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PSYCH 2E03 (70)

introduction to vision (3).docx

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Gautam Ullal

October 1 , 2012 Psych 2E03: Sensory Processes Introduction to Vision (3) Range of Accommodation - distance between far and near point - far point: 6 meters, no accommodation required - near point: 25-30cm maximum accommodation Changes with accommodation - eye balls converge: rotation - pupils constrict: pupillary constrictors are circular smooth muscles in the iris, minimizes diameter, therefore there is less light governed by iris muscles, intra-ocular muscles, organized in circular fashion, pupillary constrictors, extra-ocular muscles are skeletal muscles, and muscles which radiate from the pupil - increase in the anterior curvature of lens: lens is suspended by suspensory ligament, lens capsule that wraps around the lens, as long as the ligaments are held tight the lens is going to be flat, if the ligaments becomes loose the lens curves, lens has it’s own elastic property, suspensory ligaments are controlled by muscles inside the ciliary body. Ciliary muscles contract, suspensory ligaments become loose and anterior curvature of lens increases. Anterior changes more than posterior Ciliary muscle & suspensory ligaments of lens Change in the lens curvature with accommodation - when ciliary muscles are relaxed suspensory ligaments is pulled, lens is flatter - accommodated state: when ciliary muscles are contracted, suspensory ligament become loose and the lens becomes curved due to the elastic properties of the lens Motor Nerve for accommodation - III Cranial nerve. Oculomotor, stimulates all three processes which contribute to accommodation Oculomotor Nerve - controls all three muscles Presbyopia = “Old eye” accommodation is compromised with ageing - lens gets fixated at a particular curvature - lens is no longer flexible - lens becomes more and more flat - bifocal lens: distance and near points Cycloplegia = Paralysis of Accommodation - pupils dilate - everything looks very bright - damage to III cranial nerve (ocullomotor nerve) causes cycloplegia - cyclopegics are drugs that cause paralysis of accommodation - in cycloplegia, pupils are dilated and lens remains fixed in a relatively flat position - used for surgery on lens - atropine a cetylcholine receptor blocker causes cycloplegia lasting for a few hours, especially in the sm
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