Review Sheet 23 Anatomy of the Visual System

Chapter 14: Visual Processing: Eye and Retina

(content provided by Chieyeko Tsuchitani, Ph.D.)

Reviewed and revised 07 Oct 2020

In this affiliate you will acquire about how the visual organization initiates the processing of external stimuli. The affiliate will familiarize you with measures of visual sensation past discussing the footing of form perception, visual acuity, visual field representation, binocular fusion, and depth perception. An important attribute is the regional differences in our visual perception: the central visual field is color-sensitive, has loftier acuity vision, operates at loftier levels of illumination whereas the periphery is more sensitive at depression levels of illumination, is relatively color insensitive, and has poor visual vigil. You volition learn that the epitome is first projected onto a flattened canvass of photoreceptor cells that prevarication on the inner surface of the center (retina). The information gathered by millions of receptor cells is projected side by side onto millions of bipolar cells, which, in turn, ship projects to retinal ganglion cells. These cells encode different aspects of the visual stimulus, and thus carry contained, parallel, streams of data almost stimulus size, color, and movement to the visual thalamus.

14.1 Measures of Visual Awareness

The status of the visual organization can be determined by examining various aspects of visual sensation. For case, the ability to detect and identify small objects (i.e., visual acuity) tin be affected by disorders in the transparent media of the centre and/or visual nervous system. The inability to detect objects in specific areas of space (i.e., visual field defects) is often related to neural impairment.

Spatial Orientation and the Visual Field

The visual field is that expanse in space perceived when the eyes are in a fixed, static position looking straight ahead.

Figure 14.ane
The monocular visual field is the expanse in infinite visible to one centre. Every bit illustrated, the nose prevents the field of the correct center from roofing 180 degrees in the horizontal plane. Inset. Perimetry testing provides a detailed map of the visual field. As the nose, brow and cheeks occlude the view of the nearly nasal, superior and inferior areas, respectively, the resulting monocular visual field occupies a limited portion (colored blueish) of the potential visual infinite.

The monocular visual field (Figure 14.1)

is that expanse of space visible to one eye
tin can be mapped parametrically
- Perimetry testing provides a detailed map of the visual field. The potential visual field is described as a hemisphere. However, it does not form a perfect hemisphere as the brow, olfactory organ and cheekbones obscure the view - well-nigh prominently in the nasal hemisphere
is subdivided into two halves, the hemifields (Figure 14.ane Inset).
- A horizontal line drawn from 0° to 180° through eye of the field defines the superior & junior hemifields.
- A vertical line fatigued from 90° to 270° through center betoken defines the left & right hemifields, which are oft termed the nasal and temporal hemifields.
may be further subdivided into quadrants:
- the superior and junior nasal quadrants
- the superior and inferior temporal quadrants.
contains a bullheaded spot,
- a small area in which objects cannot be viewed
- which is located within the temporal hemifield.

Figure 14.2
The binocular visual field. Every bit our optics are angled slightly toward the nose, the monocular visual fields of the left and right eyes overlap to course the binocular visual field (colored red). Objects inside the binocular visual field are visible to each eye, albeit from different angles.

The monocular visual field (Figure fourteen.1) is adamant with 1 eye covered. The area of overlap of the visual field of 1 centre with that of the reverse eye is called the binocular field (Figure 14.2). All areas of the binocular visual field are "seen" by both eyes.

The power to locate objects in space and the ability to orient ourselves with respect to external objects are dependent upon the representation of visual space within the nervous system. The clinical examination of the visual fields almost usually used is the confrontation field exam. It defines the outer limits of our subjective visual space. Neurological disorders of the visual system can often exist localized based on the area of incomprehension within the visual field.

Visual Acuity

Visual vigil is the ability to discover and recognize small objects visually depends on the refractory (focusing) power of the eye's lens system and the cytoarchitecture of the retina.

Visual acuity is

measured under high illumination
the smallest size of a dark object in a light groundwork that tin can be correctly identified

In the clinical setting, an eye chart

is used to mensurate the patient's visual acuity.
consists of rows of blackness messages on a bright white background.
is used to mensurate visual acuity at a distance of 20 ft from the chart.
reports visual vigil every bit the ratio of the center nautical chart distance (i.due east., 20 ft) to the "normal distance" of the lowest row of letters correctly identified by the patient (e.g., row 3, which is 70 ft).

Color Vision

Color vision is the power to detect differences in the wavelengths of low-cal is chosen color vision. Clinically information technology may be tested with an Ishihara chart: a nautical chart with spots of unlike colors that are spatially organized to grade numbers that differ for ``normal" and color-blind eyes.

As mentioned to a higher place, the human has a trichromatic visual system, whereby visible colors tin can be created by a mixture of red, green and blue lights. The almost mutual form of colour blindness results in a defoliation of ruby and light-green shades (i.e., red-green colour blindness). Most cases of color blindness result from an absent or defective gene responsible for producing the reddish or light-green photopigment (protanopia, the lack of red; and deuteranopia, the lack of green). As these genes are located on the Ten chromosome, color blindness is more 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" where zilch is seen. RIGHT. Visual vigil is plotted as a function of distance (in degrees) from the center of the visual field. The curve labeled "Low-cal-adjusted" was obtained under photopic illumination levels and the curve labeled "Dark-adapted" was obtained under scotopic illumination levels.

Regional differences: There are regional differences in color sensation, visual acuity and low-illumination sensitivity inside the visual field (Effigy xiv.3).

