Ion exchange as the cause of contraction and rhythmic activity

Loeb noted that a slight amount of acid added to a NaCl solution shortens the latent period before rhythmical contractions begin.² He suggested that the acid may act like the salts he had discussed, by liberating calcium from certain organic combinations in the muscle or nerve.² Because CO2 is produced in the muscle itself, he judged this relation to be of importance.² From these facts he drew the idea that contraction follows an exchange of Na or K for Ca or Mg, or the reverse.¹ Loeb offered the idea as a suggestion drawn from these facts, not as a measured result.¹

Loeb argued that the exchange must be accompanied by a change in some physical property of the compound. His examples were surface tension, state of aggregation, viscosity and absorbing power for water.³ He held that such a change may determine or facilitate contraction.³ Loeb named these properties only as possible examples and did not identify which one changes. The latent period served as the observable link between the salt solutions and the idea.²

Other physiologists, such as Bose and Bayliss, also measured latent periods. Bose recorded latent periods in the plant Mimosa with a Resonant Recorder able to resolve a thousandth of a second.⁷ In 1913 he reported that the period is shorter under stronger stimulus but constant above a maximal one.⁵ In 1927 he reported that fatigue lengthened it from 0.10 to 0.14 second.⁸ A 1907 record showed a latent period falling from 25 to 20 minutes on repeated stimulus.⁴ Bayliss gave a latent period of 0.023 second for the cephalopod eye responding to light.⁶

Pavlov raised a warning about reading the latent period too simply. He wrote that what his own experiments had often called the latent period of a conditioned reflex was in fact a delay.⁹ He called it a preliminary inhibitory period and said it was certainly not the true latent period of the reflex.⁹ Pavlov's caution was not directed at Loeb.