Contraction properties and functional morphology of the avian stapedius muscle.

The influence of the stapedius muscle contraction on middle ear volume and acoustic impedance was investigated in the chicken, Gallus gallus. The time course of twitch responses to electrical stimulation (measured as volume and impedance changes) was found to be largely independent of the stimulus voltage, having a contraction time of 22 ms and a half-relaxation time of 22 ms. The stapedius muscle was therefore characterized as a fast twitch muscle. Slow contraction properties were also revealed: A summation of responses to repetitive stimulation beginning at 2.5 Hz and a slow decline to baseline were seen in volume and impedance change recordings. The morphological characteristics were consonant with that of a homogeneously fast muscle: Only fibres with high ATPase activity were identified and no fibres with "en grappe" or multiple innervation were observed. The slow characteristics were suggested to be due to visco-elastic elements in the middle ear. The chicken stapedius muscle is suggested to be analogous to both the stapedius and the tensor tympani of mammals.