Chapter 14: visual Processing: Eye and also Retina
Valentin Dragoi, Ph.D., department of Neurobiology and also Anatomy, The UT clinical School in ~ Houston (content detailed 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 processing of outside stimuli. The chapter will certainly familiarize girlfriend with procedures of visual sensation by discussing the communication of form perception, intuitive acuity, visual field representation, binocular fusion, and depth perception. An essential 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 periphery is more sensitive at short levels the illumination, is fairly color insensitive, and also has poor visual acuity. Friend will find out that the image is an initial projected ~ above a flattened sheet of photoreceptor cells that lie on the inner surface of the eye (retina). The info gathered by millions of receptor cell is projected next onto numerous bipolar cells, which, in turn, send projects to retinal ganglion cells. These cells encode different facets of the visual stimulus, and also thus lug independent, parallel, streams that information about stimulus size, color, and also movement come the intuitive thalamus.
14.1 steps of visual Sensation
The problem of the intuitive system deserve to be determined by examining various aspects of intuitive sensation. Because that example, the capability to detect and also identify small objects (i.e., intuitive acuity) deserve to be impacted by disorders in the transparent media the the eye and/or visual nervous system. The inability to recognize objects in specific areas of space (i.e., visual ar defects) is often related come neural damage.
You are watching: Receptor cells in the retina responsible for color vision and fine acuity are
Spatial Orientation and the intuitive Field
The visual ar is the area in space perceived once the eyes space in a fixed, static place looking right ahead.
Figure 14.1 The monocular visual ar is the area in an are visible to one eye. Together illustrated, the nose prevents the ar of the appropriate eye from extending 180 levels in the horizontal plane. Inset. Perimetry trial and error provides a in-depth map the the intuitive field. Together the nose, brow and also cheeks occlude the see of the most nasal, superior and also inferior areas, respectively, the result monocular visual ar occupies a limited portion (colored blue) the the potential visual space.
The monocular visual field (Figure 14.1)is that area of room visible to one eye deserve to be mapped parametrically Perimetry testing provides a comprehensive map the the visual field. The potential visual ar is explained as a hemisphere. However, it does not type a perfect hemisphere together the brow, nose and cheekbones obscure the see - most prominently in the nasal hemisphere is subdivided right into two halves, the hemifields (Figure 14.1 Inset). A horizontal line drawn from 0° come 180° through facility of the field specifies the remarkable & inferior hemifields. A vertical line attracted from 90° to 270° with center point defines the left & right hemifields, i m sorry are frequently termed the nasal and also temporal hemifields. might be additional subdivided right into quadrants: the superior and also inferior sleep quadrants the superior and also inferior temporal quadrants. consists of a blind spot, a little area in i m sorry objects can not be viewed which is located within the temporal hemifield.
Figure 14.2 The binocular visual field. Together our eyes room angled slightly towards the nose, the monocular visual areas of the left and also right eye overlap to form the binocular visual field (colored red). Objects in ~ the binocular visual field are visible to every eye, albeit from various angles.
The monocular visual ar (Figure 14.1) is established with one eye covered. The area that overlap that the visual ar of one eye through that of the contrary eye is dubbed the binocular field (Figure 14.2). All areas of the binocular visual field are “seen” by both eyes.
The capability to situate objects in an are and the capacity to orient ourselves through respect to exterior objects are dependent top top the representation of visual an are within the nervous system. The clinical check of the visual fields most commonly used is the confrontation ar test. It specifies the outer borders of our spatu visual space. Neurological disorders of the visual device can regularly be localized based upon the area the blindness within the visual field.
Visual acuity is the capacity to detect and also recognize little objects visually depends on the refractory (focusing) strength of the eye"s lens system and also the cytoarchitecture the the retina.
Visual acuity ismeasure under high illumination the smallest dimension of a dark thing in a light background that have the right to be correctly identified
In the clinical setting, an eye graphis provided to measure up the patient’s visual acuity. is composed of rows of black letters ~ above a shining white background. is supplied to measure up visual acuity in ~ a street of 20 ft indigenous the chart. reports intuitive acuity together the ratio of the eye chart street (i.e., 20 ft) come the “normal distance” that the lowest heat of letters correctly identified by the patient (e.g., row 3, i beg your pardon is 70 ft).
Color vision is the capacity to detect differences in the wavelengths of irradiate is called color vision. Clinically it may be tested through an Ishihara chart: a chart through spots of different colors that space spatially arranged to type numbers the differ for ``normal” and also color-blind eyes.
As stated above, the human has a trichromatic visual system, whereby visible colors have the right to be created by a mixture the red, green and blue lights. The most common form of color blindness results in a man of red and also green shades (i.e., red-green shade blindness). Most situations of shade blindness result from an absent or defective gene responsible for creating the red or eco-friendly photopigment (protanopia, the absence of red; and deuteranopia, the absence of green). Together these gene are situated on the X chromosome, color blindness is more common in males than in females.
Figure 14.3 LEFT. The visual ar of the left eye is mapped parametrically. The dark period in the temporal hemifield to represent the "blind spot" where nothing is seen. RIGHT. Intuitive acuity is plotted together a function of distance (in degrees) indigenous the center of the intuitive field. The curve labeling "Light-adapted" was obtained under photopic illumination levels and also the curve labeling "Dark-adapted" was derived under scotopic illumination levels.
