Neuromagnetic thresholds

Estimates are made of the effects which a steady magnetic field may have on the action potential. These are divided into two classes: Current distortion and inductive effects. Employing relations of the Hall Effect, it is found that for distortion of the action-potential current-pattern of the squid axon, magnetic field strengths of the order of Gauss are required. Changes in the current pattern due to inductive effects similarly imply that a magnetic field $\text{B ? (2·6/α)}$ kgauss distributed over ? 1 cm of axon would measurably perturb current flow in the action potential. The phenomenological coefficient α characterizes the asymmetry of the current pattern. For the case of a cylindrically symmetric response, α ? 0, in which case inductive perturbation of the action potential is nullified. Finite inductive effects for the case of the cylindrically asymmetric action potential are found also to be dependent on the orientation of the magnetic field with respect to the action potential. The effects of the extent of the applied magnetic field are also discussed.