Physiology of Vision (1)
Dr. Abdelrahman Mustafa
Department Basic Medical Sciences
Division of Physiology
Faculty of Medicine
Almaarefa Colleges
Learning Objecetives
By the end of this lecture, You should able to
Describe the functional anatomy of the eye
•Describe the refraction of light as it passes through the
eye to the retina, identifying the eye components that
account for refraction of light at the center of the eye and
away from the center.
•Describe the process of accommodation, contrasting the
refraction of light by the lens in near vision and in far vision.
•Describe the refractive deficits that account for myopia,
hypermetropia,, and astigmatism, and their correction by
eyeglasses
Important of vision
Definition
Visual perception is the ability to
interpret information and surroundings
from the effects of visible light reaching
the eye
Important of vision
1. Learning through written
speech
2. Maintenance of
equilibrium
Anatomy of the Eyeball
• Eye ball diameter =2.5cm
• Consist of 3 layers
1.Sclera(outer layer )
• Constitute the posterior 5/6 of the outer layer of
the eye
• It is formed of connective tissue, It is Opaque
Structure (normally it is whitish)
•
SO does not transmit light rays
• Modified interiorly to become transparent and
more curved to form the cornea
• 2.Choroid(middle layer)
• it contain many blood vessels and pigmented
•
it modified anteriority to form the ciliary body
and iris which is the colored part in front of the
lens
• The iris has central round opening called the pupil
• 3.Retina(inner layer )
• It Contains the visual receptors [rods and cones ]
that respond to the light
• Characterized by presence of an area that lacks
visual receptors called the optic disc or the blind
spot FOVEA CENTRALIS
Con
THE CILLIARY BODY:• Is circular structure consist of 3 parts
• A) Cilliary gland
• B) Cilliary muscles: inserted near to corneoscleral
junction
• C) Suspensory ligaments :Joined to lens ligiment
• It participates in accommodation , produces the
aqueous humor and maintain the lens zonules
THE IRIS :• The pigmented part of the eye
• It contain 2 types of muscles that adjust the diameter
of the pupil to control the amount of light reaching
the retina
• These muscles are :
a. dilator pupillae: contains radial muscle fibers that
cause pupillary dilatation in response to symp.
Stimulation
b. sphinctor pupillae : contains concentric muscle fibers
that cause pupillary constriction in response to
parasymp. Stimulation
• Lens
• Is a biconvex, crystallins,and transparent structure
bind suspensory ligaments for focusing light
CILNICAL :Opacity of the lens: Cataract
Con
THE RETINA :• Contains the visual receptors [rods and cones ]
that respond to the light
• The out put from the retina in form of AP passes
through the optic nerve and other structure of
the visual pathway to give perception of light
• Characterized by presence of an area that lacks
visual receptors called the optic disc or the blind
spot Temporal to the optic disc there is another
area known as the macula
• At the center of the macula is the fovea centralis
, the area with the highest visual acuity [contain
cones only
• nervous layers of the retina
(1) photoreceptor layer
(a) rods
(b) cones
(2) bipolar layer
(a) bipolar cells
(b) amacrine cells
(c) horizontal cells
(3) ganglion cell layer
Cavities of the Eye
• 1. anterior cavity
a. posterior chamber
b. anterior chamber
c. aqueous humor
e. canals of Schlemm
2. posterior cavity
a. vitreous humor
Con
• AQUEOUS HUMOR :• The clear liquid that fills the space between
the lens and the cornea
• Provides the nutrients to the a vascular
parts of the eye [the lens and cornea ] and
eliminates their waste products
• Its pressure maintains the normal convexity
of the cornea
• Increased production or decreased its
drainage leads to increased intraocular
CLINICAL: Glucoma
• VITREOUS HUMOUR :• The clear gelatinous material that fill the
space between the lens and the retina
• Consist of water , salts , sugars and collagen
fibers
• It provide structural support to the eyeball
and allows passage of light rays to the
retina
• CANALS OF SCHLEMM: for venous
drainage
Muscles of Eye Movement
Lateral rectus–moves the eye
outward, away from the nose
Medial rectus–moves the eye inward,
toward the nose
Superior rectus–moves the eye
upward and slightly outward
Inferior rectus–moves the eye
downward and slightly inward
Superior oblique–moves the eye
inward and downward
Inferior oblique–moves the eye
outward and upward
Nerve supply
•The extraocular muscles are supplied
by third, fourth
and sixth cranial nerves.
