![]() The MLR is composed of two main nuclei: the cuneiform nucleus (CnF) and the pedunculopontine nucleus (PPN). A series of studies have identified such populations, their projections, interactions and downstream targets in the mesencephalic locomotor region (MLR) and the pontomedullary reticular formation (RF) that are critically involved in the control of locomotor speed and gait ( Capelli et al., 2017 Caggiano et al., 2018 Josset et al., 2018). The advent of viral and genetic tools recently enabled experimental studies to further dissect the brainstem locomotor command circuitry into functionally distinct neuronal populations. Although the generation of locomotor oscillations and mutual interactions between oscillators controlling each limb are implemented within the spinal cord ( Grillner, 2006 Kiehn, 2006 Goulding, 2009 Grillner and Jessell, 2009 Kiehn, 2016 Boije and Kullander, 2018), both locomotor speed and interlimb coordination (gait) are controlled by several brainstem structures that transform signals from higher brain centers into meaningful commands to initiate, stop or modulate locomotor frequency and gait ( Lemon, 2008 Ryczko and Dubuc, 2013 Roseberry et al., 2016 Kim et al., 2017 Takakusaki, 2017 Brownstone and Chopek, 2018 Ferreira-Pinto et al., 2018 Gatto and Goulding, 2018). Alternatively, during chasing/hunting or escaping threats that require faster movements, animals switch to left-right synchronous gaits, such as gallop and bound ( Clarke and Still, 1999 Herbin et al., 2004 Herbin et al., 2007 Bellardita and Kiehn, 2015 Lemieux et al., 2016). During relatively slow locomotion, for example when animals explore the environment, they typically exhibit left-right asynchronous/alternating gaits, such as walk and trot. In quadrupeds, changes in locomotor speed are accompanied by changes in interlimb coordination ( Grillner, 1975 Miller et al., 1975 Hildebrand, 1989 Maes and Abourachid, 2013). To survive in changing and unpredictable environments animals need to continuously adapt their behavior including locomotor speed. We suggest that brainstem control of locomotion is mediated by two pathways, one controlling locomotor speed via connections to rhythm generating circuits in the spinal cord and the other providing gait control by targeting commissural and long propriospinal interneurons. ![]() Here, by extending our previous model, we propose a connectome of the brainstem-spinal circuitry and suggest a mechanistic explanation of the operation of brainstem structures and their roles in controlling speed and gait. Previously we suggested that commissural and long propriospinal interneurons are the main targets for brainstem inputs adjusting gait (Danner et al., 2017). However, the interactions of these brainstem centers with each other and with the spinal locomotor circuits are poorly understood. ![]() A series of recent studies identified key structures in the mesencephalic locomotor region and the caudal brainstem of mice involved in the initiation and control of slow (exploratory) and fast (escape-type) locomotion and gait. ![]()
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