Accommodation, Reduced Eye & Aqueous Humor Physiology Lecture Slideshow

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

1. Air and cornea
2. Cornea and aqveous humor
3. Aqveous humor and lens
4. 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

1. Meridional fibers
2. 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

1. Size of images of known objects, on the retina
2. Phenomena of moving parallex
3. Binocular vision– stereopsis

Depth Perception

1. 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

2. 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

3. 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