Exercise 23 Review Sheet Special Senses Anatomy of the Visual System
Chapter 14: Visual Processing: Heart and Retina
(content provided by Chieyeko Tsuchitani, Ph.D.)
Reviewed and revised 07 October 2020
In this chapter you will acquire about how the visual organisation initiates the processing of external stimuli. The affiliate will familiarize you with measures of visual awareness by discussing the footing of grade perception, visual acuity, visual field representation, binocular fusion, and depth perception. An important aspect is the regional differences in our visual perception: the cardinal visual field is color-sensitive, has loftier acuity vision, operates at high levels of illumination whereas the periphery is more sensitive at low levels of illumination, is relatively colour insensitive, and has poor visual acuity. You will learn that the paradigm is first projected onto a flattened sheet of photoreceptor cells that lie on the inner surface of the eye (retina). The information gathered past millions of receptor cells is projected next onto millions of bipolar cells, which, in plough, ship projects to retinal ganglion cells. These cells encode different aspects of the visual stimulus, and thus conduct independent, parallel, streams of information about stimulus size, colour, and move to the visual thalamus.
14.i Measures of Visual Sensation
The condition of the visual organization can be adamant by examining various aspects of visual sensation. For example, the ability to detect and identify pocket-size objects (i.e., visual vigil) can be affected by disorders in the transparent media of the centre and/or visual nervous arrangement. The inability to notice objects in specific areas of space (i.e., visual field defects) is oft related to neural impairment.
Spatial Orientation and the Visual Field
The visual field is that area in space perceived when the eyes are in a fixed, static position looking straight alee.
| Figure 14.1 |
The monocular visual field (Figure 14.1)
- is that area of infinite visible to ane centre
- can exist mapped parametrically
- Perimetry testing provides a detailed map of the visual field. The potential visual field is described as a hemisphere. Nevertheless, information technology does non form a perfect hemisphere as the brow, nose and cheekbones obscure the view - about prominently in the nasal hemisphere
- Perimetry testing provides a detailed map of the visual field. The potential visual field is described as a hemisphere. Nevertheless, information technology does non form a perfect hemisphere as the brow, nose and cheekbones obscure the view - about prominently in the nasal hemisphere
- is subdivided into two halves, the hemifields (Figure fourteen.1 Inset).
- A horizontal line drawn from 0° to 180° through center of the field defines the superior & inferior hemifields.
- A vertical line drawn from ninety° to 270° through center point defines the left & right hemifields, which are frequently termed the nasal and temporal hemifields.
- A horizontal line drawn from 0° to 180° through center of the field defines the superior & inferior hemifields.
- may be farther subdivided into quadrants:
- the superior and junior nasal quadrants
- the superior and inferior temporal quadrants.
- the superior and junior nasal quadrants
- contains a blind spot,
- a small expanse in which objects cannot be viewed
- which is located within the temporal hemifield.
- a small expanse in which objects cannot be viewed
| Figure 14.two |
The monocular visual field (Figure 14.1) is adamant with ane eye covered. The surface area of overlap of the visual field of i eye with that of the opposite heart is called the binocular field (Figure xiv.ii). All areas of the binocular visual field are "seen" by both eyes.
The ability to locate objects in space and the ability to orient ourselves with respect to external objects are dependent upon the representation of visual infinite within the nervous system. The clinical examination of the visual fields most commonly used is the confrontation field test. It defines the outer limits of our subjective visual space. Neurological disorders of the visual organization can often exist localized based on the area of blindness inside the visual field.
Visual Vigil
Visual acuity is the ability to detect and recognize small objects visually depends on the refractory (focusing) power of the eye'southward lens organisation and the cytoarchitecture of the retina.
Visual acuity is
- measured under high illumination
- the smallest size of a dark object in a light groundwork that can exist correctly identified
In the clinical setting, an eye chart
- is used to measure the patient'due south visual vigil.
- consists of rows of blackness letters on a bright white background.
- is used to measure visual vigil at a distance of 20 ft from the chart.
- reports visual vigil equally the ratio of the eye chart distance (i.e., 20 ft) to the "normal distance" of the everyman row of letters correctly identified by the patient (e.g., row 3, which is 70 ft).
Color Vision
Color vision is the ability to find differences in the wavelengths of light is called color vision. Clinically it may be tested with an Ishihara chart: a nautical chart with spots of unlike colors that are spatially organized to form numbers that differ for ``normal" and colour-blind optics.
As mentioned in a higher place, the human has a trichromatic visual system, whereby visible colors can exist created by a mixture of cherry-red, green and bluish lights. The well-nigh mutual form of colour blindness results in a confusion of red and green shades (i.e., cerise-green color blindness). Most cases of color blindness issue from an absent or defective gene responsible for producing the blood-red or dark-green photopigment (protanopia, the lack of carmine; and deuteranopia, the lack of green). As these genes are located on the 10 chromosome, color incomprehension is more mutual in males than in females.
| Effigy 14.three |
Regional differences: At that place are regional differences in color sensation, visual vigil and depression-illumination sensitivity inside the visual field (Figure 14.3).
A small "blindspot" is
- located in the temporal hemifield (Figure 14.3 Left)
- where objects cannot be seen.
Vision in the visual field middle
- operates best under high illumination.
