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 with intensities from two to six times sensory threshold (2-6T) were delivered to the lower lip. The reflex responses were monitored by surface electromyography of the ipsilateral masseter muscle. An excitatory response that was not preceded or followed by an inhibition could be evoked 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 response was statistically lower than the SLI and LLI, especially when longer pulse widths were used. Three subjects demonstrated a primarily excitatory 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 elicit excitatory responses and fibres with higher thresholds may evoke inhibitory responses. Another possible explanation is that central spatial summation could be responsible for the opening of the inhibitory and excitatory central pathways. The excitatory response may be the result of a reflex pathway similar to that evoked by activation of periodontal mechanoreceptors, and could be responsible for the load compensation mechanism during chewing and/or positioning of food. The inhibitory responses are well known, and are considered to be a protective reflex.