Zoology. Lesson: Physiology of Vision. Lesson Developer: Dr. Mahtab Zarin. College/Dept: Zoology, University of Delhi

Transcription

1 Zoology Lesson: Physiology of Vision Lesson Developer: Dr. Mahtab Zarin College/Dept: Zoology, University of Delhi Institute of Life Long Learning, University of Delhi 1

2 Table of Contents Introduction Image formation mechanisms Refraction of light rays Accommodation Change in pupil size Types of vision Errors of refraction Photochemistry of vision Rhodopsin-retinal visual cycle and excitation of rods Processing and transmission of visual impulse Visual perception Summary Exercises Glossary References Institute of Life Long Learning, University of Delhi 2

3 Learning objectives To describe the structure of eyes as a photoreception organ Pathways of Visual information from eye to brain To understand the mechanisms of image formations Role of the photoreceptors in image processing Refraction of the light rays on cornea To understand the visual perception of the object Introduction Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. Each eye with layer of receptors, lens system, and nerves act as sensory receptor for vision. Receptors of the eyes are able to detect a small portion of the vast spectrum of the electromagnetic radiation that we call visible light (Fig. 5). The wavelengths capable of stimulating the receptors of the eye-visible spectrum- are between about 400 and 700nm. Different wavelengths of light within band are perceived as different colors. Institute of Life Long Learning, University of Delhi 3

4 Fig. 1. Electromagnetic spectrum Visible light ranges in wavelength from 400 to 700nm. Source: Image Credit: This file is licensed under the Creative Commons Attribution ShareAlike 3.0 License. Value addition: Did you Know Color of Object depends on the wavelength reflected An object can absorb certain wavelengths of visible light and reflect others; the object will appear the color of wavelength that is reflected. For example, A green apple appears green because it reflects mostly green light and absorbs most other wavelengths of visible light. An object appears white because it reflects all wavelength of visible light. An object appears black because it absorbs all wavelength of visible light. Source: Principles of Anatomy & Physiology- Tortora, G.J. & Derrickson, B. Image formation mechanisms Institute of Life Long Learning, University of Delhi 4

5 To create clear vision, light reflected from objects within the visual field is focused on to the retina of each eye. The followings are the steps implicated in achieving a clear image: 1. the Refraction or bending of light through lens and cornea in eye; 2. Accommodation, the adjustment in shape of the lens; 3. Changing the size of the pupils, Constriction or narrowing Refraction of light rays Refraction is of light rays is bending of the rays at an angulated interface. It is a phenomenon that often occurs when light rays travel from a medium with a given refractive index to a medium with another at an oblique angle. At the boundary between the media, the wave's phase velocity is altered, usually causing a change in direction. Its wavelength increases or decreases but its frequency remains constant. The degree of refraction increases as a function of i) ratio of the two refractive indices of the two transparent media ii) degree of angulation between the interface and the entering wave front. Refractive index can be defined as the ratio of velocity of light in air to the velocity in a substance. Human eye is optically equivalent to a camera which has a lens system, a variable aperture in form of pupil and a retina which corresponds to photographic film. Lens system of the eye has four refractive interfaces between: a) air and anterior surface of cornea b) posterior of camera and aqueous humor c) aqueous humor and anterior surface of lens d) posterior surface of lens and vitreous humor. The refractive index of air is 1.00, cornea-1.38, aqueous humor-1.33, lens-1.40 and vitreous humor Institute of Life Long Learning, University of Delhi 5

6 Fig.2. Refractive indices of various parts of eye Source: pg ILLL in house If we consider the Lens system of the eye as one unit and add up all the refractive indices, the system would become simplified and the eye would be called as reduced eye. The reduced eye is an idealized model of the optics of the human eye. Introduced by Franciscus Donders, the reduced eye model replaces the several refracting bodies of the eye (the cornea, lens, aqueous humor, and vitreous humor) are replaced by an ideal air/water interface surface that is located 20 mm from a model retina. The single refractive surface has its central point 17mm in front of retina and a total refractive power of 59 diopters when the lens is accommodated for distant vision. About two third of the refractive power of the eye is provided by the cornea because the refractive index of cornea is markedly different from the air whereas the refractive indices of rest of lens system are not greatly different. The total refractive power of the internal lens of the eye is only 20 diopters, about one third the total refractive power of eye Institute of Life Long Learning, University of Delhi 6