- has the greatest visual acuity and color sensitivity
- is ten times improve than in the field periphery (Effigy 14.3 Right)
- represents the operation of the photopic (low-cal-adapted) subsystem
Vision in the peripheral visual field
- is more than sensitive to dim calorie-free
- operates nether low illumination.
- has fiddling color sensitivity and poor spatial vigil (Figure 14.3 Correct)
- represents the performance of the scotopic (dark-adapted) subsystem
Binocular Fusion and Depth Perception
| Figure 14.4 |
When a pencil is held an arm'south length abroad with both eyes open, most individuals volition see a single object and recognize it as a pencil. Notwithstanding, if ane quickly closes each center alternately (i.e., left middle closed, right centre opened, then right centre opened and left eye closed); you should meet the pencil "jumping" from left to right as you alternate the eye closure. This is so considering the prototype in each eye is slightly different (disparate): Notice that because each eye is located on either side of the nose, the viewing angle of each center is slightly different - especially when viewing near objects (Figure xiv.iv).
Although the area in space defined by the binocular visual field (Figure xiv.four) represents corresponding areas of the monocular visual fields, the bending at which this space is viewed by each eye is slightly different. Consequently, the images of the respective (binocular) space are slightly different in each center. The nervous system fuses these disparate binocular images to produce a single image (east.g., of the pencil located an arm's length away). The process of producing a single image from the ii disparate monocular images is chosen binocular fusion.
Clinically, binocular fusion is tested by holding up ane or ii fingers in front of the patient and asking the patient (who should be wearing corrective lenses if they are normally worn) how many fingers they see. If the patient reports seeing 4 fingers when just two are presented, the patient is unable to produce binocular fusion.
Binocular fusion permits the perception a single clear image and also provides extra cues for depth perception. That is, the binocular disparity between the 2 images is used past the nervous system to allow the perception of a three-dimensional globe where the estimate distance of an object can be determined. The nervous system cannot fuse disparate binocular images when the disparity is as well great. When corresponding areas of the normal binocular visual field are not in alignment (e.g., in strabismus where 1 center deviates from the normal position and/or is paralyzed), the nervous system cannot fuse the disparate images and gradually adapts past "ignoring" the paradigm from the deviant eye. In fact, strabismus at birth, if uncorrected, may outcome in a course of key blindness, amblyopia, where the image from the deviant middle is no longer represented at cortical levels of the nervous organization. The uncorrected, long-term amblyope is functionally blind in one middle and has poor depth perception.
14.2 The Image Forming Process
The transparent media of the eye office as a biconvex lens that refracts light entering the centre and focuses images of the external world onto the light sensitive retina.
Refraction
Recall that light rays will bend when passing from i transparent medium into another if the speed of light differs in the two media. Withal, parallel lite rays volition pass from air through a transparent trunk (east.grand., flat lens) without angle if the light rays are perpendicular to the lens surface (Figure fourteen.five, left). If the light strikes the lens surface at an angle, the calorie-free rays will be bent in a line perpendicular to the lens surface (Figure 14.5, correct).
| Figure 14.five |
A @biconvex lens, which is functionally like to the eye's lens arrangement, is flat just at its center. The surface of the expanse surrounding the center is curved and not perpendicular to parallel calorie-free rays (Effigy 14.six). Consequently, the curved surfaces of a biconvex lens will bend parallel low-cal rays to focus an epitome of the object emitting the light a short altitude behind the lens at its focal point. The image formed is articulate just if the curvature of the lens is symmetrical in all meridians and all divergent low-cal rays emitted by a point source converge at the focal signal.
| Figure 14.six |
| Effigy 14.vii |
Annotation that the greater the curvature of the lens surface the greater is its refractive power and the closer is the focused image to the lens. Note also that the image formed is inverted and left-right reversed (Figure fourteen.seven).
The prototype formed by heart's lens system is smaller than the object viewed, inverted (upside-down, Figure fourteen.half-dozen), and reversed (right-left, Figure 14.7). As the image is inverted by the lens system, the superior (top) one-half of each center's visual field is projected onto the inferior (bottom) half of each eye's retina. As well, as the lens produces a reversed image, the temporal half of each visual field is projected onto the nasal one-half of each eye's retina1. Therefore, the temporal (left) hemifield of the left eye is projected onto the nasal (right) half of the left eye's retina and the nasal (left) hemifield of correct eye is projected onto temporal (right) half of the right heart's retina. Consequently, the left hemifields of both optics are projected onto the corresponding (correct) halves of the 2 retinas. Information technology is critical that you understand the relationship between the visual field and the retinal areas and realize that corresponding halves of the two monocular visual fields are imaged on corresponding halves of the two retinas. These relationships form the neurological basis for understanding visual field defects.
Lens Accommodation
The eye must be able to change its refractive properties to focus images of both distant and nearby objects on the retina. Distant objects (greater than xxx feet or 9 meters abroad from the eye) emit or reverberate light that can exist focused on the retina in a normal relaxed middle (Figure 14.viii).
| Figure xiv.8 |
If a viewed object is brought closer to the heart, the low-cal rays from the object diverge at a greater angle relative to the middle (Figure xiv.eight). Consequently, the nearer the object of view, the greater the angle of incidence of light rays on the cornea, and the greater the refractive power required to focus the calorie-free rays on the retina. The cornea has a stock-still refractive power (i.e. it cannot change its shape). However, altering the tension of the zonules on the elastic lens capsule tin can modify the lens shape. The alter in the refractive properties of the center is chosen the accommodation or "near point" process.
