Dr. Nisreen Mansour

VISION;Refractory Media of the Eye

Dr. Nisreen Abo-elmaatyPhysiology Department

Dr. Nisreen Mansour

Visible Light

•Visible light (of wavelength ranging from 400-700 nm) only can excite photo-receptors•Infrared rays can not excite, felt only as heat ,Ultraviolet rays are filtered out by the lens of the eye

Dr. Nisreen Mansour

Basic Concepts

Refraction :• It is the bending of light rays when they travel in 2

transparent media with 2 different refractive indices, provided that they travel at angulated interface.

Refractive Index (RI) of a medium = velocity of light in air velocity of light in that medium• e.g. RI of cornea is 1.38 means = velocity of light in air / velocity of light in cornea = 1.38

Dr. Nisreen Mansour

Focal Point

Focal Length

• Parallel rays (coming from distance > 6 m) will be bent toward edges of a convex lens.

• At certain curvature of the lens, these rays will pass through a single point; Focal Point.

• Focal Length: is the distance between central part of lens & Focal point.

Dioptric power of a lens is a measure of its ability to refract light rays.

Refractive power = 1 meter / focal length = ? Dioptre.

Thus, Dioptric power is inversely related to focal length

Dr. Nisreen Mansour

The Eye Ball

• 24 mm in diameter• 3 layers + intraocular fluids & chambers

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Ocular Chambers & Fluids

The lens divides the ocular cavity into 2 chambers filled with intraocular fluids:

Aqueous Chamber Vitreous Chamber

•Anterior to lens•Divided by the iris into: anterior & posterior chambers.•Filled with Aqueous humour (clear watery fluid)

•Posterior to lens

•Filled with Vitreous humour (clear jelly-like fluid)

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Aqueous Humour• Clear transparent fluid filling ant. & post.

chambers. • Formed by the epithelial lining of ciliary

processes of ciliary body at a rate of 1-3 μl /min.

• Composition Relative to plasma, Aqueous H is: - alkaline, nearly protein-free fluid - Higher Na+ & HCO3 content - Higher vit. C, pyruvic & lactic acids - Lower glucose

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Circulation & Drainage of Aqueous humorCirculation & Drainage of Aqueous humor

Aq.H from ciliary processes → flows between suspensory Aq.H from ciliary processes → flows between suspensory ligaments → post.chamber → through pupil → ant. Chamber → ligaments → post.chamber → through pupil → ant. Chamber → through iridocorneal angle (filtration angle) →space of Fontana through iridocorneal angle (filtration angle) →space of Fontana → canal of Schlemm →aqueous veins → episcleral veins → → canal of Schlemm →aqueous veins → episcleral veins → systemic veins.systemic veins.

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Functions of Aq.H• One of refractory media of the eye

• Supply of O2 & nutrients for avascular cornea & lens.

• Buffering & removal of acid products of anaerobic metabolism of cornea & lens.

• Keeping intraocular pressure (IOP) constant by the balance between its formation & drainage.

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Vitreous Humour• Transparent gelatinous mass held together by

fibrillar network. (little flow of fluid; Vitreous body).• Filling the space between lens & retina.• Separated from lens by a very narrow Retrolental

space.• Enclosed in a thin hyaloid membrane, firmly

adherent to retina around optic disc.• Functions: - one of the refractive media of the eye. - support of intracocular structures (lens & retina). - Maintenance of spherical shape of eye.

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Medium Refractive Index (RI)

Refractive power

Air 1

Cornea 1.38 +39 Dioptres

Aqueous Humour 1.33

Lens 1.40 +20 Dioptres

Vitreous Humour 1.34

• The refractive power of cornea is mainly due to its anterior

• The Lens: half of bending occurs at its ant. surface & half at its posterior surface.

Refractive Media of the Eye

Dr. Nisreen Mansour

The Optical System of the Eye

The divergent light rays must pass through an optical system that brings them back into focus. The cornea & lens is the optical system of the eye that focus light rays onto the retina

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Cornea

• It is the transparent anterior 1/6 of outer layer.• Its thickness is 1mm at the periphery & 0.5 mm at

centre.• Richly supplied by sensory nerve endings (branches

of ophthalmic division of V cranial n).• The cornea itself is avascular, but the corneoscleral

junction is richly supplied by capillaries.• Derives its nutrition from aqueous humour

(glucose), tear film (O2) & corneoscleral capillaries.

