THE EFFECT OF EXTERNAL CALCIUM AND MAGNESIUM DEPLETION ON SINGLE NERVE FIBERS

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Abstract
The 3 types of motor axons found in the walking legs of the lobster were shown to respond differently upon exposure to Ca-free solutions. While all fiber types became more excitable initially in Ca-free solutions, only openers became spontaneously active. Fast closers showed the least reduction in rheobase value upon Ca depletion. After 5 minutes in Ca-free solution all fibers showed a rise in rheobase value, and more rapid accommodation. A natural period for spontaneous firing of opener fibers was disclosed. Following such a spontaneous discharge, low amplitude rhythmical potentials were recorded. These small potentials had the same period as the spontaneous spikes. The role of Ca ion in the excitable process was discussed. Mg ion was shown to act synergistically with Ca ion. All fiber types became spontaneously active in solutions deprived of both Ca and Mg. Subsequent hypoexcitability was more pronounced in Ca and Mg-depleted solutions than it was in only Ca-depleted solutions.