Chapter 14: visual Processing: Eye and Retina
Valentin Dragoi, Ph.D., department of Neurobiology and also Anatomy, The UT medical School in ~ Houston (content provided by Chieyeko Tsuchitani, Ph.D.) Reviewed and revised 07 Oct 2020
In this thing you will certainly learn about how the visual system initiates the handling of exterior stimuli. The chapter will certainly familiarize friend with procedures of visual sensation by discussing the basis of type perception, intuitive acuity, visual field representation, binocular fusion, and depth perception. An important aspect is the local differences in our intuitive perception: the central visual ar is color-sensitive, has high acuity vision, operates at high levels of illumination conversely, the periphery is an ext sensitive at short levels of illumination, is relatively color insensitive, and has bad visual acuity. Girlfriend will find out that the photo is an initial projected onto a flattened paper of photoreceptor cells that lie on the inner surface of the eye (retina). The info gathered by countless receptor cells is projected following onto millions of bipolar cells, which, in turn, send projects to retinal ganglion cells. These cells encode different elements of the visual stimulus, and thus bring independent, parallel, streams the information about stimulus size, color, and movement to the intuitive thalamus.
14.1 procedures of intuitive Sensation
The condition of the visual system deserve to be identified by evaluating various facets of intuitive sensation. Because that example, the ability to detect and identify tiny objects (i.e., intuitive acuity) can be influenced by obstacle in the transparent media of the eye and/or visual nervous system. The i can not qualify to recognize objects in certain areas of space (i.e., visual field defects) is often related come neural damage.
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Spatial Orientation and also the visual Field
The visual field is that area in space perceived when the eyes space in a fixed, static position looking straight ahead.
Figure 14.1 The monocular visual field is the area in room visible to one eye. Together illustrated, the nose prevents the ar of the appropriate eye from covering 180 levels in the horizontal plane. Inset. Perimetry testing provides a detailed map the the intuitive field. As the nose, brow and also cheeks occlude the check out of the many nasal, superior and also inferior areas, respectively, the result monocular visual field occupies a limited portion (colored blue) the the potential visual space.
The monocular intuitive field (Figure 14.1)is that area of room visible come one eye have the right to be mapped parametrically Perimetry trial and error provides a comprehensive map the the intuitive field. The potential visual ar is described as a hemisphere. However, that does not kind a perfect hemisphere together the brow, nose and also cheekbones obscure the view - many prominently in the nasal hemisphere is subdivided into two halves, the hemifields (Figure 14.1 Inset). A horizontal line attracted from 0° come 180° through center of the field defines the exceptional & inferior hemifields. A upright line drawn from 90° to 270° with center suggest defines the left & right hemifields, i beg your pardon are often termed the nasal and also temporal hemifields. might be further subdivided right into quadrants: the superior and inferior nasal quadrants the superior and inferior temporal quadrants. has a blind spot, a tiny area in i beg your pardon objects cannot be perceived which is located within the temporal hemifield.
Figure 14.2 The binocular intuitive field. Together our eyes room angled slightly towards the nose, the monocular visual areas of the left and also right eye overlap to type the binocular visual field (colored red). Objects in ~ the binocular visual field are visible to each eye, albeit from various angles.
The monocular visual ar (Figure 14.1) is established with one eye covered. The area that overlap the the visual field of one eye through that of the contrary eye is called the binocular ar (Figure 14.2). All locations of the binocular visual ar are “seen” through both eyes.
The ability to locate objects in room and the capacity to orient ourselves v respect to exterior objects space dependent top top the representation of visual room within the nervous system. The clinical examination of the visual fields most generally used is the confrontation field test. It specifies the outer borders of our spatu visual space. Neurological obstacle of the visual system can often be localized based on the area of blindness within the visual field.
Visual acuity is the ability to detect and also recognize small objects visually depends on the refractory (focusing) strength of the eye"s lens system and also the cytoarchitecture of the retina.
Visual acuity ismeasure under high illumination the smallest size of a dark thing in a irradiate background that have the right to be correctly figured out
In the clinical setting, an eye graphis provided to measure up the patient’s visual acuity. consists of rows of black color letters top top a bright white background. is provided to measure up visual acuity in ~ a distance of 20 ft from the chart. reports intuitive acuity as the proportion of the eye chart distance (i.e., 20 ft) come the “normal distance” that the lowest row of letters correctly figured out by the patient (e.g., row 3, which is 70 ft).
Color vision is the ability to detect differences in the wavelength of light is called shade vision. Clinically it might be tested with an Ishihara chart: a chart with spots of different colors that space spatially organized to form numbers the differ for ``normal” and also color-blind eyes.
As mentioned above, the human has actually a trichromatic visual system, whereby visible colors have the right to be developed by a mixture the red, green and also blue lights. The most common form of shade blindness results in a confusion of red and green shades (i.e., red-green shade blindness). Most instances of shade blindness result from an missing or defective gene responsible for producing the red or green photopigment (protanopia, the lack of red; and also deuteranopia, the lack of green). Together these gene are situated on the X chromosome, color blindness is much more common in males 보다 in females.
Figure 14.3 LEFT. The visual field of the left eye is mapped parametrically. The dark dot in the temporal hemifield to represent the "blind spot" wherein nothing is seen. RIGHT. Intuitive acuity is plotted as a role of distance (in degrees) from the center of the intuitive field. The curve labeled "Light-adapted" was acquired under photopic illumination levels and the curve labeling "Dark-adapted" was acquired under scotopic illumination levels.
