Spinal premotor interneurons controlling antagonistic muscles are spatially intermingled

Abstract

Elaborate behaviours are produced by tightly controlled flexor-extensor motor neuron activation patterns. Motor neurons are regulated by a network of interneurons within the spinal cord, but the computational processes involved in motor control are not fully understood. The neuroanatomical arrangement of motor and premotor neurons into topographic patterns related to their controlled muscles is thought to facilitate how information is processed by spinal circuits. Rabies retrograde monosynaptic tracing has been used to label premotor interneurons innervating specific motor neuron pools, with previous studies reporting topographic mediolateral positional biases in flexor and extensor premotor interneurons. To more precisely define how premotor interneurons contacting specific motor pools are organized, we used multiple complementary viral-tracing approaches in mice to minimize systematic biases associated with each method. Contrary to expectations, we found that premotor interneurons contacting motor pools controlling flexion and extension of the ankle are highly intermingled rather than segregated into specific domains like motor neurons. Thus, premotor spinal neurons controlling different muscles process motor instructions in the absence of clear spatial patterns among the flexor-extensor circuit components.

Spinal V1 Inhibitory Interneuron Clades Differ in Birthdate, Projections to Motoneurons and Heterogeneity

“…In the best animal we recovered 51 transsynaptically labeled interneurons with no evidence of degenerative phenotypes. These cells were found at the same locations, and in similar proportions as was reported by other groups using injections in younger animals where more cells were labeled (Stepien et al, 2010; Tripodi et al, 2011; Ronzano et al, 2022). The interneuron sample included cells in the Renshaw area (n=6 or 11.8%), LVII (15, 29.4%), dorsal horn medial LV (7, 13.7%), dorsal horn LI to LIV (20, 39.2%), and the contralateral spinal cord (3, 5.9%: 1 in LX and 2 in LVIII).…”

An alpha- to gamma-motoneurone collateral can mitigate velocity-dependent stretch reflexes during voluntary movement: A computational study

“…Thus we do not consider stretch reflex signals from II afferents, or tendon tension signals from Golgi tendon organs [37, 38]. Another key limitation is that our spinal circuit is simplified in the extreme and does not consider the divergent and convergent branching of Ia, Ib and II afferent signals to multiple homonymous and heteronymous interneuronal pathways within and across muscles and limbs, where monosynaptic inputs to even antagonist motor neuron pools are largely overlapping [37, 80, 81], and even the α, β, γ classification of MNs is evolving [82]. Future work is needed to further our investigations of the fusimotor system.…”

Motor unit modes in the calf muscles during a submaximal isometric contraction are changed by brief stretches

“…Although it is tempting to infer that correlated discharge rates reflect a command signal transmitted by descending pathways, earlier studies on motor unit synchronization emphasized the significant role of premotor interneurons in contributing to the correlated discharge times of motor units (Datta et al, 1991;Nordstrom et al, 1992). The interneurons that synapse onto motor neurons have divergent projections to motor neuron pools throughout the spinal cord, including those that innervate synergist and antagonist muscles (Ronzano et al, 2021(Ronzano et al, , 2022. These spinal premotor interneurons are presumed to coordinate activity across multiple pools of motor neurons, which may contribute to the correlated discharge rates of motor units in different muscles (Lemay et al, 2001;Mussa-Ivaldi et al, 1994;Puskár & Antal, 1997;Salmas & Cheung, 2023).…”