PHYSIOLOGY OF VISION :::
Before light can reach the rods and cones of the retina it must pass through the cornea aqueous humor pupil lens and vitreous humor The first step in vision is retinal image formation and activation of photoreceptors The resulting nerve impulses are then passed to the visual areas of the cerebral cortex
Retinal Image Formation ::
This requires four basic processes
(1) Refraction of light rays (2) accommodation of the lens (3) constriction of pupil and (4) convergence of eyes
Light rays entering the eye from the air are refracted at the anterior surface of the cornea posterior surface of cornea posterior surface of the cornea anterior surface of the lens and posterior surface of the lens The degree of refraction that takes place at each surface is very precise and such that rays fall at the fovea centralis
The lens of the eye is biconvex furthermore it has the unique ability to change the focussing power of the eye by becoming moderately curved at one moment and greatly curved the next This change in curvature of lens is known as accommodation In far vision the ciliary muscle is relaxed and the lens is flattened In near vision the ciliary muscle contracts pulling the ciliary process and choroid forward towards the lens This causes shortening thickening and bulging of the lens and thereby increasing the curvatures
Constriction of pupil occurs in response to light reflex involving autonomic nervous system and it is purely the function of smooth muscles of iris i.e. constrictor or circular muscles of iris
Human eyes are such that they focus on only one set of the object (single binocular vision ) This type of vision is possible due to the phenomenon called convergence convergence refers to the medial movement of two eye balls so that they are directed towards the object being viewed Convergence is the function of the voluntary muscles attached to the outside of the eye ball called the extrinsic eye muscles These are superior rectus inferior rectus medial rectus lateral rectus Superior oblique and inferior oblique
Stimulation of photoreceptors ::
After the formation of image on the retina it is converted into nerve impulses The steps involved in the generation of nerve potentials are activation of rhodopsin and /or iodopsin of rods and cones respectively Hyperpolarization occurs as a result of activation of these pigments in response to light These impulses are then passed through optic nerve to the thalamus Here the fibres synapse with other neurons whose axons pass to the visual areas of the cerebral cortex located in the occipital lobe
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