Fig. 2.

Incremental recruitment of V2a interneurons with increased swimming frequency. (A) Recording from a V2a interneuron displaying only subthreshold locomotor-related membrane potential oscillations (blue) and a corresponding nerve recording (black). (B) Recording from a V2a interneuron transiently recruited during the swimming episode (red) and a corresponding nerve recording (black). (C) Recording from a V2a interneuron recruited continuously during the entire swimming episode (red) and a corresponding nerve recording (black). (A–C Insets) Position of the recorded interneurons in the spinal cord. Gray bars indicate the duration of the swimming episode. IN, interneuron. (D) Incremental increase in the peak-to-trough amplitude of the locomotor-driven membrane potential oscillations as a function of swimming frequency of the V2a interneurons in A (blue circles), B (filled red circles), and C (open red circles). (E) Slopes of the change in subthreshold locomotor-driven membrane potential oscillations from all nonrecruited (blue) and transiently recruited (red) V2a interneurons. Linear-fit lines cover the entire swimming episode for nonrecruited V2a interneurons. In transiently recruited V2a the linear fit lines cover all subthreshold locomotor cycles. (F) Plot showing the number of action potentials (APs) as a function of swimming frequency. Gray circles indicate individual neurons; black circles indicate averages from all neurons).