7 Value addition: Did you Know Diopters: a measuring unit of refractive power The greater the curvature of a lens, the greater its refractive power. The refractive power of a lens is conveniently measured in diopters, the number of diopters being the reciprocal of the principal focal distance in meters. For example, a lens with a principal focal distance of 0.25m has a refractive power of 1/0.25, or 4 diopters. The human eye has a refractive power of approximately 66.7 diopters at rest. Source: Review of Medical Physiology. Ganong, William F. Accommodation The lens of human eye is biconvex i.e it is convex on both its anterior and posterior surfaces. Its ability to refract light increases as its curvature becomes greater. The change in the shape of eye lens let the focusing power of eyes adjust for far and close vision. It differs with the extent of light which is to be refracted (bent). In order to produce a sharp image on the retina, light rays from close objects must be refract more than those from far placed objects. This phenomenon is known as accommodation. Accommodation for close vision In order to focus on near objects i.e. within about 6 meters, accommodation is required and the eye must make the following adjustments Constrictions of the pupils Convergence Changing the power of lens Constrictions of the pupil: This assists accommodation by reducing the width of the beam of light entering the eye so that that it passes through the central curved part of the lens. Convergence (movement of eyeballs): Extrinsic muscles help to move the eyes. To achieve a sharp image, the eyeball has to be rotated so that they converge on the object viewed. The closer an object is to the eyes, the more eye rotation is needed to accomplish convergence. Changing the power of lens: Changes in the thickness of the lens are made to focus light on the retina. The amount of adjustment depends on the distance of the object from the eyes. When you view a close object, the ciliary muscle contracts which pulls the ciliary body Institute of Life Long Learning, University of Delhi 7

8 and choroid forward toward the lens. As a result the lens becomes more convex, which increases its focusing power. Accommodation for distant vision When objects more than 6 metres away, the ciliary muscle of the ciliary body is relaxed and lens is flatter because it is stretched in all directions (Fig.3). Fig. 3. Accommodation by human eye Source: ILLL in house Institute of Life Long Learning, University of Delhi 8

9 Fig. 4. Accommodation for near and distant object Source: ILLL in house Mechanism of accommodation and its control In young person, the lens is composed of a strong elastic capsule filled with viscous, proteinaceous but transparent fluid. When in relaxed state with no tension at all, lens assumes a spherical shape due to elastic retraction of lens capsule. Suspensory ligaments attach radially around the lens, pulls the lens towards the edges on outer surface of the eyeball. The tension on these ligaments keeps the lens relatively flat under normal conditions of eye. The ciliary muscles located laterally to the lens ligaments have two separate set of smooth muscle fibers. The contraction of either of these muscle fibers releases the ligaments tension on the lens and the lens assumes a more spherical shape. The ciliary muscle movement is controlled by parasympathetic nerve signals transmitted to the eye through the third cranial nerve from third nucleus of brain stem. Stimulation of parasympathetic nerves contracts both sets of ciliary muscles which relax the lens ligament and hence lens becomes thicker and refractive power increased. Presbyopia Institute of Life Long Learning, University of Delhi 9

10 With growing age, especially after 40, the lens grows larger and thicker and becomes less elastic because of partial denaturation of lens proteins. Hence the ability to change lens shape decreases and so is the power of accommodation. The power of accommodation decreases from about 14 diopters (in childhood) to 2 diopters at age of It becomes 0 diopters after 70 when the lens becomes totally nonaccomodating. This condition is called Presbyopia. Each eye remains focused permanently at an almost constant distance and can no longer accommodate for both near and distant vision. Bifocal glasses are used to treat this condition. Change in the pupil size The major function of the iris is to increase or decrease the amount of light that enters eye during darkness or brightness. The amount of light that enters the eye though the pupil is proportional to the area of the pupil (diameter 2 ). The pupil can become as small as 1.5mm or as large as 8 mm in diameter. Size of pupil affects the accommodation via allowing the required amount of light to enter inside the eyeball. In dim light, the pupil size increases (means dilates) to let the maximum amount of light to arrive at the light receptive retina. In bright condition, the pupil size decreases (that means it constricts). The range of image distances over which the image of an improperly focused object is acceptably sharp is called the depth of focus. Depth of focus of lens system increases with decreasing pupillary diameter. This is because with a very small aperture, almost all the rays pass through the center of the lens and the central most rays are always in focus. In other words, to have a sharp image, the pupillary diameter must be adjusted as per distance of the object from eye or the image might become blurred. Types of vision With both the eyes open, the outer region of our total visual field is perceived by only one eye known as zone of monocular vision. In the central portion, the fields from the two eyes overlap known as the zone of binocular vision (Fig 5). Parallel processing of information continues all the way to and within the cerebral cortex to the highest stages of visual neural networks. Cells in this pathway respond to electrical signals that are generated initially by the photoreceptors response to the light. Optic nerve fibres projects to several structures in the brain, the largest number passing to the thalamus) specifically toward the lateral geniculate nucleus of the thalamus, where the information such as colour, intensity, shape and movement etc. from the different ganglion cell types is kept distinct. Institute of Life Long Learning, University of Delhi 10

