Potentiation and depression of the M wave in human biceps brachii.

Abstract

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 approximately half. The enlargement of the M wave was sustained during stimulation 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 20 Hz stimulation was performed under ischaemic conditions, the M wave first enlarged and then gradually declined. After 20 Hz stimulation was discontinued, the M wave increased in size; in the ischaemic experiments the release of the cuff produced a further, rapid augmentation. In both the ischaemic and non-ischaemic experiments, the amplitudes and areas of the M waves during the recovery period became significantly larger than the resting values (range, 15-60% at the endplate zone). 4. The mean muscle fibre impulse conduction velocity decreased to less than half the resting value during 20 Hz stimulation, with or without ischaemia, and then increased above the resting value during recovery. 5. On the basis of previous experiments in animals, the augmentation of the M wave was attributed to enhanced electrogenic Na-K+ pumping, and the biceps brachii appeared to be an excellent preparation for studying the time course of this enhancement.

Relations between excitability and contractility in rat soleus muscle: role of the Na+‐K+ pump and Na+/K+ gradients

“…6). A similar potentiation of the M wave has been observed previously in vivo both in rat soleus muscles (Hicks & McComas, 1989) and in humans (Cupido et al 1996) and is thought to occur as a result of increased electrogenic Na¤-K¤ pump activity. Thus, although the force decline in the initial 30 s of the fatigue protocol cannot be ascribed to a loss of excitability of the sarcolemma, it is possible that during this period some reduction in excitability takes place in the T-tubules caused by the reduction of Na¤ and K¤ gradients in this compartment (as described above).…”

Effects of sustained stimulation on the excitability of motoneurons innervating paralyzed and control muscles

“…The area of the maximal M wave remained constant over the 90 s for both groups of subjects, suggesting that the decrease in amplitude was due to slowing of conduction of the action potentials across the fatigued muscle fibers, which was reflected by an increased duration of the potential. These changes in the maximal M wave are also consistent with sarcolemmal changes that occur with muscle activation and ischemia (8,29). The latencies of the M waves were 3.7 Ϯ 0.2 and 3.4 Ϯ 0.1 ms for SCI and control subjects, respectively (NS) at the beginning of the fatigue test and increased by ϳ1 ms for both groups of subjects by the end of the fatigue protocol.…”

Dissociation of the electrical and contractile properties in single human motor units during fatigue

“…Of course, hyperpolarization of the sarcolemmal membrane potential can only be successfully maintained by the Na/K pump for a time. 4 If the fatiguing contractions continue, the unrelenting accumulation of K+ in the interstitial spaces will eventually overwhelm the capacity of the Na/K pump, resulting in electrical propagation failure and a decline in the size of the MUAP. 7,15 However, given that fast fatigable MUs are usually recruited at higher thresholds, sustained firing probably only occurs infrequently in common daily tasks.…”