Chapter 14: visual Processing: Eye and Retina
Valentin Dragoi, Ph.D., department of Neurobiology and also Anatomy, The UT medical School in ~ Houston (content noted by Chieyeko Tsuchitani, Ph.D.) Reviewed and also revised 07 Oct 2020
In this thing you will learn about how the visual device initiates the handling of outside stimuli. The chapter will certainly familiarize you with measures of visual sensation by mentioning the basis of kind perception, intuitive acuity, visual ar representation, binocular fusion, and also depth perception. Critical aspect is the local differences in our visual perception: the central visual ar is color-sensitive, has high acuity vision, operates in ~ high level of illumination vice versa, the periphery is an ext sensitive at short levels of illumination, is fairly color insensitive, and has poor visual acuity. Girlfriend will discover that the picture is first projected top top a flattened sheet of photoreceptor cells the lie on the inner surface ar of the eye (retina). The details gathered by millions of receptor cell is projected following onto countless bipolar cells, which, in turn, send tasks to retinal ganglion cells. This cells encode different aspects of the visual stimulus, and also thus bring independent, parallel, streams of information about stimulus size, color, and movement come the intuitive thalamus.
14.1 measures of intuitive Sensation
The condition of the visual system deserve to be established by assessing various elements of visual sensation. For example, the capability to detect and also identify little objects (i.e., intuitive acuity) can be influenced by disorders in the transparent media of the eye and/or intuitive nervous system. The inability to finding objects in particular areas of space (i.e., visual field defects) is regularly related to neural damage.
You are watching: The proper sequence of eye layers from the outermost to the innermost layer is
Spatial Orientation and also the visual Field
The visual ar is the area in space perceived as soon as the eyes are in a fixed, static place looking directly ahead.
Figure 14.1 The monocular visual field is the area in space visible come one eye. Together illustrated, the nose prevents the ar of the right eye from spanning 180 levels in the horizontal plane. Inset. Perimetry experimentation provides a in-depth map the the intuitive field. As the nose, brow and cheeks occlude the check out of the most nasal, superior and inferior areas, respectively, the resulting monocular visual ar occupies a minimal portion (colored blue) of the potential visual space.
The monocular visual field (Figure 14.1)is the area of space visible to one eye deserve to be mapped parametrically Perimetry experimentation provides a detailed map of the intuitive field. The potential visual ar is described as a hemisphere. However, it does not kind a perfect hemisphere together the brow, nose and cheekbones obscure the check out - most prominently in the sleep hemisphere is subdivided into two halves, the hemifields (Figure 14.1 Inset). A horizontal line drawn from 0° come 180° through center of the field specifies the exceptional & worse hemifields. A upright line attracted from 90° to 270° with center allude defines the left & appropriate hemifields, i beg your pardon are regularly termed the nasal and temporal hemifields. may be more subdivided into quadrants: the superior and inferior nasal quadrants the superior and also inferior temporal quadrants. includes a remote spot, a tiny area in which objects can not be viewed i m sorry is located within the temporal hemifield.
Figure 14.2 The binocular intuitive field. Together our eyes are angled slightly toward the nose, the monocular visual areas of the left and right eye overlap to form the binocular visual field (colored red). Objects in ~ the binocular visual field are visible to every eye, albeit from different angles.
The monocular visual ar (Figure 14.1) is figured out with one eye covered. The area that overlap the the visual field of one eye v that of the contrary eye is called the binocular ar (Figure 14.2). All locations of the binocular visual field are “seen” by both eyes.
The ability to situate objects in room and the capacity to orient ourselves v respect to outside objects room dependent upon the depiction of visual an are within the concerned system. The clinical examination of the visual fields most typically used is the confrontation ar test. It defines the outer boundaries of our subjective visual space. Neurological disorders of the visual mechanism can regularly be localized based upon the area the blindness within the intuitive field.
Visual acuity is the ability to detect and also recognize little objects visually depends on the refractory (focusing) power of the eye"s lens system and also the cytoarchitecture of the retina.
Visual acuity ismeasure up under high illumination the smallest dimension of a dark object in a light background that deserve to be correctly determined
In the clinical setting, one eye graphis offered to measure the patient’s visual acuity. is composed of rows of black letters top top a glowing white background. is provided to measure up visual acuity at a street of 20 ft from the chart. reports intuitive acuity as the proportion of the eye chart street (i.e., 20 ft) to the “normal distance” that the lowest heat of letter correctly identified by the patient (e.g., heat 3, i m sorry is 70 ft).
Color vision is the ability to detect differences in the wavelengths of irradiate is called color vision. Clinically it might be tested v an Ishihara chart: a chart through spots of various colors that space spatially arranged to form numbers that differ because that ``normal” and also color-blind eyes.
As pointed out above, the human has a trichromatic visual system, by which visible colors can be created by a mixture the red, green and also blue lights. The most common form of color blindness results in a man of red and green shades (i.e., red-green shade blindness). Most cases of shade blindness result from an missing or defective gene responsible for producing the red or environment-friendly photopigment (protanopia, the lack of red; and also deuteranopia, the absence of green). As these genes 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 period in the temporal hemifield to represent the "blind spot" whereby nothing is seen. RIGHT. Visual acuity is plotted as a duty of street (in degrees) from the facility of the intuitive field. The curve labeling "Light-adapted" was obtained under photopic illumination levels and the curve labeling "Dark-adapted" was derived under scotopic illumination levels.
