Harmonic Analysis of Neuronal Membranes and Tissue Using SQUIDs

We report the nonlinear response of tissue and nerve fibers in the earthworm Lumbricus terrestris to sinusoidal electric field excitations. A High-
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Superconducting Quantum Interference Device (SQUID) magnetometer detects the magnetic field generated by complex currents in the animal. SQUID signals are processed on a Fast Fourier Transform (FFT) spectrum analyser. Use of the SQUID eliminates low frequency polarization impedance and field distortion that would be present near pickup electrodes. Electrical 4-probe harmonic response measurements of the excised nerve fiber are then compared to the driven Hodgkin-Huxley active transport model.