Eye physiology from guyton and halls physiology Part 1

Nishtar ken By Muhammad Ramzan UL Rehman 1

Nishtar ken By Muhammad Ramzan UL Rehman

2

PHYSIOLOGY OF EYE


EYE


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 blindness in

western hemisphere “doughnut” vision


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 LENS OUT OF THE EYE ITS POWER WILL INCREASE


MEASUREMENT OF THE REFRACTIVEPOWER OF A LENS—“DIOPTER”


REFRACTIVE ERRORS OF EYE


Normal vision

Far sightedness

Near sightedness

Refraction Errors


Myopia corrected withconcave lens

Hyperopia corrected with convex lens

Vision Correction


ACCOMMODATION It is the ability of the eye to

keep the image focused on the retina (as the distance between the eyes & the object varies)


MECHANISM OF ACCOMMODATION

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

Under resting state ciliary muscle relax keep the aperture wide.

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

Viewing an object 20 feet or more from a normal eye, the image is focused on the retina 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 contract and narrows the aperture of the ciliary body that reduces the tension on zonular fibers that suspend the lens.

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


Contraction pulls ligament forwardrelaxing tension on suspensory ligamentmaking the lens fatter

ACCOMODATION


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.


NEAR RESPONSE OR ACCOMODATION FOR NEAR VISION

The three components of near response are: 1. accommodation, 2. convergence of the eyeballs & 3. Pupillary constriction


PRESBYOPIA As a person grows older, the lens grows larger and thicker and becomes

far less elastic, partly because of progressive denaturation of the lens proteins.

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

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

It may even decreases to essentially 0 diopters at age 70 years. Treated by biconvex lenses


INTRAOCULAR FLUIDEye is filled with intraocular fluid.

Aqueous humor and Vitreous humor.

They maintain sufficient pressure in the eyeball to keep it distended.


THE INTRAOCULAR FLUID SYSTEM OF THE EYE Aqueous humor is

continually being formed and reabsorbed .

The balance between formation and reabsorption of aqueous humor regulates the total volume and pressure of the intraocular fluid.


THE MECHANISM OF FORMATION OF THE AQUEOUS HUMOR Aqueous humor is formed almost

as an active secretion by the epithelium of the ciliary processes.

Secretion begins with active transport of sodium ions into the spaces between the epithelial cells.

The sodium ions pull chloride and bicarbonate ions along with them to maintain electrical neutrality

All these ions together cause osmosis of water from the blood capillaries lying in intercellular spaces.

Resulting solution washes from the spaces of ciliary processes into the anterior chamber of eye.


OUTFLOW OF AQUEOUS HUMOR FROM THE EYE. After forming …. flows first

through the pupil into the anterior chamber of the eye.

Fluid flows anterior to the lens & into angle b/w cornea and iris.

Then meshwork of trabeculae to canal of schlemm which empties into extra ocular veins.


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 the size of RBCs.


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.


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 the help of optical instrument called tonometer.

TONOMETER


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 central plunger which push the cornea slightly inward.

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


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

Nishtar ken By Muhammad Ramzan UL Rehman

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Eye physiology from guyton and halls physiology Part 1

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