Differential effect of graded contraction of middle ear muscles on the sound transmission of the ear.

Abstract

AbstractThe middle ear muscles in cats have been brought to different degrees of contraction by electric stimulation through electrodes placed in the proper nuclei in the brain stem. The resulting change in the amplitude of the cochlear microphonic potentials produced by pure tones from 250–7000 Hz has been used to determine the change in the sound transmission of the ear. Later the muscle tension produced by the same stimulus strengths have been determined myographically. The main effect of weak contractions involving only a relatively small number of motor units was more selective than the effect of stronger muscle contractions. Contractions of the stapedius muscle involving only up to 10–15 per cent of the maximal contraction force reduced the microphonics below 2000 Hz, with most marked effect on the lowest frequencies tested. Stronger tensions produced an additional modest reduction, equally pronounced for all sound frequencies. The most marked effect of contractions of the tensor tympani, amounting to 10–15 per cent of the maximal contraction force, was a reduction of microphonics of frequencies below 750 Hz, while higher frequencies could be either slightly enhanced or reduced. Stronger contractions had a less selective effect with respect to various frequencies and gave less reduction of the sound transmission per gram tension.