Regional differences: over there are local differences in color sensation, visual acuity and also low-illumination sensitivity in ~ the visual ar (Figure 14.3).
A tiny “blindspot” islocated in the temporal hemifield (Figure 14.3 Left) whereby objects can not be seen.
Vision in the visual ar centeroperates best under high illumination. has actually the greatest 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 visual fieldis an ext sensitive come dim irradiate operates under short illumination. has tiny color sensitivity and also poor spatial acuity (Figure 14.3 Right) represents the procedure of the scotopic (dark-adapted) subsystem
Binocular blend and Depth Perception
Figure 14.4 The 2 eyes fixated on things view the object and objects in the background at slightly various angles. Consequently, the photos on the two retinas space slightly different and also must it is in "fused" through the intuitive system. The disparity in the retinal photos at the 2 eyes additionally provides binocular cues because that depth perception.
When a pencil is hosted an arm’s length away through both eye open, most people will view a single object and also recognize it as a pencil. However, if one quickly closes each eye alternately (i.e., left eye closed, ideal eye opened, then right eye opened and left eye closed); you have to see the pencil “jumping” indigenous left to best as you alternating the eye closure. This is so due to the fact that the image in every eye is slightly different (disparate): notification that since each eye is situated on either side of the nose, the viewing angle of each eye is slightly various - particularly when viewing close to objects (Figure 14.4).
back the area in an are defined by the binocular visual ar (Figure 14.4) represents corresponding locations of the monocular intuitive fields, the angle at i m sorry this an are is perceived by each eye is contempt different. Consequently, the pictures of the corresponding (binocular) space are slightly various in every eye. The nervous system fuses this disparate binocular photos to create a single image (e.g., the the pencil situated an arm’s size away). The process of creating a single image from the 2 disparate monocular photos is called binocular fusion.
Clinically, binocular combination is experiment by holding up one or 2 fingers in former of the patient and also asking the patient (who need to be put on corrective lenses if castle are normally worn) how numerous fingers lock see. If the patient reports seeing four fingers as soon as only two space presented, the patience is can not to create binocular fusion.
Binocular blend permits the tardy a solitary clear photo and likewise provides extra cues for depth perception. That is, the binocular disparity between the two images is used by the nervous system to enable the tardy of a three-dimensional people where the approximate distance of things can it is in determined. The nervous system cannot fuse disparate binocular photos when the disparity is also great. When corresponding areas of the typical binocular visual ar are no in alignment (e.g., in strabismus where one eye deviates indigenous the normal position and/or is paralyzed), the nervous device cannot fuse the different images and also gradually adapts by “ignoring” the photo from the deviant eye. In fact, strabismus at birth, if uncorrected, may an outcome in a kind of central blindness, amblyopia, where the photo from the deviant eye is no much longer represented in ~ cortical levels of the nervous system. The uncorrected, irreversible amblyope is functionally blind in one eye and has poor depth perception.
14.2 The picture Forming process
The transparent media the the eye role as a biconvex lens that refracts irradiate entering the eye and focuses photos of the external world onto the light perceptible retina.
Recall the light rays will bend once passing from one transparent tool into an additional if the speed of irradiate differs in the 2 media. However, parallel irradiate rays will pass indigenous air v a transparent human body (e.g., flat lens) there is no bending if the light rays space perpendicular to the lens surface (Figure 14.5, left). If the light strikes the lens surface at an angle, the light rays will certainly be bent in a heat perpendicular come the lens surface (Figure 14.5, right).
Figure 14.5 The food of light rays passing v a transparent lens are illustrated. LEFT: The irradiate rays space entering perpendicular to the surface ar of the lens. RIGHT: The irradiate rays room entering at an edge to the surface of the lens and are gift refracted by the lens.
A biconvex lens, i m sorry is functionally comparable to the eye"s lens system, is flat only at its center. The surface of the area surrounding the center is curved and also not perpendicular come parallel light rays (Figure 14.6). Consequently, the bent surfaces of a biconvex lens will certainly bend parallel irradiate rays come focus photo of the thing emitting the light a quick distance behind the lens at its focal length point. The image developed is clear just if the curvature the the lens is symmetry in every meridians and also all divergent irradiate rays emitted through a point source converge in ~ the focal length point.
Figure 14.6 The irradiate rays emanating from a point resource take divergent courses that get in a biconvex lens at different points along the lens surface. The lens refracts the irradiate rays pass them with each other at the focal allude some distance from the lens.
Figure 14.7 The eye"s lens system attributes like a biconvex lens and focuses an image on the retina the is inverted, left-right reversed and also smaller 보다 the thing viewed.
note that the better the curvature that the lens surface the higher is the refractive power and 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 mechanism is smaller than the thing viewed, turning back (upside-down, number 14.6), and also reversed (right-left, number 14.7). Together the photo is turning back by the lens system, the exceptional (top) fifty percent of each eye’s visual ar is projected ~ above the worse (bottom) half of every eye’s retina. Also, as the lens to produce a reversed image, the temporal half of every visual field is projected top top the nasal half of each eye’s retina1. Therefore, the temporal (left) hemifield that the left eye is projected ~ above the sleep (right) half of the left eye’s retina and the nasal (left) hemifield of best eye is projected onto temporal (right) half of the appropriate eye’s retina. Consequently, the left hemifields that both eyes space projected onto the corresponding (right) halves of the 2 retinas. The is an essential that you know the relationship in between the intuitive field and also the retinal areas and also realize that equivalent halves the the 2 monocular visual areas are imaged on corresponding halves of the two retinas. These relationships form the neurological basis for expertise visual field defects.