Regional differences: there are regional differences in shade sensation, visual acuity and low-illumination sensitivity in ~ the visual ar (Figure 14.3).
A small “blindspot” issituated in the temporal hemifield (Figure 14.3 Left) where objects can not be seen.
Vision in the visual field centeroperates ideal under high illumination. has the biggest visual acuity and color sensitivity is ten times much better than in the field periphery (Figure 14.3 Right) represents the procedure of the photopic (light-adapted) subsystem
Vision in the peripheral intuitive fieldis more sensitive to dim light operates under low illumination. has tiny color sensitivity and poor spatial acuity (Figure 14.3 Right) to represent the procedure of the scotopic (dark-adapted) subsystem
Binocular combination and Depth Perception
Figure 14.4 The two eyes fixated on things view the object and also objects in the background at slightly various angles. Consequently, the pictures on the 2 retinas are slightly different and must be "fused" by the visual system. The disparity in the retinal photos at the two eyes additionally provides binocular cues because that depth perception.
When a pencil is hosted an arm’s size away through both eyes open, most individuals will check out a single object and also recognize it as a pencil. However, if one quickly closes each eye alternately (i.e., left eye closed, best eye opened, then appropriate eye opened and also left eye closed); you have to see the pencil “jumping” indigenous left to appropriate as you alternative the eye closure. This is so due to the fact that the image in every eye is slightly various (disparate): notification that due to the fact that each eye is located on either next of the nose, the viewing angle of every eye is slightly different - specifically when viewing near objects (Figure 14.4).
return the area in space defined through the binocular visual ar (Figure 14.4) represents corresponding areas of the monocular intuitive fields, the angle at i beg your pardon this an are is perceived by each eye is contempt different. Consequently, the pictures of the equivalent (binocular) space are slightly different in every eye. The nervous system fuses these disparate binocular photos to create a solitary image (e.g., that the pencil situated an arm’s size away). The process of developing a single image indigenous the 2 disparate monocular pictures is called binocular fusion.
Clinically, binocular fusion is tested by holding up one or 2 fingers in prior of the patient and asking the patience (who should be wearing corrective lenses if lock are usually worn) how many fingers castle see. If the patience reports seeing four fingers once only two are presented, the patience is can not to produce binocular fusion.
Binocular blend permits the late a single clear image and likewise provides extra cues because that depth perception. The is, the binocular disparity in between the two images is supplied by the nervous mechanism to allow the tardy of a three-dimensional world where the approximate distance of an object can it is in determined. The nervous system cannot fuse disparate binocular pictures when the disparity is too great. When corresponding areas of the regular binocular visual field are not in alignment (e.g., in strabismus wherein one eye deviates native the normal place and/or is paralyzed), the nervous mechanism cannot fuse the different images and also gradually it is adapted by “ignoring” the picture from the deviant eye. In fact, strabismus in ~ birth, if uncorrected, may an outcome in a type of central blindness, amblyopia, whereby the picture from the deviant eye is no much longer represented in ~ cortical level of the worried system. The uncorrected, permanent amblyope is functionally blind in one eye and also has bad depth perception.
14.2 The photo Forming process
The transparent media that the eye function as a biconvex lens that refracts irradiate entering the eye and also focuses images of the external world onto the light perceptible retina.
Recall that light rays will certainly bend when passing from one transparent tool into one more if the speed of light differs in the two media. However, parallel light rays will certainly pass native air through a transparent human body (e.g., level lens) there is no bending if the light rays space perpendicular come the lens surface ar (Figure 14.5, left). If the irradiate strikes the lens surface at one angle, the irradiate rays will certainly be bending in a line perpendicular come the lens surface (Figure 14.5, right).
Figure 14.5 The course of light rays passing v a transparent lens space illustrated. LEFT: The irradiate rays are entering perpendicular to the surface of the lens. RIGHT: The irradiate rays room entering at an edge to the surface ar of the lens and also are gift refracted by the lens.
A biconvex lens, i beg your pardon is functionally comparable to the eye"s lens system, is flat only at its center. The surface of the area surrounding the center is curved and not perpendicular come parallel light rays (Figure 14.6). Consequently, the bent surfaces of a biconvex lens will bend parallel light rays to focus photo of the object emitting the light a brief distance behind the lens at its focal length point. The image developed is clear only if the curvature that the lens is symmetrical in every meridians and all divergent irradiate rays emitted through a point resource converge at the focal length point.
Figure 14.6 The light rays emanating from a point resource take divergent paths that get in a biconvex lens at various points along the lens surface. The lens refracts the light rays happen them with each other at the focal suggest some street from the lens.
Figure 14.7 The eye"s lens system attributes like a biconvex lens and focuses picture on the retina the is inverted, left-right reversed and smaller 보다 the thing viewed.
keep in mind that the higher the curvature the the lens surface ar the higher is that refractive power and also the closer is the concentrated image to the lens. Note additionally that the image formed is inverted and also left-right reversed (Figure 14.7).
The image created by eye’s lens mechanism is smaller sized than the object viewed, turning back (upside-down, figure 14.6), and also reversed (right-left, figure 14.7). Together the picture is turning back by the lens system, the superior (top) half of each eye’s visual field is projected onto the worse (bottom) fifty percent of each eye’s retina. Also, together the lens to produce a reversed image, the temporal fifty percent of every visual field is projected top top the nasal half of each eye’s retina1. Therefore, the temporal (left) hemifield the the left eye is projected ~ above the sleep (right) half of the left eye’s retina and the nasal (left) hemifield of right eye is projected onto temporal (right) half of the appropriate eye’s retina. Consequently, the left hemifields of both eyes room projected top top the corresponding (right) halves the the two retinas. That is an essential that you understand the relationship between the intuitive field and the retinal areas and realize that matching halves the the two monocular visual areas are imaged on matching halves of the two retinas. These relationships form the neurological basis for knowledge visual ar defects.