In the normal heart under resting (distant vision) conditions, the ciliary muscles are relaxed and the zonules are under tension (Figure 14.9). In this case, the lens is flattened, which reduces the refractive power of the lens to focus on distant objects. When an object is closer to the eye (i.eastward., less than thirty ft. away), accommodation occurs to affect "near vision". The ciliary muscle contracts, pulling the ciliary processes toward the lens (call up the musculus acts as a sphincter). This action releases tension on the zonules and the lens sheathing. The reduced tension allows the lens to get more spherical (i.e., increase its curvature). The increase in lens curvature increases the lens refractive power to focus on near objects. Consequently, as an object is moved closer to the viewer, his eyes conform to increase the lens curvature, which increases the refractive power of his center (Figure 14.8).
| Effigy 14.nine |
Refractive Errors of the Middle and Corrective Lenses
Presbyopia: In presbyopia, at that place is normal distance vision, but lens accommodation is reduced with age. With age, the lens loses its elasticity and becomes a relatively solid mass. During adaptation, the lens is unable to assume a more spherical shape and is unable to increase its refractive ability for almost vision (Figure xiv.10). As a result, when an object is less than xxx ft. abroad from the presbyopic viewer, the image is focused somewhere behind the retina.
| Figure xiv.ten |
A convex lens (i.e., increased refractive power) is used to correct the presbyopic heart (Figure 14.10). These lenses refract the light rays so they strike the surface of the cornea at a smaller angle. However, because the corrective lens increases the refractive power, the presbyope with convex lenses volition have problems with distance vision. Consequently, the corrective lenses are often half lenses (i.east., reading glasses) which allow the presbyope to view objects in the distance unimpeded past the convex lens.
Hyperopia: In hyperopia (Figure 14.11), the refractive power of the centre'south lens system is too weak or the eyeball too brusque. When viewing afar objects, the epitome is focused at a point across the retina.
| Figure 14.11 |
The young hyperope tin recoup past using lens adaptation, i.e., increase the refractive power of the eye's lens system (Figure fourteen.12). Nosotros call the hyperope "far-sighted" (hypermetropic) because the power of accommodation used for distance vision cannot be used for near vision.
| Figure 14.12 |
Equally the hyperope ages and becomes presbyopic, the power of accommodation is diminished. Consequently, the middle anile hyperope may have a limited range (near and far) of vision. To correct this effect of aging, the refractive power of the eye is increased with convex lenses (Effigy fourteen.12).
Myopia: In myopia (Figure fourteen.13), the refractive power of the eye'south lens system is too strong or the eyeball too long. When viewing distant objects, the image is focused at a point in forepart of retina.
| Effigy 14.xiii |
The uncorrected myopic center is "well-nigh-sighted" because it can focus unaided on nearly objects. That is, the young myope will see afar objects as blurred, poorly divers images but can encounter nearby small objects clearly (call back nearby objects emit divergent lite rays).
For distance vision, the refractive power of the myopic eye lens system is corrected with concave lenses that diverge the light rays entering the center (Effigy 14.14). Annotation that every bit the power of accommodation diminishes with age, near vision is also affected in the presbyopic-myopic center. The mature myope may crave bifocals, the upper half of the lens diverging lite rays for distance vision and the lower half with no or depression converging power for near vision.
| Figure 14.14 |
Astigmatism: An astigmatism results when the cornea surface does not resemble the surface of a sphere (eastward.g. is more oblong). In an middle with astigmatism, the image of distant and almost objects cannot exist focused on the retina (Effigy fourteen.15). Astigmatism is corrected with a cylindrical lens having a curvature that corrects for the corneal astigmatism. The cylindrical lens directs light waves through the astigmatic cornea to focus a single, clear image on the retina.
| Effigy 14.15 |
fourteen.iii The Retina
You volition now larn about the retinal neurons and the laminar structure of the retina, and the ways in which the light-sensitive receptors of the eye convert the prototype projected onto the retina into neural responses. The light sensitive retina forms the innermost layer of the eye (Figure fourteen.16).
| Effigy 14.16 |
The retina covers the choroid and extends anteriorly to just backside the ciliary body. The retina consists of neurons and supporting cells.
Components of the Retina
The retina is derived from the neural tube and is, therefore, part of key nervous system. It consists of two parts, the retinal pigment epithelium, which separates the middle, choroid coat of the eyeball from the other innermost component and the neural retina (Effigy fourteen.sixteen) – the dark pigments within the retinal pigment epithelium and choroid coat function to absorb light passing through the receptor layer, thus reducing lite besprinkle and paradigm baloney within the eye. The neural retina contains five types of neurons (Effigy 14.17): the visual receptor cells (the rods and cones), the horizontal cells, the bipolar cells, the amacrine cells, and the retinal ganglion cells.
