Understanding basic physiology of eye , Refractive errors and management

1. Special Senses
Dr MC Tayade
Associate Professor
Department of Physiology

2. Physiology of vision
Physiology of Hearing
Physiology of taste and smell

3. Physiology of vision

4. Physiology of vision – Objectives
1. Anatomy of Eyeball
2. Refractive errors
3. Physiology of vision
4. Colour vision
5. Light & Dark adaptation
6. Visual Pathway & Lesions

5. Major division of eyes :
• 1. Optical system – help to focus and form
an image on receptor cells (Retina)
• 2. Neural system – transmits optical
signals in the form of action potential
alongwith optic nerve to the visual cortex.

6. Eyeball – fluid filled spherical organ with
diameter of 24 mm
Eyeball – 3 layers
1. Fibrous Layer ( Cornea + sclera)
2. Vascular Layer ( Uvea )
3. Nervous Layer ( Retina )
1. Fibrous layer (outer )
Cornea – clear - avascular
Scleara – white –means “hard”

8. Only anterior 1/6th of eyeball is visible from
outside , which is transparent and more convex
is called – cornea.
Light rays enters eye through the cornea.
Cornea is avascular tissue.
Fibrous layer provides shape to eyeball and
protective.

9. 2. Vascular layer :
Middle vascular and pigmented uveal tract
forms choroid and cilliary body in its posterior
part and iris in its anterior part.

10. 3. Retina layer : ( Nervous layer )
Contains photoreceptors in association with
neural network .

11. Interior of eyeball is divided into :
1) Anterior chamber – aqueous humor
2) Posterior chamber – aqueous humor
3) Posterior cavity - vitreous humor
The cilliary and iris muscles are known as
intrinsic muscles of the eye.

12. Aqueous humor :
It is a protein free clear fluid from which the
lens and cornea derive their metabolic
requirements .
In cilliary process it is continuously form from
plasma.
The cilliary process form aqueous humor and
pour it into the posterior chamber .

13. Aqueous Humour :
Thin watery fluid
PH : 71 – 7.3
Contains in ant & post chambers
Formed from capillaries of the cilliary
process @ 2 cumm/ min
Low in protein – ultra filtrate
High concentration of Nacl
Low in glucose
High amout of hyaluronic acid
Viscosity : Low

14. From anterior chamber , the aqueous humor
gets absorbed by the spongy meshwork of
trabecular tissue.
Decreased these absorption leading to
increases intraocular pressure , known as
Glaucoma.

15. LENS :
It is biconvex transparent avscular highly
elastic crystaline structure held in behind the
iris and in front of vitreous body by the
suspensory ligaments.
The main function of lens is convergence of
light rays and focus them on the retina.
Development of any degree of opacity in lens
is known as cataract.
It generally occurs with age. ( Degenerative )

16. Fundus :
This is the posterior portion of interior of eye
as seen through opthalmoscope .
It is the only region in the body where
arterioles are readily visible.
It is mainly use for the diagnosis of ocular
diseases.

17. Fundus Examination

18. Macula lutea ( Yellow spot ) :
At the posterior pole of the eye there is a
yellowish pigmented spot , called macula
lutea.
Its location for fovea centralis .
Its thin ROD Free portion of the retina.
Its points of greatest visual acuity
( Sharpness of vision) & is highly
developed in Humans

22. Intraocular pressure ( IOP ) :
It is 10- 20 mmHg
The volume of lens, aqueous , vitreous , blood
inside blood vessels and elasticity of three
layers contribute to develop IOP
It maintains the shape of eyeball.
It is measured by tonometry
Increased in IOP causes glaucoma and in
long term leads to degeneration of the retina.

23. Intraocular pressure ( IOP ) :
Intraocular pressure is measured with
a tonometer as part of a comprehensive eye
examination.

24. Refractive errors of eye
Emmetropia – good vision 20/20
Myopia – nearsightedness
Hyperopia – farsightedness
Astigmatism – light does not focus to a single
point on the retina
Presbyopia – “old sight” – loss of ability to
accommodate or see up close

25. Light rays entered eye get refracted by
cornea .
Further refracted by both surfaces of lens.
Total refractive power of eye :
60 D ( Diopters)
out of which 43 D is contributed by cornea
while 17 is contributed by lens.

27. The emergent rays from posterior surface of lens
form a Real inverted and much smaller image
on retina .
But we actually we donot see inverted image
because neural processing helps the brain to
perceive visual images with upright appearance .
This phenomenon is inherent to us since birth.

29. The refractive errors may be due to :
1) Axial length of eye
2) Refractive Power
- Position of lens
- Refractive indices
- Curvature surface of cornea or the lens

32. Accommodation:
Focusing the eye to see close objects.
Lens is thin when stretched by suspensory
ligaments (low power)
When round ciliary muscle contracts, tension is
released from lens and it becomes thicker
(higher power lens).

33. When an individual change his gaze from
distant object to a near object , three
changes occurs in eye :
1) Increase in curvature of anterior surface
of the lens.
2) Convergence of visual axes
3) Constriction of pupil.

35. Astigmatism :
It is a refractive error in which parallel rays of
light from a distant object cannot converge to
a point focus on the retina due to unequal
refraction at different meridians.
It is corrected by using cylindrical lens .

37. Presbyopia :
This is a physiological phenomenon seen in
all persons at about 40 years in which the
point of vision recedes beyond normal
reading.
It occurs due to progressive loss of plasticity
of lens .
It is corrected by using a Convex lens.

38. Physiology of vision
In the dark, Na+ channels are held open by a
nucleotide called cyclic GMP (guanosine
monophosphate)
Inflow of sodium (“dark current”) triggers the
continual release of neurotransmitter .
This neurotransmitter is inhibitory- it prevents
bipolar cells from firing by hyperpolarizing
them.

39. When light strikes the retina, retinal (from
vitamin A) which is bent, straightens out,
and no longer fits into the opsin.
The two separate – this is called
bleaching.
The opsin becomes an active enzyme, that
activates other enzymes that break down
cyclic GMP.
Without cyclic GMP, the Na+ channels
close.

40. The receptor hyperpolarizes, stopping the
release of inhibitory neurotransmitter.
This decrease in inhibition allows the bipolar
cells to fire, and information is sent to the
visual cortex.
Differentiation of color is assisted by
horizontal cells.
In darkness, retinal isomerase converts trans-
retinal back to cis-retinal, which binds with
opsin forming a functional photopigment.