The eye have to be able to readjust its refractive properties to emphasis images that both distant and nearby objects top top the retina. Distant objects (greater 보다 30 feet or 9 meters away from the eye) emit or reflect irradiate that deserve to be focused on the retina in a normal relaxed eye (Figure 14.8).
Figure 14.8 The common eye at rest can focus on the retina photos of objects much more than 30 ft indigenous the eye. When an object is carried closer to the eye (i.e., less than 30 ft from the eye), the irradiate rays from the thing take much more divergent paths and each beginning the cornea through a better angle of incidence. Consequently, the photo focal allude would be beyond the retina if the eye"s lens mechanism were no adjusted. Throughout accommodation, the lens curvature increases, raising the refractive strength of the eye and also focusing the photo on the retina.
If a perceived object is brought closer come the eye, the irradiate rays from the object diverge at a higher angle relative to the eye (Figure 14.8). Consequently, the nearer the object of view, the higher the angle of incidence of irradiate rays top top the cornea, and also the greater the refractive power required to emphasis the irradiate rays on the retina. The cornea has actually a fixed refractive strength (i.e. That cannot readjust its shape). However, transforming the stress and anxiety of the zonules ~ above the elastic lens capsule can transform the lens shape. The adjust in the refractive nature of the eye is dubbed the accommodation or "near point" process.
In the normal eye under resting (distant vision) conditions, the ciliary muscles space relaxed and also the zonules room under anxiety (Figure 14.9). In this case, the lens is flattened, i beg your pardon reduces the refractive strength of the lens to focus on distant objects. When an item is closer come the eye (i.e., less than 30 ft. Away), accommodation occurs to influence “near vision”. The ciliary muscle contracts, pulling the ciliary processes toward the lens (remember the muscle acts together a sphincter). This action releases anxiety on the zonules and also the lens capsule. The reduced tension enables the lens to become an ext spherical (i.e., increase its curvature). The rise in lens curvature rises the lens refractive power to emphasis on near objects. Consequently, as an object is relocated closer come the viewer, his eye accommodate to increase the lens curvature, which boosts the refractive power of his eye (Figure 14.8).
Figure 14.9 throughout distance vision (i.e., through the eye at rest), the ciliary muscles room relaxed and the zonules room under tension. The lens is planarization by the tension on the zonules and the lens capsule. However, in the accommodation process, the ciliary muscle contract and, acting prefer a sphincter muscle, decrease the stress and anxiety on the zonules and lens capsule. The lens becomes an ext spherical with its anterior surface shifting more anteriorly right into the anterior chamber.
Refractive Errors the the Eye and Corrective Lenses
Presbyopia: In presbyopia, there is common distance vision, however lens accommodation is diminished with age. Through age, the lens loser its elasticity and also becomes a reasonably solid mass. During accommodation, the lens is can not to assume a more spherical shape and also is can not to rise its refractive power for near vision (Figure 14.10). Together a result, when an object is much less than 30 ft. Away from the presbyopic viewer, the photo is focused somewhere behind the retina.
Figure 14.10 In the presbyopic eye, once the thing is relocated closer come the eye, the lens is can not to accommodate and also the photo is focused past the retina. For the presbyopic eye a corrective lens that converges the irradiate rays (i.e., a convex lens that reduces the edge of incidence of light on the cornea) will enable the presbyopic eye to view nearby objects.
A convex lens (i.e., raised refractive power) is provided to exactly the presbyopic eye (Figure 14.10). These lenses refract the light rays for this reason they win the surface of the cornea at a smaller angle. However, since the corrective lens increases the refractive power, the presbyope with convex lenses will have problems with street vision. Consequently, the corrective lenses room often half lenses (i.e., analysis glasses) which enable the presbyope to watch objects in the street unimpeded through the convex lens.
Hyperopia: In hyperopia (Figure 14.11), the refractive power of the eye’s lens device is also weak or the eyeball also short. As soon as viewing remote objects, the image is concentrated at a allude beyond the retina.
Figure 14.11 The hyperopic eye at rest cannot focus on the retina the image of one object an ext than 30 ft from the eye. The hyperopic lens system is as well weak and the image is focused beyond the retina.
The young hyperope have the right to compensate by making use of lens accommodation, i.e., boost the refractive strength of the eye’s lens system (Figure 14.12). We contact the hyperope "far-sighted" (hypermetropic) because the power of accommodation provided for street vision can not be supplied for near vision.
Figure 14.12 If the hyperopia is no severe; the hyperopic eye deserve to use the lens accommodation procedure to rise the refractive strength of the eye for street vision.
as the hyperope ages and becomes presbyopic, the power of accommodation is diminished. Consequently, the center aged hyperope may have a limited range (near and far) that vision. To exactly this impact of aging, the refractive strength of the eye is boosted with convex lenses (Figure 14.12).
Myopia: In myopia (Figure 14.13), the refractive strength of the eye’s lens mechanism is too solid or the eyeball also long. When viewing distant objects, the photo is concentrated at a allude in prior of retina.
Figure 14.13 The myopic eye at rest cannot focus on the retina the image of one object an ext than 30 ft. From the eye. The refractive strength of the eye"s lens mechanism is too solid and the image is concentrated in front of the retina.