Retinal Layers
The retina is a laminated structure consisting of alternating layers of prison cell bodies and cell processes (Effigy 14.18).
| Figure 14.17 | |
| Figure 14.18 |
The innermost layers are located nearest the vitreous chamber, whereas the outermost layers are located adjacent to the retinal pigment epithelium and choroid. The most important layers, progressing from the outer to inner layers, are:
- the retinal paint epithelium, which provides critical metabolic and supportive functions to the photoreceptors;
- the receptor layer, which contains the light sensitive outer segments of the photoreceptors;
- the outer nuclear layer, which contains the photoreceptor cell bodies;
- the outer plexiform layer, where the photoreceptor, horizontal and bipolar cells synapse;
- the inner nuclear layer, which contains the horizontal, bipolar and amacrine cell bodies;
- the inner plexiform layer, where the bipolar, amacrine and retinal ganglion cells synapse;
- the retinal ganglion cell layer, which contains the retinal ganglion cell bodies; and
- the optic nerve layer, which contains the ganglion cell axons traveling to the optic disc.
Notice that lite passing through the cornea, lens and vitreous must laissez passer through well-nigh of the retinal layers before reaching the low-cal-sensitive portion of the photoreceptor; the outer segment in the receptor layer. Notice also that in the region of the fovea where the image of the central visual field center is focused, the retina consists of fewer layers (Figure fourteen.xix): thereby minimizing the obstacles to forming a clear image on the fovea. The area around the fovea, the surrounding macula, is thicker because it contains the jail cell bodies and processes of retinal neurons receiving information from the receptors in the fovea.
The optic disc is formed by the retinal ganglion cell axons that are exiting the retina. It is located nasal to the fovea (Effigy 14.19). This region of the retina is devoid of receptor cells and composed predominantly by the optic nerve layer. Consequently, information technology is the structural basis for the 'blind spot" in the visual field.
| Figure 14.19 |
The Photoreceptors
The human has two types of photoreceptors: the rods and cones (Figure fourteen.20). They are distinguished structurally by the shapes of their outer segments. The photopigments of the rods and cones too differ. The rod outer segment disks comprise the photopigment rhodopsin, which absorbs a wide bandwidth of light. The cones differ in the color of light their photopigments absorbs: one type of photopigment absorbs red low-cal, some other green light, and a third blue calorie-free. As each cone receptor contains only one of the three types of cone photopigment, there are three types of cones; ruby-red, greenish or blue. Each cone responds best to a specific colour of lite, whereas the rods reply all-time to white low-cal2. The rod and cone photopigments as well differ in illumination sensitivity; rhodopsin breaks down at lower light levels than that required to breakdown cone photopigments. Consequently, the rods are more sensitive - at to the lowest degree at low levels of illumination.
xiv.4 Rods and Cones Form the Footing for Scotopic and Photopic Vision
The homo visual system has ii subsystems that operate at unlike low-cal energy levels. The scotopic, night-adjusted system operates at low levels of illumination, whereas the photopic, light-adapted system operates at high levels of illumination.
| Effigy xiv.20 |
Rods are responsible for the initiation of the scotopic visual procedure. Rods
- contain the photopigment rhodopsin, which breaks down when exposed to a wide bandwidth of low-cal (i.e., it is achromatic).
- Rhodopsin is also more than sensitive to light and reacts at lower light levels than the color sensitive (chromatic) cone pigments.
- Rhodopsin is also more than sensitive to light and reacts at lower light levels than the color sensitive (chromatic) cone pigments.
- have longer outer segments, more than outer segment disks and, consequently, comprise more photopigment.
- are more sensitive to light and function at scotopic (depression) levels of illumination.
- dominate in the peripheral retina (Effigy 14.21A), which is color insensitive, has poor acuity (Figure xiv.21B), just is sensitive to low levels of illumination.
Cones are responsible for the initiation of the photopic visual process. Cones
- comprise photopigments that breakdown in the presence of a limited bandwidth of lite (i.e., cone photopigments are chromatic).
- are color sensitive.
- are less sensitive to low-cal and require high (daylight) illumination levels.
- are concentrated in the fovea (Figure 14.21A)
- in the fovea have epitome of the key visual field projected on them.
- in the fovea are responsible for photopic, light-adjusted vision (i.e., loftier visual vigil and color vision) in the central visual field (Figure 14.21B)
| Effigy 14.21 |
Biochemical processes in the photoreceptors participate in dark and light accommodation. Find when y'all enter a darkened room subsequently spending time in daylight, it takes many minutes earlier you are able to see objects in the dim low-cal. This slow increase in light sensitivity is called the dark-adaptation process and is related to the rate of regeneration of photopigments and to the intracellular concentration of calcium3. A contrasting, only faster, process occurs in high levels of illumination. When you lot are fully dark-adapted, exposure to bright light is at first blinding (massive photopigment breakdown and stimulation of photoreceptors) and is followed rapidly by a render of sight. This phenomenon, light adaptation, allows the cone response to dominate over rod responses at high illumination.
14.5 Visual Processing in the Retina
The photoreceptors exhibit a fairly high basal release of glutamate. When light strikes the photoreceptor jail cell, it initiates a biochemical procedure in the cell that reduces the release of glutamate from its axon final. The glutamate, in plough, affects the activity of the bipolar and horizontal cells, which synapse with the photoreceptor. The bipolar cells, in turn, synapse with amacrine and retinal ganglion cells. It is the axons of the retinal ganglion cells that exit the heart as the optic nervus and cease in the brain. Notice that the direct pathway for the manual of visual information from the centre to the brain includes merely the receptor jail cell, bipolar prison cell and ganglion cell. The horizontal cells attune the synaptic activity of receptor cells and, thereby, indirectly bear upon the manual of visual data by bipolar cells. Similarly the amacrine cells modulate the synaptic activity of the retinal bipolar and ganglion cells, thereby affecting the transmission of visual information by the ganglion cells.
