VISION -I

Dr.Mohammed Sharique Ahmed QuadriAssistant Professor

Department Basic Medical Sciences Faculty of Medicine Almaarefa Colleges

بسم الله الرحمن الرحيم

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Eye

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Eye• Spherical, fluid-filled structure enclosed by 3 tissue layers

– Sclera/cornea• Sclera – tough outer layer of connective tissue; forms visible white part of

the eye• Cornea – anterior, transparent outer layer through which light rays pass

into interior of eye– Choroid/ciliary body/iris

• Choroid - middle pigmented layer underneath sclera which contains blood vessels that nourish retina

• Choroid layer is specialized anteriorly to form ciliary body and iris– Retina

• Innermost coat under choroid• Consists of outer pigmented layer and inner nervous-tissue layer

– Rods and cones» Photoreceptors that convert light energy nerve impulses

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Eye • Interior consists of 2 fluid-filled cavities separated

by the lens– Posterior cavity• Larger cavity between lens and retina• Contains semifluid, jellylike substance = vitreous humor

– Important in maintaining the spherical shape of eyeball

– Anterior cavity• Anterior cavity between cornea and lens• Subdivided into

– Anterior chamber: between iris and cornea– Posterior chamber: between iris and lens

• Contains clear, watery fluid = aqueous humor– Carries nutrients for cornea and lens– Produced by capillary network within ciliary body

Formation and Drainage of Aqueous Humor

Aqueous humor is formed by capillary network in ciliary body, then drains into the canal of Schlemm, and eventually enters the blood.

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Eye • Iris – Controls amount of light entering eye– Pigment in iris is responsible for eye color– Varied flecks and lines are unique for each individual • Basis for latest identification technology

– Contains 2 sets of smooth muscle networks• Circular (or constrictor) muscle(innervated by parasympathtic)• Radial (or dilator) muscle ( innervated by sympathetic)

• Pupil – Round opening in the center of iris through which light

enters the eye– Size of opening is adjusted by variable contraction of the

iris muscles to admit more or less light as needed

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The amount of light entering the eye is controlled by the iris

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PUPILLARY LIGHT REFLEX • When light is directed into one eye, the pupil

constricts Direct light reflex.• The pupil of the other eye also constricts

consensual light reflex.

• The visual system detects and interprets light stimuli, which are electromagnetic waves.

• The eye can distinguish two qualities of light: its brightness(intensity) and its wavelength.

• For humans, the wavelengths between 400 and 700 nanometers are called visible light

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Eye• Convex structures of eye produce convergence

of diverging light rays that reach eye(bent inwards), to be focused into a point ( focal point) on retina to provide an accurate image.

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Refraction

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Eye• 2 structures most important in eye’s refractive

ability are– Cornea• Maximum refraction of light takes place at cornea-

about 42 diopters. Diopters is the unit of refractive power. • Total refractive power of eye –60D.• Remaining power comes from lens.• Refractive ability of cornea remains constant

because curvature never changes– Lens • Refractive ability can be adjusted by changing

curvature as needed for near or far vision

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Eye

Focusing on Distant and Near Light Sources

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• How strength of lens can be adjusted to focus the light from near source into a focal point on retina ?

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Accommodation:– Ability to adjust strength of lens (The process by which

the curvature of the lens is increased is called accommodation) .

– Increases strength of lens for near vision – Strength of lens depends on its shape, which is regulated

by ciliary muscle– Accomplished by action of ciliary muscle and suspensory

ligaments– Ciliary muscle: part of ciliary body– Age-related reduction in accommodation ability –

presbyopia ( elasticity of lense decreases )

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Accommodation:– When ciliary muscles are relaxed(during far vison) the

suspensory ligaments are taut and the lens is pulled into flattens shape .

– When ciliary muscles contract ( during near vision) the tension on the suspensory ligament is loosened the lens assumes a more spherical shape increases the refractive power of the lens object is focused on the retina.

– Age-related reduction in accommodation ability - presbyopia

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Mechanics of Accommodation

THE NEAR RESPONSE When an individual looks at a near object:• There is convergence of visual axes.• Accommodation & • Pupillary constriction

This 3 part response is called the near response.

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Emmetropia

FAR SOURCE FOCUSED ON RETINA WITHOUT ACCOMODATION

NEAR SOURCE FOCUSED ON RETINA WITH ACCOMODATION

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Myopia( NEAR/SHORT SIGHTEDNESS)

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Hyperopia(FAR SIGHTEDNESS)

Astigmatism• Condition in which curvature of cornea is not

uniform.• Rays passing though meridian of greater

curvature 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.

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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

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Retinal Layers

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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

References

• Human physiology by Lauralee Sherwood, seventh edition

• Text book physiology by Guyton &Hall,11th edition

• Text book of physiology by Linda .s contanzo,third edition