Excitability of Slowly Reacting Muscle

Abstract

Much attention has been given recently to the alpha excitability of Lucas 1 in the striated muscle of the frog. This excitability which gives rise to a strength-duration curve which approaches closely to its rheobase only with durations of about 200 m. sec. has been the subject of debate by Rushton, 2 Lapicque 3 and others because its time scale is so long that it does not satisfy such generalizations from experiment as Lapicque's theory of isochronism, 4 while at the same time the same tissue will give results satisfying these generalizations if a different method of stimulation is adopted. In slowly reacting muscle, similar low excitabilities are measured by any of the usual methods, but, as these correspond with the slowness of the other reactions of the muscle, the validity of their strength-duration curves with long-time scales has not been questioned. For this reason the data on these tissues are well suited to providing an answer to the question as to whether the same excitatory mechanism can be postulated in the slow tissues as in the highly excitable ones such as the frog's sciatic nerve.

It has been shown 5 that the high excitabilities and the few low excitabilities which have been studied are adequately represented by assuming that the local excitatory process is given by

equation_images

where p is the excitatory state, V is the stimulus, and k and K are constants, and that there is an independent and much faster process which appears with respect to the process of Equation (1) to be complete for all but very short stimuli and to raise or lower the threshold from a constant value h to h ± α V where α is a constant.