Dr. Nisreen Mansour

Functions of Cornea1- The most powerful refractive medium of

eye; 39 dioptres (2/3 of refractive power of eye) → formation of sharp clear image.

2- Protective for sensitive intraocular structures:

- tough.

- absorbs UV rays.

- initiator of Corneal reflex which is a protective reflex.

Dr. Nisreen Mansour

What are the causes of Corneal Transparency?

1- Anatomical Structure• Non-vascularity • Regular arrangement of collagen fibres of substantia

propria & of corneal epithelium.• Non-myelinated nerve fibres running parallel to

collagen fibres.

2- Mild degree of corneal dehydration

Achieved by an active endothelial pump → maintenance of corneal fibres packed together.

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Abnormalities of Cornea1. Corneal Opacity; - is partial loss of corneal transparency. - is a complication of corneal ulcer. - ttt: corneal grafting.2. Astigmatism; - the corneal curvature is not equal in all planes. - ttt: cylinderical or contact lens.3. Keratoconus (conical cornea); - congenital condition characterised by conical protrusion of cornea. - usually bilateral & appears at puberty. - ttt: contact lens or corneal grafting.

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Is a biconvex lens composed of a strong elastic capsule filled with viscous transparent proteinaceous fluid. Lens diameter ~ 10 mm, its thickness ~ 4 mm. The capsule is attached to ciliary body by suspensory ligaments (70), the tension of these ligaments is controlled by ciliary ms.

The LensThe Lens

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The Lens• Its transparency

is caused by uniform

arrangement of its fibres,

no blood vessels, dehydration

normal lens metabolism.

• Is avascular; derives its nutrition & O2 supply from aqueous humour.

Dr. Nisreen Mansour

Function of the Lens• The 2nd major refractive medium of the eye;

Its refractive power (20 dioptres) represents ~ 30% of total refractive power of the eye.

• The important is that the lens is the only player responsible for adjustment for distance;

It can increase its refractive power, in response to nerve signals from the brain, providing the important mechanism of Accommodation.

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Accommodation Reflex (Near Response)

• Accommodation is the ability of the eye to keep the image of an object focused on the retina as the distance between the object & retina varies

• Accommodation Reflex (Near Response) is the changes occurring in the eye as a result of retinal blurred image of the near object.

• Response: 1- Constriction of the pupils. 2- ↑ Thickness of the lens (more spherical;

becomes very convex (instead of moderate convexity).

3- Medial convergence of the eyes.

Dr. Nisreen Mansour

Dr. Nisreen Mansour

The lens becomes more spherical The lens becomes more spherical during near vision (contraction of during near vision (contraction of

ciliary muscle)ciliary muscle)

Dr. Nisreen Mansour

• Pathway of Accommodation reflex:Blurred retinal image → retinal nervous elements →

optic nerve → optic chiasma → optic tract →LGB (thalamus) → optic radiation → primary visual area; 17; occipital lobe & association visual area; 18, 19 Area 8, Frontal eye field area Superior Colliculus, midbrain

Somatic III n. supplying medial recti

Convergence

Edinger-Westphall n. of III n.

Ciliary ganglion

+ Ciliary ms. +Constrictor pupillae ms.The image falls on fovea centralis

↑ curvature of lens↑ its refractive power

↑ depth of focus↓ excess lightPrevents aberrations

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Pathway of Accommodation reflex:Pathway of Accommodation reflex:

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• Amplitude of Accommodation Is the difference between refractive power

of the lens on maximum accommodation & in far vision when the lens is fully relaxed.

It decreases gradually with age due to loss of lens elasticity & weakness of ciliary ms.

• Near Point The nearest point is the minimal distance from

the eye at which an object can be brought into focus

recedes with age (same reasons above).

Dr. Nisreen Mansour

Age

(years)

Nearest point

(cm)

Amplitude of Accommodatio

n(dioptres)

10 10 14

20 14 10

30 20 7

40 50 5

60 100 1

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PresbyopiaPresbyopia is the physiological progressive is the physiological progressive

loss of the accommodation with loss of the accommodation with age due to loss of elasticity of the age due to loss of elasticity of the lens (partly because of lens (partly because of denaturation of lens proteins), denaturation of lens proteins), until the lens becomes totally non-until the lens becomes totally non-accommodating at the age of 70 accommodating at the age of 70 years.years.

Corrected by wearing convex lens Corrected by wearing convex lens for near vision (e.g. reading).for near vision (e.g. reading).

