Accomodation is maximum in which of the following: March 2011
Correct Answer: Children
Description: Ans. B: Children As the lens gets older, the central cells become sclerosed and more compressed, thus forming a relatively hard nucleus. Consequently, the lens tends to respond less to changes in tension of the capsule (during accommodation) Thus, a normal child of 10 years is able to see a small object clearly when it is only 7 cm from the eye, while a person of 30 years of age may not see clearly at a distance less than 14 cm Accommodation It is the process by which the eye changes optical power to maintain a clear image (focus) on an object as its distance changes. Accommodation acts like a reflex, but can also be consciously controlled. Mammals, birds and reptiles vary the optical power by changing the form of the elastic lens using the ciliary body (in humans up to 15 diopters). The young human eye can change focus from distance to 7 cm from the eye in 350 milliseconds. This dramatic change in focal power of the eye of approximately 12 diopters (a diopter is 1 divided by the focal length in meters) occurs as a consequence of a reduction in zonular tension induced by ciliary muscle contraction. The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude has declined so the near point of the eye is more remote than the reading distance. When this occurs the patient is presbyopic. Once presbyopia occurs, those who are emmetropic (do not require optical correction for distance vision) will need an optical aid for near vision; those who are myopic (nearsighted and require an optical correction for distance vision), will find that they see better at near without their distance correction; and those who are hyperopic (farsighted) will find that they may need a correction for both distance and near vision. Accommodation decreases to essentially 0 dioptres at the age of 70 years. It is normally accompanied by a convergence of the eyes to keep them directed at the same point, sometimes termed accommodation convergence reflex Induced effects of accommodation When someone accommodates to a near object, they also converge their eyes and constrict their pupils. The combination of these three movements (accommodation, convergence and miosis) is under the control of the Edinger-Westphal nucleus and is referred to as the near triad.
Category:
Ophthalmology
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