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Explore the physiology of special senses, including the pupillary light reflex, accommodation reflex, and abnormalities of the crystalline lens. Learn about conditions such as aphakia, presbyopia, cataract, and errors of refraction.
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بسم الله الرحمن الرحيم ﴿و ما أوتيتم من العلم إلا قليلا﴾ صدق الله العظيم الاسراء اية 58
Physiology of Special Senses By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology Member of American Society of Physiology Physiology of Special senses, Abdelaziz Hussein
Pupillary Light Reflex Significance of light reflex Physiologically: It is a protective reflex which prevents over exposure of the retina to harmful excessive illumination Medically: It is important for diagnosis and localization of lesions Argyll-Robertson Pupil Reverse Argyll-Robertson Pupil
Causes of Pupillary Miosis and Mydriasis Physiology of Special senses, Abdelaziz Hussein
The Crystalline Lens It is avascular, transparent biconvex elastic structure Lens capsule Lens Structure Lens fibers
The Crystalline Lens Lens transparency
The Crystalline Lens Functions of the lens:
The Crystalline Lens Functions of the lens:
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
The Crystalline Lens Abnormal conditions of the lens
Aphakia • Means absence of the lens from the eye. • Causes • It may be congenital or after surgical removal. • Treated with intraocular lens
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 • Causes • This is due to degenerative changes resulting in denaturation of lens proteins. • Means loss of lens transparency • As a result of: • Ultraviolet rays (coagulation of lens protein) • Diabetes mellitus (makes the lens protein more coagulable by light) • 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 In most cases, it is due to abnormally long eyeball. 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 In most cases, it is due to abnormally short eyeball. 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
