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High frequency discharge of a fraction (f) of motor unit action potential
Abstract:
Two repetitive discharges, firing at 105 and 180 Hz, were evoked in muscles with chronic denervation. They were delayed following a motor unit action potential (M), and their maximum duration was 800 and 50 ms, respectively. The potential of both high frequency discharges was of low amplitude and short duration. It was considered to be a fraction (f) of the motor unit potential, and to depend on muscle fibres re-innervated by an axonal branch. The repetitive mechanism was tested by double stimulation. It seemed to be an ephaptic re-excitation of the axonal branch by a sprout rather than an ectopic trigger locus on the latter. The antidromic waves, associated with the repetitive discharges on the axonal branch, failed to be transmitted to the main axon. This failure, assimilated to a conduction block, was complete in the first case as M was not repetitive. It was intermittent in the second case as M was firing intermittently. Repetitive activity associated with 'terminal multifocal block' could be relevant to the fractional activation observed by others in some disease states.
Insights
Repetitive muscle discharges in denervation were studied. Ephaptic re-excitation by axonal sprouts, not ectopic triggers, caused these discharges, potentially explaining fractional activation in diseases.
Area of Science:
- Neuroscience
- Muscle Physiology
Background:
- Chronic denervation leads to muscle fiber changes and altered neural signaling.
- Understanding repetitive discharges is crucial for diagnosing neuromuscular disorders.
Observation:
- Two distinct repetitive discharges (105 Hz, 180 Hz) were observed in denervated muscles.
- These discharges followed motor unit potentials (M) and had low amplitude and short duration.
- The repetitive mechanism appeared to be ephaptic re-excitation by axonal sprouts.
Findings:
- Antidromic waves failed to transmit to the main axon, indicating a conduction block.
- This conduction block was complete or intermittent, correlating with the repetitive discharge patterns.
- The findings suggest repetitive activity and terminal multifocal block contribute to fractional activation.
Implications:
- The study provides insights into the mechanisms of abnormal muscle activation in denervation.
- Findings may be relevant for understanding fractional activation in various neuromuscular diseases.
- This research could inform diagnostic strategies for conditions involving muscle re-innervation.