A small "blindspot" is

located in the temporal hemifield (Figure 14.iii Left)
where objects cannot be seen.

Vision in the visual field eye

operates best under loftier illumination.
has the greatest visual acuity and colour sensitivity
is ten times better than in the field periphery (Effigy 14.3 Correct)
represents the operation of the photopic (calorie-free-adapted) subsystem

Vision in the peripheral visual field

is more sensitive to dim calorie-free
operates under depression illumination.
has picayune color sensitivity and poor spatial vigil (Figure xiv.3 Right)
represents the operation of the scotopic (nighttime-adapted) subsystem

Binocular Fusion and Depth Perception

Effigy fourteen.4
The two eyes fixated on an object view the object and objects in the background at slightly different angles. Consequently, the images on the two retinas are slightly dissimilar and must exist "fused" past the visual arrangement. The disparity in the retinal images at the two eyes also provides binocular cues for depth perception.

When a pencil is held an arm's length away with both optics open, most individuals will see a single object and recognize it as a pencil. However, if one apace closes each eye alternately (i.e., left center closed, right eye opened, and then right eye opened and left eye closed); you should see the pencil "jumping" from left to right as you alternate the eye closure. This is so because the image in each centre is slightly different (disparate): Notice that because each eye is located on either side of the nose, the viewing bending of each eye is slightly different - especially when viewing most objects (Figure xiv.iv).

Although the expanse in infinite defined by the binocular visual field (Figure xiv.4) represents corresponding areas of the monocular visual fields, the angle at which this space is viewed past each center is slightly unlike. Consequently, the images of the corresponding (binocular) infinite are slightly different in each eye. The nervous arrangement fuses these disparate binocular images to produce a single epitome (e.thousand., of the pencil located an arm's length away). The process of producing a single image from the 2 disparate monocular images is called binocular fusion.

Clinically, binocular fusion is tested by holding up one or two fingers in forepart of the patient and request the patient (who should exist wearing corrective lenses if they are normally worn) how many fingers they come across. If the patient reports seeing four fingers when just ii are presented, the patient is unable to produce binocular fusion.

Binocular fusion permits the perception a single clear paradigm and also provides extra cues for depth perception. That is, the binocular disparity between the two images is used past the nervous system to allow the perception of a three-dimensional globe where the estimate distance of an object tin be determined. The nervous system cannot fuse disparate binocular images when the disparity is too not bad. When corresponding areas of the normal binocular visual field are not in alignment (e.g., in strabismus where one eye deviates from the normal position and/or is paralyzed), the nervous system cannot fuse the disparate images and gradually adapts past "ignoring" the image from the deviant center. In fact, strabismus at nascence, if uncorrected, may result in a form of key blindness, amblyopia, where the paradigm from the deviant eye is no longer represented at cortical levels of the nervous system. The uncorrected, long-term amblyope is functionally blind in 1 eye and has poor depth perception.

14.2 The Epitome Forming Process

The transparent media of the eye part as a biconvex lens that refracts light entering the center and focuses images of the external world onto the light sensitive retina.

Refraction

Call back that calorie-free rays volition curve when passing from one transparent medium into some other if the speed of lite differs in the 2 media. Nonetheless, parallel lite rays volition pass from air through a transparent body (e.g., flat lens) without bending if the low-cal rays are perpendicular to the lens surface (Figure 14.5, left). If the light strikes the lens surface at an angle, the light rays will exist bent in a line perpendicular to the lens surface (Effigy 14.v, right).

Figure xiv.v
The course of light rays passing through a transparent lens are illustrated. LEFT: The light rays are entering perpendicular to the surface of the lens. RIGHT: The light rays are inbound at an bending to the surface of the lens and are existence refracted past the lens.

A @biconvex lens, which is functionally similar to the eye's lens system, is flat simply at its center. The surface of the area surrounding the center is curved and non perpendicular to parallel calorie-free rays (Figure 14.6). Consequently, the curved surfaces of a biconvex lens will bend parallel calorie-free rays to focus an image of the object emitting the light a short altitude behind the lens at its focal point. The paradigm formed is clear merely if the curvature of the lens is symmetrical in all meridians and all divergent light rays emitted by a bespeak source converge at the focal point.

Figure 14.six
The low-cal rays emanating from a point source take divergent paths that enter a arched lens at different points along the lens surface. The lens refracts the calorie-free rays bringing them together at the focal point some distance from the lens.

Figure 14.7
The center's lens organisation functions similar a biconvex lens and focuses an image on the retina that is inverted, left-right reversed and smaller than the object viewed.

Annotation that the greater the curvature of the lens surface the greater is its refractive power and the closer is the focused image to the lens. Annotation also that the prototype formed is inverted and left-right reversed (Figure 14.vii).

The image formed by eye's lens organisation is smaller than the object viewed, inverted (upside-down, Figure fourteen.vi), and reversed (right-left, Figure 14.7). Every bit the image is inverted by the lens organization, the superior (top) one-half of each eye's visual field is projected onto the junior (bottom) one-half of each middle's retina. Also, as the lens produces a reversed image, the temporal half of each visual field is projected onto the nasal one-half of each middle's retina^one. Therefore, the temporal (left) hemifield of the left eye is projected onto the nasal (correct) half of the left eye's retina and the nasal (left) hemifield of right centre is projected onto temporal (right) one-half of the right centre'south retina. Consequently, the left hemifields of both eyes are projected onto the corresponding (right) halves of the two retinas. Information technology is critical that you understand the relationship between the visual field and the retinal areas and realize that respective halves of the 2 monocular visual fields are imaged on corresponding halves of the two retinas. These relationships form the neurological basis for understanding visual field defects.