Regional differences: there are local differences in shade sensation, visual acuity and also low-illumination sensitivity within the visual field (Figure 14.3).
A little “blindspot” islocated in the temporal hemifield (Figure 14.3 Left) wherein objects can not be seen.
Vision in the visual ar centeroperates finest under high illumination. has the best visual acuity and also color sensitivity is ten times better than in the field periphery (Figure 14.3 Right) to represent the operation of the photopic (light-adapted) subsystem
Vision in the peripheral intuitive fieldis an ext sensitive to dim light operates under low illumination. has small color sensitivity and poor spatial acuity (Figure 14.3 Right) represents the procedure of the scotopic (dark-adapted) subsystem
Binocular combination and Depth Perception
Figure 14.4 The two eyes fixated on an object view the object and objects in the background at slightly different angles. Consequently, the photos on the 2 retinas are slightly different and also must be "fused" through the intuitive system. The disparity in the retinal pictures at the 2 eyes also provides binocular cues because that depth perception.
When a pencil is hosted an arm’s size away v both eye open, most individuals will watch a single object and recognize it as a pencil. However, if one promptly closes each eye alternately (i.e., left eye closed, best eye opened, then right eye opened and also left eye closed); you should see the pencil “jumping” native left to right as you alternate the eye closure. This is so due to the fact that the image in each eye is slightly various (disparate): notification that since each eye is situated on either side of the nose, the viewing angle of every eye is slightly various - specifically when viewing near objects (Figure 14.4).
although the area in room defined by the binocular visual field (Figure 14.4) to represent corresponding areas of the monocular visual fields, the edge at i m sorry this space is perceived by every eye is contempt different. Consequently, the pictures of the corresponding (binocular) room are slightly different in every eye. The nervous mechanism fuses this disparate binocular pictures to create a solitary image (e.g., of the pencil situated an arm’s size away). The process of developing a solitary image native the 2 disparate monocular images is referred to as binocular fusion.
Clinically, binocular fusion is tested by holding increase one or 2 fingers in prior of the patient and asking the patient (who must be wearing corrective lenses if castle are normally worn) how many fingers castle see. If the patient reports seeing 4 fingers once only two space presented, the patience is can not to create binocular fusion.
Binocular fusion permits the late a solitary clear photo and also provides extra cues because that depth perception. That is, the binocular disparity in between the two photos is used by the nervous system to enable the late of a three-dimensional human being where the approximate distance of things can it is in determined. The nervous mechanism cannot fuse different binocular pictures when the disparity is too great. When corresponding areas of the typical binocular visual field are not in alignment (e.g., in strabismus where one eye deviates native the normal position and/or is paralyzed), the nervous mechanism cannot fuse the disparate images and gradually adapts by “ignoring” the picture from the deviant eye. In fact, strabismus at birth, if uncorrected, may result in a kind of main blindness, amblyopia, where the photo from the deviant eye is no longer represented in ~ cortical level of the worried system. The uncorrected, long-term amblyope is functionally blind in one eye and has negative depth perception.
14.2 The image Forming process
The transparent media that the eye function as a biconvex lens the refracts irradiate entering the eye and focuses photos of the external people onto the light sensitive retina.
Recall that light rays will certainly bend as soon as passing native one transparent medium into an additional if the rate of irradiate differs in the two media. However, parallel light rays will pass native air with a transparent body (e.g., flat lens) without bending if the light rays are perpendicular come the lens surface ar (Figure 14.5, left). If the irradiate strikes the lens surface ar at one angle, the light rays will be bent in a line perpendicular come the lens surface (Figure 14.5, right).
Figure 14.5 The course of light rays passing through a transparent lens space illustrated. LEFT: The irradiate rays are entering perpendicular come the surface of the lens. RIGHT: The irradiate rays space entering in ~ an angle to the surface of the lens and are being refracted through the lens.
A biconvex lens, which is functionally comparable to the eye"s lens system, is flat only in ~ its center. The surface ar of the area bordering the facility is curved and not perpendicular come parallel light rays (Figure 14.6). Consequently, the curved surfaces of a biconvex lens will certainly bend parallel irradiate rays come focus an image of the object emitting the irradiate a quick distance behind the lens in ~ its focal length point. The image formed is clear just if the curvature the the lens is symmetrical in every meridians and all divergent irradiate rays emitted by a point resource converge at the focal distance point.
Figure 14.6 The light rays emanating from a point source take divergent paths that go into a biconvex lens at different points along the lens surface. The lens refracts the irradiate rays happen them together at the focal suggest some street from the lens.
Figure 14.7 The eye"s lens system attributes like a biconvex lens and also focuses photo on the retina the is inverted, left-right reversed and also smaller 보다 the object viewed.
note that the higher the curvature that the lens surface ar the higher is its refractive power and the closer is the focused image come the lens. Note also that the image formed is inverted and left-right reversed (Figure 14.7).
The image formed by eye’s lens device is smaller sized than the object viewed, inverted (upside-down, number 14.6), and reversed (right-left, figure 14.7). Together the picture is turning back by the lens system, the exceptional (top) fifty percent of every eye’s visual ar is projected ~ above the worse (bottom) fifty percent of every eye’s retina. Also, together the lens to produce a reversed image, the temporal fifty percent of every visual ar is projected onto the nasal half of every eye’s retina1. Therefore, the temporal (left) hemifield the the left eye is projected top top the nasal (right) half of the left eye’s retina and also the nasal (left) hemifield of ideal eye is projected onto temporal (right) fifty percent of the right eye’s retina. Consequently, the left hemifields that both eyes are projected top top the matching (right) halves that the two retinas. That is crucial that you recognize the relationship in between the intuitive field and also the retinal areas and realize that equivalent halves the the 2 monocular visual fields are imaged on corresponding halves that the 2 retinas. This relationships type the neurological communication for understanding visual field defects.
