Heterochronism in the Single Fiber Nerve-Muscle Complex of the Retrolingual Membrane

A series of measurements has been made on the excitability of the single muscle fiber preparation in the retrolingual membrane of the frog's tongue. The relation between liminal intensity and stimulus duration has been studied with the direct excitation of normal and curarized fibers and also with indirect stimulation through nerve fibers lying in the membrane. In many experiments it has been possible to carry out all 3 types of measurements while observing throughout the response of the same single muscle fiber.

The preparation is made according to the method described by Pratt, 1 which permits the use of transilluminated membranes with intact circulation. Stimulation has usually been by condenser discharges. The stimuli are applied through a localized cathode which is a capillary tube having an opening 20-80μ in diameter and which is filled with Ringer's solution.

These experiments were carried out to test the validity of La-picque's 2 “law of isochronism” that transmission of excitation from a nerve to its muscle is possible only when the “chronaxies” of the 2 tissues are equal or differ by no more than a ratio of 1:2. That a block in transmission produced by drugs is due to an overstepping of this ratio. In particular, a curari block is supposedly due to an increase in the chronaxie of the muscle.

In the present experiments it has been found that the nerve fiber always has chronaxies from 2-8 times smaller than does the muscle fiber. The chronaxie of the muscle fiber does not change upon curarization. The values of the chronaxies range from about 0.03 to 0.06° for the nerve and from 0.1 to 0.3° for the muscle fiber. Furthermore, it has been possible to obtain compound curves of the general type described by Lucas 3 and Rushton 4 but, of course, with time values entirely different from theirs and corresponding with those given above.