Lens Accommodation

The heart must be able to alter its refractive backdrop to focus images of both distant and nearby objects on the retina. Distant objects (greater than 30 feet or 9 meters away from the eye) emit or reverberate light that tin be focused on the retina in a normal relaxed centre (Figure fourteen.viii).

Figure 14.viii
The normal eye at rest tin can focus on the retina images of objects more 30 ft from the center. When an object is brought closer to the middle (i.e., less than 30 ft from the eye), the light rays from the object take more divergent paths and each enters the cornea with a greater angle of incidence. Consequently, the image focal point would exist across the retina if the eye's lens system were non adjusted. During adaptation, the lens curvature increases, increasing the refractive power of the eye and focusing the paradigm on the retina.

If a viewed object is brought closer to the heart, the light rays from the object diverge at a greater angle relative to the eye (Figure 14.eight). Consequently, the nearer the object of view, the greater the angle of incidence of light rays on the cornea, and the greater the refractive power required to focus the calorie-free rays on the retina. The cornea has a stock-still refractive ability (i.e. information technology cannot change its shape). However, altering the tension of the zonules on the elastic lens capsule tin can alter the lens shape. The change in the refractive properties of the eye is called the accommodation or "most point" process.

In the normal eye under resting (distant vision) conditions, the ciliary muscles are relaxed and the zonules are nether tension (Effigy fourteen.9). In this example, the lens is flattened, which reduces the refractive power of the lens to focus on distant objects. When an object is closer to the middle (i.due east., less than 30 ft. away), adaptation occurs to affect "almost vision". The ciliary musculus contracts, pulling the ciliary processes toward the lens (think the muscle acts as a sphincter). This activity releases tension on the zonules and the lens capsule. The reduced tension allows the lens to become more spherical (i.e., increase its curvature). The increase in lens curvature increases the lens refractive ability to focus on near objects. Consequently, as an object is moved closer to the viewer, his eyes conform to increase the lens curvature, which increases the refractive ability of his center (Figure xiv.8).

Effigy fourteen.9
During distance vision (i.e., with the centre at residue), the ciliary muscles are relaxed and the zonules are under tension. The lens is flattened by the tension on the zonules and the lens capsule. However, in the accommodation process, the ciliary muscles contract and, acting like a sphincter muscle, decrease the tension on the zonules and lens capsule. The lens becomes more spherical with its inductive surface shifting more anteriorly into the anterior chamber.

Refractive Errors of the Eye and Corrective Lenses

Presbyopia: In presbyopia, at that place is normal altitude vision, but lens accommodation is reduced with historic period. With historic period, the lens loses its elasticity and becomes a relatively solid mass. During accommodation, the lens is unable to assume a more spherical shape and is unable to increase its refractive ability for well-nigh vision (Figure 14.10). As a result, when an object is less than 30 ft. away from the presbyopic viewer, the image is focused somewhere behind the retina.

Figure xiv.10
In the presbyopic eye, when the object is moved closer to the eye, the lens is unable to arrange and the prototype is focused beyond the retina. For the presbyopic middle a corrective lens that converges the low-cal rays (i.e., a convex lens that reduces the angle of incidence of light on the cornea) will permit the presbyopic eye to view nearby objects.

A convex lens (i.e., increased refractive power) is used to right the presbyopic eye (Effigy 14.10). These lenses refract the light rays and so they strike the surface of the cornea at a smaller angle. Still, because the corrective lens increases the refractive power, the presbyope with convex lenses volition have bug with distance vision. Consequently, the corrective lenses are frequently half lenses (i.east., reading glasses) which permit the presbyope to view objects in the distance unimpeded by the convex lens.

Hyperopia: In hyperopia (Figure 14.11), the refractive power of the eye's lens arrangement is too weak or the eyeball too short. When viewing afar objects, the paradigm is focused at a indicate beyond the retina.

Figure 14.xi
The hyperopic center at residual cannot focus on the retina the image of an object more than xxx ft from the eye. The hyperopic lens system is also weak and the epitome is focused beyond the retina.

The immature hyperope can recoup past using lens accommodation, i.e., increase the refractive power of the center'south lens organization (Figure 14.12). We call the hyperope "far-sighted" (hypermetropic) considering the ability of accommodation used for altitude vision cannot be used for near vision.

Effigy fourteen.12
If the hyperopia is not severe; the hyperopic eye can use the lens adaptation process to increment the refractive power of the eye for distance vision.

As the hyperope ages and becomes presbyopic, the power of accommodation is diminished. Consequently, the middle aged hyperope may accept a limited range (near and far) of vision. To correct this effect of crumbling, the refractive power of the center is increased with convex lenses (Figure 14.12).

Myopia: In myopia (Figure 14.13), the refractive power of the eye's lens organisation is too strong or the eyeball also long. When viewing afar objects, the image is focused at a point in front of retina.

Figure xiv.thirteen
The myopic middle at balance cannot focus on the retina the image of an object more than 30 ft. from the eye. The refractive ability of the eye'due south lens organization is too potent and the paradigm is focused in front of the retina.

The uncorrected myopic middle is "near-sighted" because it tin can focus unaided on near objects. That is, the young myope will meet afar objects as blurred, poorly defined images merely can see nearby modest objects clearly (remember nearby objects emit divergent light rays).

