Regulation of Middle Ear Sound Transmission in the Nonanesthetized Rabbit

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Abstract
The regulation of sound transmission through the middle ear of nonanesthetized rabbits was quantitatively investigated by comparing the excitation of the cochlea before and after surgical inactivation of m. stapedius and/or m. tensor tympani in the stimulated ear. The crossed middle ear reflex was used as a measure of the excitation of the cochlea to stimulation with pure tones in the range from 0.5 to 8.0 kHz at intensities up to 125 dB SPL. Sounds through this frequency range were attenuated above a level of 90 to 100 dB SPL. Low frequency tones (0.5 kHz) were attenuated by all levels of activity in the m. stapedius and m. tensor tympani. High frequencies were attenuated only by m. stapedius activity at high intensity levels above reflex threshold (20–30 dB). The m. tensor tympani had no influence on high frequencies, above 2.0 kHz. Improved sound transmission due to reflex activity was not regularly found. The efficiency of the regulation was regarded as high: a 1.0 dB increase in sound level in the ear canal gave only a 0.3 dB increase in the excitation of the cochlea (i.e. the regulating efficiency of the closed‐loop system was 0.7 dB attenuation per 1.0 dB increase in stimulus sound intensity). The regulating efficiency of the open‐loop system was about 2 dB/dB. The open‐loop gain was estimated to 4 during steady‐state. The oscillations in the middle ear reflex response were found to be generated in the m. stapedius feed‐back loop regulating the sound transmission through the middle ear. It was concluded that the function of the acoustic middle ear reflexes can be described as extending the dynamic range of the cochlea and adaptation and it may also be compared with the effect of lateral inhibition.