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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9 Key excerpts on "Human Retina"
eBook - ePub- Britannica Educational Publishing, Kara Rogers(Authors)
- 2010(Publication Date)
- Britannica Educational Publishing(Publisher)
CHAPTER 3 VISION AND THE RETINAT he retina is fundamental to vision. It contains millions of light-sensitive photoreceptors that are essential to the perception of visual information. The photoreceptors of the human eye are assembled into complex networks of neurons that serve to organize incoming visual information into messages that can be interpreted by the brain.In the retina, as in other parts of the nervous system, the messages initiated in one element are transmitted, or relayed, to others. The regions of transmission from one cell to another are areas of intimate contact known as synapses. An impulse conveyed from one cell to another travels from the first cell body along a projection called an axon, to a synapse, where the impulse is received by a projection, called a dendrite, of the second cell. The impulse is then conveyed to the second cell body, to be transmitted further, along the second cell’s axon. Impulses are eventually transmitted to the optic nerve, which in turn carries the impulses to the visual centres of the brain. In this way, through the systematic transmission of electrical impulses along neurons, the information received by the retina is converted into a meaningful image.NEURON NETWORKS OF THE RETINA
The functioning cells of the retina include the photoreceptors—the rods and cones; the ganglion cells, the axons of which form the optic nerve; and cells that act in a variety of ways as intermediaries between the receptors and the ganglion cells. These intermediaries are named bipolar cells, horizontal cells, and amacrine cells.A diagram of the structure of the retina. Conditions affecting the retina can impair both central visual acuity and peripheral vision as well as alter light detection and image perception. Copyright Encyclopaedia Britannica; rendering for this edition by Rosen Educational Services.The synapses between these cells occur in definite layers, the outer and inner plexiform layers. In the outer plexiform layer the bipolar cells make their contacts, by way of their dendrites, with the rods and cones, specifically the spherules of the rods and the pedicles of the cones. In this layer, too, the projections from horizontal cells make contacts with rods, cones, and bipolar cells, giving rise to a horizontal transmission and thereby allowing activity in one part of the retina to influence the behaviour of a neighbouring part. In the inner plexiform layer, the axons of the bipolar cells make connection with the dendrites of ganglion cells, once again at special synaptic regions. (The dendrites of a nerve cell carry impulses to the nerve cell body; its axon, away from the cell body.) Here, too, a horizontal interconnection between bipolar cells is brought about, in this case by way of the axons and dendrites of amacrine cells.
eBook - ePub- Michael Peres, Michael R. Peres(Authors)
- 2013(Publication Date)
- Routledge(Publisher)
Input to the visual system occurs through the eyes. The eyes are sensory organs, and their primary role is to detect light energy, code it into fundamental “bits” of visual information, and transmit it to the rest of the brain for analysis. In the higher levels of the brain, the information bits are eventually recombined to form mental images and our conscious visual perception of the world. The visual system is perhaps the most complex of all sensory systems in humans. This is evidenced by the multiplicity of brain areas devoted to vision, as well as the complexity of cellular organization and function in each of these areas.The human eye is a globe cushioned into place within a bony orbit by extraocular muscles, glands, and fat. The extra-ocular muscles move the eyes in synchrony to allow optimal capture of interesting visual information. Eye movements can be either involuntary, reflex reactions (e.g., nystagmus, convergence during accommodation), or voluntary actions. Light rays enter the eye through a circular, clear, curved cornea, which converges them into the anterior chamber of the eye (Figure 1 ). The number of light rays allowed to pass through the rest of the eye is controlled by the iris — a contractile, pigmented structure that changes in size as a function of light intensity, distance to the object of interest, and even pain or emotional status. The iris controls not only the amount of light entering the posterior chamber of the eye, but also the focal length of this light beam, and thus the overall quality of the image achievable. After the iris, light passes through the lens, a small, onion-like structure whose different layers vary in refractive index which provides final focusing power to precisely position the visual information on the retina. The retina is the most photosensitive component of the human central nervous system and covers most of the inner, back surface of the eye. Its highly regular neuronal structure is normally organized into seven cellular and fiber layers (Figure 2
eBook - ePub- Elizabeth Allen, Sophie Triantaphillidou(Authors)
- 2012(Publication Date)
- Routledge(Publisher)
visual cortex, leads us to perceive images approximately one- to two-tenths of a second after they occur. Understanding the basic functioning of the eye leads to better design and operation of imaging systems, whether it be matching the exit pupil of a pair of binoculars to that of the eye or designing a compression system to be perceptually lossless. The visual systems of animals display incredible variation in complexity, operation and performance. Appreciation of this wide biological diversity, from the compound eye of the bee to the polarized vision of cephalopods, inspires many further advances in numerous areas.THE PHYSICAL STRUCTURE OF THE HUMAN EYE
In brief, the eye is a light-tight sphere, approximately 24 mm in diameter, whose shape is predominantly maintained by the sclera, the white part of the eye, and the vitreous humour (Figure 4.1 ). It has a lens system positioned at the front to focus light on to a photosensitive layer, the retina, which lines the rear of the eye, to form an inverted image. The lens system consists of the cornea and a crystalline lens. It is the function of the retina to convert the incoming light to electrical signals which then travel to the visual cortex and other structures via the optic nerve at the rear of the eyeball. Processing of images, their meaning and the context within which they appear is distributed throughout various parts of the brain and not presently fully understood. The visual cortex, however, is primarily responsible for perception of patterns and shapes encoded by the retina. The coloured portion of the eye, the iris, controls the amount of light entering the visual system by changing the size of the pupil
No longer available |Learn more- Hugh Foley, Margaret Matlin(Authors)
- 2015(Publication Date)
- Psychology Press(Publisher)
- The retina absorbs light rays and changes the electromagnetic information into information that can be transmitted by the neurons. The retina contains the fovea, where vision is sharpest, and the optic disc, where the absence of light receptors creates a blind spot.
