Tension and contraction time of motor units of the middle ear muscles in the cat.

Abstract

AbstractThe all‐or‐none contraction of 46 individual tensor tympani motor units and of 44 individual stapedius motor units in response to threshold stimulation of their motor neurones have been recorded. The tensor tympani was found to contain two types of twitch motor units, a fast type and a slow type. The fast type had a contraction time between 23 and 40 ms, a half relaxation time between 17 and 46 ms and a tetanic fusion rate of 75 Hz. The slow type had a contraction time between 58 and 92 ms, a half relaxation time between 52 and 100 ms and a tetanic fusion rate of 15 to 20 Hz. The stapedius muscle consisted of only one type of twitch motor units, a fast type with contraction time from 14 to 39 ms, a half relaxation time from 15 to 50 ms, and a tetanic fusion rate between 75 and 100 Hz. The number of motor units was estimated as the ratio between the average twitch tension of the muscles to supramaximal stimulation and the average twitch tension of individual motor units. On the average, the tensor tympani contained 92 fast and 40 slow twitch motor units and the stapedius 87 fast twitch motor units.