Diversity of molecularly defined spinal interneurons engaged in mammalian locomotor pattern generation

Abstract

Mapping the expression of transcription factors in the mouse spinal cord has identified ten progenitor domains, four of which are cardinal classes of molecularly defined, ventrally located interneurons that are integrated in the locomotor circuitry. This review focuses on the properties of these interneuronal populations and their contribution to hindlimb locomotor central pattern generation. Interneuronal populations are categorized based on their excitatory or inhibitory functions and their axonal projections as predictors of their role in locomotor rhythm generation and coordination. The synaptic connectivity and functions of these interneurons in the locomotor central pattern generators (CPGs) have been assessed by correlating their activity patterns with motor output responses to rhythmogenic neurochemicals and sensory and descending fibers stimulations as well as analyzing kinematic gait patterns in adult mice. The observed complex organization of interneurons in the locomotor CPG circuitry, some with seemingly similar physiological functions, reflects the intricate repertoire associated with mammalian motor control and is consistent with high transcriptional heterogeneity arising from cardinal interneuronal classes. This review discusses insights derived from recent studies to describe innovative approaches and limitations in experimental model systems and to identify missing links in current investigational enterprise.

Somatosensory corticospinal tract axons sprout within the cervical cord following a dorsal root/dorsal column spinal injury in the rat

“…In normal rats, as in primates, S1 CST terminals synapse in the (contralateral) dorsal horn primarily on to the presumed axons/dendrites of intraspinal neuronal populations (Côté, Murray, & Knikou, ; Gosgnach et al, ; Schieber, ; Ziskind‐Conhaim & Hochman, ). Target neurons may include local reflex, long and short range propriospinal (Filli & Schwab, ) and commissural interneuronal populations (Bannatyne, Edgley, Hammar, Jankowska, & Maxwell, ; Jankowska et al, ; Soteropoulos, Edgley, & Baker, ), which in turn connect with motor neurons, and/or regulate (or gate) ascending tracts (i.e., the dorsal columns, spinocerebellar and spinothalamic pathways).…”

Role of Propriospinal Neurons in Control of Respiratory Muscles and Recovery of Breathing Following Injury

“…Molecular genetic approaches to label specific developmental classes of spinal neurons have allowed investigators to begin dissecting the complex circuitry of the spinal cord (Grillner and El Manira, 2015;Lu et al, 2015;Kiehn, 2016;Gosgnach et al, 2017;Ziskind-Conhaim and Hochman, 2017;Dougherty and Ha, 2019). Developmental studies have identified 10 progenitor domains that give rise to six dorsal (dI1-6) and four ventral (V0-V3) classes of propriospinal neurons.…”

Somatosensory corticospinal tract axons sprout within the cervical cord following a dorsal root/dorsal column spinal injury in the rat

“…In normal rats, as in primates, S1 CST terminals synapse in the (contralateral) dorsal horn primarily on to the presumed axons/dendrites of intraspinal neuronal populations (Schieber, 2007;Gosgnach et al, 2017;Ziskind-Conhaim & Hochman, 2017;Côté et al, 2018). Target neurons may include local reflex, long and short range propriospinal (Filli & Schwab, 2015) and commissural interneuronal populations (Bannatyne et al, 2003;Jankowska et al, 2009;Soteropoulos et al, 2013), which in turn connect with motor neurons, and/or regulate (or gate) ascending tracts (i.e.…”