The eye should be may be to readjust its refractive properties to focus images of both far-off and surrounding objects on the retina. Distant objects (greater than 30 feet or 9 meters away from the eye) emit or reflect irradiate that deserve to be focused on the retina in a normal peaceful eye (Figure 14.8).
Figure 14.8 The normal eye at remainder can focus on the retina images of objects more than 30 ft native the eye. When things is lugged closer to the eye (i.e., much less than 30 ft indigenous the eye), the light rays from the thing take an ext divergent paths and also each start the cornea v a higher angle of incidence. Consequently, the picture focal suggest would be past the retina if the eye"s lens device were no adjusted. Throughout accommodation, the lens curvature increases, increasing the refractive power of the eye and focusing the photo on the retina.
If a regarded object is carried closer come the eye, the irradiate rays indigenous the thing diverge in ~ a higher angle family member to the eye (Figure 14.8). Consequently, the nearer the thing of view, the greater the edge of incidence of light rays on the cornea, and the better the refractive power compelled to focus the light rays ~ above the retina. The cornea has actually a addressed refractive power (i.e. The cannot readjust its shape). However, altering the anxiety of the zonules top top the elastic lens capsule can transform the lens shape. The adjust in the refractive properties of the eye is called the accommodation or "near point" process.
In the common eye under resting (distant vision) conditions, the ciliary muscles are relaxed and the zonules room under anxiety (Figure 14.9). In this case, the lens is flattened, which reduces the refractive strength of the lens to focus on distant objects. When an item is closer to the eye (i.e., much 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 as a sphincter). This activity releases anxiety on the zonules and also the lens capsule. The reduced tension enables the lens come become an ext spherical (i.e., increase its curvature). The increase in lens curvature rises the lens refractive strength to focus on near objects. Consequently, as an object is moved closer come the viewer, his eyes accommodate to increase the lens curvature, which rises the refractive strength of his eye (Figure 14.8).
Figure 14.9 throughout distance vision (i.e., through the eye in ~ rest), the ciliary muscles are relaxed and also the zonules room under tension. The lens is planarization by the stress on the zonules and the lens capsule. However, in the accommodation process, the ciliary muscle contract and, acting favor a sphincter muscle, decrease the stress and anxiety on the zonules and also lens capsule. The lens becomes an ext spherical through its anterior surface shifting much more anteriorly right into the anterior chamber.
Refractive Errors the the Eye and also Corrective Lenses
Presbyopia: In presbyopia, there is normal distance vision, however lens house is reduced with age. V age, the lens loser its elasticity and becomes a reasonably solid mass. During accommodation, the lens is unable to i think a much more spherical shape and is unable to increase its refractive power for near vision (Figure 14.10). Together a result, when an item is less than 30 ft. Far from the presbyopic viewer, the photo is focused somewhere behind the retina.
Figure 14.10 In the presbyopic eye, as soon as the object is moved closer to the eye, the lens is unable to accommodate and also the picture is focused past the retina. For the presbyopic eye a corrective lens the converges the light rays (i.e., a convex lens the reduces the edge of incidence of irradiate on the cornea) will allow the presbyopic eye to view adjacent objects.
A convex lens (i.e., increased refractive power) is used to correct the presbyopic eye (Figure 14.10). These lenses refract the light rays so they to win the surface of the cornea in ~ a smaller sized angle. However, since the corrective lens boosts the refractive power, the presbyope with convex lenses will certainly have troubles with distance vision. Consequently, the corrective lenses room often half lenses (i.e., reading glasses) which enable the presbyope to view objects in the street unimpeded by the convex lens.
Hyperopia: In hyperopia (Figure 14.11), the refractive power of the eye’s lens system is also weak or the eyeball also short. As soon as viewing distant objects, the image is concentrated at a point beyond the retina.
Figure 14.11 The hyperopic eye at remainder cannot emphasis on the retina the picture of an object more than 30 ft from the eye. The hyperopic lens device is also weak and the image is focused beyond the retina.
The young hyperope can compensate by using lens accommodation, i.e., rise the refractive power of the eye’s lens system (Figure 14.12). We speak to the hyperope "far-sighted" (hypermetropic) because the strength of accommodation supplied for distance vision cannot be offered for near vision.
Figure 14.12 If the hyperopia is not severe; the hyperopic eye deserve to use the lens accommodation process to increase the refractive power of the eye for distance vision.
as the hyperope ages and becomes presbyopic, the strength of house is diminished. Consequently, the middle aged hyperope may have actually a limited range (near and also 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 mechanism is too strong or the eyeball too long. As soon as viewing far-off objects, the image is focused at a allude in former of retina.
Figure 14.13 The myopic eye at rest cannot focus on the retina the image of an object much more than 30 ft. Native the eye. The refractive strength of the eye"s lens system is too solid and the picture is concentrated in prior of the retina.
