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