صدق الله العظيم الاسراء اية 58 Physiology of Special senses, Abdelaziz Hussein 1

صدق الله العظيم 58االسراء اية

Physiology of Special senses, Abdelaziz Hussein

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ByBy

Dr. Abdel Aziz M. HusseinDr. Abdel Aziz M. HusseinLecturer of Medical PhysiologyLecturer of Medical Physiology

Physiology of Refractive Apparatus Abdelaziz Hussein 2

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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

1. Informing the nervous system

about the external environment

3. Postural reflexes and equilibrium

2. Learning and education

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• The eye is the organ of vision which consists of 2 parts;

1. Eyeball2. Extraocular

accessory structure

EyeballEyeball

Lacrimal Lacrimal Gland Gland

EyelidEyelid Lacrimal drainage Lacrimal drainage systemsystem


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Conjunctiva

Cornea

Sclera

Iris

Ciliary Body

Choroid

Pupil

Retina

Vitreous Humour

Aqueous Humour

Lens


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• Optically, the eye is equivalent to the usual photographic camera.

• 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

• It has a lens system, a variable aperture, the pupil, & a retina that corresponds to the film.

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

Air

Medium

• When light rays strike a surface, they are either;

2. Reflected (white objects reflect all light)

1. Absorbed (black objects absorb all rays)

3. Transmitted through it, with or without refraction

• The degree of refraction depends on;

b. The refractive index of the substance.

a. The angle of incidence (incident rays striking the surface perpendicularly are not refracted)

• 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.

• It is the ratio of velocity of light in air to the velocity in the substance.

Focus or Focal point

Principal axis Nodal Point

Focal Length

Virtual Focus

Principal axis Nodal Point

= 1/ 0.1 = 10 diopters.

• 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. Convex and concave lenses2. Spherical and cylindrical lenses

Convex Lens Concave Lens

It converges the parallel rays

to one point called the focal

point or focus

It diverges light rays away

from focal point

It has real focus in behind of

the lens

It has virtual focus in front

of the lens

It is a plus lens (used in

correction of hypermetropia)

It is a minus lens (used in

correction of myopia)

Spherical Lens Cylindrical Lens

It is a part of sphere. It is a part of cylinder

It converges or diverges light

into one point.

It converges or diverges

light into one plane.

It is used for correction of

myopia and hypermetropia.

It is used for correction of

astigmatism.

Cornea Cornea

RI= 1.38RI= 1.38RP= +39 diopters RP= +39 diopters

Aqueous Humor Aqueous Humor

RI= 1.33RI= 1.33

Lens Lens

RI= 1.40RI= 1.40RP= +20 diopters RP= +20 diopters

Aqueous Humor Aqueous Humor

RI= 1.34RI= 1.34

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

Tears

O2

Aqueous humour (glucose)

Corneo-scleral junction (O2 and 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 Touch of cornea cornea

Trigeminal Trigeminal NerveNerve

Facial Facial NerveNerve

Pontine Pontine CenterCenter


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Blinking Blinking of Eyeof Eye


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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 formationMechanism 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

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

Retrolental space

Hyaloid canal

Capsule

Functions

It is avascular, transparent biconvex elastic structure

Lens Structure

Lens Lens fibers fibers

Lens Lens capsule capsule

Lens transparency

Functions of the lens:Functions of the lens:

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

Near point

The near point recedes by aging due to the decrease of the lens elasticity and ciliary muscle power

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

• 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 transparencyCauses

•This is due to degenerative changes resulting in denaturation of lens proteins.

• As a result of:a. Ultraviolet rays (coagulation of lens protein) b. Diabetes mellitus (makes the lens protein more coagulable by light)c. 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

• The condition is corrected by cylindrical lens with its longitudinal axis perpendicular to the plane to be corrected

• 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.

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صدق الله العظيم الاسراء اية 58 Physiology of Special senses, Abdelaziz Hussein 1