Transcript No Slide Title

Basic structures of the eye
Learning Objectives
• Basic Visual Optics
• Identification of the major anatomical structures
of the eye
• Functions of structures of the eye
• Blindness & ocular morbidity
PRINCIPLE OF VISION
TRANSDUCTION FUNCTION :
The retina translates the light transmission into nerve pulses,
which will be interpreted by the brain. The retina behaves like a
projection screen which receives inverse images, the cerebral
centre redresses the image.
Optical System of the Eye (60D)
• Cornea – 75% of refracting power (45D)
• Lens – 25% of refracting power (15D)
Refractive Errors
: “normal“ eye.
• Emmetropia
Axial length and refractive power of the eye fit
together
.
• Ametropia eye requiring vision correction
.
Axial length and refractive power are not matching
e.g. Myopia, Hyperopia, Astigmatism
Source of picture : http://www.e-sunbear.com/pp_page2.html
Refractive Errors
Myopia
Hyperopia
Emmetropia
Object
Image
How to get Myopia
a) Bulbus too long
b) Curvature of
cornea too steep
“Eye ball too long is the major cause of myopia”
Correcting Myopia
• Myopia means
overall power of
the eye is too
strong
• Minus lens to
reduce the power
of the optical
system
Hyperopia – Far sightedness
• Eye too short or cornea too flat
• Total power too weak : Light being bent too less
• Images far away will be less affected, but images that
are close will appear blurry.
• Farsighted individuals are constantly focusing their
eyes (Accommodating) to see objects at a distance
Correcting Hyperopia
• Hyperopia means
total power of the
eye being too weak
• Plus lens to
increase power of
the optical system
Focal Point – Plus Lens
• Light passing through a convex lens will
focus at one point called the focal point
Focal Point – Minus Lens
• An imaginary or virtual focal point
occurs in the front of a minus lens
Calculating the Power of a Lens
• 1 Diopter =
1
Focal length of 1 meter
• The focusing power of any lens in diopters
can be calculated with the formula:
D=1
f
• D = power of lens
• f = focal length in meters
Example #1
• If a lens has a focal length of 2 meters
(f = 2m), then:
– Power = 1 / 2m
– Power = 0.5
Example #2
• If lens has a focal length of 250
millimeters (f = .25m), then:
– D = 1 / 0.25 m
– 4D
Spherical Lenses (Sph)
• Have the same
curvature and same
power in all
directions
• All light passing
though a spherical
lens will focus at the
same point
Astigmatism
• Cornea has non-spherical surface
Spherical
Toroidal
Astigmatism
• Myopia - blur distant vision
• Hyperopia - blur near vision (visual fatigue)
• Astigamtism - blue at distant and near
Astigmatic Lens (Cyl)
A lens correcting astigmatism is call a toric lens (Cylinder)
Example
Rx : Pl / +2.00 x 90
Rx : Pl / -2.00 x 90
Presbyopia
• The crystalline lens losses its elasticity with age
• Results : the eye fails to focus at near
• Onset : early 40s in Asians & mid 40s in Causacians
Presbyopia
• Usually +1.00D to +2.50D additional power over
distance Rx (depends on age & reading distance)
• Reading Rx required
– Reading glasses
– Bifocals
– Multifocal
Measuring Visual Acuity
• Measured by Snellen chart
• Measure of eye’s ability to
resolve detail (resolution
power)
Expressing Visual Acuity
• Visual acuity is expressed as a fraction 20/20
or 6/6 or in decimal point (e.g. 0.8 or 1.0 etc)
• 20/20 means patient can read the 20/20 line
on chart at a distance of 20 feet
• 6/6 means patient can read the 6/6 line at a
distance of 6 meters
Visual Acuity
• 6/18 = 20/60 = 0.33
• Meaning the letter should be barely read
at 18m (60ft) away is now read at 6m
(20ft) away only
• Vision is only 33% of a normal person
• What does 6/4.5 represents ?
