October 17, 2010

USMLE 1 Question 12

A physician is explaining to his student that the process of hearing requires that amplified mechanical energy is transmitted from the middle ear to the cochlea. He claims that the process of amplification is necessary in order to amplify weak vibrations in the air before reaching the inner ear.

Which of the following factors has the greatest amplifying influence on sound?

a) the ratio of the size of the oval window to the size of the eardrum
b) the shape of the middle ear
c) the bone lever system
d) the pressure gradient between middle ear and the nasopharynx
e) the physician is wrong, amplification does not occur within the middle ear

The correct answer is A

The amplified mechanical energy transmitted from the middle ear to the cochlea is transferred into hydraulic pressure. This hydraulic pressure creates movement to the cochlear duct and to the organ of Corti (the seat of hearing). The oval window is the interface between middle and inner ear. The stirrup passes on the vibrations to the oval window, a small thin membrane covering an opening in the bony case of the cochlea.

The size of the window (15-30 times smaller than the eardrum), produces critical amplification needed to match the vibrations in the air and the fluid in the cochlea. The sound is amplified at the window another 15 to 30 times.

Amplification from outer to inner ear.

There are three distinct areas where the incoming sound vibrations are amplified on the way to the inner ear. The reason for this is to magnify weak vibrations in air so that they can establish pressure waves in liquid.

The shape of the ear canal may increase pressure times 10.

The stirrup, anvil, and hammer bones of the middle ear may nearly triple the pressure.

The arrangement between the eardrum and oval window provide up to another 30 times magnification.

The results in amplification is 800 times before the sound wave sets the liquid of the inner ear into motion.

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