POTENTIATION AND DEPRESSION OF THE M-WAVE IN HUMAN BICEPS BRACHII

1. The effects of repeated excitation on the compound action potential , or M wave, of mammalian muscle fibres have been investigated in the human biceps brachii. 2. During continuous indirect stimulation at 10 and 20 Hz the mean voltage-time area of the M wave doubled within the first minute, while the mean peak-to-peak amplitude increased by appro ximately half. The enlargement of the M wave was sustained during stim ulation at 10 Hz but not at 20 Hz. Stimulation at 3 Hz caused a small increase which was significant for M wave amplitude only 3. When the 2 0 Hz stimulation was per-formed under ischaemic conditions, the M wave first enlarged and then gradually declined. After 20 Hz stimulation w as discontinued, the M wave increased in size; in the ischaemic experi ments the release of the cuff produced a fur ther, rapid augmentation. In both the ischaemic and non-ischaemic experiments, the amplitudes a nd areas of the M waves during the recovery period became significantl y larger than the resting values (range, 15-60% at the endplate zone). 4. The mean muscle fibre impulse conduction velocity decreased to les s than half the resting value during 20 Hz stimulation, with or withou t ischaemia, and then increased above the resting value during recover y. 5. On the basis of previous experiments in animals, the augmentatio n of the hi wave was attributed to enhanced electrogenic Na+-K+ pumpin g, and the biceps brachii appeared to be an excellent preparation for studying the time course of this enhancement.