EXCITABILITY OF THE EXCISED AND CIRCULATED FROG'S SCIATIC NERVE

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Abstract
The excitability of the a fibers in the frog''s sciatic nerve during the passage of a subrheobasic constant current was measured by determining the strength of a short test shock necessary to evoke half maximal a spike potentials. The alteration of excitability in excised nerves is maximum at about 1 msec, after the application of a constant current and usually falls to a steady value other than zero. The decrease and the final value of excitability are measures of the amplitude of accommodation and electrotonus respectively. Excised nerves not exposed to Ringer''s soln. show both accommodation and electrotonus. Ringer''s soln. containing excess Ca++ reduces the amplitude of accommodation and may make it zero or negative, but excess K+ increases the amplitude of accommodation and shortens the time to half accommodation. Nerves with intact circulation in spinal animals show a negligible accommodation to one sec. after application for currents 50 to 90% of rheobasic. After the circulation is blocked or during anesthesia, the amplitude of accommodation becomes comparable to that of excised preps. In all preps, the effects at anode and cathode are not mirror images of each other. When present, accommodation is always less at the anode than at the cathode, and negative accommodation in circulated preps. and excised preps. subjected to excess Ca++ is more marked at the anode. If other expts. also demonstrate that accommodation is negligible under nearly normal physiological conditions, it should be considered as an abnormal alteration of electrotonus, excitability or threshold which is characterized by an amplitude as well as a time constant. The 2 factor theories of excitation allow this viewpoint if the minimal gradient and allied phenomena may be classified as abnormal.

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