Fasciculation potentials (FPs) may arise proximally or distally within the peripheral nervous system. We recorded FPs in the tibialis anterior using 2 concentric needle electrodes, ensuring by slight voluntary contraction and electrical nerve stimulation that each electrode recorded motor unit potentials innervated by different axons.
Time-locked FPs recorded from both electrodes, suggesting a spinal origin, were most frequent in benign fasciculation syndrome (44%) (P < .001) and amyotrophic lateral sclerosis without reinnervation (27%). Fewer time-locked FPs were found (14%) in the reinnervated tibialis anterior in amyotrophic lateral sclerosis (P < .001).
Conclusions and Relevance
We conclude that in chronic partial denervation FPs are more likely to arise distally and that FPs in benign fasciculation syndrome more frequently arise proximally.
A, Recordings from 2 adjacent sites in the tibialis anterior muscle using 2 concentric needle electrodes placed at least 1 cm apart and perpendicular to the vertical axis of the muscle. Time-locked (double arrowhead) and non–time-locked (single arrowhead) FPs are shown. B, Changing the sweep speed to 200 ms/division confirms that the FPs are time locked (double arrowhead). Repeated FPs are observed at sites 1 (*) and 2 (†). C, Slight contraction activates motor unit firing at site 1 but not at site 2, implying that the electrodes are recording electrical activity from 2 different motor units. In addition, time-locked (double arrowhead) and non–time-locked (single arrowhead) FPs are observed during muscle activation. D, Low-intensity electrical stimulation at the fibula stimulates the lower-threshold axon innervating the motor unit recorded at site 1 but not the one innervating the motor unit recorded at site 2.
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