Structure and Function of the Retina
Because the retina is extremely important in vision, we will consider the kinds of cells in the retina in some detail. Figure 3.8 (page 52) shows six general kinds of cells. We’re going to simplify the reality of the retina, because over 50 kinds of cells have been identified (Masland, 2001a, 2001b; Rodieck, 1998).Cones and rods are the two kinds of photoreceptors that transduce the light information into neural information. Cones provide our perception of color under well-lit conditions. Rods allow us to see under dimly lit conditions, but the same wavelengths that yield color perception in cones yield only black-and-white perception when these wavelengths fall on rods. The information from the cones and rods is transmitted through the other cells toward the visual area of the brain. This information passes through the bipolar cells to the next level in the chain, the ganglion cells. Ganglion cells take the information from the bipolar cells and bring it toward the brain.You can think of the chain of interconnections from the photoreceptors to the bipolar cells to the ganglion cells as a vertical chain. However, information also travels horizontally across the retina through horizontal cells and amacrine cells. We will have more to say about these horizontal interconnections later in this chapter and in Chapters 4 and 5 . For now, you should understand that various types of horizontal cells communicate with bipolar cells and other horizontal cells. Various types of amacrine cells
eBook - ePub- Mark D. Fairchild(Author)
- 2013(Publication Date)
- Wiley(Publisher)
iris is the sphincter muscle that controls pupil size. The iris is pigmented, giving each of us our specific eye color. Eye color is determined by the concentration and distribution of melanin within the iris. The pupil, which is the hole in the middle of the iris through which light passes, defines the level of illumination on the retina. Pupil size is largely determined by the overall level of illumination, but it is important to note that it can also vary with nonvisual phenomena such as arousal. (This effect can be observed by enticingly shaking a toy in front of a cat and paying attention to its pupils.) Thus it is difficult to accurately predict pupil size from the prevailing illumination. In practical situations, pupil diameter varies from about 3 to 7 mm. This change in pupil diameter results in approximately a five-fold change in pupil area and therefore retinal illuminance. The visual sensitivity change with pupil area is further limited by the fact that marginal rays are less effective at stimulating visual response in the cones than central rays (the Stiles–Crawford effect). The change in pupil diameter alone is not sufficient to explain excellent human visual function over prevailing illuminance levels that can vary over 10 orders of magnitude or more.The RetinaThe optical image formed by the eye is projected onto the retina. The retina is a thin layer of cells, approximately the thickness of tissue paper, located at the back of the eye and incorporating the visual system’s photosensitive cells and initial signal processing and transmission “circuitry.” These cells are neurons, part of the central nervous system, and can appropriately be considered a part of the brain. The photoreceptors, rods and cones, serve to transduce the information present in the optical image into chemical and electrical signals that can be transmitted to the later stages of the visual system. These signals are then processed by a network of cells and transmitted to the brain through the optic nerve. More detail on the retina is presented in “The retina.”Behind the retina is a layer known as the pigmented epithelium
eBook - ePub- Hugh J. Foley(Author)
- 2019(Publication Date)
- Routledge(Publisher)
- Cones and rods also differ in their reaction to a change from dimly lit to brightly lit conditions (light adaptation). Cones adapt rapidly, allowing us to see clearly. Rods rapidly become bleached and are not functional under brightly lit conditions.
- Ganglion cell electrical activity is studied by inserting a microelectrode near a ganglion cell, using single-cell recording techniques. The receptive fields of ganglion cells are either oncenter, off-surround or off-center, on-surround.
- Amacrine cells and horizontal cells provide lateral connections within the retina to influence the activity of the cells providing vertical connections (i.e., the photoreceptors, bipolar cells, and ganglion cells).
