Recent developments in the measurement of magnetic fields from isolated nerves and muscles (invited)

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Abstract
We have developed a Superconducting Quantum Interference Device (SQUID) magnetometer with a miniature, room-temperature toroidal pick-up coil that has sufficient sensitivity and frequency response to measure the magnetic field associated with the action potential of isolated nerves and muscles. A magnetically shielded SQUID operates in a liquid helium storage dewar so that the measurements of the weak magnetic fields can be made in a typical laboratory. The nerve or muscle fiber is immersed by Ringer’s solution and is threaded through the ferrite core of the toroidal pick-up coil so that electrical current within the fiber will thread the toroid but the return current external to the fiber will not. The time-dependent 100 pT magnetic field produced by the cellular action current induces currents in the pick-up coil that are detected by the SQUID with signal-to-noise ratios of 10 to 1 in a 2 kHz bandwidth. In the case of a single large nerve axon, this technique will allow quantitative measurement of the intracellular and transmembrane currents, the effective intracellular axial resistivity, and the time dependence of the transmembrane conductance without puncturing the nerve membrane with a microelectrode. This technique is being extended to study current flow in cardiac muscle.