The uncorrected myopic eye is "near-sighted" because it can emphasis unaided on close to objects. The is, the young myope will certainly see remote objects together blurred, poorly identified images yet can view nearby small objects plainly (remember nearby objects emit divergent irradiate rays).
For distance vision, the refractive strength of the myopic eye lens system is corrected with concave lenses the diverge the light rays start the eye (Figure 14.14). Keep in mind that as the power of accommodation diminishes with age, close to vision is also impacted in the presbyopic-myopic eye. The maturation myope might require bifocals, the upper half of the lens diverging irradiate rays for distance vision and also the lower fifty percent with no or short converging strength for close to vision.
Figure 14.14 A corrective lens the diverges irradiate rays before they get in the eye (i.e., a concave lens) will allow the myopic eye to focus the image of a distant object ~ above the retina.
Astigmatism: one astigmatism results when the cornea surface does no resemble the surface of a sphere (e.g. Is an ext oblong). In one eye with astigmatism, the photo of distant and also near objects can not be focused on the retina (Figure 14.15). Astigmatism is corrected through a cylindrical lens having actually a curvature the corrects for the corneal astigmatism. The cylindrical lens directs irradiate waves with the astigmatic cornea to emphasis a single, clear image on the retina.
Figure 14.15 The astigmatic lens is asymmetrical and has multiple focal length points, i beg your pardon produces multiple pictures of a allude source.
14.3 The Retina
You will currently learn about the retinal neurons and the laminar structure of the retina, and also the means in i beg your pardon the light-sensitive receptors of the eye convert the photo projected onto the retina into neural responses. The light perceptible retina creates the innermost class of the eye (Figure 14.16).
Figure 14.16 The eye, the three coats that the eye and the great of the retina. The retina is the innermost coat of the eye and also consists of the retinal colors epithelium and also neural retina.
The retina covers the choroid and extends anteriorly to just behind the ciliary body. The retina is composed of neurons and supporting cells.
Components of the Retina
The retina is obtained from the neural tube and is, therefore, component of central nervous system. It consists of 2 parts, the retinal colours epithelium, i m sorry separates the middle, choroid cloak of the eyeball native the various other innermost component and also the neural retina (Figure 14.16) – the dark pigments in ~ the retinal colours epithelium and choroid coat duty to absorb light passing v the receptor layer, for this reason reducing light scatter and image distortion in ~ the eye. The neural retina includes five varieties of neurons (Figure 14.17): the intuitive receptor cells (the rods and cones), the horizontal cells, the bipolar cells, the amacrine cells, and also the retinal ganglion cells.
The retina is a laminated structure consisting of alternating layers of cabinet bodies and cell procedures (Figure 14.18).
Figure 14.17 The components of the neural retina. The neural retina consists of at least five different varieties of neurons: the photoreceptors (rods and cones), horizontal cell, bipolar cell, amacrine cell and ganglion cell.
Figure 14.18 The neural retina is created by alternate layers that neuron cell bodies that appear dark and also neuron processes that show up light in Nissl stained tissue. The receptor cell synapse v bipolar and also horizontal cell in the outer plexiform layer. The bipolar cells, in turn, synapse through amacrine and ganglion cell in the inside plexiform great The axons of the retinal ganglion cells exit the eye to form the optic nerve.
The innermost class are situated nearest the vitreous chamber, whereas the outermost layers space located nearby to the retinal pigment epithelium and also choroid. The most necessary layers, advancing from the external to inner layers, are:the retinal pigment epithelium, i m sorry provides an essential metabolic and supportive attributes to the photoreceptors; the receptor layer, which includes the irradiate sensitive external segments of the photoreceptors; the outer atom layer, which contains the photoreceptor cell bodies; the outer plexiform layer, where the photoreceptor, horizontal and also bipolar cells synapse; the inner atom layer, which contains the horizontal, bipolar and amacrine cell bodies; the inner plexiform layer, whereby the bipolar, amacrine and also retinal ganglion cell synapse; the retinal ganglion cell layer, which contains the retinal ganglion cabinet bodies; and also the optic nerve layer, which contains the ganglion cabinet axons travel to the optic disc.
Notice the light passing through the cornea, lens and vitreous should pass through most of the retinal layers before reaching the light-sensitive section of the photoreceptor; the external segment in the receptor layer. Notice also the in the an ar of the fovea wherein the image of the main visual field facility is focused, the retina consists of under layers (Figure 14.19): in order to minimizing the obstacles to creating a clear image on the fovea. The area around the fovea, the bordering macula, is thicker due to the fact that it includes the cabinet bodies and also processes of retinal neurons receiving information from the receptors in the fovea.
The optic bowl is formed by the retinal ganglion cell axons that room exiting the retina. The is located nasal to the fovea (Figure 14.19). This an ar of the retina is there is no of receptor cells and also composed predominantly by the optic nerve layer. Consequently, the is the structure basis for the "blind spot" in the visual field.
Figure 14.19 The fovea the the retina and also the layers of the retina in the neighboring macula. The fovea and macula are colored together they show up when stained because that Nissl substance, i m sorry is most abundant in the neuron cabinet body.
