The acoustic reflex protects the ear from intense sounds by:

A. relaying high amplitude vibration through the helicotrema.

B. opening the oval window to transmit vibration.

C. stiffening the round tympanic membrane.

D. transmitting only low-frequency sounds to the brain.

E. contraction of the stapedius and tensor tympani muscles.

E. contraction of the stapedius and tensor tympani muscles.

The acoustic reflex protects the ear from intense sounds by contraction of the stapedius and tensor tympani muscles.

Explanation:

Although there are two middle ear muscles (tensor tympani and stapedius) in humans, only the stapedius muscle contracts in response to sound.

Contraction os stapedius causes a change in the axis of rotation of the stapes footplate, thus increasing the immitance of the middle ear system. This change in conductivity through the middle ear can be measured indirectly as changes in acoustic immitance. This reflex results in decreased sound transmission to trochlea.

Hence the answer is E. contraction of the stapedius and tensor tympani muscles.