Physiology of Vision (1)

Dr. Abdelrahman Mustafa Department Basic Medical Sciences

Division of Physiology Faculty of Medicine Almaarefa Colleges

بسم الله الرحمن الرحيم

Learning Objecetives

By the end of this lecture, You should able toDescribe 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

Important of vision 1. Learning through written

speech 2. Maintenance of

equilibrium

Definition Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye

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 lightCILNICAL :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, fourthand 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 refractive2. The aqueous humour3. The crystalline lens4. 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 pupil allows light to enter the posterior segment of the eye.

The iris constricts or dilates to adjust size of the pupil.

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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