Physiology of Refractive Apparatus of the Eye By

Physiology of Refractive Apparatus of the Eye By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology of Refractive Apparatus Abdelaziz Hussein 2

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 3

Importance of Vision 1. Informing the nervous system about the external environment 2. Learning and education 3. Postural reflexes and equilibrium 4

The Eye (Organ of Vision) 5

The Eye 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 6

The eyeball Physiology of Special senses, Abdelaziz Hussein 7

Eyeball Aqueous Humour Sclera Conjunctiva Retina Cornea Vitreous Humour Pupil Iris Ciliary Body Choroid Lens Physiology of Special senses, Abdelaziz Hussein 8

Refractive Media of the Eye And Image Formation

Camera Function of the Eye • 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 10

Light Electromagnetic radiations

Light • 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

Light

Physics of Light

Reflection, Absorption and Refraction

Reflection, Absorption and Refraction • 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

Refraction of Light • 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.

Refractive index (RI) • 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.

Refraction of Light through Convex Lens Principal axis Nodal Point Focal Length Focus or Focal point

Refraction of Light through Concave Lens Principal axis Virtual Focus Nodal Point

Refractive Power of Lens • 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.

Types of Lenses 1. Convex and concave lenses 2. Spherical and cylindrical lenses

Concave and Convex Lenses Convex Lens Concave Lens It converges the parallel rays It diverges light rays away to one point called the focal from focal 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 and Cylindrical Lenses 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 myopia and hypermetropia. astigmatism.

Refractive Media of the Eye

Refractive Media of the Eye Aqueous Humor RI= 1. 34 Aqueous Humor RI= 1. 33 Cornea Lens RI= 1. 40 RP= +20 diopters RI= 1. 38 RP= +39 diopters

Reduced or Schematic Eye 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

Reduced or Schematic Eye 29

1) The Cornea

Histology of the Cornea • 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;

Nutrition & Metabolism Significance • Maintains dehydration and transparency of the cornea Sources Corneo-scleral junction (O 2 and glucose) Tears O 2 Aqueous humour (glucose) • It contains ascorbic acid & glutathione which act as H 2 acceptors in anaerobic metabolism

Causes of Corneal Transparency 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

Factors maintaining corneal transparency 1. Vitamin A 2. Vitamin B 2 (Riboflavin) 3. Moistening of the corneal surface by tears 4. Metabolic pump of the endothelial cells

Functions of the cornea 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.

Functions of the cornea 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)

Corneal reflex Touch of cornea Trigeminal Nerve Pontine Center Blinking of Eye Facial Nerve Physiology of Special senses, Abdelaziz Hussein 37

Abnormalities of the Cornea Physiology of Special senses, Abdelaziz Hussein 38

Astigmatism • 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

Keratoconus • It is a congenital condition in which there is a conical protrusion of the cornea • It is treated by contact lens or corneal grafting

Corneal Opacity • It is a condition in which part of the cornea becomes nontransparent • It is treated by corneal grafting.

The sclera

The Sclera • 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

The Sclera Functions of Sclera: 1. It protects the delicate inner eye structures 2. It gives attachment to the external ocular muscles.

2) Aqueous Humour

The Aqueous Humour 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

The Aqueous Humour 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.

The Aqueous Humour Circulation and drainage of aqueous humour

The 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.

The Aqueous Humour 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

3) Vitreous Humour

Vitreous Body Hyaloid canal Retrolental space Capsule

Vitreous Body 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

4) The Crystalline Lens

The Crystalline Lens It is avascular, transparent biconvex elastic structure Lens Structure Lens fibers Lens capsule

The Crystalline 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

The Crystalline Lens 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

Accommodation to Near Vision 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.

Accommodation to Near vision

Range of accommodation 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

Amplitude or power of accommodation 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

Abnormal conditions of the Lens 1) Aphakia: (absence of eye lens) 2) Presbyopia: (old sight) 3) Cataract: (loss of lens transparency) 4) Errors of Refraction: myopia and hypermetropia)

Presbyopia • 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

Cataract Means loss of lens transparency Causes • This is due to degenerative denaturation of lens proteins. changes resulting in • 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.

Errors of Refraction • 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

Myopia (short-sight) • 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).

Hypermetropia (long-sight) • 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).

Astigmatism • 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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