Abstract
It has been recurrently observed that, for compound muscle action potentials (M wave) recorded over the innervation zone of the vastus lateralis, the descending portion of the first phase generally shows an “inflection” or “shoulder.” We sought to clarify the electrical origin of this shoulder-like feature and examine its implications. M waves evoked by maximal single shocks to the femoral nerve were recorded in monopolar and bipolar configurations from 126 individuals using classical (10-mm recording diameter, 20-mm inter-electrode distance) electrodes and from eight individuals using small electrodes arranged in a linear array. The changes of the M-wave waveform at different positions along the muscle fibers' direction were examined. The shoulder was identified more frequently in monopolar (97%) than in bipolar (46%) M waves. The shoulder of M waves recorded at different distances from the innervation zone had the same latency. Furthermore, the shoulder of the M wave recorded over the innervation zone coincided in latency with the positive peak of that recorded beyond the muscle. The positive phase of the M wave detected 20 mm away from the innervation zone was essentially composed of non-propagating components. The shoulder-like feature in monopolar and bipolar M waves results from the termination of action potentials at the superficial aponeurosis of the vastus lateralis. We conclude that, only the amplitude of the first phase, and not the second, of M waves recorded monopolarly and/or bipolarly in close proximity to the innervation zone can be used reliably to monitor possible changes in muscle membrane excitability.
Highlights
In the last decades, several investigations have advanced the knowledge on the electrical generation of the compound muscle action potential (M wave) (Gydikov and Kosarov, 1972; Lateva et al, 1996; Nandedkar and Barkhaus, 2007)
In the vast majority (97%) of the monopolar M waves recorded over the innervation zone, the descending portion of the first phase did not decrease at a constant rate; rather the decrease was steep at the beginning, and it slowed down temporarily for a short period, after which the decrease became steep again until the trough of the M wave
For the vast majority of the monopolar M waves recorded over the innervation zone (97%), the descending portion of the first phase showed a brief period of low slope, a feature referred to as shoulder
Summary
Several investigations have advanced the knowledge on the electrical generation (electrogenesis) of the compound muscle action potential (M wave) (Gydikov and Kosarov, 1972; Lateva et al, 1996; Nandedkar and Barkhaus, 2007). End-of-Fiber Signals in M Waves in the muscle (Lateva and McGill, 1998). The basic biphasic morphology of the M wave is normally explained by saying that the first phase mainly results from the propagation of action potentials along the muscle fibers, whereas the second phase reflects the extinction of these action potentials at the tendon (Lateva et al, 1996). The practice of identifying the first and second phases of the M wave with the propagating and non-propagating components, respectively, is a simplification of reality, as there are several factors adding complexity to the formation of the M-wave waveform. A clear visual distinction of the propagating and nonpropagating signals in the M-wave waveform may not be possible in all cases
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