Excitation wave, conduction and the nerve impulse as a gradient

Child listed a change in the conductivity of protoplasm among the ways an experimenter can influence individuation, together with the replacement of old metabolic gradients by new ones¹. In 1921 he compared a gradient that stays in place with the ascending phase of an excitation wave. The two differ because the gradient does not advance, though its length can change³. In 1924 he called physiological dominance an excitatory and transmissive relation⁶. Where no specialized path exists, he held, the excitatory change weakens with distance until it is ineffective, so dominance has a limited range⁶. He said the range is short in early embryos, very great in vertebrate nerves, and regarded by many as unlimited⁶.

Bayliss discussed the excitation wave in the electrocardiogram. The smallest difference in the time the wave reaches two points decides which becomes negative first, and he found no evidence that any component arises from a different process². Lillie reported Keith Lucas's comparison in frog muscle. The rise of the bioelectric variation took.0041 of a second at 8° and.0024 at 18°, a ratio of 1.64 that matched the ratio of propagation velocities⁵. Child described a plate row in which transmission from the apical region stopped at a cut⁴. The part beyond the cut usually began to beat with its own rhythm⁴.

Bose tested the idea that a stimulating substance carried in the transpiration current conveys the impulse in Mimosa. He cited Snow's highest rate of sap ascent, 3 mm per second, and found transmission of excitation in thin petioles as high as 400 mm per second⁷. He added that sap rises, whereas excitation travels both up and down at once⁷. He also reported that an electrotonic block, as in animal nerve, stopped transmission in the plant and that conduction returned when the block was removed⁸. From this he concluded that plant and animal impulses are essentially similar⁸.