Location of the neuromuscular junction of the human masseter muscle estimated from the low frequency component of the surface electromyogram.

The position of the neuromuscular junction of the human masseter muscle has estimated from the low frequency component of the surface electromyogram. Monopolar surface electromyograms were recorded in response to clenching from eight sites with the reference electrode placed on the tip of the nose in six healthy male subjects. Component of the slow wave was separated from the raw recordings using digital filter, and the difference of the polarity and magnitude with the sites was examined. The base line of the recordings deflected either negatively or positively depending on the recording sites. These deflections coincided with bursts of muscular action potentials. Deflection of the slow wave component was largely negative over the inferior masseter close to the mandibular ramus but shifted to positive over the superior part. The greatest magnitude of the muscular action potential coincided with the most negative deflection of the slow wave. The phase of the muscular action potential burst also reversed near the site of the polarity change of the slow wave. These findings suggest that the slow wave, as detected by this method, originates mainly from the synaptic potential at the meuromuscular junction and that this is located on the inferior part of the muscle close to the mandibular ramus.