For distance vision, the refractive power of the myopic eye lens arrangement is corrected with concave lenses that diverge the light rays entering the eye (Figure 14.14). Annotation that as the power of accommodation diminishes with age, near vision is likewise affected in the presbyopic-myopic eye. The mature myope may require bifocals, the upper one-half of the lens diverging light rays for distance vision and the lower half with no or low converging ability for near vision.

Figure xiv.14
A corrective lens that diverges lite rays before they enter the eye (i.e., a concave lens) will allow the myopic center to focus the prototype of a afar object on the retina.

Astigmatism: An astigmatism results when the cornea surface does not resemble the surface of a sphere (e.g. is more than oblong). In an eye with astigmatism, the image of distant and nearly objects cannot exist focused on the retina (Effigy fourteen.xv). Astigmatism is corrected with a cylindrical lens having a curvature that corrects for the corneal astigmatism. The cylindrical lens directs light waves through the astigmatic cornea to focus a single, clear image on the retina.

Effigy xiv.fifteen
The astigmatic lens is asymmetrical and has multiple focal points, which produces multiple images of a bespeak source.

fourteen.3 The Retina

You volition at present acquire nigh the retinal neurons and the laminar structure of the retina, and the ways in which the light-sensitive receptors of the eye convert the image projected onto the retina into neural responses. The low-cal sensitive retina forms the innermost layer of the centre (Figure 14.xvi).

Figure fourteen.16
The center, the three coats of the eye and the layers of the retina. The retina is the innermost coat of the centre and consists of the retinal pigment epithelium and neural retina.

The retina covers the choroid and extends anteriorly to just behind the ciliary trunk. The retina consists of neurons and supporting cells.

Components of the Retina

The retina is derived from the neural tube and is, therefore, function of key nervous organisation. It consists of 2 parts, the retinal pigment epithelium, which separates the heart, choroid glaze of the eyeball from the other innermost component and the neural retina (Figure 14.16) – the dark pigments within the retinal pigment epithelium and choroid glaze function to absorb low-cal passing through the receptor layer, thus reducing light scatter and epitome distortion within the eye. The neural retina contains five types of neurons (Figure fourteen.17): the visual receptor cells (the rods and cones), the horizontal cells, the bipolar cells, the amacrine cells, and the retinal ganglion cells.

Retinal Layers

The retina is a laminated structure consisting of alternating layers of prison cell bodies and cell processes (Figure fourteen.18).

Figure xiv.17
The components of the neural retina. The neural retina consists of at least v unlike types of neurons: the photoreceptors (rods and cones), horizontal cell, bipolar prison cell, amacrine cell and ganglion cell.

Figure fourteen.18
The neural retina is formed by alternate layers of neuron jail cell bodies that appear night and neuron processes that appear light in Nissl stained tissue. The receptor cells synapse with bipolar and horizontal cells in the outer plexiform layer. The bipolar cells, in turn, synapse with amacrine and ganglion cells in the inner plexiform layer The axons of the retinal ganglion cells leave the heart to form the optic nervus.

The innermost layers are located nearest the vitreous chamber, whereas the outermost layers are located adjacent to the retinal paint epithelium and choroid. The well-nigh of import layers, progressing from the outer to inner layers, are:

the retinal paint epithelium, which provides critical metabolic and supportive functions to the photoreceptors;
the receptor layer, which contains the low-cal sensitive outer segments of the photoreceptors;
the outer nuclear layer, which contains the photoreceptor cell bodies;
the outer plexiform layer, where the photoreceptor, horizontal and bipolar cells synapse;
the inner nuclear layer, which contains the horizontal, bipolar and amacrine cell bodies;
the inner plexiform layer, where the bipolar, amacrine and retinal ganglion cells synapse;
the retinal ganglion cell layer, which contains the retinal ganglion cell bodies; and
the optic nerve layer, which contains the ganglion cell axons traveling to the optic disc.

Detect that light passing through the cornea, lens and vitreous must pass through nearly of the retinal layers before reaching the light-sensitive portion of the photoreceptor; the outer segment in the receptor layer. Observe as well that in the region of the fovea where the paradigm of the central visual field center is focused, the retina consists of fewer layers (Figure fourteen.nineteen): thereby minimizing the obstacles to forming a clear image on the fovea. The area around the fovea, the surrounding macula, is thicker because information technology contains the cell bodies and processes of retinal neurons receiving information from the receptors in the fovea.

The optic disc is formed by the retinal ganglion cell axons that are exiting the retina. It is located nasal to the fovea (Figure xiv.19). This region of the retina is devoid of receptor cells and composed predominantly by the optic nerve layer. Consequently, it is the structural ground for the 'blind spot" in the visual field.

Figure 14.19
The fovea of the retina and the layers of the retina in the surrounding macula. The fovea and macula are colored as they appear when stained for Nissl substance, which is most abundant in the neuron cell trunk.

