Proximal and distal spinal neurons innervating multiple synergist and antagonist motor pools.

Camille Lancelin,Remi Ronzano,Gardave Singh Bhumbra,Robert M Brownstone,Marco Beato

doi:10.7554/elife.70858

Abstract

Motoneurons (MNs) control muscle contractions, and their recruitment by premotor circuits is tuned to produce accurate motor behaviours. To understand how these circuits coordinate movement across and between joints, it is necessary to understand whether spinal neurons pre-synaptic to motor pools have divergent projections to more than one MN population. Here, we used modified rabies virus tracing in mice to investigate premotor interneurons projecting to synergist flexor or extensor MNs, as well as those projecting to antagonist pairs of muscles controlling the ankle joint. We show that similar proportions of premotor neurons diverge to synergist and antagonist motor pools. Divergent premotor neurons were seen throughout the spinal cord, with decreasing numbers but increasing proportion with distance from the hindlimb enlargement. In the cervical cord, divergent long descending propriospinal neurons were found in contralateral lamina VIII, had large somata, were neither glycinergic, nor cholinergic, and projected to both lumbar and cervical MNs. We conclude that distributed spinal premotor neurons coordinate activity across multiple motor pools and that there are spinal neurons mediating co-contraction of antagonist muscles.

Highlights

The spinal cord is responsible for organising movement by controlling the activation pattern of motoneurons (MNs), which in turn produce appropriate patterns of muscle contractions to produce limb movement
We found divergent premotor INs distributed across the lumbar spinal cord
When homolateral limbs were targeted, we found that some premotor Long descending propriospinal neurons (LDPNs) infected from ankle extensor injections were infected from homolateral forearm muscles (FMs) injection

Summary

Introduction

The spinal cord is responsible for organising movement by controlling the activation pattern of motoneurons (MNs), which in turn produce appropriate patterns of muscle contractions to produce limb movement. Changing a joint angle, (2) stiffening a joint, and (3) relaxing a joint The concatenation of these syllables across joints within and between limbs produces behaviour (Brownstone, 2020; Wiltschko et al, 2015). To change a joint angle, MNs innervating synergist muscle fibres are activated whilst those that innervate antagonist muscle fibres are inhibited. This “reciprocal inhibition” (Eccles, 1969; Eccles et al, 1956), is mediated locally by spinal interneurons (INs) throughout the spinal cord; this syllable has been fairly well characterised, with responsible neurons identified and classified (Alvarez et al, 2005; Benito-Gonzalez and Alvarez, 2012; Sapir et al, 2004; Zhang et al, 2014)

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Discussion

Conclusion