Vision

Volgograd State Medical University Department Normal Physiology Physiology Of Vision Rodion A. Kudrin

The plan Anatomy of the eye. Optics: formation of the retinal image. Accommodation of the eye for objects at different distances. Optic defects and abnormalities. Optical defects in the ametropic eye. Visual acuity. Photoreceptors. Visual pathways.

Introduction Eyes are complex sense organs that evolved from primitive light–sensitive spot on the surfaces of invertebrates Each eye has a layer of receptors, a lens that focuses light on these receptors, and a system of nerves that conducts impulses from receptor to brain Of all sensory receptors in the body, 70% are in the eye

1. Anatomy of the eye spherical in shape 24 mm in diameter orbital cavity loosely embedded in fatty tissue protected by the eyelids

Ciliary body Suspensory ligament Posterior cavity containing vitreous humor Ciliary muscle in ciliary body Posterior chamber Anterior chamber Anterior cavity Aqueous humor Iris Canal of Schlemm

1. Anatomy of the eye Layers of the eyeball Sclera – External, fibrous tunic Choroid – Intermediate, vascular tunic iris ciliary body Retina – Internal tunic

1. Anatomy of the eye Sclera Greater part of the external surface White of the eye Anteriorly cornea transparent greater curvature than the rest of the eye

1. Anatomy of the eye Choroid Middle layer Vascular and pigmented Anteriorly, the choroids becomes modified into the: Iris ciliary body

Sympathetic stimulation Iris Cornea Flattened Weak lens Relaxed ciliary muscle Taut suspensory ligaments

1. Anatomy of the eye Retina The innermost layer Pigmented Layer Nervous Layer Macula lutea Fovea Centralis Optic nerve Optic disc area surrounding optic nerve Optic cup small depression at the center of the optic disc

Sclera Choroid Fovea Optic disc Lens Retina Pupil Iris

1. Anatomy of the eye Lens Transparent, colorless body Biconvex lens IRIS PUPIL

Sympathetic stimulation Rounded strong lens Contracted ciliary muscle Slackened suspensory ligaments

Suspensory ligaments Ciliary muscle Lens Pupillary opening in front of lens The iris is circular and pigmented. It is two layers of smooth muscle that control the amount of light passing through the pupil and into the eye.

1. Anatomy of the eye Humors The lens divide the eye cavity into an Anterior space Posterior space

2. Optics: formation of the retinal image Refraction of Light Light rays, on passing obliquely from one transparent medium to another of a different optical density, are deflected from their path rarer to denser medium denser to rarer medium

2. Optics: formation of the retinal image Image Formation by a Convex Lens Artificial lens Converging lens Diverging lens The principal axis of a lens with two spherical surfaces is a line passing through the centers of curvature

2. Optics: formation of the retinal image Focal points Light from a point on the principal axis so distant that the rays are parallel when they strike the lens, will converge at a point Focal length which is a measure of the refractive power of “strength” of the lens Diopter The unit for the refractive power of a lens which is the reciprocal of the focal length expressed in meters Refractive power depends upon the curvature of the lens surface and the refractive index of the material the lens is made of

2. Optics: formation of the retinal image Image Formation of the Eye Refracting media Reduced Eye Index of refraction (IOR)=1.333 Optical center 5 mm behind the cornea the retina is 15 mm behind the optical center Cornea to the retina is 20 mm The total refractive power is 59 D

2. Optics: formation of the retinal image A real, inverted image, smaller than the object size (object) Distance from object to optical center = size (image) Distance from image to optical center

2. Optics: formation of the retinal image The visual angle is formed at the optical center by the limiting rays from the object This angle increases as the object is placed closer to the eye

2. Optics: formation of the retinal image Retinal images are inverted they are perceived as “erect” (in the correct position) and “projected” The “righting” of the image

3. Accommodation of the eye for objects at different distances Increase curvature of the lens of the eye The lens is suspended by the zonula Ciliary muscle is relaxed -> zonule is under tension and pulls on the equator of the lens so that the lens is flattened Refractive power of the lens is decreased. Ciliary muscle contracts -> pulls the ciliary body towards the lens, relaxing the zonula. The tension which held the lens in its flattened shape having been reduced or abolished Refractive

3. Accommodation of the eye for objects at different distances This diagram shows how light from afar is bent by the stretched lens to strike the retina, and how light from a closer source is bent even more sharply by the relaxed lens to strike the retina

3. Accommodation of the eye for objects at different distances Constriction of the Pupils By constricting, the iris excludes the periphery of the lens increases the depths of focus diminishes the quantity of light entering the eye

3. Accommodation of the eye for objects at different distances Convergence of the Eyeball Visual axis are so directed that the images will be formed on the corresponding points of the retina

3. Accommodation of the eye for objects at different distances Near Points and Far points of Distinct Vision Near Point the nearest point at which an object can be distinctly seen, with full accommodation. The distance increases with age slowly in early life most rapidly in the early 40’s very slowly after 50. progressive loss of the plasticity of the lens. Presbyopia

3. Accommodation of the eye for objects at different distances In the normal eye, parallel rays are brought to focus on the retina from infinity. Object at distances greater than 20 ft. are seen distinctly without accommodation, that is, with the eye at rest. Distance of 6 meters or 20 ft. is the Far Point of the normal eye.