The Photoreceptors

The human has two types of photoreceptors: the rods and cones (Effigy 14.xx). They are distinguished structurally past the shapes of their outer segments. The photopigments of the rods and cones also differ. The rod outer segment disks contain the photopigment rhodopsin, which absorbs a wide bandwidth of light. The cones differ in the color of light their photopigments absorbs: one type of photopigment absorbs cherry light, another green lite, and a 3rd bluish calorie-free. Every bit each cone receptor contains only ane of the three types of cone photopigment, there are 3 types of cones; red, green or blue. Each cone responds best to a specific color of calorie-free, whereas the rods respond all-time to white lightⁱⁱ. The rod and cone photopigments as well differ in illumination sensitivity; rhodopsin breaks down at lower calorie-free levels than that required to breakup cone photopigments. Consequently, the rods are more sensitive - at least at low levels of illumination.

xiv.iv Rods and Cones Form the Basis for Scotopic and Photopic Vision

The man visual organization has two subsystems that operate at dissimilar low-cal free energy levels. The scotopic, night-adapted system operates at depression levels of illumination, whereas the photopic, light-adapted system operates at loftier levels of illumination.

Figure 14.20
The cone and rod photoreceptors. The photoreceptors are neurons that have a dendritic component (the outer segment) and an axonal component that forms synaptic terminals.

Rods are responsible for the initiation of the scotopic visual process. Rods

contain the photopigment rhodopsin, which breaks down when exposed to a wide bandwidth of light (i.e., it is achromatic).
- Rhodopsin is also more sensitive to light and reacts at lower light levels than the colour sensitive (chromatic) cone pigments.
take longer outer segments, more outer segment disks and, consequently, contain more photopigment.
are more sensitive to light and function at scotopic (low) levels of illumination.
dominate in the peripheral retina (Figure 14.21A), which is color insensitive, has poor acuity (Figure fourteen.21B), but is sensitive to low levels of illumination.

Cones are responsible for the initiation of the photopic visual process. Cones

contain photopigments that breakup in the presence of a limited bandwidth of light (i.e., cone photopigments are chromatic).
are colour sensitive.
are less sensitive to calorie-free and crave high (daylight) illumination levels.
are concentrated in the fovea (Effigy 14.21A)
in the fovea have paradigm of the central visual field projected on them.
in the fovea are responsible for photopic, low-cal-adjusted vision (i.east., loftier visual vigil and color vision) in the cardinal visual field (Figure xiv.21B)

Effigy xiv.21
The rods, are taller, have longer outer segments and, consequently, contain more outer segment disks and more photopigment than cones. Cone receptors are full-bodied in the fovea of the eye (at 0° eccentricity), whereas rod receptors are full-bodied in more than peripheral retina (A). Visual acuity is maximal in the central area of the visual field (at 0° eccentricity), whereas information technology is minimal in more peripheral areas (B). Notice that the location of the optic disc relative to the fovea corresponds to the location of the bullheaded spot relative to the visual field center.

Biochemical processes in the photoreceptors participate in dark and light adaptation. Notice when you enter a darkened room after spending time in daylight, information technology takes many minutes before you are able to see objects in the dim light. This boring increment in low-cal sensitivity is chosen the night-accommodation process and is related to the rate of regeneration of photopigments and to the intracellular concentration of calcium³. A contrasting, only faster, procedure occurs in high levels of illumination. When you are fully nighttime-adjusted, exposure to bright low-cal is at first blinding (massive photopigment breakup and stimulation of photoreceptors) and is followed apace by a render of sight. This miracle, light adaptation, allows the cone response to dominate over rod responses at loftier illumination.

14.5 Visual Processing in the Retina

The photoreceptors exhibit a fairly high basal release of glutamate. When calorie-free strikes the photoreceptor prison cell, it initiates a biochemical process in the cell that reduces the release of glutamate from its axon final. The glutamate, in turn, affects the activity of the bipolar and horizontal cells, which synapse with the photoreceptor. The bipolar cells, in turn, synapse with amacrine and retinal ganglion cells. It is the axons of the retinal ganglion cells that leave the eye as the optic nervus and finish in the encephalon. Notice that the direct pathway for the transmission of visual information from the eye to the encephalon includes just the receptor cell, bipolar cell and ganglion cell. The horizontal cells modulate the synaptic activity of receptor cells and, thereby, indirectly touch on the transmission of visual information by bipolar cells. Similarly the amacrine cells attune the synaptic action of the retinal bipolar and ganglion cells, thereby affecting the manual of visual data by the ganglion cells.

Bipolar Cells

Within the outer plexiform layer of the retina, approximately 125 million photoreceptor cells synapse with approximately ten 1000000 bipolar cells. A smaller number of horizontal cells as well synapse with the photoreceptor cells within the outer plexiform layer of the retina. The bipolar and horizontal cells respond to the glutamate released by the photoreceptor cells⁴.

Bipolar cells
- do not generate action potentials.
- answer to the release of glutamate from photoreceptors with graded potentials (i.e., by hyperpolarizing or depolarizing).

Bipolar cells differ based on their responses to photoreceptor stimulation.

There are at least two types of bipolar cells based on their responses to glutamate.
- The off bipolar cells are depolarized past glutamate.
- The on bipolar cells are hyperpolarized by glutamate.
The two bipolar cell types have different functional backdrop.
- The off bipolar cells function to find dark objects in a lighter groundwork.
- The on bipolar cells role to notice light objects in a darker background.

The stimulus condition that produces a depolarizing response from a bipolar cell is used to name the bipolar jail cell type.

An off bipolar cell depolarizes when the photoreceptors that synapse with information technology are in the dark (i.e., when the light is off, Figure 14.22).
An on bipolar cell depolarizes when the photoreceptors that synapse with are in the light (i.e., when the low-cal is on, Figure xiv.22). Note that the depolarization of the on bipolar cell does not issue from excitation of the presynaptic jail cell but rather from a reduction of the inhibitory action of glutamate produced by the light-induced decreased release of glutamate from the photoreceptor.

