Figure 4.

Compound synaptic responses as a function of stimulus pulse train length in phasic and tonic 2°VNs. A₁–B₄, Compound responses of single EPSPs (gray traces) in a tonic (A₁–A₄) and a phasic (B₁–B₄) 2°HC neuron after pulse-train stimulation with 0.25 (A₁, B₁), 0.5 (A₂, B₂), 1 (A₃, B₃), and 2.5 s (A₄, B₄) half-cycle length. The peak frequency was 70 Hz and pulse intensity was 1.8× T in A and 1.7× T in B. Black traces are smoothed fits of the compound responses; bottom traces show pulse trains and stimulus frequency (dashed lines), respectively; horizontal dashed lines indicate the membrane potential (−69 mV in A; −68 mV in B) and horizontal arrows indicate the lag/advance in degree of the compound response peak with respect to stimulus peak frequency (vertical dotted lines). All records are the average of 12 responses. Calibrations in A₃ and B₃ apply to all other traces. C₁, C₂, Mean (±SE) of compound response peak amplitude (C₁) and peak lag/advance relative to stimulus peak frequency (C₂) of tonic (○) and phasic 2°VNs (●) as a function of stimulus half-cycle length (bottom axis) and corresponding full stimulus cycle frequency (top axis). Amplitudes in C₁ were normalized to those obtained at a stimulus half-cycle length of 2.5 s; dotted two-sided arrows indicate half-cycle length and corresponding full-cycle frequency where response peak and stimulus peak frequency are aligned (0°; dashed horizontal line) in tonic (T) and phasic (P) 2°VNs. The number of neurons is indicated in parentheses. *p ≤ 0.05, ***p ≤ 0.0001, significance of difference between tonic and phasic neurons (Mann–Whitney U test).