The human has two varieties of photoreceptors: the rods and cones (Figure 14.20). They are distinguished structurally through the shapes of their outer segments. The photopigments the the rods and also cones additionally differ. The rod outer segment disks save the photopigment rhodopsin, i m sorry absorbs a wide bandwidth the light. The cones differ in the shade of light your photopigments absorbs: one kind of photopigment absorbs red light, an additional green light, and also a 3rd blue light. As each cone receptor consists of only one of the three varieties of cone photopigment, there are three varieties of cones; red, green or blue. Every cone responds best to a specific color of light, vice versa, the rods respond finest to white light2. The rod and also cone photopigments additionally differ in illumination sensitivity; rhodopsin breaks under at reduced light levels 보다 that forced to failure cone photopigments. Consequently, the rods are more sensitive - at the very least at low levels the illumination.
14.4 Rods and Cones kind the Basis because that Scotopic and also Photopic Vision
The human visual system has actually two subsystems that run at various light energy levels. The scotopic, dark-adapted system operates at low levels that illumination, conversely, the photopic, light-adapted system operates in ~ high level of illumination.
Figure 14.20 The cone and rod photoreceptors. The photoreceptors are neurons that have actually a dendritic ingredient (the external segment) and also an axonal ingredient that forms synaptic terminals.
Rods space responsible for the initiation the the scotopic visual process. Rodscontain the photopigment rhodopsin, which division down once exposed come a large bandwidth of irradiate (i.e., that is achromatic). Rhodopsin is also more sensitive come light and also reacts at lower light levels 보다 the shade sensitive (chromatic) cone pigments. have longer external segments, an ext outer segment disks and, consequently, contain an ext photopigment. are an ext sensitive to irradiate and role at scotopic (low) levels of illumination. overcome in the peripheral retina (Figure 14.21A), i m sorry is shade insensitive, has bad acuity (Figure 14.21B), but is perceptible to short levels the illumination.
Cones space responsible because that the initiation the the photopic intuitive process. Conessave photopigments that break down in the existence of a limited bandwidth of light (i.e., cone photopigments are chromatic). are shade sensitive. are less sensitive come light and also require high (daylight) illumination levels. are focused in the fovea (Figure 14.21A) in the fovea have image that the central visual field projected on them. in the fovea space responsible because that photopic, light-adapted vision (i.e., high visual acuity and also color vision) in the central visual ar (Figure 14.21B)
Figure 14.21 The rods, are taller, have longer external segments and, consequently, contain much more outer segment disks and more photopigment than cones. Cone receptor are concentrated in the fovea of the eye (at 0° eccentricity), whereas rod receptors are focused in more peripheral retina (A). Intuitive acuity is maximal in the main area that the visual ar (at 0° eccentricity), whereas it is minimal in more peripheral locations (B). Notice that the ar of the optic disc relative to the fovea corresponds to the ar of the remote spot loved one to the visual ar center.
Biochemical processes in the photoreceptors get involved in dark and light adaptation. Notification when you enter a darkened room after ~ spending time in daylight, it takes plenty of minutes prior to you space able to check out objects in the dim light. This slow boost in irradiate sensitivity is called the dark-adaptation process and is related to the rate of rebirth of photopigments and also to the intracellular concentration the calcium3. A contrasting, however faster, procedure occurs in high levels of illumination. As soon as you are fully dark-adapted, exposure come bright light is at an initial blinding (massive photopigment breakdown and stimulation the photoreceptors) and also is adhered to rapidly by a return the sight. This phenomenon, irradiate adaptation, allows the cone solution to overcome over stick responses at high illumination.
14.5 Visual handling in the Retina
The photoreceptors exhibit a reasonably high basal relax of glutamate. Once light strikes the photoreceptor cell, the initiates a biochemical process in the cell the reduces the release of glutamate from its axon terminal. The glutamate, in turn, affects the activity of the bipolar and horizontal cells, i m sorry synapse v the photoreceptor. The bipolar cells, in turn, synapse with amacrine and also retinal ganglion cells. The is the axons the the retinal ganglion cell that departure the eye as the optic nerve and terminate in the brain. Notice that the direct pathway because that the infection of visual details from the eye to the mind includes only the receptor cell, bipolar cell and ganglion cell. The horizontal cells modulate the synaptic activity of receptor cells and, thereby, indirectly impact the transmission of visual info by bipolar cells. An in similar way the amacrine cells modulate the synaptic activity of the retinal bipolar and also ganglion cells, thereby affecting the transmission of visual information by the ganglion cells.
Within the external plexiform layer of the retina, about 125 million photoreceptor cell synapse with about 10 million bipolar cells. A smaller number of horizontal cells also synapse through the photoreceptor cell within the outer plexiform layer of the retina. The bipolar and horizontal cells respond to the glutamate exit by the photoreceptor cells4.Bipolar cells carry out not generate action potentials. respond come the relax of glutamate indigenous photoreceptors v graded potentials (i.e., by hyperpolarizing or depolarizing).
Bipolar cell differ based upon their responses come photoreceptor stimulation.There room at the very least two species of bipolar cells based on their responses come glutamate. The off bipolar cells space depolarized by glutamate. The on bipolar cells room hyperpolarized by glutamate. The 2 bipolar cell types have various functional properties. The off bipolar cells function to detect dark objects in a lighter background. The on bipolar cells function to detect light objects in a darker background.
The stimulus problem that to produce a depolarizing response from a bipolar cabinet is used to surname the bipolar cell type.one off bipolar cell depolarizes once the photoreceptors that synapse v it are in the dark (i.e., once the light is off, number 14.22). one on bipolar cell depolarizes as soon as the photoreceptors that synapse with room in the light (i.e., as soon as the irradiate is on, figure 14.22). Keep in mind that the depolarization the the on bipolar cell does not an outcome from excitation the the presynaptic cell however rather indigenous a reduction of the inhibitory action of glutamate produced by the light-induced reduced release the glutamate indigenous the photoreceptor.