The uncorrected myopic eye is "near-sighted" because it can focus unaided on close to objects. The is, the young myope will see distant objects as blurred, poorly defined images but can view nearby tiny objects plainly (remember nearby objects emit divergent irradiate rays).
For street vision, the refractive power of the myopic eye lens device is corrected v concave lenses the diverge the irradiate rays start the eye (Figure 14.14). Note that as the strength of accommodation diminishes through age, close to vision is also influenced in the presbyopic-myopic eye. The tires myope might require bifocals, the upper half of the lens diverging irradiate rays for distance vision and also the lower half with no or low converging power for close to vision.
Figure 14.14 A corrective lens the diverges irradiate rays before they get in the eye (i.e., a concave lens) will allow the myopic eye to focus the image of a distant object ~ above the retina.
Astigmatism: an astigmatism results when the cornea surface does not resemble the surface of a ball (e.g. Is much more oblong). In an eye with astigmatism, the image of distant and also near objects cannot be focused on the retina (Figure 14.15). Astigmatism is corrected v a cylindrical lens having a curvature that corrects for the corneal astigmatism. The cylindrical lens directs light waves v the astigmatic cornea to focus a single, clear picture on the retina.
Figure 14.15 The astigmatic lens is asymmetrical and has multiple focal distance points, i m sorry 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 means in i m sorry the light-sensitive receptors of the eye convert the picture projected top top the retina into neural responses. The light sensitive retina forms the innermost great of the eye (Figure 14.16).
Figure 14.16 The eye, the three coats that the eye and the great of the retina. The retina is the innermost coat of the eye and consists that the retinal colours epithelium and neural retina.
The retina consist of the choroid and also extends anteriorly to just behind the ciliary body. The retina is composed of neurons and supporting cells.
Components the the Retina
The retina is acquired from the neural tube and is, therefore, part of central nervous system. It is composed of two parts, the retinal colours epithelium, i beg your pardon separates the middle, choroid coat of the eyeball from the various other innermost component and also the neural retina (Figure 14.16) – the dark pigments within the retinal colours epithelium and also choroid coat role to absorb light passing with the receptor layer, therefore reducing light scatter and also image distortion within the eye. The neural retina consists of five varieties of neurons (Figure 14.17): the intuitive 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 framework consisting of alternative layers of cell bodies and cell processes (Figure 14.18).
Figure 14.17 The components of the neural retina. The neural retina consists of in ~ least five 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 created by alternate layers that neuron cabinet bodies that appear dark and also neuron processes that appear light in Nissl stained tissue. The receptor cells synapse v bipolar and horizontal cells in the outer plexiform layer. The bipolar cells, in turn, synapse with amacrine and also ganglion cells in the within plexiform layer The axons the the retinal ganglion cells departure the eye to type the optic nerve.
The innermost class are situated nearest the vitreous chamber, whereas the outermost layers room located adjacent to the retinal colours epithelium and choroid. The most important layers, progressing from the outer to inner layers, are:the retinal colors epithelium, i m sorry provides crucial metabolic and also supportive functions to the photoreceptors; the receptor layer, which consists of the irradiate sensitive external segments of the photoreceptors; the outer atom layer, which includes the photoreceptor cell bodies; the outer plexiform layer, where the photoreceptor, horizontal and also bipolar cells synapse; the inner nuclear layer, which has the horizontal, bipolar and amacrine cabinet bodies; the inner plexiform layer, wherein the bipolar, amacrine and retinal ganglion cell synapse; the retinal ganglion cell layer, which consists of the retinal ganglion cell bodies; and also the optic nerve layer, which consists of the ganglion cell axons travel to the optic disc.
Notice the light passing through the cornea, lens and vitreous should pass through most of the retinal layers prior to reaching the light-sensitive portion of the photoreceptor; the external segment in the receptor layer. Notice also that in the an ar of the fovea whereby the photo of the main visual field facility is focused, the retina is composed of fewer layers (Figure 14.19): thereby minimizing the obstacles to developing a clear picture on the fovea. The area approximately the fovea, the neighboring macula, is thicker since it contains the cabinet bodies and processes of retinal neurons receiving information from the receptors in the fovea.
The optic key is developed by the retinal ganglion cabinet axons that room exiting the retina. It is located nasal come the fovea (Figure 14.19). This region of the retina is devoid of receptor cells and composed primarily by the optic nerve layer. Consequently, that is the structure basis because that the "blind spot" in the visual field.
Figure 14.19 The fovea that the retina and the class of the retina in the neighboring macula. The fovea and also macula space colored together they show up when stained because that Nissl substance, i beg your pardon is most abundant in the neuron cabinet body.
The human has two types of photoreceptors: the rods and cones (Figure 14.20). Lock are differentiated structurally through the forms of their external segments. The photopigments of the rods and also cones also differ. The rod outer segment disks save the photopigment rhodopsin, i m sorry absorbs a broad bandwidth the light. The cones different in the color of light your photopigments absorbs: one type of photopigment absorbs red light, an additional green light, and also a third blue light. As each cone receptor consists of only one of the three species of cone photopigment, there room three types of cones; red, environment-friendly or blue. Each cone responds ideal to a specific color the light, whereas the rods respond best to white light2. The rod and cone photopigments additionally differ in illumination sensitivity; rhodopsin breaks down at lower light levels than that compelled to malfunction cone photopigments. Consequently, the rods are more sensitive - at least at low levels that illumination.
