Learning Outcomes
By the end of lecture students should be able to:
- Enlist the refractive indices of various components of optical system
- Define accommodation
- Explain the mechanism of accommodation
- Explain the concept of reduced eye
- Summarize the concept of depth perception.
- Explain the process of its formation, circulation, and regulation of
aqueous humor. - Describe intraocular pressure and pathophysiology of glaucoma.
Optics of eye
- Refractive system of eye
- Composed of lens system, aperture and retina
- Lens system has 4 interfaces
- Air and cornea
- Cornea and aqveous humor
- Aqveous humor and lens
- Lens and vitreous humor
Refractive Index
- Internal index of air is 1
- Internal index of cornea is 1.35
- Internal index of aqveous humor is 1.33
- Internal index of crystalline lens is 1.40
- Internal index of vitreous humor is 1.34
- If all the refractive surfaces are algebraically added together,
then considered to be one lens —- REDUCED EYE - Single refractive surface
- Central point 17mm in front of retina
- Refractive power – 59 diopter (accommodated for distant
vision) - 2/3rd is provided by anterior surface of cornea
- 1/3rd by the lens – 20 diopter
Definition
Accommodation is the mechanism by which the eye changes
refractive power by altering the shape of lens in order to
focus objects at variable distances
Mechanism
- Refractive power of the lens can be increased from 20D—
34D - Accommodation of 14 D
- Necessary to focus the image on the retina
- Shape of the lens change
- Moderately convex to very convex
- A relaxed lens is almost spherical in shape.
- Due to the elastic retraction of the lens capsule
- Under normal resting conditions the lens is almost flat
- Lens is held in place by suspensory ligament (about 70 in
number) - Attach radially around the lens
- Pulling the lens edges towards the outer circle of the eye
ball - Attached to the anterior border of choroid and retina
- The tension on the ligaments causes the lens to remain
relatively flat
- Ciliary muscles
- At the lateral attachments of the lens ligaments to the eye
ball - Two set of smooth muscle fibers
- Meridional fibers
- Circular fibers
When
- Contract— relax the ligaments to the lens capsule—lens
assume a more spherical shape - Natural elasticity of the lens capsule
- Meridional fibers
- From the peripheral ends of suspensory ligaments to the
corneoscleral junction - Pull the insertions of lens ligaments medially towards the
edges of the cornea - Releasing the ligament tension on the lens
- Circular fibers
- Arranged circularly at the ligament
- Contract–decrease the diameter of the circle of ligament
attached—relax the ligaments
Accomodation
Ciliary Muscle— CONTRACTION
- Set for near vision
- Sphincter like action of circular fibers
- Suspensory ligament relaxes
- Exert less pull on lens, more convex
- Lens power increases
Ciliary Muscle— RELAXATION
- Eye at rest, set for far vision
- Suspensory ligament become taut
- Exert pull on lens
- Lens get flattens, less convex
- Lens power decreases to 20 diopter
Control Of Pupillary Diameter(Iris)
Circular fibers
- Sphincter like action
- Controlled by
parasympathetic nerves - Constricts pupil (Miosis)
Radial fibers
- Arranged radially
- Controlled by
sympathetic nerves
•Dilates pupil (Mydriasis)
Depth perception
Perception
The process of organizing and interpreting information, enabling us
to recognize meaningful objects and events.
Depth Perception
• is the visual ability to perceive the world in three dimensions
(3D) and perception by the distance of an object.
Binocular cues
• Based on the receipt of sensory information in three
dimensions from both eyes
Monocular cues
• Represented in two dimensions and observed with one eye
Binocular depth cues
Convergence
- Convergence of eyes occur while looking at close objects
- Divergence of eyes to look at distant objects.
Accommodation - Accommodate more to focus on closer objects
- The amount of convergence and accommodation can be
used as cues for absolute depth (for objects not too far
away from you)
Means of Depth Perception
- Size of images of known objects, on the retina
- Phenomena of moving parallex
- Binocular vision– stereopsis
Depth Perception
- Size of images of known objects, on the retina
- If a person’s height is known like 6 feet
- Distance can be determined
- Unconscious perception of size by brain– calculated automatically
- Phenomena of moving parallex
- Image of distant objects on the retina does not move with rotation of
head - Images of nearer object move with head rotation
- Relative distance of the different objects can be measured
- Binocular vision– stereopsis
- Images on two retinas are different
- Distance of two eyes is almost 2 inches
- Image closer to eye makes images on the left side of the left eye and
right side of the right eye - Farther objects forms image on the center of retina
AQUEOUS HUMOR
- Intraocular fluid
- Maintains sufficient pressure and keep the eyes distended
- Clear, colorless fluid that fills the anterior and posterior chambers of
the eye - Divided into
- Aqveous humor– freely flowing
- Vitreous humor– gelatinous mass with fibrinous network
Composition
- Water constitutes 99.9% of Aqueous humor
- Proteins (5-16mg/100ml) concentration in Aqueous humor is less
than 1% of its plasma concentration - Glucose – 75% of the plasma concentration.
