VISION -II Dr.Mohammed Sharique Ahmed Quadri Assistant Professor Department Basic Medical Sciences Faculty of Medicine Almaarefa Colleges 1

Astigmatism Condition in which curvature of cornea is not uniform. Rays passing though meridian of greater undergo greater refraction and focus in front of rays passing through meridian having lesser curvature. Hence the image at retina is not clear. Correction is by cylindrical lenses/ contact lenses.

Layers of Retina Retina – receptor containing portion of retina is actually an extension of the CNS Neural portion of retina consists of 3 layers of excitable cells – Outermost layer containing rods and cones (light sensitive ends faces away from light) – Middle layer of bipolar cells & associated interneurons – Inner layer of ganglion cells Axons of ganglion cells join to form optic nerve – Point on retina at which optic nerve leaves is the optic disc » Region often called the blind spot because no image can be detected here because of lack of rods and cones 3

Test the Existance of Blind Spot 4

Retinal Layers 5

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Eye Fovea – Pinhead-sized depression in exact center of retina – Bipolar and ganglion cells layer is pulled aside, so that light can directly strikes photoreceptors – Point of most distinct vision – Has only cones Macula lutea – Area immediately surrounding fovea – Fairly high acuity ( high concentration of cones) Macular degeneration – Leading cause of blindness in western hemisphere – “doughnut” vision – Loss of photoreceptors in macula lutea 7

Photoreceptors Rod and cone cells Consist of 3 parts – Outer segment (facing choroid) Detects light stimulus – Inner segment Contains metabolic machinery of cell – Synaptic terminal (facing bipolar cells) Transmits signal generated in photoreceptor on light stimulation to next cells in visual pathway 10

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Photoreceptors VISUAL RECEPTORS Rods :- 120 million maximum at periphery Responsible for dim light vision – (gray vision) Scotopic vision. Cones : 6 Million maximum at Fovea Centralis - Responsible for Bright light vision Acuity of vision Color vision 3 types of cones – Red /Blue/Green

Photopigments Undergo chemical alterations when activated by light Consists of 2 components – Opsin Protein that is integral part of disc plasma membrane – Retinal Derivative of vitamin A Light-absorbing part of photopigment 13

Photopigments – Rod pigment Rhodopsin – Absorbs all visible wavelengths – Cone pigments (color pigment) Respond selectively to various wavelengths of light A single photon of light can activate a rod whereas several hundred photons are required to activate a cone 14

RETINAL RECEPTORS RESPONSE TO LIGHT

Chemical response to light

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18 phototransduction

Properties of Rod Vision and Cone Vision

DARK ADAPTATION : If a person has been in brightly lighted surroundings for a long time and then moves to a dark area the retina slowly become more sensitive to light. This decline in visual threshold is known as dark adaptation. The retinal and opsins are converted back into photosensitive pigments. (It is nearly maximal in about 20 minutes).

LIGHT ADAPTATION When one passes suddenly from a dim to a brightly lighted environment, the light seems intensely & even uncomfortably bright. Then the eyes adapt to increased illumination & the visual threshold rises. This light adaptation occurs over a period of about 5 minutes. The photosensitive chemicals in both the rodes & cones are reduced to retinal & opsins (bleaching). The concentration of the photosensitive chemicals are considerably reduced.

COLOUR VISION It is the function of Cones. Cones contain colour pigments made up of Photopsin + 11 cis retinal. Young - Helmholtz Trichromatic theory of Colour Vision. 3 types of Cones are present. Each type contains a different pigment, which respond maximally to a different colour (wavelength) 3 primary colors – Blue, Green, Red.

Blue Cones respond maximally to blue colour with peak sensitivity to wavelength 440 nm. Green Cones most sensitive to green colour- 535 nm. Red Cones – red colour – 570 nm. All the other colors are sensed because of combination of these 3 types of cones.

Equal stimulation of all 3 cones causes the sensation of white color

COLOR BLINDNESS Classification of color blindness based on the Trichromatic theory. Defect in red – Prot Defect in green – Deuter Prefix Defect in blue – Trit Weakness for color – Anomaly. suffix Absence of color perception – Anopia.

Trichromats (Anomalous Trichromatic vision.) These people have all 3 types of cones present but one is weak. Weakness for red – Protanomaly Weakens for green – Deuteranomaly Weakness for blue – Tritanomaly Dichromats Have only 2 cones system. One type of cone is absent. Absence of red color – Protanopia. Absence of green color – Deutranopia Absence of blue color – Tritanopia

Monochromats Have only 1 Cone system. Very rare. Red green color blindness is most common. X linked recessive modes of inheritance. Females are carriers – males are sufferers.

Ishehara Chart 30

Left eye blind Bitemporal hemianopia Heteronymous Homonymous hemianopia with macular sparring

VISUAL CORTEX Area 17 – Primary visual area - Concerned with the appreciation of visual sensations. Area-18 – Secondary visual area/visual association area. - Concerned with correlation and integration of visual sensations. Area-19 – Occipital eye field - Concerned with movement of the eye. In each visual cortex, both the eyes are represented in alternating columns of cells called Ocular Dominance Columns. Thus corresponding regions of the retina lie side by side close to each other.