Bipolar Cells
Within the outer plexiform layer of the retina, approximately 125 1000000 photoreceptor cells synapse with approximately 10 1000000 bipolar cells. A smaller number of horizontal cells also synapse with the photoreceptor cells within the outer plexiform layer of the retina. The bipolar and horizontal cells respond to the glutamate released by the photoreceptor cellsfour.
- Bipolar cells
- exercise not generate action potentials.
- answer to the release of glutamate from photoreceptors with graded potentials (i.e., by hyperpolarizing or depolarizing).
- exercise not generate action potentials.
Bipolar cells differ based on their responses to photoreceptor stimulation.
- There are at least two types of bipolar cells based on their responses to glutamate.
- The off bipolar cells are depolarized by glutamate.
- The on bipolar cells are hyperpolarized by glutamate.
- The off bipolar cells are depolarized by glutamate.
- The two bipolar cell types accept different functional properties.
- The off bipolar cells function to detect night objects in a lighter background.
- The on bipolar cells function to discover light objects in a darker background.
- The off bipolar cells function to detect night objects in a lighter background.
The stimulus condition that produces a depolarizing response from a bipolar cell is used to name the bipolar jail cell blazon.
- An off bipolar prison cell depolarizes when the photoreceptors that synapse with it are in the dark (i.east., when the light is off, Figure 14.22).
- An on bipolar cell depolarizes when the photoreceptors that synapse with are in the light (i.e., when the light is on, Figure 14.22). Note that the depolarization of the on bipolar jail cell does non result from excitation of the presynaptic cell but rather from a reduction of the inhibitory action of glutamate produced by the low-cal-induced decreased release of glutamate from the photoreceptor.
| Figure 14.22 |
Bipolar Jail cell Receptive Field : The receptive field of a bipolar cell is defined anatomically past the location and distribution of receptor cells with which information technology makes synaptic contact.
- Each cone-bipolar cell makes straight synaptic contact with a circumscribed patch of cone receptors, which may be every bit few as 1 foveal cone. Consequently, the receptive fields of bipolar cells synapsing with cones in the fovea are extremely small and are color sensitive. The cone-bipolars may exist hyperpolarized or depolarized by glutamate and, consequently, may be on-blazon or off-type bipolar cells.
- Each rod-bipolar cell may make synaptic contact with a few to fifty or more of rod receptor cells. Consequently, the rod-bipolar prison cell receptive field is relatively large and color insensitive. All rod-bipolar cells are hyperpolarized by glutamate and, consequently, are on-type bipolar cells exclusively.
The bipolar cell receptive field is also divers physiologically as the retinal expanse which when exposed to calorie-free produces a response (i.e., depolarization or hyperpolarization) in the bipolar cell.
Bipolar cells take concentric receptive fields. Low-cal directed on the photoreceptor(s) that synapse with a bipolar jail cell produces a response from the bipolar cell called the heart response (Effigy fourteen.23). In contrast, light directed on immediately surrounding receptors produce the opposite response (Figure xiv.24).
| Figure fourteen.23 | Figure 14.24 |
When both the middle and surrounding receptor cells are illuminated with calorie-free, the on bipolar cell response to stimulation of the center receptors is reduced by stimulation of the environment receptors (Figure 14.25).
| Figure 14.25 |
Consequently, the strongest on bipolar cell response is produced when the stimulus is a light spot encircled by a nighttime ring. For the off bipolar cell, a nighttime spot encircled by a light ring produces maximal depolarization.
Horizontal Cells
Within the outer plexiform layer, the photoreceptor cells make both presynaptic and postsynaptic contact with horizontal cells.
- The horizontal cells have large receptive fields involving
- presynaptic (axonal) contact with a minor group of photoreceptors and
- postsynaptic (dendritic) contact with a larger grouping of surrounding photoreceptor cells.
- presynaptic (axonal) contact with a minor group of photoreceptors and
Past decision-making the responses of their "center" photoreceptors (based on the responses of the surrounding photoreceptors), the horizontal cells indirectly produce the bipolar prison cell receptive field surround effect. The surround outcome produced by the horizontal cell is weaker than the eye effect.
| Figure 14.26 |
The surround effect, produced by the horizontal cells, enhances brightness contrasts to produce sharper images, to make an object appear brighter or darker depending on the background and to maintain these contrasts under different illumination levels.
Retinal Ganglion Cells
Inside the inner plexiform layer, the axon terminals of bipolar cells (the ii° visual afferents) synapse on the dendritic processes of amacrine cells and ganglion cells. As in most neurons, depolarization results in neurotransmitter release by the bipolar cell at its axon terminals. Most bipolar cells release glutamate, which is excitatory to most ganglion cells (i.due east., depolarizes ganglion cells). The amacrine cells may synapse with bipolar cells, other amacrine cells or ganglion cells. Information technology is the axons of the retinal ganglion cells (the iii° visual afferents) that exit the middle to class the optic nerve and deliver visual information to the lateral geniculate nucleus of the thalamus and to other diencephalic and midbrain structures.
| Figure xiv.27 |
Ganglion Cell Response Properties. The retinal ganglion cells are the concluding retinal elements in the direct pathway from the center to the encephalon. Considering they must carry visual information some distance from the middle, they posses voltage-gated sodium channels in their axonal membranes and generate action potentials when they are depolarized past the glutamate released past the bipolar cells.
