A model for changes in middle-ear transmission caused by stapedius-muscle contractions

Abstract

We have observed in cats that when the stapedius muscle contracts, the stapes moves by as much as 60 μm, while the incus and malleus are not detectably displaced (< 1 μm). This observation provides physiological support for the hypothesis that stapedius contractions change only the impedance of the stapes. A network model has been developed to test this hypothesis. The middle ear is represented as a linear two-port network that is loaded by the impedance of the stapes and cochlea. A quantitative description of the two-port is derived from measurements of (a) acoustic input impedance (with and without load), (b) impedance of the stapes and cochlea, and (c) middle-ear transmission. With our measurements of changes in middle-ear transmission (for acoustic inputs ranging from 0.1 to 10 kHz) generated by stapedius contractions, this model is used to compute the changes in stapes impedance necessary to produce the measured changes in transmission (up to 30 dB). Results indicate the following. (1) The altered stapes impedance is representable by passive elements as its real part is positive. (2) As the magnitude of the stapes impedance increases with stronger muscle contractions both real and imaginary parts of the impedance change. (3) Changes in the stapes admittance are approximately proportional to changes in the transmission. (4) Changes in the middle-ear input impedance predicted by the model are compatible with measurements. Thus, the results support the hypothesis. [Work supported by NIH.]