14.4 Rods and Cones form the Basis for Scotopic and also Photopic Vision
The human visual system has actually two subsystems that run at various light power levels. The scotopic, dark-adapted system operates at low levels of illumination, whereas 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 a dendritic component (the external segment) and also an axonal ingredient that forms synaptic terminals.
Rods room responsible because that the initiation that the scotopic intuitive process. Rodssave the photopigment rhodopsin, which breaks down when exposed to a wide bandwidth of light (i.e., that is achromatic). Rhodopsin is also much more sensitive come light and reacts at reduced light levels than the color sensitive (chromatic) cone pigments. have longer external segments, an ext outer segment disks and, consequently, contain much more photopigment. are much more sensitive to light and function at scotopic (low) levels of illumination. conquer in the peripheral retina (Figure 14.21A), i beg your pardon is shade insensitive, has negative acuity (Figure 14.21B), however is sensitive to short levels the illumination.
Cones are responsible for the initiation that the photopic intuitive process. Hatsave on computer photopigments that malfunction in the existence of a restricted bandwidth of irradiate (i.e., cone photopigments space chromatic). are color sensitive. are less sensitive to light and also require high (daylight) illumination levels. are focused in the fovea (Figure 14.21A) in the fovea have image that the main visual ar projected on them. in the fovea space responsible for photopic, light-adapted vision (i.e., high visual acuity and also color vision) in the main visual field (Figure 14.21B)
Figure 14.21 The rods, are taller, have longer outer segments and, consequently, contain more outer segment disks and an ext photopigment 보다 cones. Cone receptor are concentrated in the fovea that the eye (at 0° eccentricity), conversely, rod receptors are concentrated in more peripheral retina (A). Intuitive acuity is maximal in the central area the the visual field (at 0° eccentricity), whereas that is minimal in an ext peripheral areas (B). Notification that the place of the optic disc relative to the fovea coincides to the place of the blind spot loved one to the visual ar center.
Biochemical processes in the photoreceptors get involved in dark and light adaptation. An alert when you go into a darkened room after ~ spending time in daylight, it takes countless minutes before you space able to watch objects in the dim light. This slow boost in light sensitivity is dubbed the dark-adaptation procedure and is concerned the price of rejuvenation of photopigments and to the intracellular concentration of calcium3. A contrasting, however faster, procedure occurs in high levels of illumination. Once you are fully dark-adapted, exposure to bright light is at very first blinding (massive photopigment breakdown and stimulation the photoreceptors) and also is adhered to rapidly by a return of sight. This phenomenon, light adaptation, allows the cone response to conquer over pole responses in ~ high illumination.
14.5 Visual processing in the Retina
The photoreceptors exhibit a fairly high basal relax of glutamate. When light strikes the photoreceptor cell, it initiates a biochemical procedure in the cell the reduces the relax of glutamate native its axon terminal. The glutamate, in turn, affect the activity of the bipolar and horizontal cells, i beg your pardon synapse through the photoreceptor. The bipolar cells, in turn, synapse through amacrine and also retinal ganglion cells. That is the axons of the retinal ganglion cells that departure the eye as the optic nerve and terminate in the brain. An alert that the straight pathway because that the transmission of visual details from the eye to the brain includes only the receptor cell, bipolar cell and ganglion cell. The horizontal cells 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 ganglion cells, in order to affecting the infection of visual details by the ganglion cells.
Within the outer plexiform class of the retina, around 125 million photoreceptor cell synapse with roughly 10 million bipolar cells. A smaller variety of horizontal cells likewise synapse v the photoreceptor cells within the external plexiform great of the retina. The bipolar and also horizontal cell respond come the glutamate released by the photoreceptor cells4.Bipolar cell do not generate activity potentials. respond come the release of glutamate native photoreceptors with graded potentials (i.e., through hyperpolarizing or depolarizing).
Bipolar cells differ based on their responses come photoreceptor stimulation.There room at the very least two varieties of bipolar cells based on their responses come glutamate. The off bipolar cells space depolarized through glutamate. The on bipolar cells space hyperpolarized through glutamate. The 2 bipolar cell types 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 condition that produce a depolarizing solution from a bipolar cabinet is offered to surname the bipolar cabinet type.an off bipolar cabinet depolarizes when the photoreceptors the synapse v it are in the dark (i.e., when the irradiate is off, number 14.22). one on bipolar cell depolarizes as soon as the photoreceptors the synapse with space in the light (i.e., as soon as the irradiate is on, number 14.22). Note that the depolarization of the on bipolar cell does not an outcome from excitation the the presynaptic cell however rather native a palliation of the inhibitory action of glutamate created by the light-induced lessened release of glutamate indigenous the photoreceptor.
Figure 14.22 once the receptor cells v which an off bipolar cabinet synapses room in the dark, the turn off bipolar cell is depolarized and also the top top bipolar cell is hyperpolarized. In contrast, once the receptor cells through which an turn off bipolar cell synapses room in the light, the off bipolar cell is hyperpolarized and the ~ above bipolar cabinet is depolarized.
