The formation of extracellular potentials over the innervation zone: Are these potentials affected by changes in fibre membrane properties?

Widespread misconceptions still exist regarding the formation of extracellular potentials over the innervation zone: the assumption (1) that the first phase of these potentials mainly reflects the process of initiation of the intracellular action potential (IAP) at the neuromuscular junction and that (2) these potentials are not sensitive to changes in fibre membrane properties (such as changes in the IAP length). Here, we examined the peculiarities of the extracellular potentials detected over the innervation region with the aim to clarify whether the changes in membrane properties are reflected in the amplitude of these potentials. These goals were addressed using a biophysical model of the IAP as well as a convolutional model of EMG generation. Also, the theoretical predictions made by these models were verified experimentally. We showed that only the initial portion of the potentials detected over the innervation zone corresponded to the origination of the IAP at the junction: the remaining (final) portion resulted from the propagation of the IAP along the fibre. It was found that, as radial distance increases, the portion of the rising phase with "propagating character" increased its duration, whereas the duration of the portion with "standing character" remained unchanged. Moreover, a lengthening of the IAP profile resulted in a distinct increase in the amplitude of the first phase of these potentials. These findings were confirmed experimentally and demonstrate that changes in fibre membrane properties influence the amplitude of surface EMG potentials recorded over the innervation zone to the same extent as they do surface potentials recorded elsewhere along the length of the muscle fibres.