Frequency-dependent recruitment of V2a interneurons during fictive locomotion in the mouse spinal cord

The principles governing the recruitment of interneurons during acceleration in vertebrate locomotion are unknown. In the mouse, the V2a spinal interneurons are dispensable for left–right coordination at low locomotor frequencies, but their function is essential for maintaining left–right coordination at high frequencies. Here we explore the mechanisms driving this frequency-dependent role using four methods to determine how V2a interneurons are recruited at different locomotor frequencies. We show that half of the V2a interneurons receive rhythmic locomotor synaptic drive, which increases with cycle frequency, recruiting more of the neurons to fire at higher frequencies. The other V2a interneurons do not receive locomotion-related synaptic drive and are not recruited into the locomotor network at any frequency. The increased role of V2a interneurons at higher locomotor frequencies arises from increased synaptic drive to recruit subthreshold oscillating V2a neurons, and not from recruitment of a second set of silent V2a interneurons.