Figure 14.22
When the receptor cells with which an off bipolar cell synapses are in the dark, the off bipolar cell is depolarized and the on bipolar cell is hyperpolarized. In contrast, when the receptor cells with which an off bipolar cell synapses are in the light, the off bipolar cell is hyperpolarized and the on bipolar jail cell is depolarized.

Bipolar Cell Receptive Field : The receptive field of a bipolar cell is defined anatomically by the location and distribution of receptor cells with which it makes 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 are extremely small and are colour sensitive. The cone-bipolars may exist hyperpolarized or depolarized by glutamate and, consequently, may be on-blazon or off-type bipolar cells.
Each rod-bipolar cell may brand synaptic contact with a few to 50 or more than of rod receptor cells. Consequently, the rod-bipolar cell receptive field is relatively big and color insensitive. All rod-bipolar cells are hyperpolarized by glutamate and, consequently, are on-type bipolar cells exclusively.

The bipolar prison cell receptive field is also divers physiologically as the retinal area which when exposed to light produces a response (i.e., depolarization or hyperpolarization) in the bipolar jail cell.

Bipolar cells have concentric receptive fields. Low-cal directed on the photoreceptor(due south) that synapse with a bipolar prison cell produces a response from the bipolar cell called the center response (Effigy 14.23). In dissimilarity, light directed on immediately surrounding receptors produce the opposite response (Figure 14.24).


Effigy 14.23 Bipolar cells take concentric receptive fields. The on bipolar cell depolarizes when the receptor cells with which it synapses are illuminated ("Low-cal On"). These center receptors (i.eastward., the ones making direct synaptic contact with the bipolar jail cell) produce the bipolar cell centre response.		Effigy 14.24 Bipolar cells accept concentric receptive fields. When the receptors surrounding the center receptors of the on bipolar receptive field are illuminated ("Light On") and the eye receptors kept in the dark, the on bipolar cell is hyperpolarized.

When both the center and surrounding receptor cells are illuminated with light, the on bipolar prison cell response to stimulation of the eye receptors is reduced by stimulation of the surroundings receptors (Effigy 14.25).

Figure fourteen.25
Bipolar cells have concentric receptive fields. When both the middle and surrounding receptors of the on bipolar cell receptive field are illuminated, the on bipolar cell depolarizes. All the same, the magnitude of the depolarization is reduced to less than the depolarization to illumination of simply the center receptors.

Consequently, the strongest on bipolar prison cell response is produced when the stimulus is a light spot encircled past a dark ring. For the off bipolar jail cell, a dark spot encircled by a lite ring produces maximal depolarization.

Horizontal Cells

Inside the outer plexiform layer, the photoreceptor cells make both presynaptic and postsynaptic contact with horizontal cells.

The horizontal cells have large receptive fields involving
- presynaptic (axonal) contact with a pocket-sized group of photoreceptors and
- postsynaptic (dendritic) contact with a larger grouping of surrounding photoreceptor cells.

By decision-making the responses of their "center" photoreceptors (based on the responses of the surrounding photoreceptors), the horizontal cells indirectly produce the bipolar cell receptive field surround issue. The environment result produced by the horizontal cell is weaker than the center effect.

Figure xiv.26
The horizontal cells make presynaptic and postsynaptic contact with photoreceptor cells. The axon terminals of a horizontal cell receives synaptic contact from 1 group of photoreceptors (colored red) and its processes make synaptic contact with surrounding photoreceptor cells (colored green).

The environs effect, produced past the horizontal cells, enhances brightness contrasts to produce sharper images, to make an object announced brighter or darker depending on the background and to maintain these contrasts under different illumination levels.

Retinal Ganglion Cells

Within the inner plexiform layer, the axon terminals of bipolar cells (the 2° visual afferents) synapse on the dendritic processes of amacrine cells and ganglion cells. Every bit in most neurons, depolarization results in neurotransmitter release by the bipolar cell at its axon terminals. Most bipolar cells release glutamate, which is excitatory to almost ganglion cells (i.eastward., depolarizes ganglion cells). The amacrine cells may synapse with bipolar cells, other amacrine cells or ganglion cells. It is the axons of the retinal ganglion cells (the 3° visual afferents) that leave the eye to form the optic nerve and deliver visual information to the lateral geniculate nucleus of the thalamus and to other diencephalic and midbrain structures.

Effigy 14.27
An off ganglion cell synapses with an off bipolar cell and produces action potentials (i.e., is excited) when the off bipolar cell is depolarized (i.e., when the light is off). In contrast, an on ganglion cell that synapses with an on bipolar cell reduces the rate at which it produces action potentials (i.e., is inhibited) when the on bipolar cell is hyperpolarized (when the lite is off).

Ganglion Cell Response Properties. The retinal ganglion cells are the final retinal elements in the straight pathway from the eye to the brain. Because they must carry visual information some distance from the centre, they posses voltage-gated sodium channels in their axonal membranes and generate action potentials when they are depolarized past the glutamate released by the bipolar cells.

The off bipolar cell (Figure 14.27, Right) volition depolarize when it is dark on its centre cones and will therefore release glutamate when it is dark on the eye of its receptive field. This will upshot in the depolarization of the retinal ganglion cells with which the off bipolar synapses and in the production of action potentials (i.e., discharges) by these ganglion cells (Effigy 14.27, Right). Consequently, the retinal ganglion cells that synapse with off bipolar cells will accept off-centre/on-surround receptive fields and are called off ganglion cells.

