A comparison of various methods of measuring the time-constant of accommodation of Nerve

Abstract

Using the nerve of sciatic-gastrocnemius preparations of winter Rana esculenta, [lambda], the time-constant of accommodation, was measured by 4 methods of excitation: (1) by exponentially rising currents, the value so obtained being taken as the standard. (2) by alternating current, supplied by a beat-tone oscillator giving a pure sine wave. The current in the output circuit was measured by a rectifier unit and microammeter. To avoid preparations with a large [lambda] and low optimum frequency, the preparation was soaked previous to the experiment in Ringer containing 5 times the normal amount of CaCl2, and the determinations were carried out at 25 [degree]C. (3) Break excitation at the anode in constant current pulses. Make excitation was avoided by cauterisation of the nerve in the cathodal region. (4) Make excitation at the cathode at the end of a gap in a constant current pulse. Here agreement with the values obtained by excitation by exponentially rising currents could be obtained only if cataelectronus was abolished by soaking in K-rich Ringer. Except in the alternating current experiments, where a short tetanus was used as an index of excitation, the index was muscle twitch. [lambda] was calculated by Hill''s formulae. With the precautions given above, at 25[degree]C the alternating current experiments gave a value for [lambda] of 5.1, compared with 5.3 for the standard; at 18-20[degree]C, break excitation in constant current pulses gave a mean value of 6, the standard a mean of 7.4; with make excitation after a gap in constant current pulse [lambda] = 5.8, the standard = 6.6. In both the latter cases agreement with the standard was obtained only when [lambda] was below 10 m sec.