate retina. The reason is that before we attempt in Chapter VI to describe the experimental results of invertebrate visual pigments, we must find a basis for our analyses. This can be found in our extensive knowledge of vertebrate visual systems. There is good reason to believe that throughout the animal world the kinds of pigment molecules and receptor structures involved in photo-sensitivity are similar. If this is true then we must expect certain features of vertebrate visual chemistry to repeat themselves in the invertebrates. With these considerations in mind, let us take a brief but very necessary look at the vertebrate retina, its visual pigments and photochemistry. The Retinal Rod The vertebrate retina consists of nine cell layers closely attached to the pigment epithelium (Figure 5.1). The nervous cell layers are the rod and cone cells, the bipolar cells, and the ganglion cells. Next to the retina is the choroid coat, a sheet of cells filled with black pigment which absorbs extra light and prevents internally reflected light from blurring the image. Toward the center of the Human Retina there is a depression, the fovea, which is the fixation point of the eye where vision is most acute. It con-tains mostly cones. In the Human Retina (Figure 3.3) there are about 1 x 10 8 retinal rods and 7 x 10 6 retinal cones (Figure 5.2) of which 4 x 10 3 are in the foveal area. The rods become more numerous as the distance from the fovea in-creases. The fovea and the region just around it, the macula lutea, are colored yellow; they contain a plant carotenoid, a xanthophyll (Figure 1.6). The rods and cones are differentiated specialized structures of the retinal cells.
eBook - PDFThe Amphibian Visual System
A Multidisciplinary Approach
- Katherine V. Fite(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
Furthermore, the first single-fiber re- cordings from a vertebrate visual system were obtained from frog ganglion-cell fibers (Hartline, 1938). Thus, many important aspects of the retinal anatomy and physiology of vertebrate visual systems have first been described in frogs. As will be discussed in this chapter, this has continued to occur. What follows is a selective review of current knowledge concerning the anatomy and physiology of the frog retina. ANATOMY The frog retina, like all other vertebrate retinas, is a thin, but com- plex, structure with three nuclear layers and two plexiform layers. The receptors are located toward the back of the eye and the more proximal layers of the retina are nearer to the front of the eye. Figure 1A* shows a light photomicrograph of a section of frog retina in which the basic layers of the retina are identified. There are approximately 2 to 3 receptors and 5 to 7 bipolar and horizontal cells for each of the 450,000 ganglion cells (Maturana et al. y 1960). Figure 2 shows the number of each type of receptor and of ganglion cells as a function of retinal location for Rana catesbeiana. There is gen- erally, an area centralis where there is an increased number of cells, but the frog has no foveal pit. This area is characterized by a thickening of the retina resulting from an increase in the number of receptors, inner nuclear layer (INL) cells, and ganglion cells, as well as an increase in the size of the inner plexiform layer (IPL). Table 1 shows the percentage of each receptor type in the retina of Rana pipiens. Receptors There are actually four types of receptor in the frog retina—two are rods and two are cones. Photomicrographs of these receptors are shown in Fig. IB.* The rods will be considered first. The red and green rods were so named due to their colored appearance in a freshly excised retina and their cylindrical, rodlike outer segments.
eBook - PDF- Edward Carterette(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
lim. membrane Optic nerve fibers Ganglion cells Inner plex. layer Inner nuclear layer (horizontal, bipolar, amacrine cells -^/ 1 ! 1 1 I 1 1 Rods >^ Nasal retina 1 1 1 1 1 1 1 1 o a ■σ c CD Fovea i ! i i 1 T r τ ' ^ v . R o d s Temporal retina 1 -Y , , , , , , , , , 60 40 20 0 20 Distance from fovea (degrees) Outer plex. layer '*'^''f^J'0& Outer nuclear layer Rods and cones ^ V P Choroid FIG. 5. Distribution of cell types across the retina. A: Density of rods and cones as a function of horizontal location across a Human Retina. The blindspot is formed by the optic nerve leaving the eye; there are no receptors in this area. (After 0ster-berg, 1935.) B: Vertical section through the fovea of a macaque monkey. C: Rep-resentative retinal sections (macaque) at increasing distances from the fovea; sec-tion on left is closest to fovea, while that on right is from peripheral retina. (Photo-micrographs from Brown et al., 1965.) convergence of receptors onto ganglion cells. In fact, the relative numbers of the cell types and their lateral ramifications make it obvious that the rule throughout the retina is considerable overlapping of pathways; in a Human Retina there are altogether about 6.5 million cones, 120 million rods, and only 1 million ganglion cells. During the past decade the electron microscope has been employed 16. VISION 335 extensively to examine the details of the synaptic organization of the sys-tem seen through the light microscope (Cohen, 1963; Dowling & Boycott, 1966; Sjöstrand, 1965). Figure 6 provides a schematic summary (not drawn to scale) of many of the observations on monkey and Human Retinas. A receptor can be divided into an outer segment, an inner segment, the cell nucleus, and a long (much longer than drawn—see Fig. 4) process ending in a synaptic terminal in the outer plexiform layer; the outer segment is divided into disks, or lamellae, which are thought to contain the photo-pigment.
