The study of jaw reflexes evoked by electrical stimulation of the lip: the importance of stimulus intensity and polarity

The aim of this study was to investigate whether reported differences in the patterns of jaw reflexes which can be evoked by electrical stimulation of the lip might be related to the intensity or polarity of the stimuli. Constant-current stimuli were applied through bipolar electrodes clipped across the lower lip of 14 subjects while EMG recordings were made from a masseter muscle. During stimulation, the subjects sustained a level of masseter activity equivalent to 10% of their maximum. The stimuli were applied as multiples of sensory threshold. The EMGs were analysed following rectification, averaging and smoothing. A sequence of inhibitory, excitatory, inhibitory and excitatory responses could be produced in the muscle by both polarities of stimuli. The latencies of these four responses were generally in the ranges 10-20, 25-40, 40-55 and 80-100 ms, respectively. These latencies, particularly for the last two responses, tended to decrease at higher intensities of stimulation. The threshold for the long-latency inhibition was significantly lower than that for the short-latency inhibition when the cathode was outside the mouth but not when it was inside the mouth. In addition, the long-latency excitation had the lowest threshold of the four responses regardless of stimulus polarity. Since nerves are excited particularly around a cathode, we interpret these results as showing that stimulation of nerves supplying the skin outside the mouth evokes predominately long-latency jaw reflexes whereas shorter latency responses can be evoked by stimulating nerves supplying oral mucosa. Furthermore, long latency excitatory reflexes seem to be the most easily evoked by stimulation of the lip.