Fig. 4

Depolarizing pulses that produce a just-detectable muscle fiber movement displace a set (“threshold”) amount of charge, which can be termed “pre-activating” charge since it must move in order to attain detectable fiber activation. a Charge movement records during muscle fiber depolarization to indicated voltages. The dashed vertical lines indicate the pulse duration needed to give a microscopically just-detectable fiber movement for shorter pulses to the same voltage. No contraction was detected in at − 55 mV. b Pulse to − 32 mV applied alone or together with indicated prepulses, which move only pre-activating charge, since no contraction was detected during the prepulses alone. Charges moved for just-detectable fiber movement (height of black dots) in the test pulses were the same with or without prepulses at the pulse durations for just-detectable fiber contraction (dashed vertical lines). The prepulses decrease the pulse duration required to reach detectable fiber contraction during the test pulse, and this decrease was equal to the time to move the prepulse (pre-activating) charge at the test pulse voltage. Reproduced, with modification from ref. [47]