Action potentials of single motor units in normal muscle.

Abstract 1. 1. By means of a simple, accurate technique of electromyography, using microelectrodes, the discrete action potentials of over 90 per cent of recorded motor units from selected channels were followed in all degrees of tension of normal muscle. 2. 2. The motor unit action potentials tended to increase in amplitude with excitation and tension during relatively isometric contractions. The increases are attributed to an increase in the number of active muscle fibers in the successive discharges of a single motor unit. 3. 3. Increase of tension by increase in frequency of discharge by the active motor units, and by recruitment of motor units previously inactive, was found to occur in weak and moderate contractions. At stronger tensions, the normal EMG was marked by the appearance of slowly discharging motor units, while the most excitable motor units tended to discharge in bursts alternating with periods of inactivity lasting up to 2 sec. 4. 4. The most excitable motor units were functionally different from the least excitable units, having smaller action potential amplitudes, and higher frequencies of discharge, with more “double discharges”. 5. 5. The least excitable motor units have not been described before. These motor units are thought to be supplied by high threshold neurones of large axone diameter. According to other authors, such neurones are selectively lost in pareses due to poliomyelitis, pressure, and ischemia. 6. 6. The slow mean rates of discharge in strong contractions may be due to conflicting states of central excitation and inhibition.

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