Investigation of the Evoked Magnetic Action Flux of Skeletal Muscle

Donald B. MacHattie

Abstract

The magnetomyogram (MMG) is a magnetic signal that may be measured external to an active skeletal muscle. It is generated by the transmembrane ion currents associated with the propagation of action potentials along the plasma membranes of the muscle fibres. Compared with other biomagnetic signals, such as the magnetocardiogram and the magnetoencephalogram, the MMG has been studied very little.

Presented in this dissertation is the development of a strategy for investigating the potential of the MMG as a research and clinical tool. Investigation of the compound magnetic action flux (MAF), the basic component of the MMG signal, was chosen in favour of direct study of the MMG. The design of a data acquisition system and experimental procedure for investigation of the MAF is also reported. A preliminary set of measurements from an excised frog gastrocnemius under controlled physiological conditions is presented. These measurements were designed to reveal the temporal and spatial characteristics of the MAF and to demonstrate the effect of several physiological variables on these characteristics. Such measurements have never before been reported.

The experimental measurements showed that the MAF from a frog gastrocnemius is a biphasic signal with an amplitude of 30 pT and a duration of 4.2 ms. The MAF was found to occur at the same time as the whole muscle action potential and well before the mechanical response of the muscle (generation of twitch force). The MAF was found to have an azimuthal component and a component that looped from end-to-end of the muscle. For submaximal stimulation, the MAF amplitude was found to increase nonlinearly with the twitch force amplitude. For supramaximal stimulation, the MAF amplitude was found to decrease with increasing muscle length. Finally, it was found that the electrical conductivity of the bath medium strongly affected the amplitude of the MAF signal.

This research has demonstrated the feasibility of performing MAF measurements in a typically "noisy" laboratory with a commercially available SQUID magnetometer. It has also shown that the MAF signal is reproducible and sensitive to some physiological variables. With these results, a greater knowledge and better understanding of the MAF has developed. Further evaluation of the potential of the MMG as a research and diagnostic tool is justified.