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

Unlabelled: The superior head of the human lateral pterygoid muscle (SHLP) may play a similar role in jaw movement as the inferior head (IHLP).
Objective: The aim was to determine whether threshold properties of single motor units (SMUs) within SHLP during jaw tasks were comparable to those identified for IHLP.
Design: In 24 human subjects, SMUs were recorded intramuscularly from computer-tomography verified sites within SHLP during standardised jaw tasks recorded by a jaw-tracking device.
Results: Of 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.
Conclusions: The 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.