Widespread innervation of motoneurons by spinal V3 neurons globally amplifies locomotor output in mice.

While considerable progress has been made in understanding the neuronal circuits that underlie the patterning of locomotor behaviours such as walking, less is known about the circuits that amplify motoneuron output to enable adaptable increases in muscle force across different locomotor intensities. Here, we demonstrate that an excitatory propriospinal neuron population (V3 neurons, Sim1 + ) forms a large part of the total excitatory interneuron input to motoneurons (∼20%) across all hindlimb muscles. Additionally, V3 neurons make extensive connections among themselves and with other excitatory premotor neurons (such as V2a neurons). These circuits allow local activation of V3 neurons at just one segment (via optogenetics) to rapidly depolarize and amplify locomotor-related motoneuron output at all lumbar segments in both the in vitro spinal cord and the awake adult mouse. Interestingly, despite similar innervation from V3 neurons to flexor and extensor motoneuron pools, functionally, V3 neurons exhibit a pronounced bias towards activating extensor muscles. Furthermore, the V3 neurons appear essential to extensor activity during locomotion because genetically silencing them leads to slower and weaker mice with a poor ability to increase force with locomotor intensity, without much change in the timing of locomotion. Overall, V3 neurons increase the excitability of motoneurons and premotor neurons, thereby serving as global command neurons that amplify the locomotion intensity.

Embryonic temporal-spatial delineation of excitatory spinal V3 interneuron diversity.

Spinal neural circuits that execute movement are composed of cardinal classes of neurons that emerged from distinct progenitor lineages. Each cardinal class contains multiple neuronal subtypes characterized by distinct molecular, anatomical, and physiological characteristics. Through a focus on the excitatory V3 interneuron class, here we demonstrate that interneuron subtype diversity is delineated through a combination of neurogenesis timing and final laminar settling position. We have revealed that early-born and late-born embryonic V3 temporal classes further diversify into subclasses with spatially and molecularly discrete identities. While neurogenesis timing accounts for V3 morphological diversification, laminar settling position accounts for electrophysiological profiles distinguishing V3 subtypes within the same temporal classes. Furthermore, V3 interneuron subtypes display independent behavioral recruitment patterns demonstrating a functional modularity underlying V3 interneuron diversity. These studies provide a framework for how early embryonic temporal and spatial mechanisms combine to delineate spinal interneuron classes into molecularly, anatomically, and functionally relevant subtypes in adults.