Threshold properties of single motor units in superior head of human lateral pterygoid muscle

The superior head of the human lateral pterygoid muscle (SHLP) may play a similar role in jaw movement as the inferior head (IHLP).ObjectiveThe aim was to determine whether threshold properties of single motor units (SMUs) within SHLP during jaw tasks were comparable to those identified for IHLP.DesignIn 24 human subjects, SMUs were recorded intramuscularly from computer-tomography verified sites within SHLP during standardised jaw tasks recorded by a jaw-tracking device.ResultsOf the 69 SMUs discriminated, 54 were active during contralateral, 52 during protrusive and 8 during ipsilateral jaw movements. The thresholds, at which SMUs commenced firing, decreased (p < 0.05) as speed of contralateral or protrusive tasks increased. The data suggest an important role for SHLP in generation and control of contralateral and protrusive jaw movements. A number of lines of evidence were consistent with functional heterogeneity within SHLP.ConclusionsThe similarities in SHLP and IHLP functional properties support the proposal that both heads should be regarded as a system of fibers acting as one muscle.