- Electrolytes:
- Na+ → similar in plasma and aqueous
- Bicarbonate ion: Concentration in PC & in AC
- Cl ion concentration than plasma and phosphate concentration than
plasma
- Ascorbic acid concentration is very high
- Various components of the coagulation and anticoagulation pathways
may be present
Functions of aqueous humor
- Brings oxygen and nutrients to cells of lens, cornea, iris
- Removes products of metabolism and toxic substances from those
structures - Provides optically clear medium for vision
- Inflates globe and provides mechanism for maintaining IOP
- High ascorbate levels protect against ultraviolet-induced oxidative
products, e.g., free radicals - Facilitates cellular and humoral responses of eye to inflammation and
infection
The blood–aqueous barrier
- Barriers to the movement of substances from the plasma to the
aqueous humor. - In the ciliary body the barriers include
- Vascular endothelium
- Stroma
- Basement membrane
- Pigmented and non-pigmented epithelium
- The blood–aqueous barrier is responsible for differences in chemical
composition between the plasma and the aqueous humor
Physiological Properties
- Volume 0.31ml
- Refractive index 1.333
- PH 7.2
- Hyperosmotic
- Rate of formation 2-3ml/min
- IOP– 12-20mmHg
- Continually being formed and reabsorbed
- Balance between formation and reabsorption regulates total volume
and pressure of the intraocular fluid
Aqueous humor formation
- Aqueous humor is produced from pars
plicata along the crests of the ciliary
processes. - Aqueous humor is derived from plasma
within the capillary network of the ciliary
processes. - Three physiologic processes contribute to
the formation and chemical composition of
the aqueous humor: - Diffusion
- Ultrafiltration
- Active secretion
Outflow of Aqueous Humor
- Flows through the pupil into the anterior chamber of the eye
- Then it flows anterior to the lens and into the angle between cornea
and iris - Then through meshwork of trabeculae finally enters the canal of
schlemm, which empties into extraocular veins
Aqueous humor dynamics
- Secreted by ciliary epithelium lining the ciliary processes
- Enters the posterior chamber.
- It then flows around the lens and through the pupil into the
AC. - It leaves the eye by two pathways at the anterior chamber
angle
Canal Of Schlemm
- A thin walled vein that extends around the eye
- Endothelial membrane fenestrated allowing all sizes particles to pass
large from protein to RBCs - Canal of Schlemm—venous blood vessel
- Rather than blood, aqueous humor normally flows into it that it is
filled only with aqueous humor rather than with blood - Drain into larger veins– aqueous veins
Tonometry
- Cornea is anesthetized with a local anesthetic and footplate of the
tonometer is placed on the cornea - Force is applied to a central plunger, and corneal portion underneath
is pressed - Amount of displacement is measured and correlated with pressure.
- Intraocular pressure is regulated by the resistance to outflow of
aqueous humor from the anterior chamber into the canal of schlemm
Glaucoma
- When debris gets plugged in trabecular meshwork outflow of fluid is
blocked and prevents reabsorption - Conditions leads to glaucoma– referred to raised intraocular pressure
- Outside the canal of schlemm and trabecular plates are phagocytic
cells - That engulf this debris
- That clear the blockage and drain the aqueous humor
- Commonly block the iridocorneal junction
- Common cause of blindness
- Pressure rise upto 60-70mmHg
- Compression of optic nerve– block axonal flow of cytoplasm
- Damage to neuronal cell bodies in optic nerve– lack of nutrition
- Compression of retinal artery at times
- Treatment
- Drops reduce secretion and enhance reabsorption
- Operative techniques to clear the blockade
Cataracts
- Common in older people
- Cloudy or opaque areas in the lens due to denaturation of proteins in
lens fibers - Proteins coagulate to form opaque areas
- Lens become opaque obliterate light transmission impairs vision
- Treatment –surgical removal of the lens and replaced by artificial one
Visual Acuity
- Light focused on retina makes A spot of 11μm
- Average diameter of foveal cone is 1.5μm(1/7th diameter of light spot)
- Person can normally distinguish two separate points that are 2μm
apart on the retina - Normal visual acuity of human eye—25 seconds of arc
- Light rays from two separate points making an angle of 25 seconds
are appreciated as two points