Physiology of Special Senses By Dr Abdel Aziz

Physiology of Special Senses By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology Member of American Society of Physiology of Special senses, Abdelaziz Hussein 2

Pupillary Light Reflex

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

Accommodation Reflex (Near Reflex)

Causes of Pupillary Miosis and Mydriasis Miosis 1. Dark adaptation. 1. Exposure to light. 2. Distant vision. 2. Accommodation reflex. 3. Emotions and pain. 3. During sleep. 4. Cutting oculomotor nerve. 4. Horner's syndrome. 5. General anesthesia during 5. General anesthesia: - Stage of excitement (2 rd). - - Stage of paralysis (4 th). During surgical stage (3 rd). 6. Sympathomimetic e. g. adrenaline and 6. Pontine haemorrhage. ephedrine. 7. Parasympatholytics e. g. atropine and 7. homatropine. Parasympathomimetic e. g. pilocarpine and eserine. 8. Alcohol intoxication and chloroform 8. Morphine poisoning Physiology of Special senses, Abdelaziz Hussein 12

The Crystalline Lens

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

The Crystalline Lens transparency Uniform arrangement of lens fibres Absence of blood vessels Dehydration maintained by an active process Constancy of its chemical composition The refractive indices of the various materials in the lens are nearly equal

The Crystalline Lens Functions of the lens: It provides about 30% of the total diopteric power of the eye. Its R. I. is 1. 4 It protects the retina by absorbing ultraviolet waves 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

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 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

The Crystalline Lens Abnormal conditions of the lens 1) Aphakia: 2) Presbyopia: 3) Cataract: 4) Errors of Refraction:

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 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.

$Errors of Refraction • Emmetropic (Normal) eye: is the eye in which parallel rays$

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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