Differences in levels of synaptic drive related to speed and V2a cell type
A) Voltage-clamp (vc) recordings of excitatory post-synaptic currents (EPSCs) received by fast (top, FD) and slow (bottom, SD) V2a-D neurons along with ventral root (vr) recordings during fictive swimming triggered by an electrical stimulus (black arrowheads, artifact blanked). Black dots indicate burst intervals for frequency measures. Grey shaded boxes indicate region expanded inset, illustrating the holding potential (dashed line, −75 mV) and individual EPSCs (tick marks). Scale bars, 25 pA, 100 ms (20 ms inset).
B) As in A, but for fast (top, FB) and slow (bottom, SB) V2a-B neurons.
C) Scatter plot of maximum EPSC amplitude (pA) as a function of input resistance (GΩ) on logarithmic x- and y-scales. Dashed logarithmic trendlines are included for illustrative purposes (Spearman’s rank correlation test, rs = −0.873, p < 0.001, n = 39).
D) Voltage-clamp recordings of inhibitory post-synaptic currents (IPSCs) at a holding potential of 10 mV from fast (top, FD) and slow (bottom, SD) V2a-D neurons, organized as detailed in panel A. Scale bars, 100 pA, 100 ms (20 ms inset).
E) As in D, but fast (top, FB) and slow (bottom, SB) V2a-B neurons.
F) Scatter plot of maximum IPSC amplitude as a function of input resistance (GΩ) on logarithmic x- and y-scales (Spearman’s rank correlation test; rs = −0.633, p < 0.001, n = 36).
G) Top panel: Box plots of the maximum EPSC per cycle as a percentage of the maximum current (max EPSC%) for fast (FD, n = 9) and slow (SD, n = 6) V2a-D neurons, organized into 4 bins based on frequency. Dashed line indicates transition between slow carangiform and fast anguilliform modes. Bottom panel: as above, but for fast (FB, n = 11) and slow (SB, n = 5) V2a-B neurons.
H) Top panel: Box plots of max EPSC% for fast and slow V2a-D neurons from panel G, collapsed into single slow (<35 Hz) and fast (>35 Hz) bins for purposes of statistical analysis (Wilcoxon signed-rank test; FD, V = 0, p < 0.01, n = 9; SD, V = 21, p < 0.05, n = 6). Bottom panel: As above but for V2a-B neurons (Wilcoxon signed-rank test; FB, V = 2, p < 0.01, n = 11; SB, V = 15, p = 0.063, n = 5). Boxed plots are superimposed on trendlines from individual neurons. Note, 1 cell each for V2a-D and V2a-B slow subtypes behaved more like fast subtypes (dashed lines).
I) Top panel: Box plots of the maximum IPSC per cycle as a percentage of the maximum current (max IPSC%) for fast (FD, n = 7) and slow (SD, n = 7) V2a-Ds organized as in panel G. Bottom panel: As above but for fast (FB, n = 12) and slow (SB, n = 6) V2a-B neurons.
J) Top panel: Box plots of max IPSC% for fast and slow V2a-D neurons from panel I, collapsed into single slow (<35 Hz) and fast (>35 Hz) bins for purposes of for statistical analysis (Wilcoxon signed-rank test; FD, V = 0, p < 0.05, n = 7; SD, V = 1, p < 0.05, n = 7). Bottom panel: as above, but for fast and slow V2a-B neurons (Wilcoxon signed-rank test; FB, V = 0, p < 0.001, n = 12; SB, V = 4, p = 0.219, n = 6).
K) Box plots of maximum excitatory and inhibitory current for FD (Wilcoxon signed-rank test; V = 0, p < 0.01, n = 9 for excitation, n = 8 for inhibition) and SD (Wilcoxon signed-rank test; V = 0, p < 0.05, n = 8 for excitation, n = 7 for inhibition) neurons.
L) As in K but for FB (Wilcoxon signed-rank test; V = 1, p < 0.001, n = 16 for excitation, n = 15 for inhibition) and SB (Wilcoxon signed-rank test; V = 0, p < 0.05, n = 6 for excitation, n = 6 for inhibition) neurons.