Pathways from the Retina to the Visual Cortex
Before exploring the details of the visual processing that takes place in the brain, we’ll first lay out a basic map of the primary route we’ll explore. Let’s begin, however, with a few caveats!What do you think is the purpose of all the processing of visual information that takes place in the retina and beyond? You may be thinking that the purpose of the visual system is to construct an accurate representation of the outside world somewhere inside your brain—some mega-pixel screen showing the scene you’re viewing. One problem with such a perspective is that you then need to posit some entity inside your brain (a homunculus, or “little person”) to view the screen. That seems unlikely! Instead, it makes more sense to think of the brain as an information-gathering device that receives its essential information from the senses. Experience interacting with the world allows the brain to construct a “world” based on the information it receives (e.g., Zeki, 2001 ), which is consistent with Theme 3 of this text.As complicated as it may seem as you read through the following descriptions, we are actually presenting you with a simplified depiction of visual processing in the brain. As a result, you may be inclined to think of visual processing as a stream of information flowing like a river to the ocean. That is certainly not the case, because many areas of the brain are interconnected with one another in a complex fashion. Thus, the visual information doesn’t flow directly from the eye through the brain. Think, instead, of your river branching off, turning upstream and joining a lake and then reconnecting with itself further upstream!
eBook - ePub- James R. Augustine(Author)
- 2016(Publication Date)
- Wiley-Blackwell(Publisher)
CHAPTER 12The Visual System- 12.1 RETINA
- 12.2 VISUAL PATH
- 12.3 INJURIES TO THE VISUAL SYSTEM
- FURTHER READING
Vision, including the appreciation of the color, form (size, shape, and orientation), and motion of objects as well as their depth, is somatic afferent sensation served by the visual apparatus including the retinae, optic nerves, optic chiasm, lateral geniculate nuclei, optic tracts, optic radiations, and visual areas in the cerebral cortex.12.1 RETINA
The photoreceptive part of the visual system, the retina, is part of the inner tunic of the eye. The retina has 10 layers, that can be divided into an outermost, single layer of pigmented cells (layer 1), the pigmented layer, and a neural part, the neural layer (layers 2–9).12.1.1 Pigmented layer1
The pigmented layer1 [Note that in this chapter, the layers of the retina are indicated as superscripts in the text] is formed by the retinal pigmented epithelium (RPE), a simple cuboidal epithelium with cytoplasmic granules of melanin. Age‐related decrease and regional variations in melanin concentration in the pigmented layer1 occur in humans. The pigmented layer1 (Fig. 12.1 ) adjoins a basement membrane adjoining choroidal connective tissue. The free surfaces of these pigmented cells are adjacent to the tips of the outer segments of specialized neurons modified to serve as photoreceptors. One pigmented epithelial cell may contact about 30 photoreceptors in the primate retina. Outer segments of one type of photoreceptor, the rods, are cylindrical whereas the outer segments of the other type, the cones, are tapering. By absorbing light and heat energy, pigmented cells protect photoreceptors from excess light. They also carry out resynthesis and isomerization of visual pigments that reach the outer segments of retinal photoreceptors. Pigmented cells demonstrate phagocytic activity, engulfing the apical tips of outer segments of retinal rods detached in the process of renewal. Age‐related accumulation of lipofuscin granules takes place in the epithelial cells throughout the pigmented layer1
eBook - ePubVisual and Non-Visual Effects of Light
Working Environment and Well-Being
- Agnieszka Wolska, Dariusz Sawicki, 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 - ePub- Phillip A. Laplante(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
3 ]After passing through the lens, light traverses the vitreous humor before reaching the retina, a neurosensory layer able to transduce light into signals and transmit those to the brain. The retina consists of several layers of cells as well as several regions with different properties. Light first passes through all layers of the retina before being transduced by the photoreceptors. The photoreceptors pass their electrical signal to a layer of bipolar cells, which in turn transmit the signal to the ganglion cells. Although the ganglion cells are located in the retina, they synapse in the LGN. Their axons are collectively called the optic nerve bundle . In between the photoreceptors and the ganglion cells, there are two layers of lateral connections, formed by horizontal cells and amacrine cells. As such, there is already a substantial amount of visual processing occurring within the retina. This processing is discussed further in the following sections.As mentioned, the retina consists of a number of regions that perform somewhat different functions and are anatomically different. The central part of the retina contains the fovea, an area of 1.5 mm in diameter. In the center of this region is a smaller disk of 0.35 mm diameter, called the foveola, which is where the density of photoreceptors is highest. As a result, this is where visual acuity is highest as well. The region outside the fovea is the peripheral retina. This region also contains the optic disk, a small area known as the blind spot.Photoreceptors
Two types of photoreceptors exist in our eyes, namely, cones and rods. These exist in different densities and fulfill different purposes.[11 ]Rods are sensitive to low light levels and allow us to detect contrast, brightness, and motion—but not color—in these conditions. The range of illumination that rod photoreceptors operate in is termed scotopic . Cones, on the other hand, operate at higher light levels, known as photopic conditions. In addition to contrast, motion, and brightness, cones are involved in seeing color (see Fig. 4 ). There exists a range of light levels under which both rods and cones are active. This range is called mesopic . The responsivities to different wavelengths for the rod and cone systems are known as the scotopic and photopic luminous efficiency , which are plotted in Fig. 5
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