Transcript refractive media

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‫االسراء اية ‪58‬‬
‫‪1‬‬
‫‪Physiology of Special senses, Abdelaziz‬‬
‫‪Hussein‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Medical Physiology
Physiology of Refractive Apparatus Abdelaziz Hussein
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Vision is a complex process through which an image of
the external environment is formed on the retina of the
eye, and then conducted as a nerve impulse to the brain
where it is interpreted and recognized
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1. Informing the
nervous system
about the external
environment
2. Learning
and education
3. Postural reflexes and equilibrium
4
5
Lacrimal
Gland
• The eye is the organ
of vision which
consists of 2 parts;
1. Eyeball
2. Extraocular
accessory
structure
Eyeball
Eyelid
Physiology of Special senses, Abdelaziz
Hussein
Lacrimal drainage
system
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Physiology of Special senses, Abdelaziz
Hussein
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Aqueous
Humour
Sclera
Conjunctiva
Retina
Cornea
Vitreous
Humour
Pupil
Iris
Ciliary Body
Choroid
Lens
Physiology of Special senses, Abdelaziz
Hussein
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• Optically, the eye is equivalent to the usual photographic camera.
•
It has a lens system, a variable aperture, the pupil, & a retina that
corresponds to the film.
• The optical or lens system (refractive media) of the eye consists of
the cornea, aqueous humor, lens, and vitreous humor.
• This system is responsible for refraction of light that fall on it onto a
point on the retina
Physiology of Special senses, Abdelaziz
Hussein
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Electromagnetic
radiations
• Light is a form of radiant energy, consisting of electromagnetic
waves
• Its velocity in air it is 300,000 km/second, but it is much
slower through transparent solids & liquids e.g. in the glass it is
200.000 Km/sec
• The wave-length of visible light to the human eye ranges from
400 to 750 nm
• The ultraviolet rays are less than 400 nm and infrared radiations
are more than 750 nm.
• These radiations are not visible, but ultraviolet rays cause
darkening of skin and infrared rays cause heating of skin
•
When light rays strike a surface, they are either;
1. Absorbed (black objects absorb all rays)
2. Reflected (white objects reflect all light)
3. Transmitted through it, with or without refraction
Air
Medium
•
The degree of refraction depends on;
a. The angle of incidence (incident rays striking the surface
perpendicularly are not refracted)
b. The refractive index of the substance.
• It is the ratio of velocity of light in air to the velocity in the
substance.
• It is 1.0 for air, 1.5 for glass, and 1.33 for water
• Light rays passing from air to a denser medium (has high RI) are
refracted towards the perpendicular line, while those passing from
a dense medium to air are refracted away from the perpendicular
line.
Principal axis
Nodal Point
Focal Length
Focus or
Focal point
Principal axis
Virtual Focus
Nodal Point
• The power (or strength) of lenses is measured in diopters
e.g. the power of a lens having a focal length 10 cm or 0.1 meter
= 1/ 0.1 = 10 diopters.
1. Convex and concave lenses
2. Spherical and cylindrical lenses
Convex Lens
Concave Lens
It converges the parallel rays It diverges light rays away
to one point called the focal from focal point
point or focus
It has real focus in behind of It has virtual focus in front
the lens
of the lens
It is a plus lens (used in It is a minus lens (used in
correction of hypermetropia)
correction of myopia)
Spherical Lens
It is a part of sphere.
Cylindrical Lens
It is a part of cylinder
It converges or diverges light It converges or diverges
into one point.
light into one plane.
It is used for correction of It is used for correction of
myopia and hypermetropia.
astigmatism.
Aqueous Humor
RI= 1.34
Aqueous Humor
RI= 1.33
Cornea
Lens
RI= 1.40
RP= +20 diopters
RI= 1.38
RP= +39 diopters
This is an eye with simplified optics. In this eye, the overall refractive
media (power) during rest is represented by a single convex lens
placed 15-17 mm in front of the retina with total refractive power + 59
diopters
The image formed on the retina by refractive media is a true small
inverted image which is corrected in position by cerebral cortex
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•The cornea is the transparent anterior 1/6 of outer coat of
the eyeball.
•It is 11 mm in diameter & 1.0 mm in thickness.
•5 layers;
Significance
• Maintains dehydration and transparency of the cornea
Sources
Corneo-scleral junction
(O2 and glucose)
Tears
O2
Aqueous humour
(glucose)
• It contains ascorbic acid & glutathione which act as H2 acceptors
in anaerobic metabolism
Anatomic factors
1. Regular & uniform arrangement of the epithelial
cells & lamellae.
2. Absence of blood vessels & myelinated nerve
fibers
Physical factors
1. The refractive index of various layers of cornea is
the same.
2. Relative corneal dehydration by osmotic pump
and metabolic pump
1. Vitamin A
2. Vitamin B2 (Riboflavin)
3. Moistening of the corneal surface by tears
4. Metabolic pump of the endothelial cells
1. It acts as a powerful convex
lens (+ 39- 43 diopters)
having 70% of total diopteric
power of the eye.
2. The regular curvature of the
corneal surface helps the
formation of sharp clear images
on the retina.
3. Protection of inner structures of the eyeball by
a. Its fibrous structure
b. It absorbs ultraviolet rays that fall on the eye (protect the
retina)
c. The corneal reflex (touch of the cornea by any foreign body
e.g. piece of cotton results in reflex blinking of both eyes)
Touch of
cornea
Trigeminal
Nerve
Pontine
Center
Blinking
of Eye
Facial
Nerve
Physiology of Special senses, Abdelaziz
Hussein
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Physiology of Special senses, Abdelaziz
Hussein
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• It is a condition in which the curvatures of the cornea are
not the same in all directions (planes).
