October 17, 2010

Q&A: Physiology of the Eye

When you look at an object you see it because light reflects off the object and enters your eye. The eyes receive light from many different directions and distances. To be seen, all this light must focus on the comparatively tiny area of the retina. This means the eyes have to bend light from different angles and directions.

This article is for Medical Students & Professionals
This is a Question & Answer revision article designed for medical students and professionals preparing for the PLAB, MRCP or USMLE examinations. They are based on actual questions from these examinations. You may find the Anatomy Of The Eye article more useful, or one of our many articles on Diseases & Conditions, Medical Syndromes, Health & Wellness or Home Remedies.
In this article:
The visual system
MCQ: clinical scenario
MCQ: answer
MCQ: explanation

The visual system

The cornea is the first part of the process of focusing what you look at into an image on the back of the eye. Firstly, light passes through the transparent cornea. Most bending of light occurs here. Light then travels through the pupil and hits the lens. The lens also bends light, increasing the amount focused on the highly specialised cells of the retina.

MCQ: clinical scenario

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

MCQ questions & answers on medicalnotes.info

MCQ: answer

The correct answer is E

MCQ: explanation

Ciliary Process (see image below):
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.

Ciliary muscle

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.

1). George Mather: The Eye. Available online: http://www.lifesci.sussex.ac.uk/home/George_Mather/Linked%20Pages/Physiol/The%20Eye.html

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