The eye need to be may be to adjust its refractive properties to focus images the both far-off and nearby objects top top the retina. Remote objects (greater 보다 30 feet or 9 meters far from the eye) emit or reflect light that can be focused on the retina in a normal relaxed eye (Figure 14.8).
Figure 14.8 The common eye at remainder can focus on the retina photos of objects much more than 30 ft from the eye. When things is lugged closer to the eye (i.e., much less than 30 ft native the eye), the irradiate rays from the thing take much more divergent paths and each enters the cornea v a better angle that incidence. Consequently, the image focal point would be past the retina if the eye"s lens system were not adjusted. Throughout accommodation, the lens curvature increases, raising the refractive strength of the eye and also focusing the photo on the retina.
If a regarded object is carried closer come the eye, the light rays indigenous the object diverge at a better angle loved one to the eye (Figure 14.8). Consequently, the nearer the thing of view, the higher the edge of incidence of light rays on the cornea, and also the higher the refractive power compelled to focus the light rays on the retina. The cornea has actually a fixed refractive strength (i.e. It cannot readjust its shape). However, changing the stress of the zonules on the elastic lens capsule can change the lens shape. The change in the refractive properties of the eye is referred to as the house or "near point" process.
In the common eye under resting (distant vision) conditions, the ciliary muscles are relaxed and also the zonules are under stress (Figure 14.9). In this case, the lens is flattened, i m sorry reduces the refractive strength of the lens to emphasis on distant objects. When an object is closer come the eye (i.e., much less than 30 ft. Away), accommodation wake up to affect “near vision”. The ciliary muscle contracts, pulling the ciliary processes toward the lens (remember the muscle acts together a sphincter). This activity releases tension on the zonules and the lens capsule. The diminished tension allows the lens come become an ext spherical (i.e., boost its curvature). The boost in lens curvature boosts the lens refractive strength to emphasis on close to objects. Consequently, as an object is moved closer to the viewer, his eye accommodate to boost the lens curvature, which rises the refractive power of his eye (Figure 14.8).
Figure 14.9 during distance vision (i.e., v the eye at rest), the ciliary muscles room relaxed and also the zonules space under tension. The lens is flattened by the anxiety on the zonules and the lens capsule. However, in the house process, the ciliary muscle contract and, acting prefer a sphincter muscle, diminish the tension on the zonules and lens capsule. The lens becomes more spherical v its anterior surface ar shifting an ext anteriorly into the anterior chamber.
Refractive Errors the the Eye and also Corrective Lenses
Presbyopia: In presbyopia, over there is normal distance vision, however lens accommodation is reduced with age. With age, the lens loser its elasticity and also becomes a fairly solid mass. During accommodation, the lens is unable to i think a much more spherical shape and is can not to rise its refractive strength for near vision (Figure 14.10). As a result, when an item is less than 30 ft. Away from the presbyopic viewer, the picture is concentrated somewhere behind the retina.
Figure 14.10 In the presbyopic eye, as soon as the thing is moved closer come the eye, the lens is can not to accommodate and the image is focused beyond the retina. For the presbyopic eye a corrective lens that converges the irradiate rays (i.e., a convex lens the reduces the angle of incidence of irradiate on the cornea) will allow the presbyopic eye come view adjacent objects.
A convex lens (i.e., increased refractive power) is offered to correct the presbyopic eye (Figure 14.10). These lenses refract the light rays so they win the surface ar of the cornea at a smaller angle. However, since the corrective lens increases the refractive power, the presbyope v convex lenses will certainly have problems with distance vision. Consequently, the corrective lenses room often half lenses (i.e., analysis glasses) which allow the presbyope to see objects in the street unimpeded by the convex lens.
Hyperopia: In hyperopia (Figure 14.11), the refractive power of the eye’s lens device is as well weak or the eyeball too short. When viewing distant objects, the picture is focused at a allude beyond the retina.
Figure 14.11 The hyperopic eye at remainder cannot emphasis on the retina the image of an object an ext than 30 ft indigenous the eye. The hyperopic lens device is too weak and also the picture is focused beyond the retina.
The young hyperope can compensate by making use of lens accommodation, i.e., increase the refractive power of the eye’s lens device (Figure 14.12). We contact the hyperope "far-sighted" (hypermetropic) because the strength of accommodation used for street vision can not be provided for close to vision.
Figure 14.12 If the hyperopia is no severe; the hyperopic eye can use the lens accommodation procedure to rise the refractive power of the eye for distance vision.
together the hyperope ages and also becomes presbyopic, the power of house is diminished. Consequently, the middle aged hyperope may have a limited range (near and also far) that vision. To correct this result of aging, the refractive power of the eye is raised with convex lenses (Figure 14.12).
Myopia: In myopia (Figure 14.13), the refractive strength of the eye’s lens device is too solid or the eyeball too long. Once viewing remote objects, the photo is focused at a point in former of retina.