Figure 14.22 once the receptor cells with which an turn off bipolar cabinet synapses are in the dark, the turn off bipolar cabinet is depolarized and the top top bipolar cabinet is hyperpolarized. In contrast, when the receptor cells with which an off bipolar cell synapses room in the light, the off bipolar cell is hyperpolarized and the ~ above bipolar cell is depolarized.
Bipolar cell Receptive Field: The receptive ar of a bipolar cabinet is defined anatomically through the location and distribution that receptor cells through which it renders synaptic contact.Each cone-bipolar cell makes direct synaptic contact with a circumscribed patch of cone receptors, which may be as couple of as one foveal cone. Consequently, the receptive areas of bipolar cell synapsing with cones in the fovea are extremely small and are shade sensitive. The cone-bipolars might be hyperpolarized or depolarized through glutamate and, consequently, may be on-type or off-type bipolar cells. every rod-bipolar cell might make synaptic contact with a few to fifty or an ext of pole receptor cells. Consequently, the rod-bipolar cell receptive field is relatively big and color insensitive. All rod-bipolar cells space hyperpolarized by glutamate and, consequently, space on-type bipolar cells exclusively.
The bipolar cabinet receptive ar is additionally defined physiologically as the retinal area which as soon as exposed to irradiate produces a solution (i.e., depolarization or hyperpolarization) in the bipolar cell.
Bipolar cells have actually concentric receptive fields. Irradiate directed on the photoreceptor(s) the synapse v a bipolar cabinet produces a solution from the bipolar cell dubbed the center an answer (Figure 14.23). In contrast, irradiate directed on instantly surrounding receptors produce the opposite solution (Figure 14.24).
Figure 14.23 Bipolar cells have actually concentric receptive fields. The ~ above bipolar cell depolarizes once the receptor cells v which the synapses are illuminated ("Light On"). These center receptors (i.e., the persons making straight synaptic call with the bipolar cell) create the bipolar cell facility response.
Figure 14.24 Bipolar cells have concentric receptive fields. When the receptors neighboring the center receptors that the on bipolar receptive ar are illuminated ("Light On") and the facility receptors maintained in the dark, the on bipolar cell is hyperpolarized.
When both the center and also surrounding receptor cells space illuminated through light, the on bipolar cell an answer to stimulation of the facility receptors is decreased by stimulation the the surround receptors (Figure 14.25).
Figure 14.25 Bipolar cells have actually concentric receptive fields. As soon as both the center and surrounding receptor of the ~ above bipolar cabinet receptive ar are illuminated, the ~ above bipolar cell depolarizes. However, the magnitude of the depolarization is reduced to less than the depolarization to illumination of just the facility receptors.
Consequently, the strongest on bipolar cell response is created when the stimulus is a light spot encircled by a dark ring. Because that the off bipolar cell, a dark point out encircled through a light ring produce maximal depolarization.
Within the outer plexiform layer, the photoreceptor cells make both presynaptic and postsynaptic contact with horizontal cells.The horizontal cells have big receptive fields involving presynaptic (axonal) call with a small group the photoreceptors and postsynaptic (dendritic) call with a larger team of surrounding photoreceptor cells.
By managing the responses of their “center” photoreceptors (based on the responses of the surrounding photoreceptors), the horizontal cells indirectly produce the bipolar cell receptive ar surround effect. The surround effect produced by the horizontal cell is weaker 보다 the center effect.
Figure 14.26 The horizontal cells do presynaptic and also postsynaptic contact with photoreceptor cells. The axon terminals the a horizontal cabinet receives synaptic contact from one group of photoreceptors (colored red) and also its processes make synaptic call with surrounding photoreceptor cell (colored green).
The surround effect, created by the horizontal cells, improves brightness contrasts to develop sharper images, to make an object appear brighter or darker depending upon the background and to maintain these contrasts under different illumination levels.
Retinal Ganglion Cells
Within the within plexiform layer, the axon terminals that bipolar cells (the 2° visual afferents) synapse ~ above the dendritic processes of amacrine cells and ganglion cells. Together in most neurons, depolarization results in neurotransmitter relax by the bipolar cabinet at the axon terminals. Most bipolar cells release glutamate, i m sorry is excitatory to most ganglion cells (i.e., depolarizes ganglion cells). The amacrine cells might synapse v bipolar cells, other amacrine cell or ganglion cells. The is the axons of the retinal ganglion cells (the 3° visual afferents) that departure the eye to kind the optic nerve and also deliver visual information to the lateral geniculate nucleus of the thalamus and also to other diencephalic and midbrain structures.
Figure 14.27 An off ganglion cabinet synapses with an turn off bipolar cell and produces action potentials (i.e., is excited) when the off bipolar cabinet is depolarized (i.e., when the irradiate is off). In contrast, an on ganglion cell the synapses through an ~ above bipolar cell reduces the rate at which the produces action potentials (i.e., is inhibited) once the ~ above bipolar cell is hyperpolarized (when the irradiate is off).
Ganglion Cell response Properties. The retinal ganglion cells space the final retinal aspects in the straight pathway from the eye come the brain. Since they must lug visual info some street from the eye, they posses voltage-gated sodium networks in their axonal membranes and generate activity potentials once they space depolarized by the glutamate exit by the bipolar cells.
