EFFECT OF IONIC ENVIRONMENT ON EXCITABILITY AND ELECTRICAL PROPERTIES OF FROG SINGLE NERVE FIBER

Abstract

Single fibers dissected free from the sciatic nerve of Rana pipiens were studied by using the air gap technique of Tasaki. Two nodes were exposed; one, the test node, was immersed in a varied solution, the other, the contrast node, was immersed in a pool of Ringer''s solution. Potassium lack in the test node increased both the membrane potential and the threshold (rheobase); in the contrast node there was an increase in threshold and retarded conduction. Potassium excess in the test node decreased the membrane potential and increased the threshold. In the contrast node there was decrease in threshold, plus extinction of action potential of the test node. Sodium depletion induced both an increase of membrane potential and threshold of the test node, plus extinction of action potential in addition to increase of threshold of contrast node. Calcium lack caused no membrane potential change and a marked decrease of threshold of the test node. There was no change in threshold of contrast node, and no delayed increase in threshold of test node even after 70 minutes repetitive firing. Calcium excess induced no membrane potential change, but gave a sharp rise in threshold of the test node, with no change in threshold of contrast node. Cooling the test node from 10[degree] to -1[degree] compared to contrast node kept at 10[degree] caused reduced voltage, and increased duration of test node action potential. Conduction ceased at 1[degree]C, test node action potential becoming a large local response. It is concluded that excitability rheobase voltage and membrane potential are not directly related. A possible mechanism of excitation involving the removal of Ca ions from membrane surface sites by the cathodal voltage is cited.