Figure 14.13 The myopic eye at remainder cannot emphasis on the retina the image of one object an ext than 30 ft. Indigenous the eye. The refractive strength of the eye"s lens device is too strong and the photo is concentrated in prior of the retina.
The uncorrected myopic eye is "near-sighted" due to the fact that it can focus unaided on near objects. The is, the young myope will certainly see far-off objects as blurred, poorly identified images but can check out nearby little objects clearly (remember adjacent objects emit divergent light rays).
For street vision, the refractive power of the myopic eye lens device is corrected through concave lenses the diverge the irradiate rays start the eye (Figure 14.14). Keep in mind that together the strength of house diminishes v age, near vision is also affected in the presbyopic-myopic eye. The maturation myope may require bifocals, the upper half of the lens diverging irradiate rays for distance vision and also the lower half with no or short converging power for near vision.
Figure 14.14 A corrective lens the diverges irradiate rays prior to they enter the eye (i.e., a concave lens) will enable the myopic eye to emphasis the photo of a far-off object top top the retina.
Astigmatism: an astigmatism results as soon as the cornea surface does no resemble the surface of a round (e.g. Is much more oblong). In an eye through astigmatism, the photo of distant and near objects cannot be concentrated on the retina (Figure 14.15). Astigmatism is corrected through a cylindrical lens having a curvature the corrects for the corneal astigmatism. The cylindrical lens directs irradiate waves v the astigmatic cornea to emphasis a single, clear image on the retina.
Figure 14.15 The astigmatic lens is asymmetrical and also has multiple focal points, which produces multiple photos of a point source.
14.3 The Retina
You will now learn around the retinal neurons and also the laminar structure of the retina, and also the methods in i m sorry the light-sensitive receptor of the eye convert the photo projected top top the retina right into neural responses. The light perceptible retina develops the innermost class of the eye (Figure 14.16).
Figure 14.16 The eye, the three coats that the eye and the layers of the retina. The retina is the innermost coat of the eye and consists that the retinal pigment epithelium and also neural retina.
The retina consists the choroid and also extends anteriorly to simply behind the ciliary body. The retina is composed of neurons and supporting cells.
Components the the Retina
The retina is obtained from the neural tube and is, therefore, part of main nervous system. It is composed of 2 parts, the retinal pigment epithelium, i m sorry separates the middle, choroid cloak of the eyeball from the various other innermost component and the neural retina (Figure 14.16) – the dark pigments in ~ the retinal colors epithelium and choroid coat role to absorb irradiate passing v the receptor layer, thus reducing light scatter and image distortion in ~ the eye. The neural retina consists of five types of neurons (Figure 14.17): the visual receptor cell (the rods and also cones), the horizontal cells, the bipolar cells, the amacrine cells, and the retinal ganglion cells.
The retina is a laminated structure consisting of alternate layers of cell bodies and also cell processes (Figure 14.18).
Figure 14.17 The materials of the neural retina. The neural retina is composed of at least 5 different types of neurons: the photoreceptors (rods and cones), horizontal cell, bipolar cell, amacrine cell and also ganglion cell.
Figure 14.18 The neural retina is formed by alternating layers of neuron cabinet bodies that show up dark and also neuron procedures that show up light in Nissl stained tissue. The receptor cells synapse with bipolar and horizontal cells in the external plexiform layer. The bipolar cells, in turn, synapse through amacrine and ganglion cell in the inner plexiform great The axons the the retinal ganglion cells departure the eye to type the optic nerve.
The innermost great are situated nearest the vitreous chamber, vice versa, the outermost layers are located adjacent to the retinal pigment epithelium and choroid. The most essential layers, advancing from the outer to inner layers, are:the retinal colours epithelium, which provides crucial metabolic and also supportive functions to the photoreceptors; the receptor layer, which contains the light sensitive external segments that the photoreceptors; the outer nuclear layer, which contains the photoreceptor cell bodies; the outer plexiform layer, where the photoreceptor, horizontal and also bipolar cell synapse; the inner atom layer, which consists of the horizontal, bipolar and also amacrine cell bodies; the inner plexiform layer, where the bipolar, amacrine and also retinal ganglion cells synapse; the retinal ganglion cabinet layer, which has the retinal ganglion cabinet bodies; and the optic nerve layer, which has the ganglion cabinet axons traveling to the optic disc.
Notice the light passing through the cornea, lens and also vitreous have to pass through most of the retinal layers prior to reaching the light-sensitive portion of the photoreceptor; the external segment in the receptor layer. Notification also that in the region of the fovea where the image of the main visual field center is focused, the retina is composed of under layers (Figure 14.19): thereby minimizing the obstacles to forming a clear photo on the fovea. The area around the fovea, the bordering macula, is thicker because it consists of the cabinet bodies and processes the retinal neurons receiving information from the receptor in the fovea.
The optic key is formed by the retinal ganglion cabinet axons that space exiting the retina. That is located nasal come the fovea (Figure 14.19). This an ar of the retina is there is no of receptor cells and composed predominantly by the optic nerve layer. Consequently, the is the structural basis for the "blind spot" in the visual field.
Figure 14.19 The fovea the the retina and the great of the retina in the surrounding macula. The fovea and macula room colored together they show up when stained for Nissl substance, which is most abundant in the neuron cabinet body.
