N is the cause of limited somatic firing rates in response to axonal pulse trains, PD173074 solubility rather than features of the somatic membrane, this would predict that direct somatic excitation could produce firing rates that exceed the maximum following frequency of conducted APs. We tested this hypothesis in C-type neurons in which the characteristic of slow following frequencies provides the best conditions to identify differences in somatic versus T-junction rates. Specifically, we compared the maximal rate at which 20 somatic APs could be evoked by current injection through the recording electrode to the maximal rate of impulse conduction through the T-junction (i.e. the following frequency), determined in random sequence for each neuron. Thirty-seven of 42 (88 ) C-type neurons had a maximal firing frequency during somatic stimulation that exceeded the following frequency (Fig. 5B). Although injury increased following frequency for C-type neurons, maximal somatic firing rates during direct stimulation were not different between injured SNL5 and Control neurons. This indicates that the increased order GW 4064 ability of C-type neurons to conduct trains of APs after injury is due to altered properties at the T-junction rather than effects on excitability of the somatic membrane. Additional evidence that the T-junction regulates following frequency independent of somatic excitability is the lack of relationship between following frequency and the rheobase current necessary to initiate an AP by direct somatic depolarization via current injection through the recording electrode (R2 = 0.01, P = 0.13, n = 188). Further support for the view that events in the axon control following frequency comes from a significant inverse relationship between CV slowing during the train andFigure 5. Site and mechanism of impulse propagation failure A, testing maximum axonal firing rate in C-type units in excised dorsal roots. A sample trace (CV = 1.03 m s-1 ) during stimulation (90 Hz) of the dorsal root segment at the end transected from the spinal cord and recorded from fibres teased from the end transected from the DRG. The first 3 and last 7 APs (arrows) of the train are shown. Stimulation rate is 90 Hz, and the conduction latency is 27.1 ms for the first AP and 29.7 ms for the 20th AP. Summary data from this and other C-type units (n = 18) are shown in the right panel, in which open circles represent recordings in which the rate is assigned as the highest observed following frequency prior to the AP being occluded by the stimulus artefact at higher frequencies, thus making these data possible underestimates. Filled circles represent data for which failure was directly observed at the next higher frequency. The central indicator bars represent the median value for the entire group. B, comparison of the maximum rate at which APs in C-type neurons can propagate through the axonal T-junction (`Axon’), versus the maximum rate at which the same neuron’s soma can fire all of a train of 20 APs during direct somatic current injection (`Soma’). Conduction through the T-junction was confirmed by the generation of a depolarization, including either an AP or anincomplete electrotonic depolarization, during axonal stimulation. The central indicator bars represent the median value. Main effect of Injury Group P < 0.01; main effect of Stimulation Site P < 0.001; significant post hoc comparisons are shown by connecting brackets, P < 0.05. C, sensitivity of following frequency to modulation of.N is the cause of limited somatic firing rates in response to axonal pulse trains, rather than features of the somatic membrane, this would predict that direct somatic excitation could produce firing rates that exceed the maximum following frequency of conducted APs. We tested this hypothesis in C-type neurons in which the characteristic of slow following frequencies provides the best conditions to identify differences in somatic versus T-junction rates. Specifically, we compared the maximal rate at which 20 somatic APs could be evoked by current injection through the recording electrode to the maximal rate of impulse conduction through the T-junction (i.e. the following frequency), determined in random sequence for each neuron. Thirty-seven of 42 (88 ) C-type neurons had a maximal firing frequency during somatic stimulation that exceeded the following frequency (Fig. 5B). Although injury increased following frequency for C-type neurons, maximal somatic firing rates during direct stimulation were not different between injured SNL5 and Control neurons. This indicates that the increased ability of C-type neurons to conduct trains of APs after injury is due to altered properties at the T-junction rather than effects on excitability of the somatic membrane. Additional evidence that the T-junction regulates following frequency independent of somatic excitability is the lack of relationship between following frequency and the rheobase current necessary to initiate an AP by direct somatic depolarization via current injection through the recording electrode (R2 = 0.01, P = 0.13, n = 188). Further support for the view that events in the axon control following frequency comes from a significant inverse relationship between CV slowing during the train andFigure 5. Site and mechanism of impulse propagation failure A, testing maximum axonal firing rate in C-type units in excised dorsal roots. A sample trace (CV = 1.03 m s-1 ) during stimulation (90 Hz) of the dorsal root segment at the end transected from the spinal cord and recorded from fibres teased from the end transected from the DRG. The first 3 and last 7 APs (arrows) of the train are shown. Stimulation rate is 90 Hz, and the conduction latency is 27.1 ms for the first AP and 29.7 ms for the 20th AP. Summary data from this and other C-type units (n = 18) are shown in the right panel, in which open circles represent recordings in which the rate is assigned as the highest observed following frequency prior to the AP being occluded by the stimulus artefact at higher frequencies, thus making these data possible underestimates. Filled circles represent data for which failure was directly observed at the next higher frequency. The central indicator bars represent the median value for the entire group. B, comparison of the maximum rate at which APs in C-type neurons can propagate through the axonal T-junction (`Axon'), versus the maximum rate at which the same neuron's soma can fire all of a train of 20 APs during direct somatic current injection (`Soma'). Conduction through the T-junction was confirmed by the generation of a depolarization, including either an AP or anincomplete electrotonic depolarization, during axonal stimulation. The central indicator bars represent the median value. Main effect of Injury Group P < 0.01; main effect of Stimulation Site P < 0.001; significant post hoc comparisons are shown by connecting brackets, P < 0.05. C, sensitivity of following frequency to modulation of.
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