EVOCATION OF EITHER EXCITATORY OR INHIBITORY REFLEX RESPONSES IN HUMAN MASSETER MUSCLE BY ELECTRICAL-STIMULATION OF THE LIP AT VARYING INTENSITIES

Electrical stimuli at 1 Hz with pulse widths of 0.05, 0.1 and 1 ms wit h intensities from two to six times sensory threshold (2-6T) were deli vered to the lower lip. The reflex responses were monitored by surface electromyography of the ipsilateral masseter muscle. An excitatory re sponse that was not preceded or followed by an inhibition could be evo ked in seven out of ten subjects at intensities below 5T at all pulse widths. At higher stimulus intensities, the excitation disappeared and /or was preceded by a short-latency inhibition (SLI) or a long-latency inhibition (LLI). The electrical threshold for the excitatory respons e was statistically lower than the SLI and LLI, especially when longer pulse widths were used. Three subjects demonstrated a primarily excit atory response, whereas four had a more pronounced inhibitory response . It was concluded that separate populations of myelinated fibres may be responsible for the responses: the lowest-threshold fibres may elic it excitatory responses and fibres with higher thresholds may evoke in hibitory responses. Another possible explanation is that central spati al summation could be responsible for the opening of the inhibitory an d excitatory central pathways. The excitatory response may be the resu lt of a reflex pathway similar to that evoked by activation of periodo ntal mechanoreceptors, and could be responsible for the load compensat ion mechanism during chewing and/or positioning of food. The inhibitor y responses are well known, and are considered to be a protective refl ex.