11 Fig.5. Types of Vision Source: CC Table 1. Difference between Monocular and Binocular vision S.No. Binocular/ Stereoscopic vision Monocular Vision 1. Gives a three-dimensional Monocular vision provides a limited Institute of Life Long Learning, University of Delhi 11

12 image produced by the fusion of two different views of an object on each retina 2. In an animal with stereoscopic vision (e.g. humans), due to the overlapping of visual fields of both eyes, two eyes can focus on the same object on retina. Though the two eyes simultaneously produce two images with little difference, the brain resolves this into a three dimensional impression which therefore gives rise to stereoscopic vision. 3. It gives a better judgement of the distance of the object 4. Especially important to predatory animals, such as eagles, hawks, owls and cats, as good judgement of distance is necessary to capture prey. 5. Animals with stereoscopic vision have both eyes at the front of the head. However, the field of vision of animals with stereoscopic vision is narrow. two-dimensional view. The visual fields of animals with monocular vision have little or no overlapping. It helps to detect enemies from a wide range of directions. Prey such as fishes, rabbits and grasshoppers usually have monocular vision. Animals with monocular vision have eyes at the side of the head (laterally placed eyes). The field of vision of animals with monocular vision is wide, but has restricted stereoscopic vision. Institute of Life Long Learning, University of Delhi 12

13 6. Errors of refraction Emmetropia (normal vision): An eye is said to be emmetropic if it can form a sharp image of a distant object (at more than 6 meters) with all its ciliary muscles relaxed. However to view the nearby objects, the eye must contract its ciliary muscle and thereby provide appropriate degrees of accommodation. Myopia (near-sightedness): It is a type of refractive error where image is formed in front of retina for the light rays coming from distant object when all the ciliary muscles of eyes are relaxed. This is the inability to see faraway objects clearly but have clear nearsightedness. This can either be due to excessive curvature of refractive lens (refractive myopia) or when the eyeball is longer in reference to the focusing power of the lens (also known as axial myopia). Myopia is corrected by using a concave lens that results in the deviation (divergence) of light rays prior to reaching the cornea. Hyperopia (far-sightedness): It is a type of refractive error where image is formed beyond retina for light rays coming from nearby objects when all the ciliary muscles of eyes are relaxed. This is the inability to see near objects clearly but have clear farsightedness. This can be either due to the weak focusing power as the length of the eyeball is short (also known as axial hyperopia). This may also result from a cornea or crystalline lens with not enough curvature (refractive hyperopia). It is corrected by convex lenses which cause light rays to congregate (converge) before striking the cornea. Institute of Life Long Learning, University of Delhi 13

14 Astigmatism: This is the defects of vision which occurs when the surface of cornea or lens is not smoothly spherical. Due to this condition, some portions of image are out of the focus, thereby the vision become blurred or altered. Cylindrical lenses can be used to correct this condition. Fig. 6. Refraction abnormalities and their correction Source: ILLL in house Photochemistry of vision Once the eye is accommodated to see an object, all the transparent parts contribute to converge the rays of light to form an image on the retina. An inverted image is formed on Institute of Life Long Learning, University of Delhi 14

15 retina and the photoreceptor cells of retina finally perceive the image to convey it to the CNS. The major events that follow are: 1. Rhodopsin-retinal visual cycle and excitation of rods 2. Processing and conduction of visual sensation, performed through the image processing cells of retina and visual pathway, and 3. Perception of vision involves a role of visual cortex and associated regions of cerebral cortex. Rhodopsin-retinal visual cycle and excitation of rods Visual cycle When light energy is absorbed by rhodopsin, the rhodopsin begins to decompose within a fraction of seconds. Photoactivation of electron in the retinal component leads to instantaneous conversion of cis form of retinal (angulated molecule) to all-trans form (straight form). Because of geometric isomerization of retinal, the protein component scotopsin cannot fit into the reactive site anymore. Hence it pulls away from the scotopsin. The immediate product is bathorhodopsin (partially split combination of all -trans retinal and scotopsin). Being highly unstable, bathorhodopsin decays into lumirhodopsin. Subsequently it decays to metarhodopsin I followed by metarhodopsin II and finally much more slowly into completely split products scotopsin and all-trans retinal. So this portion of visual cycle (separation of opsin and retinal) is termed as photodecomposition and the rhodopsin or photopigment is known to be bleached through the action of lightor rhodopsin bleaching. Metarhodopsin II also called as activated rhodopsin excites the electrical changes in rods and the rods then transmit the image to CNS through optic nerve. This process is called Phototransduction. Reformation of rhodopsin: rhodopsin is reformed by conversion of all-trans retinal to 11-cis retinal in presence of enzyme retinal isomerase. Once formed 11-cis retinal automatically combines with scotopsin to form rhodopsin which remains stable until it is decomposed by absorption of photon. the bleaching of the rhodopsin takes place Institute of Life Long Learning, University of Delhi 15