eBook - PDFDiscovering Behavioral Neuroscience
An Introduction to Biological Psychology
- Laura Freberg(Author)
- 2018(Publication Date)
- Cengage Learning EMEA(Publisher)
Any single visual interneuron, such as a bipolar cell, receives input from one or several photoreceptors located in a specific area on the retina. That area is referred to as the interneuron’s receptive field (Hartline, 1938). You can think about the retina as an overlapping mosaic of receptive fields (see ● Figure 6.14). If a pinpoint light is directed to the retina, it is possible to identify which interneurons are responding to the light by recording their activity. A light stimulus must fit within a cell’s receptive field to influence its activity. The cell is “blind” to any light falling outside its receptive field on the retina. Let’s imagine that we are doing a single-cell recording from one bipolar cell to map its receptive field. When we shine a pinpoint of light into our participant’s eye, we find that our bipolar cell depolarizes. When we turn the light off, the cell returns to its normal resting status. If we move our light a little bit to the side, then the cell ● Figure 6.14 Visual Receptive Fields Visual interneurons, including bipolar and ganglion cells (modeled by the spheres), will only respond to light falling on photoreceptors located in the interneuron’s receptive field (modeled by the overlapping circles on the image). Light falling outside an interneuron’s receptive field has no effect on its activity. The cell is essential-ly blind to that light. NS Photograph/Shutterstock.com receptive field A location on the retina at which light affects the activity of a particular visual interneuron. Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
eBook - PDFColor Imaging
Fundamentals and Applications
- Erik Reinhard, Erum Arif Khan, Ahmet Oguz Akyuz, Garrett Johnson(Authors)
- 2008(Publication Date)
- A K Peters/CRC Press(Publisher)
The activity of horizontal cells and, therefore, indirectly photoreceptors and bipolar cells, is modulated by further feedback from signals originating in the in-ner plexiform layer. This allows photoreceptors to neuronally adapt to light, in addition to the non-linear response mechanisms outlined in Section 4.3.3. This 224 4. Human Vision feedback mechanism also affords the possibility of color coding the response of bipolar cells [609]. As a result, the notion that retinal processing of signals is strictly feed-forward is false, as even the photoreceptors appear to receive feed-back from later stages of processing. 4.3.7 Ganglion Cells Ganglion cells form the connectivity between the retina and the rest of the brain. Their input, taken from both bipolar cells and amacrine cells, is relayed to the lateral geniculate nucleus through the ganglion’s axons which form the optic nerve bundle. The predominant ganglion cell types in the Human Retina are ON-center and OFF-center [430]. An ON-center ganglion cell is activated when a spot of light falls on the center of its receptive field. The opposite happens when light falls onto the periphery of its receptive field. For OFF-center ganglion cells, the activation pattern is reversed with activation occurring if light falls on the periphery; illumi-nation of the center of its receptive field causing inhibition. Thus, the receptive fields of these ganglion types are concentric, with center-surround organization (ON-center, OFF-surround and vice versa) [632]. In the fovea, the receptive fields of ganglion cells are much smaller than in the rest of the retina. Here, midget ganglion cells connect to midget bipolar cells in a one-to-one correspondence. Each red and green cone in the fovea (but not blue cone) connects to one ON and one OFF midget bipolar cell, and these are in turn connected to an ON and OFF ganglion cell, thereby continuing the ON and OFF pathways.
eBook - PDF- A.C. Damask(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
They concluded that: (1) the disk membranes can actively accumulate C a 2 + in the dark and light, (2) the rate of accumula-tion is nearly three times greater in light than in dark, and (3) the disk membranes release C a 2 + as a consequence of light absorption and visual pigment bleaching. Reviews of subsequent experiments on the role of calcium are given in Barlow and Fatt (1977). This specific role of C a 2 + is undergoing intensive investigation with some agreement (Wormington and Cone, 1978), some disagreement (Bertrand et al., 1978; Flaming and Brown, 1979), or that calcium is a co-factor (Arden and Low, 1978). 214 6. VISION STRUCTURE OF THE PRIMATE RETINA There are over 100 million primary receptors in the human eye, about 95% of which are rods and the remainder cones. These make connections through a variety of cells within the retina to ganglion cells which send only about 1 million optic nerve fibers to the brain. Thus, the brain receives only a convergence of visual information, most of the integration being performed within the cells of the retina. The cells of the retina are arranged in five layers, and detailed micro-scope studies are best interpreted by means of a schematic. Figure 6.53 shows such a drawing. Note that, as mentioned earlier, light passes through the neural cells to reach the chromophore, i.e., from the bottom of the page toward the top. It is seen that the pathway between the receptors and the ganglion cells is much more complicated than a simple line through the FIG. 6.53 Semischematic diagram of the connections among neural elements in the primate retina that were identified as of 1966. R, rod; C, cone; MB, midget bipolar nerve cell; RB, rod bipolar; FB, flat bipolar; H, horizontal cell; A, amacrine cell; MG, midget ganglion; and DG, diffuse ganglion. The regions where the cells are contiguous are synapses. [From Dowling and Boycott (1966).]
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