The on bipolar jail cell (Figure fourteen.28, Left) will depolarize when at that place is low-cal on its centre cones and will therefore release glutamate when information technology is calorie-free on the center of its receptive field. This will result in the depolarization of the retinal ganglion cells with which the on bipolar synapses and in the product of action potentials (i.east., discharges) by these ganglion cells (Effigy xiv.28, Left). Consequently, the retinal ganglion cells that synapse with on bipolar cells will have on-eye/off-surround receptive fields and are called on ganglion cells.

In short, the receptive fields of the bipolar cells with which the retinal ganglion cell synapses determine the receptive field configuration of a retinal ganglion cell.

The retinal ganglion cells provide information of import for detecting the shape and move of objects.

In the primate middle, there are 2 major types of retinal ganglion cells, Type One thousand and Type P cells, that process information well-nigh different stimulus properties.

Figure 14.28
Left: The on ganglion cell synapses with an on bipolar jail cell and produces action potentials (i.e., is excited) when the on bipolar cell is depolarized (i.e., when the light is on). Right: In contrast, an off ganglion jail cell that synapses with an off bipolar cell reduces the rate at which it produces activity potentials (i.e., is inhibited) when the off bipolar cell is hyperpolarized (when the light is on).

Type P retinal ganglion cells are colour-sensitive object detectors.

The P ganglion prison cell(s)

outnumber the G-ganglion cells, past approximately 100 to 1 in the primate retina
makes synaptic contact with 1 to a few cone bipolars that are innervated past cone receptors in the macula fovea
is colour sensitive
has a small concentric receptive field
produces a sustained, slowly adapting response that lasts as long as a stimulus is centered on its receptive field.
produces weak responses to stimuli that motion across its receptive field.

The slowly adapting response of the Blazon P retinal ganglion jail cell is all-time suited for signaling the presence, color and elapsing of a visual stimulus and is poor for signaling stimulus movement.

Type G retinal ganglion cells are colour-insensitive motion detectors.

The M ganglion cell

is much larger than P ganglion cells
synapses with many bipolar cells
is colour insensitive
has a large concentric receptive field
is more sensitive to small middle-environs effulgence differences
responds with a transient, rapidly adapting response to a maintained stimulus.
responds maximally, with high discharge rates, to stimuli moving across its receptive field.

The rapidly adapting responses of Type M ganglion cells are best suited for signaling temporal variations in, and the movement of, a stimulus.

The axons of the M and P retinal ganglion cells travel in the retina optic nerve cobweb layer to the optic disc where they leave the eye. Nigh of the axons travel to and terminate in the lateral geniculate nucleus of the thalamus.

Amacrine Cells

Amacrine cells synapse with bipolar cells and ganglion cells and are similar to horizontal cells in providing lateral connections between similar types of neurons (e.k., they may connect bipolar cells to other bipolar cells)⁵. They differ from horizontal cells, all the same, in also providing ''vertical" links betwixt bipolar and ganglion cells.

Amacrine jail cell types. There are 20 or more types of amacrine cells based on their morphology and neurochemistry. The roles of three types have been identified. One type

is responsible for producing the motion sensitive (rapidly adapting) response of the Type Grand ganglion cells.
enhances the heart-surround event in ganglion cell receptive fields.
connects rod bipolar cells to cone bipolar cells, thus assuasive ganglion cells to respond to the entire range of light levels, from scotopic to photopic.

Convergence of Inputs and Visual Acuity

Depression convergence of cones to cone bipolar cells and low convergence of cone bipolar cells to P-retinal ganglion cells produce high visual acuity in the central visual field.

Recall that

visual vigil and color vision are greatest in the fundamental visual field.
the image of the central visual field is projected onto the fovea.
the cones are concentrated in the fovea, whereas the rods predominate in the peripheral retina.
there is low convergence of foveal cones onto macular bipolar cells, equally low every bit ane cone receptor to one bipolar jail cell.

In addition, the cones in the fovea are of smaller diameter than those in the periphery of the retina, which allows for a greater packing density of foveal cones. The loftier packing density of cones and the low convergence of cones onto bipolar cells in the macula support higher visual vigil in the central visual field. Consequently, the foveal cones, macular bipolar cells and the P-retinal ganglion cells are responsible for photopic, light-adjusted vision in the central visual field. In contrast, the college convergence of the rods onto peripherally located bipolar cells and of peripheral bipolar cells onto amacrine cells forms the basis for the poor visual vigil merely high lite sensitivity of scotopic vision.

xiv.5 Clinical Manifestations of Retinal Dysfunction

The chemic and physical integrity of the retina is essential for normal visual office. Abnormalities in the claret supply and retinal pigment epithelium outcome in retinal dysfunctions.

Vitamin A deficiency can cause permanent incomprehension. An adequate supply of photopigments is necessary to sustain photoreceptors. The supply of all-trans retinal as a photopigment breakdown product is bereft to maintain adequate photopigment production. Vitamin A can exist oxidized into all-trans retinal, and is, therefore, critical in the synthesis of photopigment. In the center, it is the retinal paint epithelium that stores vitamin A. The retinal pigment epithelium is also the site of the oxidization of vitamin A into all-trans retinal and conversion of all-trans retinal into 11-cis-retinal. Vitamin A cannot be synthesized by the body and must be ingested. It is establish in blood and stored in the liver and retinal pigment epithelium. Vitamin A deficiency, which can consequence from liver impairment (e.one thousand., from alcoholism or hepatitis), produces degeneration of photoreceptors with visual symptoms get-go presenting equally "night blindness" (i.e., extremely poor vision under depression illumination).