3. Accommodation of the eye for objects at different distances Refractive Power and Amplitude of Accommodation The refractive power of a lens is usually expressed in terms of its principal focal distance or focal length. A lens with a focal distance of 1 meter is taken as a unit and is designated as having a refractive power of 1 diopter (D). The refractive power of the lens is expressed in terms of the reciprocal of their focal distances measured in meters lens with a principal focal distance of 0.10 meter is a lens of 10D, and one with a focal distance of 0.2 meters is a lens of 5D

4. Optic Defects and Abnormalities Optic Defects of the Emmetropic (Normal Eye) Spherical Aberration Chromatic Aberration Blind Spot

4. Optic Defects and Abnormalities Spherical Aberration In the optical lens, the marginal rays are focused in front of the focus of the central rays: thus blurring the image Corrected: Constriction of the iris Greater optical density of the nucleus of the lens with respect to the cortex

4. Optic defects and abnormalities Chromatic Aberration This is due to different dispersion of the light rays by the lens, according to their wavelength Chromatic aberration is most marked to wavelengths at the end of the spectrum

4. Optic defects and abnormalities Blind Spot Optic nerve enters the eye has no cones and no rods This produces a blind spot in the visual field The blind spot is 15 degrees to the temporal side of the visual field

5. Optical defects in the ametropic eye Emmetropia Ametropia

5. Optical defects in the ametropic eye Myopia Hyperopia or Hypermetropia Presbyopia: or “Old-Sightedness” Astigmatism

5. Optical defects in the ametropic eye Myopia Without accommodation come to a focus in front of the retina due the eyeball is too long the lens is too thick The far point is nearer than infinity All distant objects appear blurred Its near point is nearer than that of an emmetropic eye with equal amplitude of accommodation. Thus the term “nearsightedness” For distant vision, the remedy is the use of concave lenses

5. Optical defects in the ametropic eye Hyperopia or Hypermetropia Parallel rays of light without accommodation are focused behind the retina, that is, the retina is reached by the rays before they come to focus The uncorrected hyperope may see distant objects clearly only by the use of his accommodation The near point is greater than 10 cm The term “far-sightedness” refers mainly to the excessive distance of the near point Correction is by the use of convex lenses

5. Optical defects in the ametropic eye Presbyopia or “Old-Sightedness” A decrease in the amplitude of accommodation as a consequence of aging The near point of distinct vision recedes farther and farther from the eye until near is difficult or impossible All properly corrected eyes will become presbyopic at about the same time, at an age approximately 45

5. Optical defects in the ametropic eye Astigmatism An error of refraction due to the uneven curvature of the cornea The corneal surface is not spherical, so there is a meridian of least curvature and meridian of greatest curvature at right angle to the first Rays falling on the greatest curvature are focused earlier than those falling on the least curvature Correction is by the use of cylindrical lenses

6. Visual acuity Visual acuity is the sharpness with which details and contours of objects are perceived and constitutes the basis for form or object vision The zone immediately surrounding the fovea possesses the next greater capacity for detailed vision Visual acuity diminishes further towards the periphery The fovea is specialized for detailed vision in four ways: the cones are more slender and densely packed it is rod free blood vessels and nerves detour around it, and the cellular layers are deflected to the side, removing the scattering of light each cone is connected to one ganglion cell

6. Visual acuity Measurement of Visual Acuity Visual acuity is usually expressed in terms of minimum separable the smallest distance by which two lines may be separated without appearing as a single line. The angle that these two lines subtend at the eye is called the visual angle , which is one minute for the normal eye. Visual acuity can also be expressed in terms of minimum visible , the narrowest line or the finest thread that can be discriminated from a homogenous background.

6. Visual acuity Factors Modifying Visual Acuity Dependent upon Stimulus Brightness of object in contrast with dark background Intensity of illumination Size of object Dioptric Factors Spherical aberration Chromatic aberration Error of refraction Composition of light (monochromatic light improves visual acuity by decreasing chromatic aberration) Retinal Factors. The fovea centralis is adapted for acutest vision

6. Visual acuity Snellen’s Test Chart: Consists of 9 lines of letters in which the letters in each line are smaller than those in the previous line. The chart is viewed at a distance of 20 ft., or 6 m If at 20 ft. the individual reads the letters of the line marked 20, visual acuity is 20/20 which is considered normal If the individual can read only the line marked 100 (which a normal individual can read at 100 ft), his visual acuity is 20/100

7. Photoreceptors Signal transduction pathway is by which the energy of a photon signals a mechanism in the cell that leads to its electrical polarization. This polarization ultimately leads to either the transmittance or inhibition of a neural signal that will be fed to the brain via the optic nerve.

