Cellular magnetic fields: fundamental and applied measurements on nerve axons, peripheral nerve bundles, and skeletal muscle.

We review the fundamental origins of biomagnetic fields in terms of ionic currents flowing at the cellular level. Mathematical models provide the link between the macroscopic fields and the microscopic electrophysiological sources. The single cell view is then expanded to include more complex systems such as nerve and muscle bundles. We provide an overview of the two most promising methods to measure the magnetic fields from these systems, we discuss the capabilities and limitations of the techniques based on comparisons with conventional electric methods, and we show that the direct measurement of action currents and the ability to scan along nonuniform samples are of prime importance. We present a number of interesting applications for basic research under laboratory conditions, including measurements of the time course of electrophysiological changes following a crush injury to a nerve and the spatial and temporal dependence of action currents as they propagate away from the motor endplate zone of a single motor unit in skeletal muscle. We conclude by discussing the potential applications in the clinic, including the intraoperative assessment of neuroma-in-continuity and the long-term monitoring of nerve regeneration and degenerative neuromuscular disorders.