Representative recordings of successive AP trains in a fast-twitch EDL muscle fiber. Two electrodes were inserted in the fiber at close proximity (∼70 µm): one electrode was used for injection of square current pulses, and the other electrode recorded the membrane potential. (A) The current injection protocol consisted of low amplitude (±10–20 nA) 75-ms square pulses, which were used for Gin determinations. These were followed by a train of AP trigger pulses (for 3 ms at 300 nA), here delivered at 15 Hz. The protocol was repeated until 4,998 trigger pulses had been injected. Recovery of Gin was assessed 1 min after the stimulation ceased. (B–D) Recordings of the membrane potential during the 1st, the 32nd, and the 85th current injection protocol. (E) An AP train 1 min of recovery after the fiber had fired 4,998 APs. (F) Enlargement of the resting membrane voltage deflections (ΔV) that are underlined in B–E. Because Gin and Gm are inversely related to ΔV, an increase in ΔV reflects reduced Gin and Gm and vice versa.