7. Photoreceptors In humans, the visual system uses millions of photoreceptors to view, perceive, and analyze the visual world. Moreover, the photoreceptor is the only neuron in humans capable of phototransduction (with an exception being the recently discovered photosensitive ganglion cell). All photoreceptors in humans are found in the outer nuclear layer in the retina at the back of each eye, while the bipolar and ganglion cells that transmit information from photoreceptors to the brain are in front of them.

7. Photoreceptors This arrangement requires two specializations: a fovea in each retina (for high visual acuity) and a blind spot in each eye, where axons from the ganglion cells can go back through the retina to the brain. Humans have two types of photoreceptors: rods and cones. Both are neurons that transduce light into a change in membrane potential through the same signal transduction pathway

7. Photoreceptors Rods and cones differ in a number of ways. The most important difference is in their relative sensitivity: rods are sensitive to very dim light, cones require much brighter light. The most notable being the absence of rods in the fovea. They differ in shape: rods are long and slender; cones are short and tapered. Both rods and cones contain light-sensitive pigments. All rods have the same pigment; cones are of three types, each type containing a different visual pigment. The four pigments are sensitive to different wavelengths of light, and in the case of the cones these differences form the basis of our color vision.

7. Photoreceptors The receptors respond to light through a process called bleaching. In this process a molecule of visual pigment absorbs a photon, or single package, of visible light and is thereby chemically changed into another compound that absorbs light less well, or perhaps differs in its wavelength sensitivity. In virtually all animals, from insects to humans and even in some bacteria, this receptor pigment consists of a protein coupled to a small molecule related to vitamin A, which is the part that is chemically transformed by light.

7. Photoreceptors 1. Incident light 2. Structural change in the retinine of photopigment 3. Conformational change of photopigment 4. Activation of transduction 5. Activation of phosphodiestrase 6. Decreased intracellular cGMP 7. Closure of Na+ channels 8. Hyperpolarization 9. Decreased release of synaptic transmitter (glutamate) 10. Response in bipolar cells and other neural elements

8. Visual pathways There are three main visual pathways in the central nervous system of vertebrates. The most commonly discussed pathway is the Thalamofugal Pathway, necessary for visual distinction of form and colour, as well as visual motion perception. The Tectofugal Pathway, on the other hand, is primarily involved in the processes necessary for visual orientation and spatial attention, and neurons within this neural circuit are frequently found to be sensitive to visual motion stimuli. The final pathway, the Accessory Optic System, is a subcortical pathway necessary for the perception of self-motion and gaze stabilization.

8. Visual pathways This scheme shows the flow of information from the eyes to the central connections of the optic nerves and optic tracts, to the visual cortex. Area V1 is the region of the brain which is engaged in vision .

Responses in the visual pathways and cortex The lateral geniculate nucleus: "Geniculate" means knee-shaped, and it is a pretty accurate description of the LGN. The stripes are actually layers, and there should be six of them in most parts of the LGN. Each layer receives inputs from a different eye: 3 layers for the left eye and 3 layers for the right. There is a second aspect of organization in the LGN. The outer 4 layers are composed of small cells, and correspondingly, receive inputs from the small ganglion cells of the retina. These layers are called the parvocellular layers . The magnocellular layers , on the other hand, are composed of large cells and receive their input from large ganglion cells.

Responses in the visual pathways and cortex On to cortex: The neurons in the LGN send their axons directly to V1 (primary visual cortex, striate cortex, area 17) via the optic radiations . This highway of visual information courses through the white matter of the temporal and parietal lobes. Once the axons reach V1, they terminate primarily in a single sub-layer of cortex. Brodmann first subdivided the cortex into over 50 areas. These areas are known today to correspond to functionally distinct areas - area 17 is primary visual cortex, for example.

Primary visual cortex The koniocortex (sensory type) located in and around the calcarine fissure in the occipital lobe. receives information directly from the lateral geniculate nucleus. highly specialized for processing information about static and moving objects and is excellent in pattern recognition. divided into six functionally distinct layers Layer 4, which receives most visual input from the lateral geniculate nucleus (LGN), is further divided into 4 layers(4A, 4B, 4Cα, and 4Cβ). Sublamina 4Cα receives most magnocellular input from the LGN layer 4Cβ receives input from parvocellular pathways. Axons from the interlaminar region end in layers 2 and 3.

Primary visual cortex Simple cells-respond only when the bar of light have a particular orientation. Complex cells-respond maximally when a linear stimulus is moved laterally without a change in its orientation. Simple and complex cells are called feature detectors due to their functions. So, primary visual cortex segregates information about color from that concerned with the form and movement, combines the input from two eyes and converts the visual world into short line segments of various orientation.

Other cortical areas V2, V3, VP-larger visual fields V3A-motion V5/MT-motion;put to control of movement V8-concerned with color vision in human. LO-recognition of large objects.

Vision

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