Abstract
In a previous report, experiments were described purporting to test for the presence of an electro-magnetic field surrounding the segment of nerve bearing a nerve impulse. 1 Considerations arising from the bioelectric circuit hypothesis of nerve transmission lead to the conclusion that no electro-magnetic field should exist owing to the cancellation of the magnetic field accompanying the component of the circuit external to the axis cylinder by the magnetic field accompanying the component within the cylinder. The experiments indicated, in fact, that, within the sensitivity of the measuring instrument, no electro-magnetic field was present.
The passage of a nerve impulse involves, however, not only the existence of local bioelectric currents through the electrolytic media within and without the axis cylinder, but the existence of electrical lines of force which originate at any point of high potential and terminate at a point of lower potential. In the case of the nerve impulse the zone of depolarization will be characterized by a low electrical potential with respect to the 2 intact zones on either side of it. Hence there will be electrical lines of force originating in the two intact regions and terminating in the depolarized region. If a detecting instrument is interposed in the space containing these lines of force, then an electric charge is induced upon the instrument and may be detected with a suitable recorder.
A detector placed alongside a conducting nerve will convey to the recording instrument 2 peaks of such induced potential; one peak will occur when the depolarized region is approaching the detector, the other when it has passed under it and is receding from the detector. If, however, the nerve is crushed for a length of several centimeters and the detector is placed at the boundary of the intact and crushed region, then but one peak will be observed.
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