Hich the next AP in a train is initiated (aRMP) according to the interstimulus interval. Rates that place the following APs initiation to the left of the dotted arrows will result in progressive depolarization of the aRMP as the fast AHP diminishes. Slow rates will have AP initiations to the right of the dotted arrow, and result in progressive hyperpolarization of the aRMP. The scale bars apply to both neurons.2012 The Authors. The Journal of Physiology 2012 The Physiological SocietyCCG. Gemes and othersJ Physiol 591.AHP at the moment of the initiation of the next AP. Because the early AHP typically diminishes with SB 202190 web repetition but the late AHP expands (Fig. 6), we predicted that AP initiation in trains with short interstimulus intervals will occur at a progressively more depolarized V m , while repetition at slower rates will result in APs being initiated during the slow AHP that is expanding during the train, producing hyperpolarization of the aRMP. We confirmed this by comparing patterns of aRMP in individual SIS3 web neurons at different stimulation rates (Fig. 8A and B), which showed a shift from hyperpolarization to depolarization with increasing rates. Finally, Amir and Devor identified depolarization of inactive sensory neuron somata during stimulation of adjacent fibres, termed `cross-depolarization’. We sought evidence of this phenomenon as a possible contribution to depolarization in our recordings. We observed, however, that in the absence of conducted APs, V m recovered to the pre-train RMP along the trajectory of the AHP (Fig. 8C), suggesting that only active events for the recorded neuron contribute to shifts in aRMP. It is unlikely that the shift in aRMP accounts for the eventual failure of AP propagation through the T-junction as failure evolves in the context of both depolarizing and hyperpolarizing patterns. We nonetheless asked whether the pattern of change of the aRMP during a train regulates the frequency at which the conduction fails. Neurons with a depolarizing pattern of aRMP during a train had higher following frequencies compared with those thathad unchanging or hyperpolarizing patterns, for both Ai neurons (208 ?15 vs. 85 ?14 s-1 , P < 0.001) and Ao neurons (374 ?15 vs. 203 ?14 s-1 , P < 0.001). However, the relationship between firing rate and the pattern of aRMP shift described above dictates that neurons with fast following frequencies will show a depolarizing pattern as a result of those firing rates, so cause and effect cannot be distinguished on this basis. Further evidence that the pattern of aRMP shift does not regulate conduction failure is provided by the findings that NS309 and niflumic acid altered the following frequency but had no effect on the pattern of aRMP during trains (data not shown).Role of membrane resistanceAn established mechanism by which Ca2+ -activated channels may modulate membrane excitability is by decreasing membrane resistance, which shunts current and causes failure of AP propagation at critical sites such as the T-junction. We examined this in the somata of 17 A-type neurons by comparing input resistance (Rin ) after single APs and after trains of 20 APs at following frequency. Rin was calculated from the membrane voltage change induced by a brief hyperpolarizing current injection (0.5 nA, for 7 ms). A larger and more sustained decrease in Rin developed during a train of APs than after a single AP (Fig. 9), which was consistent regardless of whether the neuron showed a depolarizing pat.Hich the next AP in a train is initiated (aRMP) according to the interstimulus interval. Rates that place the following APs initiation to the left of the dotted arrows will result in progressive depolarization of the aRMP as the fast AHP diminishes. Slow rates will have AP initiations to the right of the dotted arrow, and result in progressive hyperpolarization of the aRMP. The scale bars apply to both neurons.2012 The Authors. The Journal of Physiology 2012 The Physiological SocietyCCG. Gemes and othersJ Physiol 591.AHP at the moment of the initiation of the next AP. Because the early AHP typically diminishes with repetition but the late AHP expands (Fig. 6), we predicted that AP initiation in trains with short interstimulus intervals will occur at a progressively more depolarized V m , while repetition at slower rates will result in APs being initiated during the slow AHP that is expanding during the train, producing hyperpolarization of the aRMP. We confirmed this by comparing patterns of aRMP in individual neurons at different stimulation rates (Fig. 8A and B), which showed a shift from hyperpolarization to depolarization with increasing rates. Finally, Amir and Devor identified depolarization of inactive sensory neuron somata during stimulation of adjacent fibres, termed `cross-depolarization'. We sought evidence of this phenomenon as a possible contribution to depolarization in our recordings. We observed, however, that in the absence of conducted APs, V m recovered to the pre-train RMP along the trajectory of the AHP (Fig. 8C), suggesting that only active events for the recorded neuron contribute to shifts in aRMP. It is unlikely that the shift in aRMP accounts for the eventual failure of AP propagation through the T-junction as failure evolves in the context of both depolarizing and hyperpolarizing patterns. We nonetheless asked whether the pattern of change of the aRMP during a train regulates the frequency at which the conduction fails. Neurons with a depolarizing pattern of aRMP during a train had higher following frequencies compared with those thathad unchanging or hyperpolarizing patterns, for both Ai neurons (208 ?15 vs. 85 ?14 s-1 , P < 0.001) and Ao neurons (374 ?15 vs. 203 ?14 s-1 , P < 0.001). However, the relationship between firing rate and the pattern of aRMP shift described above dictates that neurons with fast following frequencies will show a depolarizing pattern as a result of those firing rates, so cause and effect cannot be distinguished on this basis. Further evidence that the pattern of aRMP shift does not regulate conduction failure is provided by the findings that NS309 and niflumic acid altered the following frequency but had no effect on the pattern of aRMP during trains (data not shown).Role of membrane resistanceAn established mechanism by which Ca2+ -activated channels may modulate membrane excitability is by decreasing membrane resistance, which shunts current and causes failure of AP propagation at critical sites such as the T-junction. We examined this in the somata of 17 A-type neurons by comparing input resistance (Rin ) after single APs and after trains of 20 APs at following frequency. Rin was calculated from the membrane voltage change induced by a brief hyperpolarizing current injection (0.5 nA, for 7 ms). A larger and more sustained decrease in Rin developed during a train of APs than after a single AP (Fig. 9), which was consistent regardless of whether the neuron showed a depolarizing pat.
