Eye is a special sense organ

1. EYE PHYSIOLOGY
DR. Saima Zareen

2. LEARNING
OBJECTIVE
S:
Describe optics &
fluid system of eye

4. FLUID SYSTEM OF EYE
• Eyes filled with intraocular fluid to maintain
sufficient pressure to keep the eye ball
distended
• Divide into
1. AQUEOUS HUMOR
2. VITREOUS HUMOR

6. VITREOUS HUMOR(vitreous body)
• Present in: posterior segment of eye ball
• Transparent medium with refractive index of
1.34
• Consistency: gelatinous mass
• Composed of fine fibrillar network, held
together by elongated proteoglycan molecules.
• Avascular
• Water & dissolved substances can slowly

7. AQUEOUS HUMOR
⮚Colorless, transparent fluid present in
anterior segment of eye
⮚Volume: 0.31 ml
⮚Refractive index of aqueous humor is
1.33

8. MECHANISM OF SECRETION:
• Secreted by ciliary processes
• It is continuously being formed and absorbed. The
balance between formation and absorption
regulates the total volume and pressure of
intraocular fluid
• First an ultrafiltrate of plasma is formed in stroma
of ciliary processes
• Next, there is active secretion by epithelium of
ciliary processes which begins with active
transport of Na ions into the spaces between
epithelial cells.
• The Na ions pull Cl & HCO3 to maintain

9. ⮚Ion movement is followed by osmosis of
water from capillaries, the resulting solution
washes from the ciliary processes into
anterior segment of eye.
⮚Nutrients like amino acids, ascorbic acid
and glucose are transported across ciliary
epithelium by active transport or facilitated
diffusion.

11. OUT FLOW AND DRAINAGE OF
AQUEOUS HUMOR:
Fluid flows anterior to lens , into the
angle b/w cornea & iris, then into
network of trabeculae finally into canal of
schlemm

13. FUNCTIONS OF AQUEOUS HUMOR:
• Maintains intraocular pressure
• Transport vitamin c in front section as
antioxidant

14. INTRAOCULAR PRESSURE
• Pressure inside eye
• Normal value: 15mmHg (12-20)
MEASUREMENT OF INTRAOCULAR
PRESSURE:
The pressure is measured clinically by tonometry
REGULATION OF INTRAOCULAR PRESSURE:
Remains constant in normal eye 15mmHg (±
2mmHg)
At about 15 mmHg, rate of drainage of fluid is

16. GLAUCOMA
⮚One of the most common cause of blindness
⮚A degenerative disease in which there is
loss of retinal ganglion cells
⮚Pathological increase in intraocular
pressure causes visual damage.
⮚Pressure rise can be up to 60-70 mmHg.
⮚Pressure above 25-30mmHg can cause loss
of vision when maintained for long periods

18. REFRACTIVE POWER:
• How much is the ability to bend the light
rays
• Measured in diopters
• Is equal to 1/ focal length of lens in
meters

21. THE REDUCED EYE
• All the refractive surface are added together
and considered as a single lens- reduced
eye
• This is useful in simple calculations
• In reduced eye a single refractive surface is
considered to exist with its central point
17mm in front of retina and total refractive
power of 59 diopters

22. ACCOMMODATION
• Ability to adjust the refractive power of
lens, when one looks at a near object
(less than 6m distance)
• Stimulus is blurring of vision
• Regulated by negative feed back
mechanisms that automatically adjust the
refractive power of lens to achieve highest
degree of visual acuity

24. WHY THERE IS NEED TO ACCOMMODATE?

25. MECHANISM OF
ACCOMMODATIO
N
When a person is looking at a
distant object, the ciliary
muscle is relaxed, the
suspensory ligaments are
taut and anterior surface of
lens relatively flat.
With relaxed ciliary muscle,
the refractive of lens is less
(20 diopters).

26. • When the gaze is shifted from distant
object to near object, the ciliary muscle
contracts.
• Releases the tension on suspensory
ligaments-become relax
• Lens assume a more spherical/globular in
shape-refractive power of lens is increased
and image of the object is formed on the
retina

28. • In addition, there is
1. Constriction of pupil and
2. Convergence of visual axis.
• The three changes are collectively called
the “near response”

29. Accommodation is parasympathetic reflex

30. PRESBYOPIA
SENILE LONG SIGHTEDNESS Loss of ability to
accommodate in old age is known as presbyopia
• Maximum ability of accommodation is seen in young
children at age of 10 years
• As a person grows older, the lens becomes less
elastic because of progressive denaturation of lens
proteins
• The power of accommodation decreases to 2
diopters by age of 45-50 years; it then reaches to
zero by the age of 70

31. • Once the state of presbyopia is reached,
eye remain focused permanently at a
constant distance and can not
accommodate for both near and far vision
• Treated by bifocal glasses

32. NEAR
POINT
OF
VISION
The nearest point to the eye at
which an object can be brought into
clear focus by accommodation is
called the near point if vision
At the age of 10, the near point of
vision is 9cm
Near point recedes with age. By
age of 60years it is at a distance of
83cm

33. EMMETROPIA (Normal vision)
• Eye is considered to be
“emmetropic” or normal, if the
parallel light rays from distant
objects are in sharp focus on
retina when ciliary muscle is
completely relaxed
• This means that all distant
objects are seen clearly with
ciliary muscle relaxed in an
emmetropic eye .

34. ERRORS OF REFRACTION
AMMETROPIA:
1. HYPEROPIA (FAR SIGHTEDNESS)
2. MYOPIA (SHORT SIGHTEDNESS)
3. ASTIGMATISM

35. HYPEROPIA
• Also known as long sightedness/far sightedness
• Subject’s distant vision is better than near vision-
blurring of near vision
• Light rays from distant objects are focused behind the
retina
• If hypermetropia is slight, can be compensated by
accommodation
• All new born have hypermetropia of +4 diopters
• As eye ball grows, the defect disappears in most of the
children
• Complains of headache
• Correction: convex lens

37. MYOPIA
NEARSIGHTEDNESS
• Near vision is better than far vision
• Difficulty in seeing distant objects
• The image of distant the object is form in front of
retina
• Near vision is not defective because light rays from
near objects are diverging
• Can be congenital or acquired
• Excessive close work during activities such as reading
accelerates development of myopia
• Correction : concave lens

39. ASTIGMATISM
• Refractive error of eye that causes the visual image in one
plane to focus at a different distance from that of plane at
right angles.
• Results from non uniform curvature of cornea
• Cornea has different refractive power in two
planes/curvatures
• An astigmatic lens would be a surface like the an egg
placed by its side
• In common type, vertical curvature is more than horizontal
curvature
CORRECTION:
• Find a spherical lens by trial and error method that corrects
the focus in one of the two planes

41. CATARACT:
• Lens become opaque/cloudy
• Seen in old age or under certain
pathological conditions
• Metabolic defects, diabetes mellitus,
radiation, toxic agents, injury and systemic
diseases can result in early cataract
formation
• Treatment is lens implantation