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

 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

 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

Refraction Errors
Normal vision
Far sightedness
Near sightedness

Vision Correction
Myopia corrected with
concave lens
Hyperopia corrected
with convex lens

 It is the ability of the eye to
keep the image focused on the
retina (as the distance between
the eyes & the object varies)

 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
Nishtar ketn h i rBdy M nuhearmvmead nRaumzcalne UuL sR eihnma nt he brain stem. 16

Contraction pulls
ligament forward
relaxing tension on
suspensory ligament
making the lens fatter
ACCOMODATION

 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.

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

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

 Eye is filled with
intraocular fluid.
 Aqueous humor and
Vitreous humor.
They maintain
sufficient pressure in
the eyeball to keep it
distended.

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.

 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.

 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.

 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.

 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.

 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

 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 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
32

Eye physiology from guyton and halls physiology Part 1

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