The human has two varieties of photoreceptors: the rods and cones (Figure 14.20). They are identified structurally by the forms of their external segments. The photopigments of the rods and cones additionally differ. The rod outer segment disks save on computer the photopigment rhodopsin, i m sorry absorbs a vast bandwidth of light. The cones differ in the color of light your photopigments absorbs: one type of photopigment absorbs red light, an additional green light, and also a 3rd blue light. Together each cone receptor contains only one of the three species of cone photopigment, there space three types of cones; red, environment-friendly or blue. Each cone responds ideal to a details color that light, vice versa, the rods respond finest to white light2. The rod and cone photopigments additionally differ in illumination sensitivity; rhodopsin breaks under at reduced light levels 보다 that required to failure cone photopigments. Consequently, the rods are an ext sensitive - at the very least at low levels of illumination.
14.4 Rods and also Cones kind the Basis because that Scotopic and also Photopic Vision
The human being visual system has actually two subsystems that operate at different light power levels. The scotopic, dark-adapted system operates at short levels that illumination, whereas the photopic, light-adapted system operates in ~ high level of illumination.
Figure 14.20 The cone and also rod photoreceptors. The photoreceptors room neurons that have a dendritic ingredient (the external segment) and also an axonal ingredient that develops synaptic terminals.
Rods space responsible for the initiation the the scotopic intuitive process. Rodscontain the photopigment rhodopsin, which breaks down once exposed come a wide bandwidth of light (i.e., that is achromatic). Rhodopsin is also much more sensitive to light and also reacts at reduced light levels 보다 the shade sensitive (chromatic) cone pigments. have longer outer segments, much more outer segment disks and, consequently, contain an ext photopigment. are an ext sensitive to light and function at scotopic (low) level of illumination. dominate in the peripheral retina (Figure 14.21A), i m sorry is shade insensitive, has poor acuity (Figure 14.21B), however is perceptible to low levels the illumination.
Cones space responsible because that the initiation of the photopic intuitive process. Conessave on computer photopigments that failure in the existence of a restricted bandwidth of light (i.e., cone photopigments room 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 actually image of the main visual field projected ~ above them. in the fovea are responsible for photopic, light-adapted vision (i.e., high intuitive acuity and also color vision) in the main visual ar (Figure 14.21B)
Figure 14.21 The rods, space taller, have actually longer outer segments and, consequently, contain more outer segment disks and an ext photopigment 보다 cones. Cone receptors are concentrated in the fovea of the eye (at 0° eccentricity), vice versa, rod receptor are concentrated in an ext peripheral retina (A). Visual acuity is maximal in the central area that the visual field (at 0° eccentricity), whereas it is minimal in more peripheral areas (B). Notification that the place of the optic disc family member to the fovea corresponds to the ar of the remote spot family member to the visual ar center.
Biochemical procedures in the photoreceptors get involved in dark and also light adaptation. Notice when you get in a darkened room after spending time in daylight, that takes many minutes before you room able to view objects in the dim light. This slow rise in irradiate sensitivity is called the dark-adaptation procedure and is regarded the rate of rebirth of photopigments and to the intracellular concentration that calcium3. A contrasting, yet faster, process occurs in high levels of illumination. When you are totally dark-adapted, exposure come bright irradiate is at first blinding (massive photopigment break down and stimulation that photoreceptors) and is adhered to rapidly through a return that sight. This phenomenon, irradiate adaptation, allows the cone solution 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. As soon as light strikes the photoreceptor cell, the initiates a biochemical procedure in the cell the reduces the release of glutamate indigenous its axon terminal. The glutamate, in turn, affects the task of the bipolar and also horizontal cells, i beg your pardon synapse with the photoreceptor. The bipolar cells, in turn, synapse with amacrine and retinal ganglion cells. The is the axons of the retinal ganglion cell that exit the eye together the optic nerve and also terminate in the brain. An alert that the straight pathway because that the infection of visual info from the eye to the brain includes only the receptor cell, bipolar cell and ganglion cell. The horizontal cell modulate the synaptic activity of receptor cell and, thereby, indirectly influence the transmission of visual info by bipolar cells. Similarly the amacrine cells modulate the synaptic activity of the retinal bipolar and also ganglion cells, in order to affecting the infection of visual info by the ganglion cells.
Within the outer plexiform class of the retina, around 125 million photoreceptor cell synapse with about 10 million bipolar cells. A smaller number of horizontal cells also synapse v the photoreceptor cell within the outer plexiform layer of the retina. The bipolar and horizontal cells respond come the glutamate released by the photoreceptor cells4.Bipolar cells execute not generate action potentials. respond to the relax of glutamate indigenous photoreceptors through graded potentials (i.e., by hyperpolarizing or depolarizing).
Bipolar cells differ based on their responses to photoreceptor stimulation.There space at least two varieties of bipolar cells based on their responses to glutamate. The off bipolar cells space depolarized by glutamate. The on bipolar cells are hyperpolarized by glutamate. The two bipolar cell species have various functional properties. The off bipolar cells duty to finding dark objects in a lighter background. The on bipolar cells role to detect light objects in a darker background.
The stimulus problem that produces a depolarizing an answer from a bipolar cabinet is used to surname the bipolar cabinet type.an off bipolar cell depolarizes as soon as the photoreceptors that synapse with it room in the dark (i.e., as soon as the light is off, number 14.22). an on bipolar cell depolarizes when the photoreceptors the synapse with room in the irradiate (i.e., when the light is on, figure 14.22). Keep in mind that the depolarization of the on bipolar cell does not result from excitation that the presynaptic cell however rather from a reduction of the inhibitory action of glutamate produced by the light-induced decreased release of glutamate from the photoreceptor.
