Accommodation reflex

The accommodation reflex is a reflex actionof the eye, in response to focusingon a near object, then looking at distant object (and vice versa). It is dependent on cranial nerveII(afferent limbof reflex), higher centres and cranial nerve III.

Image:Focus in an eye2.png A near object (for example, a computer screen) appears large in the field of vision, and the eye receives lightfrom wide angles. When focusing on a near object, the pupilconstricts in order to prevent diverging light rays from hitting the periphery of the retinaand resulting in a blurred image. As the pupil constricts, the lensbecomes more sphericalto allow for the diverging light rays.

Theories of mechanism

• Helmholtz - Hermann von Helmholtzproposed his theory of accommodation in the middle of the 19th century. When viewing a far object, the circularly arranged ciliary musclerelaxes causing the lens zonules to pull on the lens, flattening it. When viewing a near object, the ciliary muscles contract causing the lens zonules to slacken which allows the lens to spring back into a thicker, more convex, form.
• Schachar

See also

• Accommodation (eye)
• Ciliary muscle
• Presbyopia

External links

• Accommodation
• Exam of cranial nerve II- accommodation is usually tested as a part of this exam.

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It uses material from the http://en.wikipedia.org/wiki/Accommodation+reflex Wikipedia article Accommodation reflex.