1. This report is concerned with the question of whether the alpha and delta groups of myelinated A fibres show conduction failure at different temperatures. 2. The experiments were done on cat saphenous nerve in vitro. Stimuli were applied to both ends of the nerve and biphasic recordings were taken adjacent to an 11 mm segment of nerve, whose temperature was varied. Before cooling commenced, the stimuli were adjusted so that the action potential which passed through the cold zone and was recorded, collided with the action potential initiated at the opposite end of the nerve. 3. Upon cooling the nerve, it was always observed that the delta peak of the action potential which had been previously occluded by collision reappeared at a temperature at which the alpha peak remained occluded. 4. The reappearance of the delta peak was reversible upon warming the nerve and was not affected by increasing the interstimulus interval. 5. The mean temperature for reappearance of the delta peak was 13·5° C, for reappearance of the alpha peak, 5·3° C. 6. In any given nerve, the blocking temperature was replicable and was dependent on the temperature of the cooled segment rather than the gradient between that segment and the remainder of the nerve. 7. We conclude that in cat saphenous nerve, the delta group of myelinated A fibres shows conduction failure at a higher temperature than does the alpha group.
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