Ocular structures
• Part 1: Surrounding ocular structures
• Part 2: Intra-ocular structures
Surrounding Structures
Structures around the eye :
•
•
•
•
•
Bony Orbit: protection
Extra-ocular muscles / Orbital fat
Eyelids: protection
Eyelashes: protection
Conjunctiva
External eye
• Thin mucus membrane covering the sclera
• Acts as a lining for upper and lower lids
Conjunctiva
• Thin mucus membrane
extending from the limbus
posteriorly covering the anterior
part of the sclera
• Then the conjunctiva is
reflected at the fornix to cover
the inside of the upper and
lower lids
• The folding of conjunctiva
between the eye ball and eye
lid creates a bag like structure
called the conjunctival sac
Tearfilm
• Produced by the tear & other glands
• A layer of water, oils and nutrients that
flow over the exposed surfaces of the eye
• Approximately .05ml/1.0ml of tears are
produced each day
• Virtually no tears are produced during
sleep
Tear film structure & function
Layer
Lipid
(oily)
Dimension
Source
0.1micron Meibomian
glands (eye
lid)
Function
Prevents
evaporation
Creates smooth
optical surface
Nutrition to
cornea
Aqueous 7 microns
(water)
Lacrimal
glands
Mucin
0.02-0.05
(mucous) microns
Goblet cells Stability of tear
(Conjunctiva) film
Part 2: The Eye
ANATOMY OF THE EYE
ANTERIOR SEGMENT
lens
POSTERIOR SEGMENT
Sclera
Choroid
Iris
Aqueous
Humour
Retina
Visual axis
Macula
vitreous
Cornea
Optic
nerve
+
Blood
vessels
Cornea
Structure
– Transparent; No blood vessels
– Transition to sclera via the limbus (location of incision)
– Endothelium critical to transparency (viscoelastics for protection)
• The shape of the cornea directly impacts visual acuity
• Function: major refractive structure of the eye (+45D)
• Pathology: Loss of transparency, irregularity
Corneal structure
1. Epithelium
2. Bowman’s
Membrane
3. Stroma
4. Descemet’s
Membrane
5. Endothelium
Limbus
• Located at junction of cornea & sclera
• 1.5mm wide
Sclera
• “The White of the Eye”
• Interwoven dense tissue
– Tendons of extra-ocular muscles blend with
it for insertion
Uvea
The middle layer of eye, consisting of 3 structures:
– Iris (11 – 13 mm) and Pupil (3 – 4 mm)
– Ciliary Bodies & Zonules
– Choroid
Iris & pupil
• Circular shaped diaphragm
containing 2 sets of muscles to
control the pupil size. (circular
& radial muscles)
• With pigment & non-pigmented
cells. Many pigments = 'brown
eyes'; some pigments = 'green
eyes‘; very few pigments =
'blue eyes‘
• The pupil should be equally big
& round and respond both to
light & accommodation
Uvea: Ciliary Body
– Contraction of ring shape
ciliary muscle causes the
ciliary body to move anteriorly.
– Also reduces the size of the
lumen & releases the tension
of the zonules (suspensory
ligament linking the ciliary
body with the lens) & the
crystalline lens becomes
thicker.
– This will increase the total
refractive power of the eye for
near vision. This process is
called accommodation.
Crystalline Lens
Parts of the human lens
• Lens capsule
• Sub-capsular epithelium (at
the backside of the anterior
capsule)
• Cortex
• Nucleus
Lens
• Biconvex transparent body
– Curvature : Ant = 10mm; Post = 6 mm (widely varies)
• Lens grows with age
– Diameter : 6mm at birth to 9 mm in adult
– Thickness : 3.5 mm at birth to 5.0 mm at age 80
– Weight : 65mg in infant to 270mg at age 80
• Capsule
– Body’s thickest basement membrane (thickest where
zonules attached)
– Basically collagen fibres in a mucopolyasccharide matrix
Lens
• Subcapular Lens epithelium
– simple cuboidal in appearance, keep multiplying &
elongates as they migrate from central region to the
peripheral of the lens capsule.
• Lens cortex – elongated columnar cells from
epithelium (the lens is like an onion – with layers)
– bulk of the substance of lens
– As the newer cells (fibres) are formed, they will
compress the older cell to the nucleus of the lens
• Nucleus
– Central hardened structure
– Increases in size with age
Zonules
• Zonules fibres pass from the basement
membrane of the unpigemted ciliary epithelial
cell to the lens capsule
• Zonules fibres are 1 – 10 micron thick & are
formed of collagen filaments about 20nm in
diameter
• There are 2 major groups of zonular fibres
(anterior & posterior capsular fibre sheets)
Aqueous humor
• fills anterior chamber (0.34 ml)
– Composition not quite but similar
to plasma (less protein, higher
ascorbate, pyruvate, & lactate
than plasma)
– nourishes lens and other tissues
• Produced by ciliary body
• escapes at the corneo-scleral
junction at the anterior angle of
the eye (the angle between the
iris and cornea) through the
trabecular meshwork into the
small blood vessels
• Removal rate : 2 – 3 uL / min
Uvea: Choroid
• Thin, spongy, highly vascular, dark brown, layer.
• Important to provide nutrition to the inner eye (retina)
Retina
• Inner layer of eye containing the sensory
receptors required for transmission of light
• The retina can be divided into10 layers & contains
120 millions photoreceptors & over 1 Million nerve
fibres. It is a highly active structure and require a
lot of nutrient supply – mainly from the choroid.
