THE REACTANCE OF NERVE AND THE EFFECT UPON IT OF ELECTRICAL CURRENTS

Abstract

The "capacity" of nerve is discussed in terms of the polarizability of its interfaces to ions. A network of capacity and resistance is described which in 1 arm of a Wheatstone bridge balances the reactance of nerve in the opposite arm, when the Pb electrodes are in contact with intact tissue. The distribution of polarizability in nerve is inferred to be different from the distribution of the reactance in this network, but to involve more than 1 component, such that no single unit of 1 capacity and resistance will compensate nerve in a bridge. By means of the visible cathode ray oscillograph record it is feasible to measure the reactance of a stretch of nerve mounted on electrodes with a galvanic current of short duration, to differentiate qualitatively between its different components of polarizability, and to measure its resistance to direct current, and to alternating currents of (practically) infinite frequency. The reactances of nerve trunks, and of roots with thin connective tissue sheaths, are found to be similar in distribution. The effect of the sheath is chiefly due to its resistance, in series transversely, and in shunt longitudinally, to the nerve fibers. The effect of injury under 1 or both electrodes, by heating, by crushing, or by repeated electric shocks, is to reduce 1 component chiefly of the nerve''s reactance, with little effect on the others. Injury in the inter-polar stretch reduces a different component. The action potential is decreased to extinction, and the threshold is raised, progressively, through injury by electrical shocks, considerably before the transverse polarizability of the nerve under the electrodes is abolished; further injury by shocks of the same strength then further reduces the polarizability to approximately zero at the region injured. Certain apparatus suitable for the measurement of resistance and capacity with the oscillograph as a null instrument is described in an appendix.