The external action potential of a nerve or muscle fibre in an extended medium

The external electrical field of a uniform nerve or muscle fibre has been computed for various distances from the fibre. The calculated amplitudes, shapes, and frequency spectra of the action potentials are described. The results are in reasonable accord with records in the literature. It is predicted that, however small the fibre, a detectable action potential will be obtained if a suitable electrode is brought close to the membrane. This is unlikely to occur in skin (as opposed to muscle) if the electrode is inserted at random. An amplifier of bandwidth 2 to 3 octaves, with variable central frequency, is recommended. It is pointed out that the strongest frequency present may not be a genuine repetition rate, and may be determined either by the time-course of the membrane action potential, or by the ratio of propagation velocity to distance from the electrode, or by a combination of these. The alpha rhythm of the E.E.G. probably represents a repetition rate, but the origin of the other rhythms remains in doubt.