• The retina has a average thickness of 200um, 130
um in the centre of fovea to 550um at the margin
of the fovea. Total surface a
• The retina is considered the extension of the
brain.
Chorio-retinal structures
1. ILM
2. NFL
3. Ganglion cell layer
4. Inner Plexiform layer
5. Inner nuclear layer
6. Outer plexiform layer
7. Outer nuclear layer
8. ELM
9. Photoreceptor (outer segment)
10. Pigment Epithelium
11. Bruch’s membrane
12. Choriocapillaris
13. Choroid
Green = 10 layers of the retina
Inter-connections of cells in the retina
1. ILM
2. NFL
3. Ganglion cell layer
4. Inner Plexiform layer
5. Inner nuclear layer
6. Outer plexiform layer
7. Outer nuclear layer
8. ELM
9. Photoreceptor
10. Pigment Epithelium
11. Bruch’s membrane
12. Choriocapillaris
13. Choroid
The retina
Indirect Ophthalmoscopy
Direct Ophthalmoscopy
Ora Serratta
Examination of the retina
Direct ophthalmoscopy
• No pupil dilation
• Magnified view
• Can not exam the
periphery of the retina
• More use in examination of
the macular & Optic nerve
head
Indirect ophthalmoscopy
• Pupil dilation required
• No / low magnification
• Give a wider view of the
retina (upto Ora Serrata)
• Provide a stereoscopic
(3D) image
• Use in thorough exam of
the retina (retinal
degeneration or
detachment)
Retina
• High myopia is highly associated with retinal
degeneration
• Elongation of eyeball stretches on the retina resulting
in thinning of the retina especially in the midperipheral area resulting in choroidal & retinal
degeneration
• Thinning and degenerative retina may develop into
retinal holes / tears which allows liquid in the vitreous
to enter the sub-retinal space and resulting in retinal
detachment
• High myopic patients should have an annual dilated
retinal examination by a Eye Care Practitioner using
an indirect ophthalmoscope
Retina – Macula Area & Fovea
• Light entering eye focuses on an indentation in retina
called the macula lutea
• Contains the greatest density of cones - responsible
for central vision
Retina: macula & fovea
• Macula (macula lutea and fovea
centralis).
– At posterior pole of eye
– It is called the Macular Lutea because it
contains a lot of Lutein – a yellow substance
which is a form of Vitamin A to protect the
macular from free radical damages due to
high energy radiation
• Spot of most acute vision.
• Small shallow depression caused by
almost complete absence of inner retinal
layers.
• At center rods absent and only cones.
The Retina
The retina contains about 120 million rods and cones
- Cones are more concentrated at the macula
- Rods reach their maximum density at about 20° from the
fovea
Source of picture : http://www.webvision.med.utah.edu/photo2.html
Optic Nerve
• Carries visual impulses from the
retina to the brain
• Consist of 1 Million Nerve Fibres
Right
Optic track
Left
Optic Nerve
Optic Chiasma
Image on the L side of the visual field (red) is focused on the R side of the retina
in both eyes (red). All the nerve fibre in the nasal retina of the L eye (carrying the
signal from the L visual field) cross over to join the temporal retina fibre of the R
eye (also carrying signal of the L visual field). Then they travel together to the R
visual cortex via the R Optic track (Left visual field is interpreted by the R side of
the brain). Similarly, image on the R visual field will be sent to the L side of the
brain.
Optic Nerve Optic papilla (optic nerve, optic disc)
• site of the exit of optic nerve fibers
– 3 mm medial to the posterior pole of eye
• Neuro-sensory elements lacking so 'blind spot'
Vitreous Body
Vitreous Body
• Fills the posterior chamber (4ml)
• Give support & shape to the eye ball
• Composition
– Colorless near structure-less, gelatinous mass
– 99% water in a very fine network of collagen fibers
• The vitreous fibre condensed on the surface
forming a 100um thick cortical vitreous
• Degeneration of the vitreous can result in floaters
(patient seeing black dots or thread like shadows
floating around) or even traction to the retina
• Vitrectomy is a surgical procedure
to remove the problematic vitreous
Common Terminology related to anatomy
• Superior/inferior, temporal/nasal,
lateral/medial Anterior/posterior
• Clock hours reference
• OS:left eye:LE ; OD:right eye:RE; OU:both
eyes
• Unilateral / Bilateral
• Extra-ocular, intra-ocular, retro-ocular
End
For Further Queries Contact :
Ms. Priyanka Singh
Head – Optometry Service
Email – [email protected]