The off bipolar cabinet (Figure 14.27, Right) will certainly depolarize once it is dark on its center cones and also will because of this release glutamate as soon as it is dark top top the facility of its receptive field. This will result in the depolarization the the retinal ganglion cells v which the off bipolar synapses and also in the production of action potentials (i.e., discharges) by these ganglion cell (Figure 14.27, Right). Consequently, the retinal ganglion cells the synapse through off bipolar cells will have off-center/on-surround receptive fields and are dubbed off ganglion cells.
The on bipolar cell (Figure 14.28, Left) will certainly depolarize as soon as there is irradiate on its facility cones and will as such release glutamate as soon as it is irradiate on the center of its receptive field. This will result in the depolarization of the retinal ganglion cells through which the on bipolar synapses and in the manufacturing of activity potentials (i.e., discharges) by this ganglion cell (Figure 14.28, Left). Consequently, the retinal ganglion cells that synapse v on bipolar cells will have on-center/off-surround receptive fields and also are called on ganglion cells.
In short, the receptive areas of the bipolar cells v which the retinal ganglion cell synapses determine the receptive ar configuration of a retinal ganglion cell.
The retinal ganglion cells provide information important for detecting the shape and also movement that objects.
In the primate eye, there room two major types of retinal ganglion cells, form M and form P cells, that procedure information about different economic stimulation properties.
Figure 14.28 Left: The top top ganglion cabinet synapses through an ~ above bipolar cell and also produces action potentials (i.e., is excited) once the on bipolar cabinet is depolarized (i.e., once the irradiate is on). Right: In contrast, an turn off ganglion cell that synapses with an turn off bipolar cell reduces the rate at which it produces action potentials (i.e., is inhibited) when the off bipolar cabinet is hyperpolarized (when the light is on).
Type p retinal ganglion cells room color-sensitive thing detectors.
The ns ganglion cell(s)outnumber the M-ganglion cells, by roughly 100 come 1 in the primate retina makes synaptic contact with one come a few cone bipolars that are innervated through cone receptors in the macula fovea is color sensitive has a small concentric receptive ar to produce a sustained, slowly adapting response the lasts as long as a economic stimulation is centered on its receptive field. produce weak responses to stimuli the move throughout its receptive field.
The progressively adapting an answer of the form P retinal ganglion cabinet is ideal suited for signaling the presence, color and also duration that a visual stimulus and is negative for signaling stimulus movement.
Type M retinal ganglion cells room color-insensitive movement detectors.
The M ganglion cellis much bigger than p ganglion cells synapse with many bipolar cell is shade insensitive has a large concentric receptive field is much more sensitive to tiny center-surround brightness distinctions responds through a transient, rapidly adapting response come a kept stimulus. responds maximally, through high discharge rates, come stimuli moving across its receptive field.
The rapidly adapting responses of type M ganglion cell are ideal suited because that signaling temporal variations in, and also the activity of, a stimulus.
The axons of the M and P retinal ganglion cells travel in the retina optic nerve fiber layer come the optic disc wherein they departure the eye. Most of the axons travel to and terminate in the lateral geniculate cell core of the thalamus.
Amacrine cells synapse with bipolar cells and ganglion cells and also are similar to horizontal cell in offering lateral connections between comparable types the neurons (e.g., they may attach bipolar cells to various other bipolar cells)5. They different from horizontal cells, however, in likewise providing ‘’vertical” links in between bipolar and ganglion cells.
Amacrine cabinet types. There room 20 or an ext types of amacrine cells based upon their morphology and also neurochemistry. The roles of three types have been identified. One kindis responsible for creating the activity sensitive (rapidly adapting) response of the form M ganglion cells. enhances the center-surround result in ganglion cell receptive fields. connects rod bipolar cells to cone bipolar cells, thus enabling ganglion cell to respond to the entire variety of irradiate levels, native scotopic come photopic.
Convergence the Inputs and Visual Acuity
Low convergence of cones to cone bipolar cells and also low convergence that cone bipolar cell to P-retinal ganglion cells create high intuitive acuity in the central visual field.
Recall thevisual acuity and also color vision are biggest in the main visual field. the photo of the main visual ar is projected ~ above the fovea. the hat are concentrated in the fovea, conversely, the rods predominate in the peripheral retina. over there is low convergence that foveal cones top top macular bipolar cells, together low together one cone receptor come one bipolar cell.
In addition, the cap in the fovea room of smaller sized diameter than those in the periphery of the retina, which permits for a higher packing thickness of foveal cones. The high packing density of cones and the low convergence that cones onto bipolar cell in the macula support higher visual acuity in the central visual field. Consequently, the foveal cones, macular bipolar cells and also the P-retinal ganglion cells room responsible because that photopic, light-adapted vision in the central visual field. In contrast, the greater convergence of the rods onto peripherally located bipolar cells and of peripheral bipolar cells onto amacrine cells develops the basis for the poor visual acuity yet high irradiate sensitivity that scotopic vision.
14.5 Clinical Manifestations that Retinal Dysfunction
The chemical and also physical integrity of the retina is essential for common visual function. Abnormalities in the blood supply and also retinal pigment epithelium an outcome in retinal dysfunctions.
Vitamin A deficiency can reason permanent blindness. An adequate supply of photopigments is important to sustain photoreceptors. The it is provided of all-trans retinal as a photopigment malfunction product is poor to preserve adequate photopigment production. Vitamin A can be oxidized right into all-trans retinal, and also is, therefore, critical in the synthesis of photopigment. In the eye, it is the retinal pigment epithelium the stores vitamin A. The retinal pigment epithelium is also the website of the oxidization of vitamin A into all-trans retinal and conversion of all-trans retinal right into 11-cis-retinal. Vitamin A can not be synthesized by the body and also must it is in ingested. The is uncovered in blood and also stored in the liver and also retinal pigment epithelium. Vitamin A deficiency, i beg your pardon can result from liver damages (e.g., from alcoholism or hepatitis), produces degeneration of photoreceptors with visual symptoms very first presenting together “night blindness” (i.e., extremely negative vision under low illumination).
