Temperature Effects on Neuromuscular Transmission (Opener Muscle of Crayfish, Astacus Leptodactylus)

Abstract

In experiments on the opener muscle of the third walking legs of crayfish (Astacus leptodactylus) it was found that the mechanical tension developed in response to repetitive stimulation of the single motor axon increases over the entire temperature range from 30 down to 0°C. In contrast to this, the tension elicited by depolarizing single muscle fibres decreases with decreasing temperature; the threshold for excitation-contraction coupling is not significantly altered. With decreasing temperature the resting potential decreases (up to 2 mV/°C) but the amplitude and decay time of the e.p.s.p.'s increase. The time constant, λ, of e.p.s.p. decay has a Q10 of less than −2 in the range above 15 °C but reaches a value of −7 between 10 and 0°C. This pattern of temperature dependence is fully accounted for by a parallel change of membrane resistance and its reciprocal, the membrane conductance. The corresponding activation energies computed from λ-values approximate 3 kcal/mol at high temperature and 46 kcal/mol in the low temperature range. The combined effects of a lowered resting potential, an increased amplitude, and especially an increased decay time of e.p.s.p.s result in a drastic enhancement of the depolarization reached during summation of e.p.s.p.s as the temperature is decreased. These effects overcompensate the declining effectiveness of excitation-contraction coupling so that the entire process of neuromuscular transmission becomes more and more effective as the temperature declines. In order to reach the same tension lower frequencies of nerve stimulation are needed at lower temperatures.