Figure 14.22 as soon as the receptor cells v which an off bipolar cabinet synapses are in the dark, the off bipolar cell is depolarized and the ~ above bipolar cabinet is hyperpolarized. In contrast, when the receptor cells with which an off bipolar cell synapses are in the light, the off bipolar cabinet is hyperpolarized and the ~ above bipolar cell is depolarized.
Bipolar cell Receptive Field: The receptive ar of a bipolar cabinet is identified anatomically through the location and also distribution that receptor cells with which it provides synaptic contact.Each cone-bipolar cell makes direct synaptic contact with a circumscribed patch of cone receptors, which might be as couple of as one foveal cone. Consequently, the receptive fields of bipolar cell synapsing with cones in the fovea space extremely little and are shade sensitive. The cone-bipolars might be hyperpolarized or depolarized by glutamate and, consequently, might be on-type or off-type bipolar cells. each rod-bipolar cell might make synaptic contact with a few to fifty or more of pole receptor cells. Consequently, the rod-bipolar cabinet receptive ar is relatively big and color insensitive. All rod-bipolar cells space hyperpolarized by glutamate and, consequently, space on-type bipolar cells exclusively.
The bipolar cell receptive ar is likewise defined physiologically together the retinal area which when exposed to irradiate produces a response (i.e., depolarization or hyperpolarization) in the bipolar cell.
Bipolar cells have actually concentric receptive fields. Light directed on the photoreceptor(s) that synapse with a bipolar cabinet produces a response from the bipolar cell referred to as the center response (Figure 14.23). In contrast, light directed on automatically surrounding receptors develop the opposite solution (Figure 14.24).
Figure 14.23 Bipolar cells have concentric receptive fields. The top top bipolar cell depolarizes once the receptor cells through which it synapses room illuminated ("Light On"). These center receptors (i.e., the persons making direct synaptic contact with the bipolar cell) produce the bipolar cell facility response.
Figure 14.24 Bipolar cells have concentric receptive fields. Once the receptors neighboring the center receptors of the ~ above bipolar receptive field are illuminated ("Light On") and the center receptors preserved in the dark, the on bipolar cabinet is hyperpolarized.
When both the center and surrounding receptor cells room illuminated through light, the on bipolar cell solution to stimulation that the facility receptors is reduced by stimulation of the surround receptor (Figure 14.25).
Figure 14.25 Bipolar cells have actually concentric receptive fields. When both the center and surrounding receptor of the ~ above bipolar cabinet receptive ar are illuminated, the top top bipolar cabinet depolarizes. However, the magnitude of the depolarization is diminished to less than the depolarization to illumination of only the facility receptors.
Consequently, the strongest on bipolar cell an answer is created when the economic stimulation is a light spot encircled by a dark ring. For the off bipolar cell, a dark clues encircled through a irradiate ring produces maximal depolarization.
Within the outer plexiform layer, the photoreceptor cells do both presynaptic and also postsynaptic call with horizontal cells.The horizontal cell have huge receptive fields involving presynaptic (axonal) call with a little group the photoreceptors and also postsynaptic (dendritic) contact with a larger group of neighboring photoreceptor cells.
By regulating the responses of their “center” photoreceptors (based on the responses of the neighboring photoreceptors), the horizontal cell indirectly create the bipolar cabinet receptive field surround effect. The surround effect created by the horizontal cabinet is weaker 보다 the facility effect.
Figure 14.26 The horizontal cells do presynaptic and also postsynaptic call with photoreceptor cells. The axon terminals the a horizontal cell receives synaptic call from one team of photoreceptors (colored red) and its procedures make synaptic call with surrounding photoreceptor cells (colored green).
The surround effect, produced by the horizontal cells, boosts brightness contrasts to create sharper images, come make things appear brighter or darker relying on the background and also to keep these contrasts under different illumination levels.
Retinal Ganglion Cells
Within the within plexiform layer, the axon terminals that bipolar cell (the 2° intuitive afferents) synapse on the dendritic processes of amacrine cells and ganglion cells. As in most neurons, depolarization results in neurotransmitter relax by the bipolar cabinet at the axon terminals. Many bipolar cells relax glutamate, i beg your pardon is excitatory to many ganglion cell (i.e., depolarizes ganglion cells). The amacrine cells may synapse through bipolar cells, other amacrine cell or ganglion cells. The is the axons the the retinal ganglion cell (the 3° intuitive afferents) that departure the eye to type the optic nerve and also deliver visual information to the lateral geniculate cell nucleus of the thalamus and also to various other diencephalic and midbrain structures.
Figure 14.27 An off ganglion cabinet synapses v an off bipolar cell and produces action potentials (i.e., is excited) as soon as the turn off bipolar cell is depolarized (i.e., as soon as the light is off). In contrast, an on ganglion cell the synapses v an on bipolar cabinet reduces the price at which the produces action potentials (i.e., is inhibited) as soon as the top top bipolar cell is hyperpolarized (when the irradiate is off).
Ganglion Cell solution Properties. The retinal ganglion cells space the last retinal facets in the straight pathway indigenous the eye come the brain. Since they must bring visual details some street from the eye, they posses voltage-gated sodium channels in your axonal membranes and generate activity potentials once they room depolarized through the glutamate exit by the bipolar cells.