The off bipolar cell (Figure fourteen.27, Right) will depolarize when it is dark on its center cones and volition therefore release glutamate when it is dark on the heart of its receptive field. This will consequence in the depolarization of the retinal ganglion cells with which the off bipolar synapses and in the product of action potentials (i.e., discharges) by these ganglion cells (Figure 14.27, Correct). Consequently, the retinal ganglion cells that synapse with off bipolar cells will have off-center/on-surroundings receptive fields and are called off ganglion cells.
The on bipolar prison cell (Figure fourteen.28, Left) will depolarize when there is light on its center cones and will therefore release glutamate when it is light on the center of its receptive field. This will event in the depolarization of the retinal ganglion cells with which the on bipolar synapses and in the production of action potentials (i.east., discharges) past these ganglion cells (Effigy xiv.28, Left). Consequently, the retinal ganglion cells that synapse with on bipolar cells will have on-center/off-environs receptive fields and are called on ganglion cells.
In short, the receptive fields of the bipolar cells with which the retinal ganglion cell synapses determine the receptive field configuration of a retinal ganglion cell.
The retinal ganglion cells provide data of import for detecting the shape and motility of objects.
In the primate eye, there are two major types of retinal ganglion cells, Type G and Blazon P cells, that procedure information near different stimulus backdrop.
| Effigy xiv.28 |
Blazon P retinal ganglion cells are colour-sensitive object detectors.
The P ganglion cell(s)
- outnumber the M-ganglion cells, by approximately 100 to 1 in the primate retina
- makes synaptic contact with one to a few cone bipolars that are innervated by cone receptors in the macula fovea
- is color sensitive
- has a small concentric receptive field
- produces a sustained, slowly adapting response that lasts as long as a stimulus is centered on its receptive field.
- produces weak responses to stimuli that move across its receptive field.
The slowly adapting response of the Type P retinal ganglion cell is best suited for signaling the presence, color and elapsing of a visual stimulus and is poor for signaling stimulus movement.
Blazon M retinal ganglion cells are color-insensitive motion detectors.
The M ganglion cell
- is much larger than P ganglion cells
- synapses with many bipolar cells
- is color insensitive
- has a large concentric receptive field
- is more than sensitive to modest center-surround brightness differences
- responds with a transient, quickly adapting response to a maintained stimulus.
- responds maximally, with loftier discharge rates, to stimuli moving across its receptive field.
The rapidly adapting responses of Type M ganglion cells are best suited for signaling temporal variations in, and the movement of, a stimulus.
The axons of the Chiliad and P retinal ganglion cells travel in the retina optic nervus fiber layer to the optic disc where they leave the eye. Near of the axons travel to and stop in the lateral geniculate nucleus of the thalamus.
Amacrine Cells
Amacrine cells synapse with bipolar cells and ganglion cells and are similar to horizontal cells in providing lateral connections betwixt like types of neurons (e.grand., they may connect bipolar cells to other bipolar cells)5. They differ from horizontal cells, however, in also providing ''vertical" links between bipolar and ganglion cells.
Amacrine prison cell types. There are 20 or more than types of amacrine cells based on their morphology and neurochemistry. The roles of three types accept been identified. One blazon
- is responsible for producing the movement sensitive (rapidly adapting) response of the Blazon M ganglion cells.
- enhances the heart-surround effect in ganglion cell receptive fields.
- connects rod bipolar cells to cone bipolar cells, thus allowing ganglion cells to reply to the entire range of light levels, from scotopic to photopic.
Convergence of Inputs and Visual Acuity
Depression convergence of cones to cone bipolar cells and low convergence of cone bipolar cells to P-retinal ganglion cells produce high visual acuity in the central visual field.
Call up that
- visual acuity and color vision are greatest in the fundamental visual field.
- the image of the central visual field is projected onto the fovea.
- the cones are concentrated in the fovea, whereas the rods predominate in the peripheral retina.
- there is low convergence of foveal cones onto macular bipolar cells, equally low as ane cone receptor to one bipolar prison cell.
In improver, the cones in the fovea are of smaller diameter than those in the periphery of the retina, which allows for a greater packing density of foveal cones. The high packing density of cones and the low convergence of cones onto bipolar cells in the macula support higher visual acuity in the primal visual field. Consequently, the foveal cones, macular bipolar cells and the P-retinal ganglion cells are responsible for photopic, light-adapted vision in the fundamental visual field. In contrast, the higher convergence of the rods onto peripherally located bipolar cells and of peripheral bipolar cells onto amacrine cells forms the ground for the poor visual acuity but high light sensitivity of scotopic vision.
14.5 Clinical Manifestations of Retinal Dysfunction
The chemical and concrete integrity of the retina is essential for normal visual function. Abnormalities in the blood supply and retinal pigment epithelium result in retinal dysfunctions.
