Decoupling of timescales reveals sparse convergent CPG network in the adult spinal cord

Marija Radosevic,Henrik Lindén,Alex Willumsen,Peter C Petersen,Mikkel Vestergaard,Rune W Berg

doi:10.1038/s41467-019-10822-9

Abstract

During the generation of rhythmic movements, most spinal neurons receive an oscillatory synaptic drive. The neuronal architecture underlying this drive is unknown, and the corresponding network size and sparseness have not yet been addressed. If the input originates from a small central pattern generator (CPG) with dense divergent connectivity, it will induce correlated input to all receiving neurons, while sparse convergent wiring will induce a weak correlation, if any. Here, we use pairwise recordings of spinal neurons to measure synaptic correlations and thus infer the wiring architecture qualitatively. A strong correlation on a slow timescale implies functional relatedness and a common source, which will also cause correlation on fast timescale due to shared synaptic connections. However, we consistently find marginal coupling between slow and fast correlations regardless of neuronal identity. This suggests either sparse convergent connectivity or a CPG network with recurrent inhibition that actively decorrelates common input.

Highlights

During the generation of rhythmic movements, most spinal neurons receive an oscillatory synaptic drive
We recorded from pairs of spinal neurons (n = 66 pairs) either motor neurons (MNs) or interneurons (INs) located in the lumbar region of adult turtles
Discussion substantial progress has been made in describing spinal cell types and projection patterns[2,4,32,42,43,44,45] remarkably little is known about connectivity in motor circuits

Summary

Introduction

During the generation of rhythmic movements, most spinal neurons receive an oscillatory synaptic drive. We use multi–electrode arrays to measure population activity to determine the pairwise spike–spike correlation as an additional indicator for shared synaptic input, under same assumptions In both approaches, we found a consistent decoupling between the slow rate modulation and the fast synaptic activity, even for pairs belonging to the same module. Active decorrelation is a mechanism observed, e.g., in the neocortex, by which correlated input due to shared connectivity is partially cancelled by inhibition[23,24,25,26] The latter interpretation, if true, implies a role of inhibition in motor circuits, which is fundamentally different from the previously assumed role of inhibition in the spinal cord

Methods

Results

Conclusion