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At least 19 recordsLinked to original sources
PubMed1956

The effect of external calcium and magnesium depletion on single nerve fibers.

The three types of motor axons found in the walking legs of the lobster were shown to respond differently upon exposure to calcium-free solutions. While all fiber types became more excitable initially in calcium-free solutions, only openers became spontaneously active. Fast closers showed the least reduction in rheobase value upon calcium depletion. After 5 minutes in calcium-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 calcium ion in the excitable process was discussed. Magnesium ion was shown to act synergistically with calcium ion. All fiber types became spontaneously active in solutions deprived of both calcium and magnesium. Subsequent hypoexcitability was more pronounced in calcium- and magnesium-depleted solutions than it was in only calcium-depleted solutions.

Calcium↗
PubMed1959

Effects of calcium lack on action potential of motor axons of the lobster limb.

The effects of external calcium deprivation on certain characteristics of the action potential of the lobster motor axon have been studied. Upon exposure to calcium-free solution the spike amplitude is rapidly decreased within a few minutes and is followed by a slow linear decline. The rates of spike rise and fall are proportionally reduced more than the spike but follow similar time courses during calcium lack. Associated with these phenomena are the loss in the normal slow spike repolarization process, the development of a large and lengthy undershoot, and the appearance of a high degree of refractoriness. The mean increase in the refractory period is 525 per cent upon 10 minutes' exposure to calcium-free solution. These effects are completely reversible upon returning the axons to normal solution. These results are compared to similar effects of calcium deprivation on frog myelinated axons and squid and lobster giant axons recently observed by other workers.

Action Potentials↗
PubMed1959

The effect of oxytocics on the "Ca-deficient" uterus. A measure of oxytocic potency.

If the excised, parturient rabbit uterus is repeatedly treated with a Krebs solution free from Ca, its tension in a tetanus drops gradually, and in 15 to 30 minutes becomes zero. We call such a uterus "Ca-deficient." The uterus in this condition has a high threshold, it is non-propagating, "inexcitable," fails to respond to oxytocics in a characteristic fashion, but retains maximum contractility. As Ca is gradually restored to the Krebs, these lost qualities return in a graded fashion and tension of the tetanized uterus becomes a log function of the [Ca]. If the [Ca] is kept low, i.e. 1/10 to 1/20 of the normal, tetanic tension is small but steady, and the preparation offers a full scale of tension increment for the measurement of oxytocic potency. Keeping the stimulus and the [Ca] constant, excitability (measured by tension increment) is a log function of the drug concentration. The recovery of excitability by restoring Ca to the Ca-deficient uterus is strongly temperature-dependent. The Ca-deficient uterus is a useful preparation for the study of the mechanism of regulation. When its excitability is partially recovered by Ca, the electrically stimulated uterus becomes an excellent tool for the quantitative measurement of oxytocic potency.

Animals↗