Abstract
A nerve poisoned by yohimbine exhibits, after a single response, a recovery curve of excitability characterized by a refractory period, a supernormal period, and a subnormal period. 1 Of these the first 2 have been recognized as existing in unpoisoned nerve and have been brought into approximate relationship with the parts of the action-potential known as the spike and negative after-potential. A subnormal period has been recognized in unpoisoned nerve but only after the nerve has been tetanized, in which case the subnormality is associated with the positive after-potential. 2 The latter association suggests that the effect of yohimbine is to augment the process responsible for the positive after-potential. Such being the case, the apparent absence of a subnormal period following a single response in unpoisoned nerve would be interpreted as due to the small size of the positive after-potential which there exists.
A few experiments sufficed to demonstrate that the potential is augmented as was anticipated. The nerves (isolated sciatic of Rana pipiens) were treated in all cases as in the excitability experiments, and their potentials recorded on a cathode ray oscillograph after amplification with a direct-current amplifier, the latter being necessary to avoid distortion of potentials of the length in question.
The progress of the potential-change can best be followed in connection with Fig. 1. All parts of the figure start from a potential-level reached after a long period (15 minutes or more) of freedom from activity. Activity is induced by a single break induction shock and starts with the spike potential, which throws the spot far off the record. The actual record starts with the negative after-potential and is continued by the positive. In the initial unpoisoned state (Fig. 1 A) the positive after-potential reaches a maximum of about 5μν and is not distinguishable for more than about 0.6 sec.
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