AN ANALYSIS OF THE LOW-VOLTAGE ELEMENTS OF THE ACTION-POTENTIAL WAVE IN NERVE

Abstract

By a direct method of analysis the low-voltage elements of the action-potential wave in nerve were studied, and the voltage and duration of the negative after-potential were approximately determined. The main observations were: (1) In resting nerve the low-voltage elements are very much reduced or absent; after activation of the tissue by a period of repetitive stimulation they become highly developed so that the electrical discharge from the negative after-potential may be 4-10 times as great as that from the initial high-voltage "spike." (2) The negative after-potential and the "after positivity" are very much reduced at low temp. (5[degree] C. or below), even though the nerve is continually activated by repetitive stimulation. (3) During O-lack the low-voltage elements, both positive and negative, disappear after 1-2 hrs., so that the action potential wave consists of the high-voltage phase alone. The latter may continue for more than an hr. after the suppression of the low-voltage phase. Upon readmission of air the low-voltage elements soon reappear, and become very highly developed, often rising far above the values previously observed in the fresh nerve. The high-voltage phase shows no such behavior, merely returning to, or nearly to, its original value. (4) Direct analysis of the electrical discharge from 2 waves, separated by an interval (10 o) just longer than the relative refractory period, shows that while the high-voltage phases of the 2 waves come through without diminution, the low voltage elements interfere over all, or nearly all, of their extent, so that the electrical discharge from the negative after-potentials of the 2 waves together is less than double that given by a single wave alone. The differing behaviors of these 2 elements of the action potential wave are interpreted to mean that they are the electrical signs of 2 separate processes. It is concluded that the low-voltage after-potentials are the electrical sign of an active second process, restorative in nature, which builds the nerve back to its original state, possibly by replenishing the stock of some material whose breakdown is responsible for the high-voltage phase. This first process, underlying the high-voltage phase, is relatively anaerobic, and slightly affected by various physical and physiological changes. The second process is relatively aerobic, and is sensitively related to a variety of factors. This conception is similar to the idea long ago proposed by Hering in explanation of the "after-positivity" after repetitive stimulation, but differs from that suggested recently by Levin, who believes that the after-potentials arise from an accumulation of chemical metabolites during activity. The authors believe that the mere presence of such products has no effect upon the action potential wave, although they may affect the injury potential, but that the after-potentials arise only when these substances are being removed, or, more probably, when a new store of precursor substance is being formed, by direct chemical action. A possible relationship between such chemical changes and the physical state of the membrane is suggested, according to which fluctuations in the pH of the tissue affect the dissociation state of an ampholyte near its isoelectric point, and so alter the value of the membrane potential by changing the membrane charge.