Summary
The ionic requirement for the action potentials recorded from the neurohaemal tissue on the lateral branch of the median nerve inCarausius morosus has been studied using extracellular electrodes. Sodium-free, magnesium-free, or calcium-free salines produce irreversible block of the action potentials following prolonged exposure to the nerves. Reducing the sodium concentration to 4 mM has little effect on the amplitude of the action potentials, whilst increasing the sodium concentration to 100 mM reduces the amplitude by 50%. Neither tetrodotoxin nor procaine has any effect on these action potentials.
Reducing the magnesium concentration to 1 mM increases the amplitude of the action potentials, whilst increasing the concentration of magnesium reduces the amplitude.
The amplitude of the action potentials is linearly related to the log of the external calcium concentration, and the action potentials are blocked by both cobalt ions and lanthanum ions.
It is concluded that calcium is the major charge carrier of the inward current in these neurosecretory axons which is the first report of calcium dependent action potentials in a nerve axon. Furthermore, small amounts of sodium and magnesium are necessary to maintain electrical activity. Magnesium is a competitive inhibitor of the calcium currents.
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We are grateful to the Science Research Council for financial support, and to Mrs. J. Birch for the printing of the electron micrographs.
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Orchard, I., Osborne, M.P. The effects of cations upon the action potentials recorded from neurohaemal tissue of the stick insect. J. Comp. Physiol. 118, 1–12 (1977). https://doi.org/10.1007/BF00612333
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DOI: https://doi.org/10.1007/BF00612333