16 in the presence of light, whereas the regeneration event occurs in the absence of light This completes the visual cycle. Under equilibrium state, the rate at which the photopigments are being bleached must be equal to the rate at which they are regenerated. A. Institute of Life Long Learning, University of Delhi 16

17 B. Fig.7 a. Diagrammatic representation of Rhodopsin-retinal visual cycle 7 b. Rhodopsin-retinal visual cycle through flowchart Source: ILLL in house Value addition: Did you Know?? Heading text: Why Vitamin A deficiency causes Night blindness? Body text: The severe deficiency of Vitamin A in a person causes night Blindness. Vitamin A is normally present in both cytoplasm of rod and pigment layer of retina. Therefore Vitamin A is normally always available to form new retinal in retina. Deficiency of Vitamin A leads to severe depression of system for formation of retinal and rhodopsin. Due to lack of rhodopsin and retinal, the person is unable to see in Institute of Life Long Learning, University of Delhi 17

18 dim light as the light is too low to excite reaction. Source: Textbook of Medical Physiology. Guyton and Hall Excitation of rods When rhodopsin is activated by light, it causes excitation of rod. Excitation of rod causes increased negativity of intrarod membrane potential, which is a state of hyperpolarization (inside the negativity is more as compared to normal condition). Rhodopsin increases the negativity by decreasing the rod membrane conductance for sodium ions in the outer segment of the rod leading to hyperpolarization of entire rod membrane. Photoreceptor cells are unusual cells in that they depolarize in response to absence of stimuli or scotopic conditions (darkness). In photopic conditions (light), photoreceptors hyperpolarize to a potential of -60mV. It is this 'switching off' that activates the next cell and sends an excitatory signal down the neural pathway. Value addition: Interesting to know Heading text: Visual Transduction Body text: The entire phenomenon of transforming light rays (refracted) into nerve impulse is called as phototransduction. Visual phototransduction is a process by which light is converted into electrical signals in the rod cells, cone cells and photosensitive ganglion cells of the retina of the eye. In the light Light transduces the visual pigment via the following enzyme cascade: photons rhodopsin activated rhodopsin (metarhodopsin II) opsin activates the regulatory protein a GTP binding protein (transducin) transducin dissociates from its bound GDP, and bind GTP, then the alpha subunit of transducin dissociates from the beta and gamma subunits, with the GTP still bound to the alpha subunit alpha subunit-gtp complex activates cgmp-phosphodiesterase (an enzyme hydrolyzing cgmp) breaks down cgmp to 5'-GMP- lowers the concentration of cgmp and therefore the sodium channels close Closure of the sodium channels causes hyperpolarization of the cell due to the ongoing potassium current Hyperpolarization of the cell causes voltage-gated calcium channels to close calcium level in the photoreceptor cell drops the amount of the neurotransmitter glutamate that is released by the cell also drops ( This is because calcium is required Institute of Life Long Learning, University of Delhi 18

19 for the glutamate-containing vesicles to fuse with cell membrane and release their contents). Fig. 8a. Visual transduction in light In the dark, a steady current flows into the open channels, carried mainly by Na ions, constituting a dark current {composed mainly of the influx of the Na+ component (80%) however, a Ca2+ component (15%) and a Mg2+ component (5%)} that partially depolarizes the photoreceptor cell. Dark current keeps the cell depolarized at about -40 mv. Thus, the depolarized photoreceptor releases neurotransmitter (the amino acid glutamate) from its synaptic terminals upon second-order neurons in the dark. On light stimulation the rhodopsin molecules are isomerized to the active form, the above cascade ensues, leading to closure of the cation channels of the photoreceptor membrane, stopping the dark current and causing the photoreceptor cell membrane to hyperpolarize and cease neurotransmitter release to second-order neurons. Institute of Life Long Learning, University of Delhi 19

