Multielectrode Study of the Territory of a Motor Unit

Abstract

Summary.: By means of multielectrodes containing twelve leads along a 25 mm distance (Fig. 1) the spread of action potentials from a motor unit was studied over a cross section of the brachial biceps in man. The territory of the motor unit could be determined from the analysis of a single cross section of the muscle, since its fibres pass uninterrupted from tendon to tendon. The mutual position of two multielectrodes inserted perpendicularly to each other into the muscle was determined with an accuracy of 0.2 mm by recording with one multielectrode subthreshold pulses sent through successive leads of the other multielectrode (Fig. 2). The action potentials of a motor unit spread over an approximately circular area as evidenced by experiments in which the two multielectrodes traversed the centre of the motor unit (Fig. 4) and other experiments in which one of the multielectrodes was situated at the periphery of the motor unit (Fig. 5). Potentials with spike components exceeding 50 μV were confined to an average area four to six mm in diameter (Fig. 6 A). These potentials had a protracted terminal component of low amplitude and if they were recorded outside the site of innervation an initial positive component as well. Outside the area in which spike potentials occurred the potentials had a low amplitude, a short duration and a negative sign (Figs. 3, 4, 5). On account of volume conduction the action potentials were picked up in a larger area than occupied by the fibres of the motor unit. By means of the relationship between spike amplitude and distance from the potential source (Fig. 1 C) the average territory of the motor unit was determined to be four to six mm (minimum two and maximum ten mm) (Fig. 6 B). This area allows space for the fibres of ten overlapping motor units. On the same lead of the multielectrode action potentials from maximally six different motor units were recorded. Their potentials were often similar in shape so that they could be identified only by their discharge frequency (Fig. 9). The muscle fibres of as many as three of the intermingling motor units overlapped entirely (Fig. 8). The temporal dispersion of the spikes recorded on the different leads of the multielectrode was identical for these overlapping motor units (Fig. 7). The results are discussed with respect to their bearing on the parameters of the motor unit potential: The action potential of the motor unit arises from the summation of temporally dispersed potentials originating from fibre groups at a mutual distance of 0.3 to 1 mm. The amplitude of the motor unit potential is largely determined by the distance of the recording electrode from the fibre group closest to the electrode. The shape of the potential derives from the way in which the action potentials of two to three of these fibre groups summate. The duration of the potential is determined by the temporal dispersion of the component potentials due to the spatial dispersion of the end plates in the innervation zone.