ELECTROTONIC POTENTIALS ELICITED BY THRESHOLD STIMULI FROM SHEATH-FREE NERVE AT DIFFERENT TEMPERATURES

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Abstract
Electrotonic potentials induced by threshold rectangular current stimuli of various durations were recorded at the extrapolar edge of the stimulating cathode in frog sciatic nerve stripped of perineural sheath. Curves of potential decline subsequent to break of the stimulus exhibit an identical configuration with respect to time after a 0.1 msec. has elapsed. On relative translation of the curves along the time axis it is found that the curves coincide, if time of stimulus break for the brief stimuli occurs 0.1 msec. or so prior to that for the rheobasic stimuli. These data are consistent with the suggestion that attainment of a critical membrane voltage is sufficient for initiation of a propagated response. The apparent difficulties involved in obtaining coincidence for the voltage-time curves were resolved on noting that for very brief stimuli,steady state voltage is not attained across the distributed membrane capacity. Voltage must overshoot to a critical threshold value over a local cathodal region after stimulus break before it can subside at the rate characteristic of the rheobasic decline, whose time function is relatively independent of the previous history of the system. Electrotonic potentials elicited by a brief subthreshold conditioning shock and a subsequent threshold testing shock eventually decline along an identical time course in each instance, regardless of conditioning shock intensity or interval between stimuli. These data provide further evidence which strongly supports the conclusion that local attainment of a critically outwardly directed potential difference across the distributed membrane capacity is sufficient to initiate a propagated response. Time limits are such that accommodation can be neglected. Cooling of the preparation through 10[degree] or 15[degree] C has little or no effect on the time parameters of either electrotonic potential or excitation. When retarding effects are appreciable, as they are unaccountably in some expts., there remains the close functional relation between electrotonic potential and excitability.
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