•The third cranial nerve
•(Oculomotor) supplies the superior,
medial and inferior recti and inferior
oblique muscles.
•The fourth cranial nerve (Trochlear)
supplies the superior oblique and
•The sixth nerve (Abducent) supplies
the lateral rectus muscle.
•CLINICAL : Paralysis of These Nerve=
Squint
The formation of images on the retina
involves three processes:
• 1. refraction of light rays
• 2. accommodation of the lens
• 3. constriction of the pupil
Con
Refraction is the change in
direction of a wave due
to a change in its speed
when a ray of light
crosses from one
material to another of
different density
• Refractive media of the
eye :• There are 4 refractive
media in the eye :
1. The cornea its refractive
2. The aqueous humour
3. The crystalline lens
4. The vitreous humour
Accomodation
• Accommodation :
• Is the process by which the optical system of the eye is adjusted
to see the near objects clearly
• Light rays are refracted by the cornea and lens to be focused on
the retina
Con
•
•
•
•
•
•
•
•
Accommodation :
On looking to a distant object [more than 6 meter]
The light rays are coming in parallel
Ciliary muscle is relaxed and the lens is flat
The rays are brought to a focus on the retina
On looking to a near object [close than 6meter ] :
Light rays are diverging
They will be focused behind the retina , this is
prevented by the near response which consist of :
1. Increased convexity of the lens [accommodation ]
2. Convergence of visual axis
3. Pupillary constriction
From the Eye Sight website of student Kyle Keenan at Steton Hall University.
Constriction of the Pupil
• 1. part of accommodation reflex
• 2. limits peripheral light
The iris constricts or dilates to adjust size of the pupil.
The pupil allows light to enter the posterior segment
of the eye.
Convergence of the Eyes
ERRORS OF REFRACTION
• Lens: Refracts (bends) light
• Focuses precise image on the retina (fovea)
through accommodation (changing thickness)
Myopia
•
•
•
•
Eyeball too long
Distant objects focused in front of retina
Cannot see far
Image striking retina is blurred
Correction:
• Concave lens or
• laser surgery to slightly flatten the cornea
Hypermetropia
•
•
•
•
Eyeball too short, lens too thin or too stiff.
Nearby objects are focused behind retina.
Needs some accommodation .
Image striking the fovea is blurred.
Correction:
• Convex lens
Astigmatism
• Irregular Curvature in parts of the cornea or
lens
• Causes blurry image
• This may be corrected by specially ground
lenses which( cylindrical ) compensate for
the irregularity or laser surgery.
Q1
• The structure that Modified interiorly to
become transparent and more curved to form
the cornea is
• A)Iris
• B)Choroid
• C)Retina
• D)Sclera
Q2
•
•
•
•
•
Cataract defined as :
A)Opacity of the lens
B) Increase the intraocular pressure
C) Errors of refraction
D)Paralysis of abducent nerve
Q3
• On looking to a distant object ,Which event
DOSE NOT occur
• A) The light rays are coming in parallel
• B)Ciliary muscle is relaxed and the lens is flat
• C)The rays are brought to a focus on the retina
• D) Convergence of visual axis
Q4
•
•
•
•
•
The cause of myopia is
A)Eyeball too short
B)lens too thin
C)Lens too stiff
D)Eye ball to long
References
• Human physiology by Lauralee Sherwood, 7th
edition
• Text book physiology by Guyton &Hall,12th
edition
• Text book of physiology by Linda .s
contanzo,third edition
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