The morphology of single muscle fibre potentials - Part II: experimental findings

Studies dealing with single fibre action potentials (SFAPs) have been more interested in obtaining quantitative data of certain parameters of the SFAP waveform than in the analysis of its morphologic features. The characterization of the SFAP morphology is highly valuable as it will allow to obtain information about in vivo intracellular action potentials (IAPs). However, the SFAP final portion is highly sensitive to distant electrical activity, as shown in Part I of this study. The present paper, Part II, is aimed at analysing the morphologies found in human SFAPs and deriving data of the associated IAPs. It was found that, for most SFAPs (97%), the declining negative phase starts decreasing steeply up to a certain point (slope-discontinuity point), from which it returns more slowly towards the baseline. This return may be further slowed down due to the contamination from distant potentials, but the slope-discontinuity point seems to be an intrinsic feature of human SFAPs. The third phase of SFAPs was, either absent (65%), or of rather small amplitude and prolonged duration. The slope-discontinuity point was apparent for the SFAPs of larger amplitude and vanished gradually as the SFAPs got smaller. Several lines of evidence strongly suggest that the spike duration of human IAPs should be about 1.0 ms.