Bioelectric properties of sciatic nerves will be measured before, during, and after exposure to a uniform 9.0-Tesla (90,000 Gauss) static magnetic field produced by a superconducting magnet. These properties include the amplitude and conduction velocity of maximal action potentials, the absolute and relative refractory periods that follow the passage of a maximal action potential, and the threshold for neural excitation. The frog sciatic nerve will be used for these electrophysiological measurements because of its extremely stable and reproducible bioelectrical properties. Changes of less than 5% in the action potential amplitude or conduction velocity during exposure to strong magnetic fields can be measured quantitatively. Initial experiments will involve electrophysiological measurements on isolated frog sciatic nerves exposed to a 9.0 Tesla field for 4 hr. For those bioelectrical parameters that exhibit a response to a 9.0 Tesla field, measurements will be made at progressively lower field levels to define the minimum magnetic flux density capable of producing a measurable perturbation. Exposures will be made with the long axis of the sciatic nerve oriented in both a parallel and a perpendicular configuration relative to the lines of magnetic flux, thereby permitting the detection of any dependence on nerve orientation in the interaction of an applied magnetic field with the ionic conduction currents responsible for impulse propagation. The results of these studies will be relevant to diagnostic radiology, insofar as they will define the potential effects on nerve tissue of the ultrahigh magnetic fields that have been proposed for use in magnetic resonance spectroscopy with human subjects. In addition, the electrophysiological data will be used to test and to refine the extant theoretical models of magnetic field interactions with the ionic currents responsible for nerve bioelectric activity.