Bipolar cell Receptive Field: The receptive ar of a bipolar cabinet is characterized anatomically by the location and also distribution of receptor cells with which it provides synaptic contact.Each cone-bipolar cell makes direct synaptic contact with a circumscribed patch of cone receptors, which may be as few as one foveal cone. Consequently, the receptive fields of bipolar cells synapsing with cones in the fovea room extremely small 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. every rod-bipolar cell may make synaptic contact with a couple of to fifty or an ext of stick receptor cells. Consequently, the rod-bipolar cell receptive ar is relatively large and color insensitive. Every rod-bipolar cells room hyperpolarized through glutamate and, consequently, are on-type bipolar cells exclusively.
The bipolar cell 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 ~ above the photoreceptor(s) the synapse with a bipolar cell produces a an answer from the bipolar cell dubbed the center solution (Figure 14.23). In contrast, light directed on automatically surrounding receptors produce the opposite an answer (Figure 14.24).
Figure 14.23 Bipolar cells have actually concentric receptive fields. The on bipolar cabinet depolarizes as soon as the receptor cells v which that synapses room illuminated ("Light On"). These facility receptors (i.e., the persons making direct synaptic contact with the bipolar cell) develop the bipolar cell center response.
Figure 14.24 Bipolar cells have concentric receptive fields. When the receptors surrounding the facility receptors that the ~ above bipolar receptive field are illuminated ("Light On") and the center receptors retained in the dark, the top top bipolar cabinet is hyperpolarized.
When both the center and surrounding receptor cells are illuminated v light, the on bipolar cell solution to stimulation the the center receptors is decreased by stimulation the the surround receptors (Figure 14.25).
Figure 14.25 Bipolar cells have actually concentric receptive fields. As soon as both the center and also surrounding receptor of the ~ above bipolar cell receptive ar are illuminated, the top top bipolar cell depolarizes. However, the size of the depolarization is reduced to much less than the depolarization come illumination of only the center receptors.
Consequently, the strongest on bipolar cell solution is developed when the stimulus is a light spot encircled by a dark ring. Because that the off bipolar cell, a dark clues encircled by a irradiate ring produces maximal depolarization.
Within the outer plexiform layer, the photoreceptor cells make both presynaptic and postsynaptic contact with horizontal cells.The horizontal cells have big receptive fields involving presynaptic (axonal) contact with a tiny group of photoreceptors and postsynaptic (dendritic) call with a larger team of neighboring photoreceptor cells.
By regulating the responses of your “center” photoreceptors (based on the responses of the bordering photoreceptors), the horizontal cells indirectly develop the bipolar cabinet receptive field surround effect. The surround effect created by the horizontal cell is weaker than the center effect.
Figure 14.26 The horizontal cells make presynaptic and also postsynaptic contact with photoreceptor cells. The axon terminals of a horizontal cell receives synaptic contact from one group of photoreceptors (colored red) and its procedures make synaptic call with surrounding photoreceptor cells (colored green).
The surround effect, produced by the horizontal cells, enhances brightness contrasts to create sharper images, to make an object appear brighter or darker depending upon the background and also to maintain these contrasts under various illumination levels.
Retinal Ganglion Cells
Within the inner plexiform layer, the axon terminals of bipolar cells (the 2° visual afferents) synapse ~ above the dendritic processes of amacrine cells and ganglion cells. Together in most neurons, depolarization results in neurotransmitter relax by the bipolar cell at the axon terminals. Many bipolar cells relax glutamate, which is excitatory to many ganglion cell (i.e., depolarizes ganglion cells). The amacrine cells might synapse through bipolar cells, various other amacrine cells or ganglion cells. It is the axons of the retinal ganglion cells (the 3° intuitive afferents) that departure the eye to kind the optic nerve and also deliver visual details to the lateral geniculate nucleus of the thalamus and to other diencephalic and also midbrain structures.
Figure 14.27 An off ganglion cabinet synapses through an off bipolar cell and also produces action potentials (i.e., is excited) as soon as the turn off bipolar cell is depolarized (i.e., once the irradiate is off). In contrast, one on ganglion cell that synapses v an on bipolar cabinet reduces the price at which it produces activity potentials (i.e., is inhibited) when the top top bipolar cabinet is hyperpolarized (when the light is off).
Ganglion Cell solution Properties. The retinal ganglion cells room the final retinal elements in the straight pathway native the eye to the brain. Since they must lug visual information some street from the eye, they posses voltage-gated sodium networks in your axonal membranes and generate action potentials when they room depolarized by the glutamate released by the bipolar cells.
The off bipolar cell (Figure 14.27, Right) will depolarize as soon as it is dark top top its center cones and will because of this release glutamate once it is dark top top the facility of that receptive field. This will result in the depolarization that the retinal ganglion cells v which the turn off bipolar synapses and also in the production of action potentials (i.e., discharges) by these ganglion cells (Figure 14.27, Right). Consequently, the retinal ganglion cells the synapse v off bipolar cells will have off-center/on-surround receptive fields and are dubbed off ganglion cells.