Vitamin A deficiency can cause permanent incomprehension. An adequate supply of photopigments is necessary to sustain photoreceptors. The supply of all-trans retinal equally a photopigment breakup product is insufficient to maintain adequate photopigment production. Vitamin A tin be oxidized into all-trans retinal, and is, therefore, critical in the synthesis of photopigment. In the eye, it is the retinal pigment epithelium that stores vitamin A. The retinal paint epithelium is too the site of the oxidization of vitamin A into all-trans retinal and conversion of all-trans retinal into 11-cis-retinal. Vitamin A cannot exist synthesized by the body and must be ingested. Information technology is plant in blood and stored in the liver and retinal pigment epithelium. Vitamin A deficiency, which tin can result from liver damage (e.g., from alcoholism or hepatitis), produces degeneration of photoreceptors with visual symptoms first presenting every bit "nighttime incomprehension" (i.e., extremely poor vision under low illumination).
Retinitis pigmentosa is an inherited disorder in which in that location is a gradual and progressive failure to maintain the receptor cells. One form involves the production of defective opsin that usually combines with 11-cis retinal to course rhodopsin. Consequently, the rods do not comprise sufficient rhodopsin and do not part as the low illumination receptors. A symptom of this status is "night blindness" and loss of peripheral vision. In this form of retinitis pigmentosa, the cones receptors function ordinarily and cardinal vision remains intact. Other forms of retinitis pigmentosa that affect the cones may progress to destroy key vision.
Macular Degeneration. The leading cause of blindness in the elderly is age-related macular degeneration. The dry form of macular degeneration involves intraocular proliferation of cells in the macular area (i.eastward., in the fovea and the immediately surrounding retinal areas). In the wet form of macular degeneration, the capillaries of the choroid coat invade the macular area and destroy receptor cells and neurons. In both forms, the visual loss is in the central visual field and the patient volition complain of blurred vision and difficulty reading. Laser surgery is the nigh mutual treatment for the wet form but has the disadvantage of destroying normal retinal cells. Information technology also may not exist effective in preventing cell proliferation following treatment.
Retinal detachment. When the neural retina is torn away from the retinal paint epithelium (e.g., by a blow to the eye), there is a loss of vision in the area of detachment. The loss of vision results because the neural retina is dependent on the retinal pigment epithelium for 11-cis retinal, nutrients and photoreceptor integrity. The retinal pigment epithelium supplies glucose and essential ions to the neural retina, helps support the photoreceptor prison cell outer segment, removes outer segment disks shed past the receptor cells, and converts retinol and stores vitamin A for photopigment resynthesis. Lasers may be used to weld the detachment to foreclose it from increasing in size. However, the detached and welded areas are functionally blind.
Diabetic retinopathy. The pathological process in diabetic retinopathy involves microaneurysms and punctate hemorrhages in the retina. The tiny swollen blood vessels and/or bleeding in the underlying choroid coat damage the receptor cells and retinal neurons and consequence in blindness in the regions afflicted. Lasers may be used to seal swollen and/or leaking blood vessels.
14.6 Summary
This chapter described the stimulus (light) backdrop that are of import for the visual perception of our external environment, such every bit colour, brightness, color and effulgence contrasts (for course perception and visual vigil), visual field representation, binocular fusion and depth perception. Remember that there are regional differences in visual perception: the fundamental visual field is color-sensitive, has loftier acuity vision and operates at high levels of illumination (i.e., operates with the photopic, light-adjusted subsystem). In dissimilarity, the visual field periphery is more sensitive at low levels of illumination, is relatively colour insensitive and has poor visual vigil (i.e., operates with the scotopic, dark-adapted, subsystem). The chapter also described how the lens system of the center produces an image on the retina of light emitted past or reflected off objects in space. The image is a smaller, inverted, and reversed picture of the object. Keep in mind that the image projected onto the retina is, in fact, projected onto a flattened sheet of receptor cells that line the inner surface of the eye. The following chapter volition draw the role of the visual receptors and other retinal neurons in converting the visual image into an array of neural activity.
The chapter too reviewed the retinal neurons and the laminar structure of the retina. The image projected onto the retina is distributed over a mosaic of photoreceptors. Light free energy projected onto each photoreceptor is converted into receptor membrane potential changes by a process that involves photosensitive pigments and cyclic nucleotide-gated ion channels in the photoreceptor outer segment. The phototransduction process converts light energy into photoreceptor membrane potential changes that produce a chemical indicate (the release of glutamate), which results in membrane potential changes in the postsynaptic bipolar and horizontal cells. The receptor substrate for scotopic and photopic vision lies in differences between the rod and cone receptors.
