1st Lecture on Physiology of Eye by Dr. Roomi

7/28/2019 1st Lecture on Physiology of Eye by Dr. Roomi

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Eye


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Eye

Macula lutea Area immediately

surrounding fovea Fairly high acuity

Fovea CENTRALIS Pinhead-sized depression

in exact center of retina Point of most distinct

vision Has only cones

Macular degeneration Leading cause of blindnessin western hemisphere

doughnut vision


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The Eye as a Camera

Total refractive power

of reduced eye: 59 d

Ant. Surface of cornea

provides: 40 d Lens within eye

provides: 19 d

IF WE BRING THE LENSOUT OF THE EYE ITS

POWER WILL INCREASE


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Measurement of the Refractive

Power of a LensDiopter


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REFRACTIVE ERRORS OF EYE


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Normal vision

Far sightedness

Near sightedness

Refraction Errors


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Myopia corrected with

concave lens

Hyperopia corrected

with convex lens

Vision Correction


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Accommodation

It is the ability of the

eye to keep the image

focused on the retina

(as the distancebetween the eyes & the

object varies)


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MECHANISM OF ACCOMMODATION

Accommodation results from contraction of the ciliary muscle, which is like asphincter muscle.

Under resting state ciliary muscle relax keep the aperture wide.

Relaxation of ciliary muscle apply tension on suspensary ligaments which pulls thelens taut.

Viewing an object 20 feet or more from a normal eye, the image is focused on theretina and the lens is in its more flat or least convex form.

As the object moves closer to the eyes the muscles of the ciliary body contractand narrows the aperture of the ciliary body that reduces the tension on zonularfibers that suspend the lens.

When tension is reduced, lens become more rounded and convex as a result of itsinherent elasticity.

Changes in the shape of the lens permit accommodation

The ciliary muscle is controlled almost entirely by parasympathetic nerve signals

transmitted to the eye through the third cranial nerve from the third nerve

nucleus in the brain stem.


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Contraction pullsligament forward

relaxing tension on

suspensory ligament

making the lens fatter

ACCOMODATION


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Near Point

The nearest point to the eye at which an

object can be brought into clear focus by

accommodation is called near point of vision.

Normally, it is 25 cm in young persons.

It shifts away from eyes in presbyopia.


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NEAR RESPONSE or accomodation for near

vision

The three components of near response are:

1. accommodation,

2. convergence of the eyeballs &

3. Pupillary constriction


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Presbyopia

As a person grows older, the lens grows larger and thicker andbecomes far less elastic, partly because of progressivedenaturation of the lens proteins.

The ability of the lens to change shape decreases with age.

The power of accommodation decreases from about 14diopters in a child to less than 2 diopters by the time a personreaches 45 to 50 years

It may even decreases to essentially 0 diopters at age 70years.

Treated by biconvex lenses


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Eye is filled withintraocular fluid.

Aqueous humor and

Vitreous humor.

They maintain sufficientpressure in the eyeball to

keep it distended.

intraocular fluid


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The intraocular fluid system of the Eye

Aqueous humor is

continually being formed

and reabsorbed .

The balance betweenformation and

reabsorption of aqueous

humor regulates the

total volume andpressure of the

intraocular fluid.


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The mechanism of formation of the

Aqueous humor

Aqueous humor is formed almost asan active secretion by the epitheliumof the ciliary processes.

Secretion begins with active transportof sodium ions into the spaces

between the epithelial cells. The sodium ions pull chloride and

bicarbonate ions along with them tomaintain electrical neutrality

All these ions together cause osmosis

of water from the blood capillarieslying in intercellular spaces.

Resulting solution washes from thespaces of ciliary processes into theanterior chamber of eye.


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Outflow of aqueous humor from the eye.

After forming . flows

first through the pupilinto the anteriorchamber of the eye.

Fluid flows anterior tothe lens & into angleb/w cornea and iris.

Then meshwork oftrabeculae to canal ofschlemm which emptiesinto extra ocular veins.


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Canal of Schlem

It is a thin walled vein that extends circumferentially

all around the eye.

Its endothelial membrane is permeable to large

protein molecules and particulate matter up to thesize of RBCs.


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Canal of Schlem

It is a venous vessel but contains only aqueous

humor instead of blood.

Small veins drain aqueous humor from the canal of

schlemm in to larger veins of the eye. These small veins are known as aqueous veins.


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Regulation of Intraocular Pressure

IOP remains constant in the

normal eye, which is

15mmHg (12-20) It is determined by

resistance to outflow of

aqueous

It can be measured with thehelp of optical instrument

called tonometer.

TONOMETER


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Tonometery

Cornea is anesthetized with a local anesthetic

Footplate of the Tonometer is placed on the

cornea.

A small force is then applied to a centralplunger which push the cornea slightly inward.

The amount of displacement is recorded onthe scale of the tonometer and this iscalibrated in terms of intraocular pressure.


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Glaucoma

Glaucoma is one of the causes of blindness.

It is a disease of the eye in which the intraocular

pressure becomes pathologically high sometimes

rising acutely to 60 to 70 mm Hg Pressures above 25 to 30 mm Hg can cause loss of

vision when maintained for long periods

1st Lecture on Physiology of Eye by Dr. Roomi

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