THE TIME FACTOR IN ELECTRICAL EXCITATION

Abstract

Summary.: The rate of development of the excitatory process in tissues may conveniently be characterised by a measure, the “Excitation Time,” which was formerly known as “Chronaxie.” This constant also measures roughly the rate of other processes in the tissue, e.g. action‐potential wave, conduction velocity, rate of contraction, etc. probably because all these processes are limited by the development of electrical states in the tissue.The excitation time of muscle (but not medullated nerve) is very largely dependent on the size of the electrodes used. This is easily explained in terms of a physical theory of excitation. If nerves are stimulated by the penetration of current through the nodes of Ranvier, these will constitute unvarying pore electrodes and account for the relative independence of nerve upon electrode size.According to Lapicque, paralysis by curare and other similar conditions is due to a great increase in chronaxie of the muscle, which initially was the same as that of the nerve. This standpoint, which is fundamental in Lapicque's school, is criticised in detail. In the first place, the evidence upon which the theory rests is inadequate, In the second, further work by Lapicque's school has rendered the theory so complicated that it is now of doubtful practical value. Lastly, the experiments of other workers appear to make the theory untenable.In the light of the rejection of Lapicque's views and of the dependence of excitation time upon the nature of the electrodes, it is necessary to review the significance of the measure, and to modify the technique of its determination. Some practical aspects are discussed, and it is pointed out that at present the chief application to biology is in the analysis of multiple excitabilities (e.g. muscle with nerve twigs, tonic and phasic muscles, etc.). But it must be emphasised that in this analysis it is essential to determine the whole strength‐duration curve if the results are to be significant.