DR YAHYA AL-FALKI MD KKU-COLLEGE OF MEDICINE OPHTHALMOLOGY-UNDERGRADUATE LECTURE SERIES Ocular Physiology

Objectives To be familiar with normal functions of ocular structures. Clinical applications.

Eye Lids The eye lids form the anterior –most part of the visual system. Anatomically consists of two parts. Frequent blink is essential for corneal protection. Eye lids contribute to the tear film. They are parts of facial expressions.

Tear Film It is highly specialized and well-organized Moist film. It covers the ocular surface. The rate of secretion is 1-2/minute

The Tear Film The Tear Film is composed of three layers: 1-Superficial Lipid Layer. 2-Middle Aqueous Layer. 3-Posterior Mucin Layer.

Functions Smooth optical surface. Cleans the ocular surface. Lubrication Antimicrobial activity.

Lacrimal System  Tear secretion balanced with drainage.

Conjunctiva  It is a mucous membrane.  It comprises epithelium,basement membrane and stroma.  It contributes to the tear film.

Cornea Clear refractive surface. Protective barrier. Thinnest centrally and thicker peripherally. 5 histological layers. Epithelium and endothelium are lipophilic.

Corneal Drug Permeability Factors contribute to drug permeability: 1-Lipid Solubility. 2-Water Solubility.

Cornea Corneal Transparency -Partially dehydrated. -Regular orientation of stromal collagen. - It is avascular. Corneal shape is maintained by structural rigidity and intra ocular pressure.

Corneal Decompensation Endothelial cell count is lower than the threshold number. Severe endothelial damage= irreversible corneal oedema

Aqueous Humour It is clear colorless solution. It produced by ciliary body. It is different from plasma. It leaves the eye by trabecular route.

Uveal Tract The pupil is dynamic structure. Contraction of ciliary body permits accommodation and trabecular outflow. Choroid has highest ocular blood flow.

Iris&Pupil The human pupil is circular aperture situated near the center of the iris. Pupillary diameter regulated by: 1-The sphincter. 2-The dialator. contraction of the sphincter makes the pupil smaller. Contraction of the dialator enlarges the pupil.

Sclera Tough Outer Coat. Comlete Sphere. It is thickest around and thinnest just posterior to It is avascular. It is opaque. Low metabolic demand.

Lens It is atransparent biconex strucure. It has low water and high protein. It is relatively hypoxic. It has the ability to change shape. It has antioxidant mechanisms.

Accommodation Cascade Ciliary muscle contraction Zonules relaxe Lens becomes more spherical refractive power

Presbyopia The most common refractive disorder of older people. It is due to stiffness of the aging lens. In emmetropes it is manifested at 40 what about myopes and hypermetropes ???

Glucose Metabolism Lens is avascular and surrounded by aqueous and vitreous. Glucose is metabolized through: 1-Glycolytic Pathway 2-Krebs Cycle 3-hexose Monophosphate Shunt. 4-Sorbitol Pathway.

Diabetic Cataract Glucose concentration in the aqueous is similar to that of plasma. In diabetes there is increased aqueous sugar. Glucose will be converted to sorbitol. Sorbitol accumilation in the lens will lead to diabetic cataract.

Vitreous 80% of ocular volume. It is transparent gel. It has shock-absorbing capacity. The ageing vitreous becomes progressively liquefied.

Retina Transparent light – transforming structure. It comprises photoreceptors interneurones ganglion cells retinal pigment epithelium The main function is phototransduction.

Photochemistry of Vision There are two photosensetive cells. The cones contain apigment known as idoopsin. The rods contain apigment called rhodopsin. Light absorbed by the photoreceptor pigments creating photochemical reactions. The photochemical product will initiate electrical signals.

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