The off bipolar cell (Figure 14.27, Right) will depolarize when it is dark ~ above its facility cones and will thus release glutamate when it is dark top top the facility of the receptive field. This will result in the depolarization of the retinal ganglion cells with which the turn off bipolar synapses and in the production of action potentials (i.e., discharges) by these ganglion cells (Figure 14.27, Right). Consequently, the retinal ganglion cells the synapse through off bipolar cell will have actually off-center/on-surround receptive fields and are referred to as off ganglion cells.
The on bipolar cabinet (Figure 14.28, Left) will depolarize when there is light on its facility cones and also will as such release glutamate when it is irradiate on the center of that is receptive field. This will result in the depolarization that the retinal ganglion cells through which the top top bipolar synapses and in the production of action potentials (i.e., discharges) by this ganglion cell (Figure 14.28, Left). Consequently, the retinal ganglion cells the synapse through on bipolar cells will have on-center/off-surround receptive fields and are dubbed on ganglion cells.
In short, the receptive fields of the bipolar cells through which the retinal ganglion cabinet synapses determine the receptive field configuration of a retinal ganglion cell.
The retinal ganglion cells provide information important for detecting the shape and movement of objects.
In the primate eye, there are two significant types the retinal ganglion cells, kind M and form P cells, that procedure information around different economic stimulation properties.
Figure 14.28 Left: The ~ above ganglion cabinet synapses through an top top bipolar cell and also produces action potentials (i.e., is excited) once the ~ above bipolar cell is depolarized (i.e., when the irradiate is on). Right: In contrast, an off ganglion cell the synapses through an turn off bipolar cabinet reduces the price at which that produces activity potentials (i.e., is inhibited) once the off bipolar cell is hyperpolarized (when the light is on).
Type ns retinal ganglion cells are color-sensitive object detectors.
The ns ganglion cell(s)outnumber the M-ganglion cells, by roughly 100 come 1 in the primate retina provides synaptic contact with one come a few cone bipolars that room innervated through cone receptor in the macula fovea is shade sensitive has a small concentric receptive ar produce a sustained, slowly adapting response that lasts as lengthy as a stimulus is centered on that is receptive field. produce weak responses to stimuli the move throughout its receptive field.
The gradually adapting response of the form P retinal ganglion cabinet is best suited for signaling the presence, color and duration that a visual stimulus and is negative for signaling economic stimulation movement.
Type M retinal ganglion cells space color-insensitive activity detectors.
The M ganglion cellis much bigger than ns ganglion cell synapses with numerous bipolar cell is shade insensitive has a big concentric receptive ar is more sensitive to little center-surround brightness differences responds with a transient, rapidly adapting response come a kept stimulus. responds maximally, through high discharge rates, to stimuli moving throughout its receptive field.
The swiftly adapting responses of form M ganglion cells are ideal suited for signaling temporal sports in, and the motion of, a stimulus.
The axons of the M and P retinal ganglion cells take trip in the retina optic nerve fiber layer to the optic disc wherein they exit the eye. Many of the axons travel to and terminate in the lateral geniculate cell core of the thalamus.
Amacrine cells synapse through bipolar cells and also ganglion cells and are similar to horizontal cell in providing lateral relations between comparable types that neurons (e.g., they may affix bipolar cell to other bipolar cells)5. They different from horizontal cells, however, in additionally providing ‘’vertical” links in between bipolar and also ganglion cells.
Amacrine cabinet types. There room 20 or more types of amacrine cells based upon their morphology and also neurochemistry. The functions of three varieties have been identified. One formis responsible for creating the activity sensitive (rapidly adapting) an answer of the form M ganglion cells. boosts the center-surround impact in ganglion cell receptive fields. connects stick bipolar cells to cone bipolar cells, thus allowing ganglion cells to respond come the entire range of irradiate levels, indigenous scotopic to photopic.
Convergence that Inputs and also Visual Acuity
Low convergence the cones to cone bipolar cells and also low convergence of cone bipolar cells to P-retinal ganglion cells create high visual acuity in the main visual field.
Recall thatvisual acuity and also color vision are biggest in the main visual field. the picture of the central visual field is projected onto the fovea. the cap are focused in the fovea, whereas the rods predominate in the peripheral retina. there is low convergence of foveal cones top top macular bipolar cells, together low together one cone receptor come one bipolar cell.
In addition, the hat in the fovea space of smaller sized diameter 보다 those in the perimeter of the retina, which permits for a greater packing density of foveal cones. The high packing density of cones and the low convergence that cones ~ above bipolar cell in the macula support higher visual acuity in the main visual field. Consequently, the foveal cones, macular bipolar cells and also the P-retinal ganglion cells are responsible because that photopic, light-adapted vision in the main visual field. In contrast, the higher convergence the the rods onto peripherally located bipolar cells and also of peripheral bipolar cells onto amacrine cells develops the basis for the negative visual acuity yet high irradiate sensitivity of scotopic vision.
14.5 Clinical Manifestations the Retinal Dysfunction
The chemical and also physical integrity of the retina is crucial for normal visual function. Abnormalities in the blood supply and also retinal colors epithelium an outcome in retinal dysfunctions.
