Electrophysiological evidence of doubly innervated branched muscle fibers in the human brachioradialis muscle.
OBJECTIVE
Motor-unit action potentials (MUAPs) with unstable satellite (late-latency) components are found in EMG signals from the brachioradialis muscles of normal subjects. We analyzed the morphology and blocking behavior of these MUAPs to determine their anatomical origin.


METHODS
EMG signals were recorded from the brachioradialis muscles of 5 normal subjects during moderate-level isometric contractions. MUAP waveforms, discharge patterns, and blocking were determined using computer-aided EMG decomposition.


RESULTS
Twelve MUAPs with unstable satellite potentials were detected, always two together in the same signal. Each MUAP also had a second unstable component associated with its main spike. The blocking behavior of the unstable components depended on how close together the two MUAPs were when they discharged.


CONCLUSIONS
The latencies and blocking behavior indicate that the unstable components came from branched muscle fibers innervated by two different motoneurons. The satellite potentials were due to action potentials that traveled to the branching point along one branch and back along the other. The blockings were due to action-potential collisions when both motoneurons discharged close together in time.


CONCLUSIONS
Animal studies suggest that branched muscle fibers may be a normal characteristic of series-fibered muscles. This study adds to our understanding of these muscles in humans.
Lateva
Zoia C
ZC
Rehabilitation Research and Development Center, VA Palo Alto Health Care System, Palo Alto, CA 94304, USA. lateva@roses.stanford.edu
McGill
Kevin C
KC
D000200:Action Potentials / Q000502:physiology; D000328:Adult; D001369:Axons / Q000502:physiology / Q000648:ultrastructure; D003625:Data Collection; D004576:Electromyography / Q000379:methods; D005260:Female; D005542:Forearm / Q000294:innervation / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D009469:Neuromuscular Junction / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D012815:Signal Processing, Computer-Assisted