Slow (red), intermediate (green), fast (light blue) and non-patterned (purple) V0v interneurons show different activity patterns during locomotion. (A) Recording from a V0v interneuron that was active already at a slow swimming frequency and remained recruited at high frequencies. (B) Recording from a V0v interneuron that was not active at slow swimming frequencies and became recruited only at intermediate and high frequencies. (C) Recording from a V0v interneuron that only displayed subthreshold membrane potential oscillations that did not reach the firing threshold at slow and intermediate swimming frequencies. (D) Recording from a V0v interneuron whose activity was non-patterned by swimming activity. (E) Recruitment frequency was significantly different between the three subtypes of V0v interneurons with patterned membrane potential oscillations during swimming activity (one-way ANOVA, F(2, 28) = 123.3, p<0.0001; followed by Tukey’s procedure as post hoc; graph displays means and SEM; n = 17 from 16 preparations, 9 from 8 preparations and 5 from 5 preparations for slow, intermediate and fast, respectively). (F) The amplitude of the membrane potential oscillations of the three (slow, intermediate and fast) rhythmically active V0v interneurons varied with swimming frequency. (G) The membrane oscillation amplitude increased significantly between slow (<3 Hz) and intermediate (>5 Hz) swimming frequencies for the V0v interneurons recruited at intermediate and fast swimming frequencies (Student’s paired t-test; p=0.035 for slow, p=0.0029 for intermediate, p=0.018 for fast; n = 5 from 5 preparations, 8 from 8 preparations, and 9 from 7 preparations for slow, intermediate and fast, respectively). (H) There is no topographic organization of the somata of the different V0v interneurons in the spinal motor column. There was no significant difference (one-way ANOVA, F(3, 115) = 2.129, p>0.05) between the soma size of the different sub-classes (slow: 40.4 ± 1.5; intermediate: 44.5 ± 3.0; fast: 41.8 ± 1.2; and unpatterned: 53.3 ± 9.9).