• It is treated by cylindrical or contact lenses
• It is a congenital condition in which there is a
conical protrusion of the cornea
• It is treated by contact lens or corneal grafting
• It is a condition in which part of the cornea becomes
nontransparent
• It is treated by corneal grafting.
• It constitutes the posterior
opaque 5/6 of the outer coat of
the eye.
• It is covered anteriorly by the
conjunctival membrane.
• Normally, it is whitish in adults
& bluish in infants & young
children.
• It is opaque due to marked
irregularity of its fibres
Functions of Sclera:
1. It protects the delicate inner
eye structures
2. It gives attachment to the
external ocular muscles.
Transparent colourless alkaline fluid that fills the anterior and
posterior chambers of the eye
Mechanism of formation
It is formed continuously by the Ciliary epithelium by facilitated
diffusion and active transport mechanisms at a rate 1-2 µL/min
Composition
Nearly protein free (contains about 100-200 mg/L).
Na+ concentration is higher plasma
Vitamin C concentration is 10-20 times higher than
plasma
Bicarboante concentration is higher than
plasma.
Circulation and drainage of aqueous humour
Circulation and drainage of aqueous humour
• After its formation it flows between the suspensory
ligaments of the lens into the posterior chamber.
• Then, it passes through the pupil into the anterior chamber.
• Lastly, it passes through the irido-corneal junction (filtration
angle) into the spaces of Fontana to the canal of Schlemm
which encircles the anterior chamber at the cornea-scleral
junction.
• Finally the aqueous passes from the canal of Schlemm to
the aqueous veins to the systemic veins.
• There is a balance between its rate of production and the
rate of drainage.
Functions
1. It is one of the important refractive media of the eye
2.It nourishes the avascular cornea and lens.
3.It also buffers the acid produced by the anaerobic
metabolism of the cornea and lens.
4.It has a mechanical function to keep the eyes rigid and
to maintain its refractory power.
5.It maintains the intraocular pressure constant by means
of its steady formation and drainage
Hyaloid canal
Retrolental space
Capsule
Functions
It is one of the refractive media of the
eye.
It supports the retina
It supports the crystalline lens and
prevents it from falling back
It maintains the spherical shape of the
eye
It is avascular, transparent biconvex elastic structure
Lens Structure
Lens
fibers
Lens
capsule
Lens transparency
1. Uniform arrangement of lens fibres
2. Absence of blood vessels
3. Dehydration maintained by an active
process
4. Constancy of its chemical composition
5. The refractive indices of the various
materials in the lens are nearly equal
Functions of the lens:
1. It provides about 30% of the total diopteric power
of the eye. Its R.I. is 1.4
2. It protects the retina by absorbing ultraviolet waves
3. Accommodation to near vision: the lens enables
the eye to see far and near objects clearly by the
mechanism of accommodation that changes its
power from 20 to 32 D
It is the process by which the optical system of the eye is adjusted
to see the near objects. It includes;
1. Medial convergence of both eyes.
2. Miosis of both eyes.
3. Increase lens convexity of both eyes.
Def
It is the distance between
the far point of distinct
vision (normally infinity)
and the near point of
distinct vision
Far point
The near point recedes by
aging due to the decrease
of the lens elasticity and
ciliary muscle power
Near point
Def
It is the difference between the power of the lens when
accommodation is relaxed for far vision and its power when fully
action in near vision
Age
(in year )
Near point
(cm)
Amplitude
accommodation
(diopters)
10
7
14.0
20
10
10.0
30
14
7.0
40
22
4.5
60
100
1.0
1) Aphakia: (absence of eye lens)
2) Presbyopia: (old sight)
3) Cataract: (loss of lens
transparency)
4) Errors of Refraction: myopia and
hypermetropia)
• Means failure of accommodation to near objects due to gradual
diminution of lens elasticity with advancing age
Causes
• It is due to loss of elasticity or sclerosis of lens capsule or
suspensory ligaments or weakness of the ciliary muscles.
• It is corrected by convex lens for near vision only
Means loss of lens transparency
Causes
• This is due to degenerative
denaturation of lens proteins.
•
a.
b.
c.
changes
resulting
in
As a result of:
Ultraviolet rays (coagulation of lens protein)
Diabetes mellitus (makes the lens protein more coagulable by light)
Old age (senile cataract) (glutathione is absent from the lens)
• Cataract is treated by
removal of the lens.
• Emmetropic (Normal) eye:
is the eye in which parallel
rays converge to a focus on
the retina.
• Ammetropic eye: is the eye
in which parallel rays can not
converge to a focus on the
retina
• It is a condition in which parallels rays converge to point in front of the
retina
Causes
1. In most cases, it is due to abnormally long eyeball.
2. Occasionally, it is due to abnormally great
curvature of cornea or lens.
• Objects can be brought nearer to the
eye to be seen distinctly.
• The condition is corrected by biconcave
lens (Divergent lens).
• It is a condition in which parallels rays converge to point behind the
retina
Causes
1. In most cases, it is due to abnormally short eyeball.
2. Occasionally, it is due to abnormally small curvature
of cornea or lens.
• The condition is corrected by biconvex
lens (convergent lens).
• It is a condition in which the curvatures of the cornea or to less extent
the lens are not the same in all planes
• so that rays fall on the eye are not focused in one focus on the
retina but some rays in one plane are focused on the retina while
those in other plane do not
• This causes blurring of vision.
• The condition is corrected
by cylindrical lens with its
longitudinal axis
perpendicular to the plane to
be corrected
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