Retinitis pigmentosa is an inherited disorder in which there is a gradual and progressive failure to maintain the receptor cells. One form involves the production of defective opsin that normally combines with 11-cis retinal to form rhodopsin. Consequently, the rods practice not contain sufficient rhodopsin and do not office every bit the low illumination receptors. A symptom of this condition is "nighttime blindness" and loss of peripheral vision. In this class of retinitis pigmentosa, the cones receptors function normally and central vision remains intact. Other forms of retinitis pigmentosa that affect the cones may progress to destroy primal vision.

Macular Degeneration. The leading cause of blindness in the elderly is historic period-related macular degeneration. The dry grade of macular degeneration involves intraocular proliferation of cells in the macular area (i.east., in the fovea and the immediately surrounding retinal areas). In the wet course of macular degeneration, the capillaries of the choroid coat invade the macular area and destroy receptor cells and neurons. In both forms, the visual loss is in the central visual field and the patient volition complain of blurred vision and difficulty reading. Laser surgery is the near common treatment for the wet class merely has the disadvantage of destroying normal retinal cells. It also may not be effective in preventing cell proliferation following treatment.

Retinal detachment. When the neural retina is torn away from the retinal paint epithelium (e.g., by a accident to the eye), there is a loss of vision in the area of detachment. The loss of vision results because the neural retina is dependent on the retinal paint epithelium for eleven-cis retinal, nutrients and photoreceptor integrity. The retinal pigment epithelium supplies glucose and essential ions to the neural retina, helps support the photoreceptor cell outer segment, removes outer segment disks shed by the receptor cells, and converts retinol and stores vitamin A for photopigment resynthesis. Lasers may be used to weld the detachment to prevent it from increasing in size. However, the detached and welded areas are functionally blind.

Diabetic retinopathy. The pathological process in diabetic retinopathy involves microaneurysms and punctate hemorrhages in the retina. The tiny swollen blood vessels and/or bleeding in the underlying choroid coat damage the receptor cells and retinal neurons and result in incomprehension in the regions affected. Lasers may be used to seal swollen and/or leaking blood vessels.

fourteen.6 Summary

This chapter described the stimulus (calorie-free) properties that are important for the visual perception of our external environs, such as colour, brightness, color and brightness contrasts (for form perception and visual acuity), visual field representation, binocular fusion and depth perception. Remember that there are regional differences in visual perception: the central visual field is color-sensitive, has high acuity vision and operates at high levels of illumination (i.e., operates with the photopic, light-adapted subsystem). In contrast, the visual field periphery is more sensitive at low levels of illumination, is relatively colour insensitive and has poor visual acuity (i.e., operates with the scotopic, dark-adapted, subsystem). The chapter also described how the lens system of the eye produces an image on the retina of lite emitted past or reflected off objects in space. The paradigm is a smaller, inverted, and reversed moving-picture show of the object. Keep in listen that the epitome projected onto the retina is, in fact, projected onto a flattened canvass of receptor cells that line the inner surface of the centre. The following affiliate will describe the function of the visual receptors and other retinal neurons in converting the visual image into an assortment of neural activeness.

The affiliate likewise reviewed the retinal neurons and the laminar structure of the retina. The paradigm projected onto the retina is distributed over a mosaic of photoreceptors. Light energy projected onto each photoreceptor is converted into receptor membrane potential changes by a process that involves photosensitive pigments and cyclic nucleotide-gated ion channels in the photoreceptor outer segment. The phototransduction procedure converts light free energy into photoreceptor membrane potential changes that produce a chemical betoken (the release of glutamate), which results in membrane potential changes in the postsynaptic bipolar and horizontal cells. The receptor substrate for scotopic and photopic vision lies in differences between the rod and cone receptors.

In the primate eye, the information gathered past 125 meg receptor cells converges on 10 million bipolar cells, which, in turn, converge on 1 million retinal ganglion cells. The degree of convergence from receptors to bipolar cell and bipolar cells to ganglion jail cell differs regionally within the retina. In the peripheral retina, the convergence can be fifty or more rod receptors to one bipolar cell, which increases the sensitivity to dim lights but decreases the spatial acuity of the peripheral bipolar jail cell. In addition, these peripheral bipolar cells are color insensitive. The M-ganglion cells receive input from many peripheral bipolar cells, take large receptive fields, are sensitive to small brightness contrasts and are color insensitive. They likewise generate transient responses and are uniquely sensitive to changes in illumination levels and movement. In contrast, the bipolar cells in the macula synapse with few foveal-cone receptors, which maintain the spatial resolution of the densely packed cones. Such macular bipolar cells accept small receptive field centers, are color sensitive but must operate at high illumination levels. Each P-ganglion cell synapses with few macular bipolar cells and is colour sensitive, but less sensitive to dim "white" calorie-free and to minor effulgence contrasts. The P ganglion cells have smaller receptive fields than the M ganglion cells and respond with sustained discharges to maintained stimuli. Every bit the Thousand ganglion cells and P ganglion cells reply to different aspects of the visual stimulus, they are described to exist encoding and carrying independent, parallel, streams (M-stream and P-stream) of information about stimulus size, color, and movement.

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