Satellite Potentials of Motor Unit Action Potentials in Normal Muscles: A New Hypothesis for their Origin

Lateva ZC, McGill KC. Satellite potentials of motor unit action potentials in normal muscles: a new hypothesis for their origin. Clin Neurophysiol, 110: 1625-1633, 1999.

A satellite potential is a late component of the motor-unit action potential (MUAP) that occurs in pathologic muscle and, rarely, in normal muscle. We investigated the physiological mechanisms responsible for satellites in normal muscle by relating the latencies of MUAP features to the timing of the underlying electrical events. Methods: We analyzed 21 MUAPs with satellite potentials that had been recorded using monopolar needle electrodes from brachial biceps and tibialis anterior muscles in 10 normal subjects. We estimated the endplate-to-electrode, endplate-to-tendon, and satellite propagation times from the latencies, with respect to the MUAP onset, of the MUAP spike, terminal wave, and satellite. Results: Satellite latencies ranged from 8.8 to 32 ms, too long to be explained by mechanisms involving regenerating axons or atrophic muscle fibers. The spike-to-satellite latencies approximated either twice the spike-to-terminal-wave latency (17 MUAPs) or twice the onset-to-terminal-wave latency (4 MUAPs). Conclusions: These results are consistent with the hypothesis that satellite potentials are due to retrograde propagation in a non-innervated muscle fiber that is in electrical contact with an innervated muscle fiber at one muscle/tendon junction. Such a configuration could arise as a result of longitudinal muscle-fiber splitting.