Firing patterns of rabbit masseter motoneurons during evoked rhythmic activation

Objective: Examine the behavior of masseter motoneurons in adult (> 6 month old) male and female rabbits to determine whether differences in their firing patterns could explain sex differences in the proportions of muscle fibers of different phenotypes.  Methods: We produced rhythmic activation of the masticatory muscles in urethane-anesthetized animals by stimulation of the cortical masticatory area, and recorded EMG discharges from nine locations in the masseter muscle using selective wire electrodes.  Isometric forces produced were recorded simultaneously with a multi-axis force moment sensor attached to the rostral mandible.  Action potentials produced by single motor units were extracted from EMG records by thresholding and template matching methods.  Interspike intervals were determined for identified units during each cycle of activation and used to determine initial and median firing rates and mean burst durations.  Results: Most masseter motor units fire between 20 and 60 Hz and for durations of 50-250 msec. No discrete division of units into continuously active (tonic) and briefly active (phasic) was evident. An inverse relationship (r=-0.72, p<0.001) between motor unit firing rate and the duration of firing was found.  In this respect, masseter motor units studied correspond to type FR and S units identified by others (Hennig & Lomo, Nature 314:164-166, 1985).  A few units fired slowly (<30 Hz) AND for short duration (<150 msec).  Nearly half of the units identified as type FR fired as initial doublets.  In general, units from male rabbits fired faster, for shorter durations, and with more initial doublets than units from females.  Conclusion: We interpret these findings to support the hypothesis that sex differences in masseter muscle fiber phenotype proportions may be a reflection of androgen-mediated sex differences in the firing properties of their motoneurons.  Supported by DE11536 from the USPHS.