20 Fig. 8b. Visual transduction in dark Deactivation of the phototransduction cascade GTPase Accelerating Protein (GAP) interacts with the alpha subunit of transducin, and causes it to hydrolyse its bound GTP to GDP, and thus halts the action of phosphodiesterase, stopping the transformation of cgmp to GMP. Click here to see an animation of phototransduction (Quicktime movie): Source: Textbook of Medical Physiology. Guyton and Hall Image credit: ILLL in house Institute of Life Long Learning, University of Delhi 20

21 Value addition: Video Vision - Light and Neuronal Activity <iframe width="420" height="315" src="// frameborder="0" allowfullscreen></iframe> Source: You tube Photochemistry of color vision Different cones are sensitive to different colors of light. The absorption characteristics of the pigments in the three types of cones blue, green and red show peak absorbencies at light wavelengths of 445, 535 and 570 nm, respectively which helps the retina to differentiate the colors. The nervous system interprets the color on the basis of ratio of stimulation by monochromatic light with a particular wavelength. For example, a monochromatic blue light with a wavelength of 450 nm stimulates the red cones to a stimulus 0, green cones to stimulus value 0 and blue cones to stimulus value 97. This set of ratios 0:0:97 will be interpreted as blue by nervous system. Institute of Life Long Learning, University of Delhi 21

22 Fig. 9. Absorption of spectra by cones Source: ILLL in house Value addition: Did you Know?? Heading text: Color Blindness Body text: When a single group of color-receptive cones is missing from the eye, the person is unable to distinguish some colors from others. This disorder is called Color blindness. Depending on the type of cone missing, the color blindness can be of following types: 1. Protanope: a person with loss of red cones. 2. Deuteranope: a person lacks green cones. If either of these cones are missing, the person is unable to distinguish red, orange, yellow and green color and is therefore called as red-green color blindness. This is a sex-linked disorder that occurs exclusively in males because genes in the female X chromosome code for respective cones. Red- green color blindness. red- green color blindness is present predominantly in men, affecting 1 out of 12. Color blindness in women is much more rare (1 out of 200) since males only have one X chromosome (in females, a functional gene on only one of the two X chromosomes is sufficient to yield the required photopigments). At high light intensities, as in daylight vision, most people 92% of the male population and over 99% of the female population have normal color vision. However, there are several types of defects in color vision that result from mutations in the cone pigments. The most common form of colour blindness, red- green color blindness. Colour blindness results through recessive mutation in one or more genes encodes for the cone pigments. Genes encoding the red and green cone pigments are positioned very close to each other on the X chromosome, whereas the gene encoding the blue chromophore is located on chromosome 7. Ishihara charts are used for determining color blindness. Institute of Life Long Learning, University of Delhi 22

23 Fig. 10. Ishihara charts Source: Textbook of Medical Physiology. Guyton and Hall Image credit: This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license. Processing and conduction of visual impulse Role of various retinal cells in processing and conduction of visual impulse 1. Rods and cones transmit signals to the outer plexiform layer, where they synapse with bipolar cells and horizontal cells. 2. Horizontal cells transmit signals horizontally in the outer plexiform layer from rods and cones to bipolar cells. Institute of Life Long Learning, University of Delhi 23

24 3. Bipolar cells transmits signals vertically from rods, cones and horizontal cells to inner plexiform layer where they synapse with ganglion cells and amacrine cells. 4. Amacrine cells transmit signals in two directions, either directly from bipolar cells to ganglion cells or horizontally to other amacrine cells or from axons of bipolar cells to dendrites of ganglion cells in the inner plexiform layer.. 5. Ganglion cells transmit output signals from retina through the optic nerve into the brain. 6. Interplexiform cell transmits signals in the retrograde direction from the inner plexiform layer to outer plexiform layer. This prevents lateral spread of visual signals by horizontal cells in outer plexiform layer and also controls degree of contrast. Visual Pathway for cones Visual pathway for cones to ganglion cells is different from rods. The neurons and nerve fibers that conduct visual signals for cones are larger than the ones for rods and hence the transmission of the signal is two to five times faster than rods. The visual pathway for cones is shown in the fig. Neurotransmitters Neurotransmitter glutamate is released by both rods and cones at their synapses with bipolar cells. Transmission of signals The ganglion cells are the only retinal neuron that always transmits visual signals by action potential rest of the retinal cells conduct by electrotonic conduction (direct flow of electric current, not action potential from point of excitation to output synapses) to other cells of the retina namely horizontal cells, amacrine cells, and ganglion cells. The ganglion cells pass on the visual signals in the form of action potential to the neurons of lateral geniculate body and soon after to the primary visual cortex. Visual image is decoded and interpreted in both serial and parallel fashion. Perception of vision This is an intricate assimilation of senses of light, form, contrast and color of object. The accessible visual field created in the retina and cortex is used to predetermine this visual information of an object image. 1. Sense of light Institute of Life Long Learning, University of Delhi 24