The on bipolar cabinet (Figure 14.28, Left) will certainly depolarize when there is irradiate on its facility cones and will thus release glutamate once it is light on the facility of that receptive field. This will result in the depolarization of the retinal ganglion cells through which the ~ above bipolar synapses and also in the manufacturing of activity potentials (i.e., discharges) by this ganglion cells (Figure 14.28, Left). Consequently, the retinal ganglion cells that synapse v on bipolar cells will have actually 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 cell synapses determine the receptive ar configuration that a retinal ganglion cell.
The retinal ganglion cells provide information vital for detecting the shape and also movement that objects.
In the primate eye, there room two major types of retinal ganglion cells, type M and kind P cells, that procedure information about different stimulus properties.
Figure 14.28 Left: The ~ above ganglion cabinet synapses with an top top bipolar cell and produces activity potentials (i.e., is excited) when the on bipolar cabinet is depolarized (i.e., as soon as the light is on). Right: In contrast, an turn off ganglion cell that synapses v an off bipolar cell reduces the price at which that produces activity potentials (i.e., is inhibited) when the off bipolar cell is hyperpolarized (when the irradiate is on).
Type p retinal ganglion cells room color-sensitive thing detectors.
The ns ganglion cell(s)outnumber the M-ganglion cells, by about 100 come 1 in the primate retina provides synaptic call with one come a couple of cone bipolars that room innervated by cone receptor in the macula fovea is shade sensitive has a tiny concentric receptive ar to produce a sustained, slowly adapting response the lasts as long as a economic stimulation is focused on that receptive field. to produce weak responses to stimuli that move throughout its receptive field.
The progressively adapting an answer of the kind P retinal ganglion cabinet is finest suited because that signaling the presence, color and duration that a intuitive stimulus and also is negative for signaling economic stimulation movement.
Type M retinal ganglion cells are color-insensitive movement detectors.
The M ganglion cabinetis much larger than ns ganglion cells synapse with many bipolar cell is shade insensitive has a huge concentric receptive ar is much more sensitive to tiny center-surround brightness distinctions responds v a transient, rapidly adapting response to a maintained stimulus. responds maximally, through high discharge rates, to stimuli moving across its receptive field.
The quickly adapting responses of type M ganglion cells are finest suited because that signaling temporal sport in, and also the motion of, a stimulus.
The axons of the M and also P retinal ganglion cells travel in the retina optic nerve fiber layer come the optic disc whereby they departure the eye. Many of the axons travel to and terminate in the lateral geniculate cell nucleus of the thalamus.
Amacrine cells synapse with bipolar cells and also ganglion cells and are comparable to horizontal cell in providing lateral relations between comparable types that neurons (e.g., they may connect bipolar cells to other bipolar cells)5. They different from horizontal cells, however, in additionally providing ‘’vertical” links between bipolar and ganglion cells.
Amacrine cabinet types. There are 20 or an ext types that amacrine cells based on their morphology and also neurochemistry. The roles of three varieties have to be identified. One formis responsible for creating the motion sensitive (rapidly adapting) an answer of the form M ganglion cells. boosts the center-surround result in ganglion cell receptive fields. connects stick bipolar cell to cone bipolar cells, thus allowing ganglion cell to respond to the entire range of light levels, native scotopic to photopic.
Convergence that Inputs and Visual Acuity
Low convergence the cones come cone bipolar cells and also low convergence the cone bipolar cell to P-retinal ganglion cells develop high intuitive acuity in the main visual field.
Recall thatvisual acuity and also color vision are greatest in the main visual field. the image of the main visual field is projected onto the fovea. the cones are focused in the fovea, whereas the rods predominate in the peripheral retina. over there is low convergence the foveal cones onto macular bipolar cells, as low together one cone receptor come one bipolar cell.
In addition, the hat in the fovea space of smaller sized diameter than those in the perimeter of the retina, which enables for a better packing density of foveal cones. The high packing thickness of cones and also the low convergence that cones top top bipolar cells in the macula support greater visual acuity in the main visual field. Consequently, the foveal cones, macular bipolar cells and the P-retinal ganglion cells room responsible for photopic, light-adapted vision in the central visual field. In contrast, the greater convergence that the rods ~ above peripherally situated bipolar cells and also of peripheral bipolar cells onto amacrine cells forms the basis because that the poor visual acuity however high irradiate sensitivity the scotopic vision.
14.5 Clinical Manifestations the Retinal Dysfunction
The chemical and also physical traltoalsimce.org of the retina is crucial for normal visual function. Abnormalities in the blood supply and retinal pigment epithelium result in retinal dysfunctions.
Vitamin A deficiency can reason permanent blindness. An adequate supply that photopigments is crucial to sustain photoreceptors. The supply of all-trans retinal as a photopigment malfunction product is poor to keep adequate photopigment production. Vitamin A have the right to be oxidized right into all-trans retinal, and is, therefore, an important in the synthesis of photopigment. In the eye, the is the retinal colours epithelium the stores vitamin A. The retinal colours epithelium is additionally the website of the oxidation of vitamin A into all-trans retinal and conversion the all-trans retinal into 11-cis-retinal. Vitamin A can not be synthesized by the body and also must it is in ingested. That is uncovered in blood and stored in the liver and also retinal colours epithelium. Vitamin A deficiency, i beg your pardon can result from liver damage (e.g., indigenous alcoholism or hepatitis), to produce degeneration of photoreceptors with visual symptoms an initial presenting as “night blindness” (i.e., extremely poor vision under low illumination).
