Neuronal activity in the isolated mouse spinal cord during spontaneous deletions in fictive locomotion: insights into locomotor central pattern generator organization

Key points The organization of the spinal circuitry responsible for the generation of locomotor rhythm and control of locomotion in mammals is largely unknown, though several types of spinal interneurons involved in the rodent locomotor network have been identified. Ventral root recordings of spinal motoneurons during fictive locomotion in the isolated mouse spinal cord show spontaneous deletions of activity. The majority of deletions in the isolated neonatal mouse spinal cord are non-resetting: they do not change the phase of subsequent motor cycles. Flexor and extensor motoneurons express asymmetric responses during deletions: flexor deletions are accompanied by tonic ipsilateral extensor activity, while extensor deletions do not perturb rhythmic ipsilateral flexor activity. Non-resetting deletions on one side of the cord do not perturb rhythmic activity on the other side of the cord and can occur in isolated hemicords. We have characterized the activity of motoneurons and identified interneurons during spontaneous motor deletions. The motoneurons and a subset of V2a interneurons fall silent during non-resetting motor deletions while a second subset of V2a interneurons and commissural interneurons continue unperturbed rhythmic firing. This allowed us to suggest their involvement at different levels of the locomotor network operation. We have developed a computational model of the central pattern generator that reproduces, and proposes a mechanistic explanation for, our experimental results. The model provides novel insights into the organization of spinal locomotor networks. Abstract We explored the organization of the spinal central pattern generator (CPG) for locomotion by analysing the activity of spinal interneurons and motoneurons during spontaneous deletions occurring during fictive locomotion in the isolated neonatal mouse spinal cord, following earlier work on locomotor deletions in the cat. In the isolated mouse spinal cord, most spontaneous deletions were non-resetting, with rhythmic activity resuming after an integer number of cycles. Flexor and extensor deletions showed marked asymmetry: flexor deletions were accompanied by sustained ipsilateral extensor activity, whereas rhythmic flexor bursting was not perturbed during extensor deletions. Rhythmic activity on one side of the cord was not perturbed during non-resetting spontaneous deletions on the other side, and these deletions could occur with no input from the other side of the cord. These results suggest that the locomotor CPG has a two-level organization with rhythm-generating (RG) and pattern-forming (PF) networks, in which only the flexor RG network is intrinsically rhythmic. To further explore the neuronal organization of the CPG, we monitored activity of motoneurons and selected identified interneurons during spontaneous non-resetting deletions. Motoneurons lost rhythmic synaptic drive during ipsilateral deletions. Flexor-related commissural interneurons continued to fire rhythmically during non-resetting ipsilateral flexor deletions. Deletion analysis revealed two classes of rhythmic V2a interneurons. Type I V2a interneurons retained rhythmic synaptic drive and firing during ipsilateral motor deletions, while type II V2a interneurons lost rhythmic synaptic input and fell silent during deletions. This suggests that the type I neurons are components of the RG, whereas the type II neurons are components of the PF network. We propose a computational model of the spinal locomotor CPG that reproduces our experimental results. The results may provide novel insights into the organization of spinal locomotor networks.