25 It is responsiveness to the light. The lowest brightness needed to induce a feeling of light is termed as the light minimum. It can be determined when the eye is dark adapted for minimum of minutes. The visual adaptation is the adjustment that occurs within the system of vision due to exposure to broad range of illumination for normal functioning of eye. Two types of adaptation involved in this event: Dark adaptation (alteration in low light), and Light adaptation (alteration to high light). Dark adaptation It is the ability of the eye to adapt itself to decreasing illumination. If we go from sun-shine into a dim light, we can only perceive the objects in dim light after many minutes have passed. In course of this time interval, eye is adapting to low light. As described earlier, vision can only be supplied by the rods in the darkened room. Therefore, rods are used more in low level of light (i.e. in scotopic vision) and cones in high level of light (i.e. in photopic vision). If the person is fully dark adapted, his eye s retina is many times (approx. one lakh times) more responsive to brightness. If somebody has delayed dark adaptation, that means he may has one of the diseases of rods, for example, retinitis pigmentosa and vitamin A deficiency. Light adaptation When we go suddenly from a dark to a bright light or sun-shine, the light seems very intense and still uncomfortably bright till the eyes are adapted to the higher level of light and the visual threshold rises. This event in which retina adapts itself to bright illumination is termed as light adaptation. Initially, the eye is extremely sensitive to light as rods are overwhelmingly activated, and the visual image too bright and has poor contrast. However, the rhodopsin, is soon inactivated (or bleached) through the high illumination- the rods become unresponsive so that only the less- susceptible cones are operating and the image becomes less and more clear. Other mechanism of light and dark adaptation In bright light, the circular muscle of the iris contract, causing constriction of the pupil (decrease in the size). In the dim light, the radial muscles of the iris contract, causing an Institute of Life Long Learning, University of Delhi 25

26 dilation of the pupil (increase in size) (Fig 11). Parasympathetic neurons stimulate the circular muscle while sympathetic neurons stimulate the radial muscle of the iris. Fig. 11: Autonomic Control of Pupillary Size In bright, pupil constricts as circular muscles of iris contract and in dim light, pupil dilates as radial muscles contract Source: OpenStax College. "Autonomic Reflexes and Homeostasis." OpenStax- CNX. June 5, Image Credit: This file is licensed under the Creative Commons Attribution ShareAlike 3.0 License. Institute of Life Long Learning, University of Delhi 26

27 .2. Sense of Form It is the capability to distinguish between the shapes of the objects. Cones play a main role in this process. So, the sense of object shape is most sharp at the fovea centralis, where it has highest number of cones and reduces very quickly towards the periphery. In medical observation, determination of the threshold of distinguishing two spatially apart targets (a role of the fovea centralis) is called visual acuity. Major differences exist between the peripheral retina and central retina. Near fovea, fewer rods and cones converge on each on each optic fiber and the rods and cones also become slender. These effects progressively increase the acuity of vision. At fovea centralis, there are nearly slender cones and no rods are present and also nearly same number of optic nerve leads from this part. Hence this region has high degree of visual acuity. The peripheral retina on the other hand has much greater sensitivity to weak light. Fig. 12. Difference in neural organization of retina at periphery and at fovea Source: ILLL in house Institute of Life Long Learning, University of Delhi 27

28 3. Sense of Contrast This is the capability of the eye to recognize minor changes in the luminance between areas that are not separated by exact boundaries. Loss of contrast sensitivity causes mild fogginess of the vision. Contrast sensitivity is affected by a variety of factors for example, age, refractive errors, glaucoma, amblyopia, diabetes, optic nerve diseases and lenticular changes. Additionally, contrast sensitivity may be weakening still in the presence of normal visual acuity. 4. Sense of Colour This is the ability of the eye to distinguish diverse colours transmitted through light of different wavelengths. Colour vision results from different combinations of the three types of cones (red, green and blue) and thus better experienced in photopic vision (Fig 13). In low level of light (scotopic vision), you see all colours as grey and this phenomenon is termed Purkinje shift. Fig.13. Simulated appearance of a red geranium and foliage in normal bright-light (photopic) vision, dusk (mesopic) vision, Institute of Life Long Learning, University of Delhi 28

