SPINAL CIRCUITS FOR LOCOMOTOR ADAPTATION

Animals’ ability to execute and adapt movements relies on neural networks in the spinal cord known as central pattern generators (CPGs), which regulate the rhythm and pattern of muscle contractions. CPGs include six neuronal populations, V0, V1, V2a, V2b, V3 and dI6, that differ in developmental origin, neurotransmitter phenotype and connectivity. Dysfunctions within these populations affect locomotor execution.
We used an intersectional chemogenetic approach to transiently enhance (hM3D) or inhibit (hM4D) the activity of selected CPG populations and assessed the resulting changes in biomechanical strategies during motor tasks requiring either speed (treadmill) or postural adaptation (narrow beams). High-speed videos tracked with DeepLabCut were used to extract kinematic parameters from 15 body landmarks, while EMG recordings from flexor (tibialis) and extensor (gastrocnemius) muscles were used to infer kinetic features.
Activation of V2a neurons shortened the stride and induced a crouched posture, conversely silencing of V2a neurons extended the stride and elongated the posture. Intriguingly, V2a activation also caused paw collisions and stuttering during beam, but not treadmill, locomotion. Preliminary EMG analyses suggest that V2a neuron activation, even on wider beams, increases flexor/extensor co-contraction, resembling the kinetic pattern observed during narrow beam locomotion. The paw collisions and muscle co-contraction driven by V2a activation, along with postural changes, suggest that these excitatory neurons are critical for real-time processing of proprioceptive feedback during demanding locomotion.
Together, these findings indicate that V2a neurons promote adaptive biomechanical strategies during locomotion and may play a key role in integrating proprioceptive feedback during posturally demanding tasks.