Retinitis pigmentosa is an inherited disorder in which over there is a gradual and progressive failure to maintain the receptor cells. One form involves the production of defective opsin that generally combines through 11-cis retinal to kind rhodopsin. Consequently, the rods carry out not contain adequate rhodopsin and do not function as the low illumination receptors. A symptom of this condition is “night blindness” and also loss that peripheral vision. In this kind of retinitis pigmentosa, the hat receptors duty normally and central vision continues to be intact. Other develops of retinitis pigmentosa that impact the cones may progress come destroy central vision.
Macular Degeneration. The leading reason of remote in the yonsi is age-related macular degeneration. The dry form that macular degeneration involves intraocular proliferation of cells in the macular area (i.e., in the fovea and the automatically surrounding retinal areas). In the wet form the macular degeneration, the capillaries of the choroid coat attack the macular area and destroy receptor cells and also neurons. In both forms, the intuitive loss is in the main visual field and the patient will complain the blurred vision and an obstacle reading. Laser surgical treatment is the most common treatment for the wet type but has actually the disadvantage of ruining normal retinal cells. It additionally may not be effective in avoiding cell proliferation complying with treatment.
Retinal detachment. Once the neural retina is torn far from the retinal pigment epithelium (e.g., by a punch to the eye), over there is a ns of vision in the area of detachment. The ns of vision results since the neural retina is dependency on the retinal pigment epithelium because that 11-cis retinal, nutrients and also photoreceptor integrity. The retinal pigment epithelium provides glucose and also essential ions to the neural retina, helps support the photoreceptor cell outer segment, clears outer segment disks shed by the receptor cells, and converts retinol and stores vitamin A because that photopigment resynthesis. Lasers might be offered to weld the detachment to avoid it from enhancing in size. However, the detached and welded areas are functionally blind.
Diabetic retinopathy. The pathological procedure in diabetic retinopathy requires microaneurysms and punctate hemorrhages in the retina. The tiny swollen blood vessels and/or bleeding in the basic choroid coat damage the receptor cells and retinal neurons and result in blindness in the regions affected. Lasers may be supplied to seal swollen and/or leaking blood vessels.
This chapter explained the stimulus (light) properties that are crucial for the visual perception that our exterior environment, such as color, brightness, color and also brightness contrasts (for form perception and also visual acuity), visual field representation, binocular fusion and depth perception. Remember that there are local differences in visual perception: the central visual ar is color-sensitive, has actually high acuity vision and operates at high levels of illumination (i.e., operates v the photopic, light-adapted subsystem). In contrast, the visual ar periphery is an ext sensitive at low levels that illumination, is relatively color insensitive and also has bad visual acuity (i.e., operates with the scotopic, dark-adapted, subsystem). The chapter also described exactly how the lens system of the eye produces picture on the retina of irradiate emitted through or reflected off objects in space. The photo is a smaller, inverted, and reversed picture of the object. Save in mind the the photo projected top top the retina is, in fact, projected onto a flattened sheet of receptor cells that line the inner surface of the eye. The following chapter will define the duty of the visual receptors and also other retinal neurons in converting the visual picture into variety of neural activity.
The chapter likewise reviewed the retinal neurons and the laminar framework of the retina. The image projected top top the retina is spread over a mosaic that photoreceptors. Light energy projected onto every photoreceptor is converted into receptor membrane potential alters by a process that entails photosensitive pigments and also cyclic nucleotide-gated ion networks in the photoreceptor external segment. The phototransduction process converts light energy into photoreceptor membrane potential transforms that produce a chemical signal (the release of glutamate), which outcomes in membrane potential transforms in the postsynaptic bipolar and also horizontal cells. The receptor substrate because that scotopic and also photopic vision lies in differences in between the rod and also cone receptors.
See more: Why Is My Rabbit Pulling Her Fur Out ? 6 Reasons Why! 6 Reasons Why!
In the primate eye, the details gathered by 125 million receptor cells converges on 10 million bipolar cells, which, in turn, converge ~ above 1 million retinal ganglion cells. The level of convergence from receptors to bipolar cell and also bipolar cells to ganglion cell differs regionally within the retina. In the peripheral retina, the convergence can be fifty or more rod receptor to one bipolar cell, which increases the sensitivity to dim lights however decreases the spatial acuity the the peripheral bipolar cell. In addition, this peripheral bipolar cells are color insensitive. The M-ganglion cells get input from numerous peripheral bipolar cells, have big receptive fields, are sensitive to small brightness contrasts and are shade insensitive. They additionally generate transient responses and also are uniquely perceptible to alters in illumination levels and also movement. In contrast, the bipolar cells in the macula synapse with few foveal-cone receptors, which keep the spatial resolution the the densely packed cones. Such macular bipolar cell have little receptive ar centers, are color sensitive but must operate 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 little brightness contrasts. The p ganglion cells have smaller receptive fields than the M ganglion cells and respond with continual discharges to preserved stimuli. Together the M ganglion cells and P ganglion cell respond come different aspects of the visual stimulus, castle are defined to it is in encoding and also carrying independent, parallel, streams (M-stream and P-stream) that information about stimulus size, color, and movement.