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Abstract It has long been supposed that the mechanism of propagation is the stimulation of the inactive region just in front, by the advancing actionpotential wave. Plausible as is this theory, it is only within the last year that evidence has been published which puts the matter beyond doubt. Blair and Erlanger (1936b) obtained suggestive evidence by blocking a single impulse with calcium or anodal polarization. They found that this blocked impulse left a transitory state of enhanced conductivity which allowed a second impulse to be propagated. The nature of this enhancement was studied independently and in greater detail by Hodgkin (1937). He used cold or pressure to block the first impulse, and found that the lowering of threshold on the farther side of the block had the same spatial and temporal distribution as had the spread of the action current. Moreover, the action current spread had the same space-time distribution as that of a subthreshold current applied from an external circuit, and so arranged as to have the same time course as the action-potential wave.
W. A. H. Rushton (Fri,) studied this question.