Electrophysiological study of size and fibre distribution of motor units in the human masseter and temporal muscles.

Probably related to their unique function, human jaw muscles have a specialized muscle-fibre composition different from that of limb and trunk muscles. In jaw muscle from healthy young adults, there were large groups of densely-packed fibres of the same histochemical type, a finding which indicates re-innervation when seen in limb muscles. The number of muscle fibres per motor unit, the local distribution of muscle fibres within motor units and the neuromuscular transmission in human masseter and temporal muscles were examined by single-fibre EMG and macro EMG. One hundred and forty-seven low-threshold motor units in four healthy subjects were recorded. The first dorsal interosseus muscle of the hand was similarly measured. In the jaw and hand muscles the low-threshold motor units were slightly smaller, than in large arm and leg muscles. The temporal muscle differed from the masseter in having significantly-higher fibre density, which may be explained by the higher frequency of small type-II fibres in the temporal causing closer packing of the type-I, low-threshold motor units. The fibre concentration within motor units was similar to that of normal limb muscles without any sign of grouping of fibres from the same motor unit. Normal motor end-plate transmission was another argument against re-innervation.