29 and night (scotopic) vision illustrate the Purkinje effect or Purkinje shift Source: ic_mesopic_scotopic.jpg Value addition: Did you Know?? Heading text: How brain interprets a visual signal This figure shows how the brain uses mapping to make sense of visual information from the eye. The green numbers in the figure correspond to the following steps: Rays of light (blue) reflected off of an image are focused through the lens onto the back of the eye, forming an upside-down image on the retina. On the retina, those photocells that are hit by light from the image are activated. These photocells are shown in white in this figure. Photocells that do not receive any reflected light are not activated, and are shown in this figure. Thus, we can think of the image as a pixellate map of activated and nonactivated photocells on the retina. A nerve (gold) from each photocell connects to a particular location in the visual cortex of the brain. The photocells that are activated (white) send a nerve impulse to the brain, while the photocells that are not activated (black) do not send any impulse to the brain. (Only a small sample of the nerves are shown in this figure.) The brain, when it receives a collection of nerve signals from the eye, interprets where each signal comes from, and reconstructs the pixellate map. Institute of Life Long Learning, University of Delhi 29

30 The brain then interprets the pixellate map as an image. Fig. 14. How brain interprets a visual signal Source: Written for permission Value addition: Video Recap the concepts of vision physiology through this video: <iframe width="420" height="315" src="// frameborder="0" allowfullscreen></iframe> Source: You tube Institute of Life Long Learning, University of Delhi 30

31 Summary Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The wavelengths capable of stimulating the receptors of the eye-visible spectrumare between about 400 and 700nm. Different wavelengths of light within band are perceived as different colors. Refraction, accommodation and changing of pupil size aids in achieving a clear image of an object. Human eye is optically equivalent to a camera which has a lens system, a variable aperture in form of pupil and a retina which corresponds to photographic film. About two third of the refractive power of the eye is provided by the cornea because the refractive index of cornea is markedly different from the air whereas the refractive indices of rest of lens system are not greatly different. The total refractive power of the internal lens of the eye is only 20 diopters, about one third the total refractive power of eye The change in the shape of biconvex lens of human eye let the focusing power of eyes adjust for far and close vision. When objects more than 6 metres away, the ciliary muscle of the ciliary body is relaxed and lens is flatter because it is stretched in all directions. Stimulation of parasympathetic nerves contracts both sets of ciliary muscles which relax the lens ligament and hence lens becomes thicker and refractive power increased. With growing age, especially after 40, the lens grows larger and thicker and becomes less elastic because of partial denaturation of lens proteins. Hence the ability to change lens shape decreases and so is the power of accommodation. The major function of the iris is to increase or decrease the amount of light that enters eye during darkness or brightness. With both the eyes open, the outer region of our total visual field is perceived by only one eye known as zone of monocular vision. In the central portion, the fields from the two eyes overlap known as the zone of binocular vision. Myopia, hyperopia and astigmatism are the refractive disorders of eye. An inverted image is formed on retina and the photoreceptor cells of retina finally perceive the image to convey it to the CNS through following steps: Rhodopsin-retinal visual cycle and excitation of rods Institute of Life Long Learning, University of Delhi 31

32 Processing and conduction of visual sensation, performed through the image processing cells of retina and visual pathway, and Perception of vision involves a role of visual cortex and associated regions of cerebral cortex. The absorption characteristics of the pigments in the three types of cones help the retina to differentiate the colors. The nervous system interprets the color on the basis of ratio of stimulation by monochromatic light with a particular wavelength. Processing and conduction of visual impulse is carried out by various cells of retina: rods, cones, bipolar cells, amacrine cells and ganglion cells which finally transfer the signal to brain via optic nerve. Perception of vision is an intricate assimilation of senses of light, form, contrast and color of object. The accessible visual field created in the retina and cortex is used to predetermine this visual information of an object image. Institute of Life Long Learning, University of Delhi 32

33 Exercises A. Multiple Choice questions 1. The total refractive power of the internal lens of the eye is A. 20 diopters B. 6 diopters C. 40 diopters D. 60 diopters 2. The change in the shape of eye lens to let the focusing power of eyes adjust for far and close vision is brought about by A. Iris B. Ciliary muscle C. Extraocular muscle D. Retina 3. The condition where each eye remains focused permanently at an almost constant distance and can no longer accommodate for both near and distant vision is called A. Emmetropia B. Presbyopia C. Hyperopia D. Myopia 4. The condition where image is formed beyond retina for light rays coming from nearby objects when all the ciliary muscles of eyes are relaxed is called A. Emmetropia B. Presbyopia C. Hyperopia D. Myopia 5. Photoactivation of electron in the retinal component leads to instantaneous conversion of A. cis form of retinal to all-trans form. B. cis form of retinol to all-trans form. C. all-trans form of retinal to cis form. D. all-trans form of retinol to cis form. 6. Excitation of rod causes increased negativity of intrarod membrane potential, which is a state of A. Hyperpolarization B. Depolarization Institute of Life Long Learning, University of Delhi 33

