October 17, 2010

USMLE 1 Question 11

When our eyes attempt to focus on a close object what action is typically required?

a) increase in the tension of the lens capsule
b) the lens becomes less convex
c) the sympathetic system is stimulated
d) the zonnules of zinn contract
e) the ciliary muscles contract

The correct answer is E

The smooth muscle of the ciliary body consists of both radial and circular fibres. When we wish to focus on some close object we must increase the power of our optics. This process is called accommodation.

The out of focus retinal image triggers the parasympathetic system which contracts the ciliary muscle. The muscle moves forward and inwards; consequently the zonnules of zinn relax, decreasing the tension in the lens capsule which becomes more convex, increasing the lens' power.

This is an important junction where the iris and the sclera meet. Close by is the circular canal of Schlemm, which runs around the eye just below the limbus. Aqueous humour is exuded from secretory cells just below the pigment epithelium in the cauliflower-like ciliary processes. The aqueous humour drains through the Zonnules of Zinn to the posterior chamber and through the pupil to the anterior chamber. The fibrous Zonnules of Zinn, which support the lens, are attached to the valleys between the ciliary processes.

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