In the primate eye, the information gathered past 125 million receptor cells converges on ten million bipolar cells, which, in turn, converge on ane million retinal ganglion cells. The caste of convergence from receptors to bipolar cell and bipolar cells to ganglion cell differs regionally within the retina. In the peripheral retina, the convergence can be fifty or more than rod receptors to i bipolar cell, which increases the sensitivity to dim lights only decreases the spatial acuity of the peripheral bipolar cell. In add-on, these peripheral bipolar cells are color insensitive. The M-ganglion cells receive input from many peripheral bipolar cells, have large receptive fields, are sensitive to minor brightness contrasts and are color insensitive. They too generate transient responses and are uniquely sensitive to changes in illumination levels and movement. In contrast, the bipolar cells in the macula synapse with few foveal-cone receptors, which maintain the spatial resolution of the densely packed cones. Such macular bipolar cells have small receptive field centers, are color sensitive but must operate at high illumination levels. Each P-ganglion cell synapses with few macular bipolar cells and is color sensitive, but less sensitive to dim "white" light and to small-scale brightness contrasts. The P ganglion cells have smaller receptive fields than the 1000 ganglion cells and respond with sustained discharges to maintained stimuli. Every bit the Yard ganglion cells and P ganglion cells reply to dissimilar aspects of the visual stimulus, they are described to be encoding and conveying independent, parallel, streams (M-stream and P-stream) of information about stimulus size, color, and motion.
Test Your Cognition
- Question 1
- A
- B
- C
- D
- Due east
All of the following is characteristic of the cornea of the eye EXCEPT:
A. Cataracts are formed when it is damaged.
B. Information technology is devoid of blood vessels.
C. It receives oxygen from the tear moving picture
D. Nutrients are provided by the aqueous sense of humour
E. Its refractive ability is fixed for altitude vision
All of the post-obit is characteristic of the cornea of the eye EXCEPT:
A. Cataracts are formed when information technology is damaged. This respond is Right!
Cataracts form when the lens is damaged.
B. It is devoid of blood vessels.
C. Information technology receives oxygen from the tear film
D. Nutrients are provided by the aqueous sense of humour
E. Its refractive power is fixed for distance vision
All of the following is characteristic of the cornea of the eye EXCEPT:
A. Cataracts are formed when it is damaged.
B. It is devoid of blood vessels. This answer is INCORRECT.
The cornea is devoid of claret vessels.
C. It receives oxygen from the tear moving picture
D. Nutrients are provided by the aqueous sense of humour
E. Its refractive power is fixed for altitude vision
All of the following is characteristic of the cornea of the eye EXCEPT:
A. Cataracts are formed when it is damaged.
B. It is devoid of blood vessels.
C. Information technology receives oxygen from the tear motion-picture show This answer is Incorrect.
Equally the cornea is devoid of blood vessels, it must receive oxygen from the tear film.
D. Nutrients are provided by the aqueous humour
Due east. Its refractive ability is fixed for distance vision
All of the post-obit is characteristic of the cornea of the center EXCEPT:
A. Cataracts are formed when it is damaged.
B. It is devoid of blood vessels.
C. It receives oxygen from the tear film
D. Nutrients are provided by the aqueous humor This answer is INCORRECT.
As the cornea is devoid of blood vessels, information technology gets nutrients from the aqueous humor.
E. Its refractive power is fixed for distance vision
All of the following is characteristic of the cornea of the eye EXCEPT:
A. Cataracts are formed when it is damaged.
B. It is devoid of blood vessels.
C. It receives oxygen from the tear film
D. Nutrients are provided past the aqueous humor
E. Its refractive power is fixed for distance vision This reply is INCORRECT.
The cornea's shape is fixed; consequently so is its refractive ability.
- Question 2
- A
- B
- C
- D
Which of the following business relationship for the ability of rod bipolar cells to detect and signal light at lower illumination levels than cone bipolar cells?
A. Rods are more concentrated in the fovea than the cones.
B. The rod-biplar cells projections are denser than the cone-bipolar cells projections.
C. Rods have thicker outer segments than the cones.
D. Photopigments in rods are broken downwardly by the narrowest bandwidth of lite.
Which of the post-obit business relationship for the power of rod bipolar cells to detect and signal low-cal at lower illumination levels than cone bipolar cells?
A. Rods are more concentrated in the fovea than the cones. This respond is Incorrect.
B. The rod-biplar cells projections are denser than the cone-bipolar cells projections.
C. Rods have thicker outer segments than the cones.
D. Photopigments in rods are broken down by the narrowest bandwidth of low-cal.
Which of the post-obit account for the ability of rod bipolar cells to find and signal light at lower illumination levels than cone bipolar cells?
A. Rods are more than full-bodied in the fovea than the cones.
B. The rod-biplar cells projections are denser than the cone-bipolar cells projections. This reply is CORRECT!
C. Rods accept thicker outer segments than the cones.
D. Photopigments in rods are broken downwardly past the narrowest bandwidth of low-cal.
Which of the post-obit account for the ability of rod bipolar cells to detect and bespeak light at lower illumination levels than cone bipolar cells?
A. Rods are more concentrated in the fovea than the cones.
B. The rod-biplar cells projections are denser than the cone-bipolar cells projections.
C. Rods have thicker outer segments than the cones. This answer is INCORRECT.
D. Photopigments in rods are broken downwards by the narrowest bandwidth of light.
Which of the post-obit account for the ability of rod bipolar cells to detect and signal lite at lower illumination levels than cone bipolar cells?
A. Rods are more full-bodied in the fovea than the cones.
B. The rod-biplar cells projections are denser than the cone-bipolar cells projections.
C. Rods take thicker outer segments than the cones.
D. Photopigments in rods are broken downward by the narrowest bandwidth of light. This respond is INCORRECT.
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Source: https://nba.uth.tmc.edu/neuroscience/m/s2/chapter14.html
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