SOME FACTORS INFLUENCING THE PLATEAUFORMATION IN Co-TREATED OR Ni-TREATED SINGLE MYELINATED NERVE FIBRES

Abstract

In Co-treated or Ni-treated myelinated nerve fibers, the effect of various agents on the plateau of the action current was investigated. Lithium could serve as a substitute for sodium in these fibers, too. Complete replacement of sodium with lithium reversibly produced an increase in the rheobase and only a slight curtailment of the plateau. By replacement of NaCl with choline chloride, the plateau duration was curtailed. Spike height was reduced proportionally with the reduction in sodium concentration, while the curtailment of plateau duration was saturated under higher concentrated choline. By replacement of NaCl with sucrose, both the spike height and the plateau duration were diminished linearly with proportion to the logarithm of sodium concentration. The plateau duration was more sensitive to changes in sodium concentration than the spike height. Co or Ni ion counteracted the depolarizing action of potassium ion. High concentrated potassium prolonged the plateau duration without the depolarization. On the contrary, when it depolarized the nodal membrane, the plateau duration was curtailed. The reduction of potassium concentration did not cause any appreciable change in the plateau duration. The deficiency of calcium ion in the surrounding media produced a decrease in the rheobase, reduction in the spike height and prolongation of the plateau duration. Co or Ni ion augmented the spike height diminished by Ca-deficiency and increased the lowered rheobase. The increase in Ca concentration curtailed the plateau duration. Narcotics, such as urethane or cocaine curtailed the plateau duration. As Co or Ni ion counteract the reducing effect of narcotics on the spike height, the curtailment sometimes occurred without appreciable change in the spike height under the action of their suitable concentrations. Cate-lectrotonus curtailed the plateau duration, increasing its slope. On the other hand, under anelectrotonus its slope became slower, but its duration was not changed. Brief inward current applied during the plateau abolished the portion of the action potential following the application at a certain strength in all-or-none manner. The threshold intensity depended mainly upon the relative position of the pulse on the action potential. Much stronger pulses were needed in the early stage of the plateau than in its later stage. During the relative refractory period, the spike height recovered as rapidly as in the normal fiber. However, the plateau duration did not completely recover until the end of the supernormal phase which became apparent after application of these metal ions. Some characters of the excitation mechanism of the nodal membrane were discussed briefly.