Figure 3.

Compound synaptic responses as a function of stimulus train peak frequency in phasic and tonic 2°VNs. A₁–B₃, Compound responses formed by summation of single EPSPs (gray traces) in a tonic 2°PC (A₁–A₃) and a phasic 2°AC (B₁–B₃) neuron after pulse-train stimulation with a peak frequency of 40 (A₁, B₁), 70 (A₂, B₂), and 100 Hz (A₃, B₃). The black traces are smoothed fits of the compound responses; bottom traces show pulse trains (half-cycle duration, 2.5 s) and stimulus frequency modulation (dashed lines), respectively; horizontal dashed lines indicate the resting membrane potential (−68 mV in A; −67 mV in B) and horizontal arrows the lag/advance of the compound response peak with respect to stimulus peak frequency (vertical dotted lines). Synaptic responses in A₁–B₃ are the averages of 12 single sweeps. Calibrations in A₃ and B₃ apply to A₁, A₂ and B₁, B₂, respectively; open arrowheads in B₂ and B₃ mark the truncation of the compound response. C, D, Population mean of compound responses (C) and smoothed fits (D) evoked by pulse trains with 70 Hz peak frequency and 2.5 s half-cycle length in tonic (n = 14) and phasic 2°VNs (n = 67). The gray bands in D indicate ±SE of the mean, the dotted curves indicate the stimulus modulation and the horizontal dashed lines the mean resting membrane potential of −69 mV in tonic and −68 mV in phasic 2°VNs. E, Advance (in degrees) of the compound response peak relative to stimulus peak frequency (mean ± SE) in phasic (●) and tonic 2°VNs (○) as a function of stimulus peak frequency. **p ≤ 0.001, ***p ≤ 0.0001, significant difference between tonic and phasic 2°VNs (Mann–Whitney U test). F₁, F₂, Average amplitude of individual EPSPs evoked by the single pulses of the train with peak frequencies of 40 (□), 70 (○), and 100 Hz (▵) in tonic (F₁) and phasic 2°VNs (F₂). Amplitudes were normalized to the size of the first EPSP. Plots of ±SE were omitted for clarity; vertical gray bars mark the timing of the stimulus peak frequency.