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

By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology Physiology of Refractive Apparatus Abdelaziz Hussein2

3 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 4

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The eye is the organ of vision which consists of 2 parts; 1.Eyeball 2.Extraocular accessory structure Eyeball Lacrimal Gland Eyelid Lacrimal drainage system Physiology of Special senses, Abdelaziz Hussein 6

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8 Conjunctiva Cornea Sclera Iris Ciliary Body Choroid Pupil Retina Vitreous Humour Aqueous Humour Lens

Physiology of Special senses, Abdelaziz Hussein 10 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 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 axisNodal Point Focal Length

Virtual Focus Principal axisNodal 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 lenses 2.Spherical and cylindrical lenses

Convex LensConcave 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 LensCylindrical 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 RI= 1.38 RP= +39 diopters Aqueous Humor RI= 1.33 Lens RI= 1.40 RP= +20 diopters Aqueous Humor RI= 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 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 H 2 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 B 2 (Riboflavin) 3.Moistening of the corneal surface by tears 4.Metabolic pump of the endothelial cells

1.It acts as a powerful convex lens ( 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 Facial Nerve Pontine Center Physiology of Special senses, Abdelaziz Hussein 37 Blinking of 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 Nearly protein free (contains about mg/L).Na + concentration is higher plasma Vitamin C concentration is times higher than plasma Bicarboante concentration is higher than plasma.

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 eye2.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

Retrolental space Hyaloid canal Capsule

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 Functions

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

Lens transparency 1. Uniform arrangement of lens fibres2. 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:Functions of the lens: 1. It provides about 30% of the total diopteric power of the eye. Its R.I. is 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 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)

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 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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