Some Features of the Acoustic Middle-ear Muscle Reflex of Rats

Abstract

The acoustic reflex was studied on rats which have a wide audible frequency range extended to ultrasonic sound. Reflex activities were registered by means of EMG from the middle-ear muscles and of acoustic impedance measurment at the eardrum. Transmission loss in the middle-ear due to the reflex was estimated from decrease in amplitude of CM recorded at the round window or in the scala tympani of the cochlear basal turn. The reflex threshold of the tensor tympani muscle was lowest for sounds with frequency of 3-5 kHz and it elevated steeply at lower and higher frequenies. The stapedius reflex threshold was higher than that of the tensor reflex by ca 10 dB. The threshold for ipsilaterally applied sounds was several dB lower than that for the contralateral at frequency lower than 5 kHz, above which they were nearly same. The reflex suppressed most efficiently the middle-ear transmission of sound of several hundred Hz and the transmission loss decreased with elevation of the sound frequency up to 20 kHz. The loss could not be observed at frequencies above this, though the acoustic impedance was slightly changed in some frequencies between 20-30 kHz.