Vitamin A deficiency can reason permanent blindness. An adequate supply the photopigments is vital to sustain photoreceptors. The supply of all-trans retinal together a photopigment failure product is inadequate to preserve adequate photopigment production. Vitamin A can be oxidized right into all-trans retinal, and also is, therefore, an important in the synthetic of photopigment. In the eye, that is the retinal pigment epithelium the stores vitamin A. The retinal pigment epithelium is also the website of the redox of vitamin A into all-trans retinal and also conversion that all-trans retinal into 11-cis-retinal. Vitamin A cannot be synthesized through the body and must be ingested. That is uncovered in blood and stored in the liver and also retinal pigment epithelium. Vitamin A deficiency, i beg your pardon can result from liver damages (e.g., indigenous alcoholism or hepatitis), produces degeneration of photoreceptors with visual symptoms first presenting as “night blindness” (i.e., extremely bad vision under short illumination).
Retinitis pigmentosa is one inherited disorder in which there is a gradual and also progressive fail to maintain the receptor cells. One kind involves the production of defective opsin that usually combines with 11-cis retinal to type rhodopsin. Consequently, the rods execute not contain adequate rhodopsin and do not role as the low illumination receptors. A symptom the this condition is “night blindness” and loss of peripheral vision. In this kind of retinitis pigmentosa, the cones receptors duty normally and central vision stays intact. Other develops of retinitis pigmentosa that influence the cones may progress to destroy main vision.
Macular Degeneration. The leading cause of blindness 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 also the immediately 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 visual loss is in the central visual field and the patient will complain of blurred vision and an obstacle reading. Laser surgery is the most typical treatment because that the wet form but has actually the disadvantage of ruining normal retinal cells. It additionally may no be effective in avoiding cell proliferation adhering to treatment.
Retinal detachment. Once the neural retina is torn away from the retinal colours epithelium (e.g., by a blow to the eye), over there is a ns of vision in the area of detachment. The ns of vision results because the neural retina is dependency on the retinal pigment epithelium because that 11-cis retinal, nutrients and photoreceptor integrity. The retinal pigment epithelium supplies glucose and also essential ion to the neural retina, helps support the photoreceptor cell outer segment, clears outer segment disks burned by the receptor cells, and converts retinol and stores vitamin A for photopigment resynthesis. Lasers may be used to weld the offhanded to avoid it from boosting in size. However, the detached and also welded locations are functionally blind.
Diabetic retinopathy. The pathological process in diabetic retinopathy involves microaneurysms and also punctate hemorrhages in the retina. The small swollen blood vessels and/or bleeding in the basic choroid coat damage the receptor cells and also retinal neurons and result in blindness in the areas affected. Lasers may be offered to seal swollen and/or leaking blood vessels.
This chapter described the stimulus (light) properties that are essential for the intuitive perception the our external environment, such together color, brightness, color and brightness contrasts (for kind perception and also visual acuity), visual field representation, binocular combination and depth perception. Remember that there are regional differences in intuitive perception: the main visual field is color-sensitive, has high acuity vision and operates in ~ high level of illumination (i.e., operates through the photopic, light-adapted subsystem). In contrast, the visual ar periphery is much more sensitive at short levels the illumination, is relatively color insensitive and has negative visual acuity (i.e., operates v the scotopic, dark-adapted, subsystem). The chapter also described how the lens system of the eye produces picture on the retina of light emitted by or reflected turn off objects in space. The picture is a smaller, inverted, and reversed picture of the object. Save in mind that the photo projected top top the retina is, in fact, projected ~ above a flattened paper of receptor cells that line the inner surface of the eye. The complying with chapter will explain the function of the visual receptors and also other retinal neurons in converting the visual image into variety of neural activity.
The chapter additionally reviewed the retinal neurons and also the laminar framework of the retina. The image projected onto the retina is spread over a mosaic of photoreceptors. Light power projected onto each photoreceptor is converted into receptor membrane potential changes by a procedure that involves photosensitive pigments and cyclic nucleotide-gated ion channels in the photoreceptor external segment. The phototransduction procedure converts light power into photoreceptor membrane potential transforms that produce a chemical signal (the relax of glutamate), which results in membrane potential alters in the postsynaptic bipolar and horizontal cells. The receptor substrate because that scotopic and photopic vision lies in differences between the rod and also cone receptors.
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In the primate eye, the info gathered by 125 million receptor cell converges on 10 million bipolar cells, which, in turn, converge on 1 million retinal ganglion cells. The degree of convergence from receptor to bipolar cell and bipolar cell to ganglion cell differs regionally in ~ the retina. In the peripheral retina, the convergence deserve to be fifty or much more rod receptors to one bipolar cell, which boosts the sensitivity to dim lights however decreases the spatial acuity that the peripheral bipolar cell. In addition, these peripheral bipolar cells are color insensitive. The M-ganglion cells obtain input from many peripheral bipolar cells, have big receptive fields, are sensitive to small brightness contrasts and also are color insensitive. They also generate transient responses and also are uniquely perceptible to alters in illumination levels and also movement. In contrast, the bipolar cell in the macula synapse with few foveal-cone receptors, which preserve the spatial resolution that the densely packed cones. Such macular bipolar cells have small receptive ar centers, are color sensitive but must run at high illumination levels. Each P-ganglion cell synapse with couple of macular bipolar cells and is color sensitive, yet less perceptible to dim “white” light and to small brightness contrasts. The ns ganglion cells have smaller receptive fields than the M ganglion cells and respond with sustained discharges to maintained stimuli. Together the M ganglion cells and also P ganglion cells respond come different aspects of the intuitive stimulus, lock are defined to be encoding and carrying independent, parallel, streams (M-stream and also P-stream) of information around stimulus size, color, and also movement.