Regional distribution of putative rhythm-generating and pattern-forming components of the mammalian locomotor CPG

- Location and projections of neonatal mouse locomotor CPG interneurons are mapped.
- Functional subtypes are identified via firing pattern during non-resetting deletions.
- Putative rhythm generators are clustered medially and display many local projections.
- Putative pattern formers extend long axons toward lateral motoneurons.
- Functional subtypes show distinct distribution and morphology within the spinal cord.

The ventromedial spinal cord of mammals contains a neural network known as the locomotor central pattern generator (CPG) which underlies the basic generation and coordination of muscle activity during walking. To understand how this neural network operates, it is necessary to identify, characterize, and map connectivity among its constituent cells. Recently, a series of studies have analyzed the activity pattern of interneurons that are rhythmically active during locomotion and suggested that they belong to one of two functional levels; one responsible for rhythm generation and the other for pattern formation. Here we use electrophysiological techniques to identify locomotor-related interneurons in the lumbar spinal cord of the neonatal mouse. By analyzing their activity during spontaneous deletions that occur during fictive locomotion we are able to distinguish between those likely to belong to the rhythm-generating and pattern-forming levels, and determine the regional distribution of each. Anatomical tracing techniques are also employed to investigate the morphological characteristics of cells belonging to each level. Results demonstrate that putative rhythm-generating cells are medially located and extend locally projecting axons, while those with activity consistent with pattern formation are located more laterally and send axonal projections to the lateral edge of the spinal cord, in the direction of the motoneuron pools. Results of this study provide insight into the detailed anatomical organization of the locomotor CPG.