34 C. Hypopolarization D. Neutralization 7. Three types of cones blue, green and red show peak absorbencies at light wavelengths of A. 445, 535 and 570 nm B. 535, 445 and 570 nm C. 570, 535 and 445nm D. 570, 445 and 535nm 8. Sense of object shape is most sharp at the fovea centralis, where it has highest number of A. Cones B. Rods C. Bipolar cells D. Amarcrine cells B. Short answer type questions 1. Define the following: a. Rhodopsin b. Visual acuity c. Astigmatism d. Accomodation e. Presbyopia 2. Differentiate between: a. Rods and cones b. Monocular and Binocular vision c. Myopia and hyperopia d. Photopic and scotopic vision C. Long answer type questions 3. Explain the neural pathways of vision. 4. Describe photochemical changes for initiation of vision. 5. What are differences between dark and light adaptation? 6. Explain the role of visible spectrum in colour vision. 7. Write short notes on the followings: Color vision Rhodopsin bleaching Institute of Life Long Learning, University of Delhi 34

35 8. How and why the changes occur in size of pupil? 9. Explain accommodation for near vision and far vision. 10. Describe the refraction abnormalities and how they are corrected. 11. Explain the role of rod and cones and in vision. Institute of Life Long Learning, University of Delhi 35

36 Glossary Accommodation: It is the process to create a sharp image on the retina by changing the thickness of lens as light rays from near objects must be bent more than those from distant objects. Amacrine cell: A type of cell in the retina that connects to the bipolar cells near the outer synaptic layer and provides the basis for early image processing within the retina. Binocular vision: The centre of the field of object view is binocular when it is seen by both eyes that is termed as binocular vision. Bipolar cell: Cell type in the retina that connects the photoreceptors to the retinal ganglionic cells. Colour vision: It is created by the three types of cones (red, green and blue) and so that we can able to see colored object in bright light. Cones: It is the photoreceptor which allow us see in brighter lights which stimulate cones to produce colour vision. Fovea centralis: It is the exact center of the retina at which visual stimuli are focused for maximal acuity, where the retina is thinnest, at which there is nothing but photoreceptors. Hyperopia: The condition where image is formed beyond retina for light rays coming from nearby objects when all the ciliary muscles of eyes are relaxed. Lens: The lens is a highly flexible, round and biconvex body, located just behind the pupil and iris and helps focus images on the retina to facilitate clear vision. Myopia: The condition where image is formed in front of retina for light rays coming from distant objects when all the ciliary muscles of eyes are relaxed. Photoreceptors: Photoreceptors are specialized cells in which light rays falls on objects are transmitted to nerve impulses. Phototransduction: It is a phenomenon of translation of light rays into nerve impulse is known as phototransduction. Presbyopia The condition where each eye remains focused permanently at an almost constant distance and can no longer accommodate for both near and distant vision. Refraction: Bending of light rays when it travelling through a medium in called refraction. Rod: It is the photoreceptor which allow us see in dim light and we can see only black, white, and all shades of gray in between. Visual acuity: It is the property of vision related to the sharpness of focus, which varies in relation to retinal position Institute of Life Long Learning, University of Delhi 36

37 References 1. Tortora, G.J. & Grabowski, S. Principles of Anatomy & Physiology. 13th Edition, p Moyes, C. D. and Schulte, P. M. (2006). Principles of Animal Physiology, p Hill, R. W., Wyse, G. A. and Anderson, M. (2006). Animal Physiology. p Randall, D., Burggren W. and French, Kathleen (2001). Eckert Animal Physiology. 5. Widmaier, E.P., Raff, H. and Strang, K.T. (2008). Vander s Human Physiology, XI Edition, McGraw Hill. 6. Guyton, A.C. and Hall, J.E. (2011). Textbook of Medical Physiology, XII Edition, Harcourt Asia Pvt. Ltd./W.B. Saunders Company. 7. Ganong, William F. Review of Medical Physiology. XXI Edition. Mc Graw Hill 8. Textbook of Physiology by Prof. A.K. Jain. 9. Anatomy And Physiology: In health and illness. Ross and Wilson (Tenth Edition) 10. Rushton, W. A. H. (1 June 1966). "Densitometry of pigments in rods and cones of normal and color defective subjects" (PDF). Investigative Ophthalmology 5 (3): PMID Weblinks Institute of Life Long Learning, University of Delhi 37