Stimulation pulse characteristics and electrode configuration determine site of excitation in isolated mammalian skeletal muscle: implications for fatigue.

We examined whether electrical field stimulation with varying characteristics could excite isolated mammalian skeletal muscle through different sites. Supramaximal (20-V, 0.1-ms) pulse stimulation with transverse wire or parallel plate electrodes evoked similar forces in nonfatigued slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles from mice. d-tubocurarine shifted the twitch force-stimulation strength relationship toward higher pulse strengths with both electrode configurations in soleus muscle, suggesting that weaker pulses excite muscle via neuromuscular transmission. With wire stimulation, movement of the recording electrode along the muscle caused a delay between the stimulus artifact and the peak of the action potential, consistent with action potential propagation along the sarcolemma. TTX abolished all contractions evoked with 20-V, 0.1-ms pulses, suggesting that excitation occurred via voltage-dependent Na+ channels and, hence, muscle action potentials. TTX did not prevent force development with > or = 0.4-ms pulses in soleus or 1-ms pulses in EDL muscle. Furthermore, myoplasmic Ca2+ (i.e., the fura 2 ratio) and sarcomere shortening were greater during tetanic stimulation with 2.0-ms than with 0.5-ms pulses in flexor digitorum brevis fibers from rats. TTX prevented all shortening and Ca2+ release with 0.5-ms, but not 2.0-ms, pulses, indicating that longer pulses can directly trigger Ca2+ release. Hence, proper interpretation of mechanistic studies requires precise understanding of how muscles are excited; otherwise, incorrect conclusions can be made. Using this new understanding, we showed that disrupted propagation of action potentials along the surface membrane is a major cause of fatigue in soleus muscle that is focally and continuously stimulated at 125 Hz.