Reflex inhibition in single motor units of the human lateral pterygoid muscle

Reflex electromyographic (EMG) responses to electrical stimulation in the human lateral pterygoid muscle are ambiguous, possibly due to previous methods of EMG analysis. In a rigorous paradigm, single motor unit (SMU) activity in 30 units was recorded in the inferior head of the lateral pterygoid muscle in 5 subjects. Single pulses of 1 ms duration at 6T intensity were delivered via bipolar electrodes attached to the gingiva near the maxillary canine. The firing frequency of each SMU was constrained to 10 or 15 Hz by biofeedback techniques. Stimuli were delivered at known positions relative to a series of preselected spikes. Successive stimuli were delivered sequentially by increasing increments of 1 ms. SMUs were activated consistently by incisal clenching and jaw opening against resistance applied to the chin. When the SMU prestimulus firing frequency was constrained at 10 Hz, each unit displayed an inhibitory effect which was highly dependent on the stimulus position. In units with a constrained firing frequency of 15 Hz, no effect was observed. These findings qualitatively resemble responses observed in the human masseter muscle.