CHRONAXIE

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Abstract
If the time scales are properly chosen the laws describing conduction, contraction, and excitation apply to a great variety of tissues. "The chronaxie, selected by Lapicque as the most convenient measure of the speed of excitation, has been widely employed as a ''measure of excitability'' without due regard for its arbitrary and empirical character. Originally identical with the ''excitation time'' of Lucas, it is now restricted to determinations made with certain types of electrodes and undo conditions which yield a ''canonical'' strength-duration curve. Neither the canonical formula of Lapicque nor any of the many other theoretical or empirical formulae which have been proposed have proved entirely satisfactory. The phenomena of excitation appear too complex to be characterized adequately by a single simple chronological index. An approximate isochronism usually exists between a skeletal muscle and its motor nerve. Lapicque''s theory of strict isochronism and his associated theory of curarization are not substantiated by the experimental evidence or by theoretical analysis, in spite of the fact that Rush ton''s major criticisms have been met by the recognition of the ''a effect.'' The latter depends on the sizes of excitable fiber and of electrode. Isochronism does not seem to be a necessary condition for neuromuscular transmission, nor heterochronism for ''curarization.'' The effectors of the autonomic nervous system, whether ''iterative'' or ''non-iterative'' are apparently excited by a neurohumoral mechanism. Since the theory of isochronism is based on the assumption of an electrical mechanism of transmission, it should not be applied to the autonomic effectors. Empirically these systems are heterochronous. Explanation of central nervous function on the basis of isochronism and of ''subordination'' is purely speculative. The evidence for subordination, i.e., a change in the characteristics of peripheral nerves dependent on influences other than nerve impulses emanating from the central nervous system, is reviewed. The changes do not appear to involve primarily the time factor of excitability, but seem to be akin to anelectrotonus. The theoretical significance of these changes is obscure. The demonstration of the phenomenon experimentally depends upon a recognition of its characteristic variability and of its sensitivity to the state of activity of the centers. The use of chronaxie as a clinical index of excitability should be placed upon a completely empirical basis, and as much attention given to standardized determinations of the rheobase as to the chronaxie itself.".

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