External Field of the Nerve Impulse

Abstract

Since the time of Hermann, 1 the conception of the nerve impulse as a self-propagating electric stimulus has become more and more widely accepted. Within the last few years several experiments have been reported purporting to test the consequences of this view of the impulse as an electrical circuit with two longitudinal resistive components (the transverse ones do not concern us here), one within the core of the axon itself, the other existing outside the core and involving the surrounding media. 2-4

An additional possible consequence of the local circuit theory concerns the electro magnetic properties of the field surrounding the region occupied by the advancing impulse. If the theory is true, the magnetic field surrounding the external component of the circuit should be oriented in a manner which is opposed to that of the internal component, with the result that, owing to the extreme with the result that, owing to the extreme smallness of the core's diameter, almost complete cancellation of the magnetic fields will result. (Fig. 1) If, therefore, a suitable detecting instrument is placed outside the nerve itself, no magnetic field should be observed.

Apparatus. The detecting instrument was a toroidal coil made of thirty turns of No. 28 wire wound longitudinally about a cylindrical core made of a stack of Numetal plates with a 3/32” hole through the center. The coil was 1 cm long. This was inbedded in a “Lucite” cylinder (2 cm in diameter) and carefully insulated with “Amhenol dope” in such wise that there could be no leakage between the contents within the aperture and the wire. The coil was tested for electrical leakage before and after each experiment. The coil leads were inducted into matching transformer whose output was fed into an cording instrument was a commercial Dumont oscillograph.