Innervation Zone and Propagation Velocity in Human Muscle.1

Summary. The innervation of the brachial biceps was found to be localised to a narrow band in the proximal third of the short head and in the distal third of the long head. Counts of terminal nerve endings showed that 67 per cent of all endings were situated within 10 per cent of the muscle length. The action potential recorded with an electrode placed within the zone of innervation had an abrupt onset, and with two or more electrodes placed in the innervation zone near fibres of the same motor unit, the initial deflection of the action potential was simultaneous within 0.5 msec. This was seen up to electrode distances of 40 mm, along the longitudinal axis of the muscle, and indicates that the end plates of the same unit are scattered over this area. It excludes furthermore that propagation over the thin terminal nerve branches contributes to the action potential duration. When care was taken to place the electrodes distal to the site of innervation and an identical threshold to electrical stimulation had ensured that the electrodes led off from the same muscle fibres, the propagation velocity of the spike of the action potential released by voluntary effort varied over a narrow range. The mean value was 4.7 m/sec. with a maximal variation of 1.3 m/sec. within the same muscle (36.57° C). The small variation in velocity was unexpected in view of the threefold variation in diameter, but could be accounted for by the influence of the external resistance which was shown to be localised mainly in the interspaces between inactive fibres in situ. The propagation velocity during voluntary effort exceeded the velocity of electrically activated fibres by 0.7 m/sec. Differences in action potential duration during voluntary effort can mainly be attributed to the spatial dispersion of the site of innervation for the different fibres of the motor unit.