Retinitis pigmentosa is an inherited disorder in which there is a gradual and progressive fail to maintain the receptor cells. One form involves the manufacturing of defective opsin that generally combines v 11-cis retinal to form rhodopsin. Consequently, the rods perform not contain sufficient rhodopsin and also do not function as the short illumination receptors. A symptom the this problem is “night blindness” and also loss that peripheral vision. In this form of retinitis pigmentosa, the hat receptors duty normally and central vision stays intact. Other creates of retinitis pigmentosa that affect the cones may progress come destroy central vision.
Macular Degeneration. The leading reason of remote in the yonsei is age-related macular degeneration. The dry form the macular degeneration entails intraocular proliferation of cell in the macular area (i.e., in the fovea and the instantly surrounding retinal areas). In the wet form the macular degeneration, the capillaries of the choroid coat attack the macular area and destroy receptor cells and also neurons. In both forms, the intuitive loss is in the central visual field and also the patient will complain of blurred vision and challenge reading. Laser surgery is the most usual treatment because that the wet kind but has actually the disadvantage of damaging normal retinal cells. It likewise may not be reliable in avoiding cell proliferation complying with treatment.
Retinal detachment. Once the neural retina is torn far from the retinal colors epithelium (e.g., by a blow to the eye), there is a loss of vision in the area that detachment. The ns of vision results since the neural retina is dependence on the retinal pigment epithelium for 11-cis retinal, nutrients and also photoreceptor integrity. The retinal pigment epithelium offers glucose and essential ion to the neural retina, helps support the photoreceptor cell external segment, removes outer segment disks shed by the receptor cells, and also converts retinol and stores vitamin A for photopigment resynthesis. Lasers might be used to weld the detachment to prevent it from raising in size. However, the detached and welded locations are functionally blind.
Diabetic retinopathy. The pathological procedure in diabetic retinopathy entails microaneurysms and punctate hemorrhages in the retina. The small swollen blood vessels and/or bleeding in the basic choroid coat damages the receptor cells and retinal neurons and an outcome in blindness in the areas affected. Lasers may be provided to seal puffy and/or leaking blood vessels.
This chapter explained the stimulus (light) properties that are essential for the intuitive perception the our outside environment, such as color, brightness, color and brightness contrasts (for type perception and also visual acuity), visual ar representation, binocular blend and depth perception. Remember the there are regional differences in intuitive perception: the central visual ar is color-sensitive, has actually high acuity vision and also operates in ~ high levels of illumination (i.e., operates through the photopic, light-adapted subsystem). In contrast, the visual field periphery is an ext sensitive at short levels of illumination, is reasonably color insensitive and has poor visual acuity (i.e., operates v the scotopic, dark-adapted, subsystem). The chapter likewise described just how the lens system of the eye produces picture on the retina of light emitted by or reflected off objects in space. The picture is a smaller, inverted, and also reversed snapshot of the object. Store in mind that the picture projected onto the retina is, in fact, projected onto a flattened sheet of receptor cells that line the inner surface ar of the eye. The complying with chapter will explain the duty of the intuitive receptors and also other retinal neurons in converting the visual photo into selection of neural activity.
The chapter additionally reviewed the retinal neurons and the laminar framework of the retina. The picture projected onto the retina is spread over a mosaic the photoreceptors. Light power projected onto each photoreceptor is converted right into receptor membrane potential changes by a process that requires photosensitive pigments and also cyclic nucleotide-gated ion channels in the photoreceptor outer segment. The phototransduction process converts light power into photoreceptor membrane potential changes that develop a chemistry signal (the relax of glutamate), which outcomes in membrane potential transforms in the postsynaptic bipolar and also horizontal cells. The receptor substrate because that scotopic and also photopic vision lies in differences between the rod and also cone receptors.
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In the primate eye, the details gathered by 125 million receptor cell converges on 10 million bipolar cells, which, in turn, converge ~ above 1 million retinal ganglion cells. The degree of convergence from receptor to bipolar cell and also bipolar cell to ganglion cell differs regionally in ~ the retina. In the peripheral retina, the convergence have the right to be fifty or much more rod receptors to one bipolar cell, which rises the sensitivity come dim lights but decreases the spatial acuity the the peripheral bipolar cell. In addition, this peripheral bipolar cells are shade insensitive. The M-ganglion cells receive input from many peripheral bipolar cells, have large receptive fields, space sensitive to small brightness contrasts and also are color insensitive. They likewise generate transient responses and are uniquely perceptible to alters in illumination levels and also movement. In contrast, the bipolar cells in the macula synapse with couple of foveal-cone receptors, which preserve the spatial resolution of the densely pack cones. Together macular bipolar cell have tiny receptive field centers, are shade sensitive yet must run at high illumination levels. Every P-ganglion cell synapses with few macular bipolar cells and also is color sensitive, but less sensitive to dim “white” light and also to tiny brightness contrasts. The p ganglion cells have actually smaller receptive fields than the M ganglion cells and respond with continual discharges to kept stimuli. Together the M ganglion cells and P ganglion cell respond come different elements of the visual stimulus, lock are described to it is in encoding and also carrying independent, parallel, streams (M-stream and P-stream) the information about stimulus size, color, and movement.