Dr. Nisreen Mansour

• Errors of Refraction In normal eye (emmetropic), parallel rays

from distant objects converge to a focus on the retina, provided that the ciliary ms. is completely relaxed.

If this does not occur → an error of refraction

• Types of refractive errors

1. Hypermetropia (long sight).

2. Myopia (short sight).

3. Astigmatism.

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Myopia (short sight)

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Hypermetropia (Long sight)

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Errors of RefractionErrors of Refraction

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Myopia Hypermetropia Astigmatism

DefecDefectt

Focus in Focus in front of retinafront of retina

Focus behind Focus behind retinaretina

Focus of rays in Focus of rays in one plane is at one plane is at different different distance from distance from that of rays in that of rays in another plane.another plane.

CauseCause -long eye ball-long eye ball

-Strong lens-Strong lens

-short eye ball-short eye ball

- Weak lens- Weak lens

- Unequal - Unequal curvature of curvature of cornea or lens.cornea or lens.

tttttt Biconcave Biconcave lens (diverge)lens (diverge)

Biconvex lens Biconvex lens ((↑↑refractive refractive power)power)

Cylindrical lens Cylindrical lens in suitable planein suitable plane

Contact lens.Contact lens.

Dr. Nisreen Mansour

• During near vision, which of the following is false?

a) The tension in the suspensory ligaments is increased

b) The tension in the lens capsule is decreased

c) The lens becomes more round

d) There is an increased signal to ciliary muscle

Dr. Nisreen Mansour

Dr. Nisreen Mansour

Control of Pupillary DiameterControl of Pupillary Diameter

The diameter of the pupil varies between 1.5 mm (bright light) – 8 mm (darkness).This achieved by the presence of 2 smooth muscles;1-Constrictor pupillae ms.; its fibres arranged in a circular manner around pupil. Receives motor postgang.para-sympathetic supply along short Ciliary nerves (pregang.from EW n of III relay in ciliary ganglion).2- Dilator pupillae ms.; its fibres arranged radially around pupil.Receives post gang. sympathetic from SCG (pregang. arise in 1, 2 Thoracic segments.

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Light ReflexLight Reflex

StimulusStimulus: exposure of one eye to light.: exposure of one eye to light. ResponseResponse: Ipsilateral constriction of pupil (: Ipsilateral constriction of pupil (Direct Direct

Light Reflex)Light Reflex) & Contralateral pupillary & Contralateral pupillary constriction (constriction (Indirect or consensual Light ReflexIndirect or consensual Light Reflex).).

PathwayPathway: light : light →→ retinal nervous elements retinal nervous elements →→ optic optic nerve nerve →→optic chiasma optic chiasma →→ optic tract optic tract →→ ipsilateral ipsilateral pretectal nucleus in midbrainpretectal nucleus in midbrain →→ along tectonuclear along tectonuclear tract around tract around aqueduct of Sylviusaqueduct of Sylvius →→ to to EW nuclei EW nuclei (parasymp n of III) of both sides(parasymp n of III) of both sides →→ motor pregang. motor pregang. to ciliary ganglion to ciliary ganglion →→ post ganglionic along short post ganglionic along short ciliary nerves ciliary nerves →→ + bilateral constrictor pupillae ms. + bilateral constrictor pupillae ms. →→ bilateral constriction of pupils. bilateral constriction of pupils.

Dr. Nisreen Mansour

Dr. Nisreen Mansour

Argyll-Robertson PupilArgyll-Robertson Pupil Non-reactive to light but reactive to Non-reactive to light but reactive to

accommodation.accommodation. Due to damage of tectonuclear tract (by Due to damage of tectonuclear tract (by

syphilis or syringomyelia of aqueduct of syphilis or syringomyelia of aqueduct of Sylvius).Sylvius).

Reverse Argyll-Robertson PupilReverse Argyll-Robertson Pupil Non-reactive to accommodation but reactive Non-reactive to accommodation but reactive

to light.to light. Due to lesion in occipitotectal tract. Due to lesion in occipitotectal tract.

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The Visual FieldThe Visual Field

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a. Blindness of ipsilat. Eye

(anopia of left eye.

b→ bitemporal hemianopia (heteronymous hemianopia)

c→homonymous hemianopia

Lesion area 17(d)→ homonymous hemianopia of opposite side with macular sparing

Dr. Nisreen MansourDr. Nisreen Mansour

GOOD LUCKGOOD LUCK