Motor modularity and perturbation analyzed using stochastic dynamic operators

Trevor Shawn Smith

doi:10.17918/00010848

Although a significant amount of research has elucidated many questions in motor control, how the spinal cord organizes motor task inputs into muscle activations remains uncertain. In the motor modularity model of movement, dynamic 'building blocks' are combined to compactly construct most motor activity. A motor 'module' can be defined as a neural element evoking stereotyped motor drive, extracted from kinetic or biological features (e.g., synergist muscle EMG). Spinal motor modules can be studied in experimental preparations which separate the spinal cord from descending influences. In the spinal frog, the hindlimb wiping reflex has been studied as a model motor behavior composed of the adjusted gains and amplitudes of three modules. During a wiping reflex, the spinal cord must control when modules are recruited, at what strengths, and how modules coordinate the motor pools to generate behavior. Apparently-modular motor behavior can also be evoked from the spinal cord via intraspinal microstimulation (ISMS). Exogenously activating the spinal cord with ISMS drives motor activity and combinations of ISMS-evoked responses can resemble spinal reflexes, suggesting a common neural facilitation. Previously, spike-triggered averaging has identified synergy-associated spinal interneurons which are active during modular pulses. Here, we challenged the spinal capacity to execute modular motor behavior by providing perturbations, including ISMS, before, during, and after the execution of the wiping reflex. Consistent with historical data, ISMS usually evoked a pulse comprising a single component in both force trajectory and muscle activations. In most instances, the ISMS-induced motor activity resolves quickly and is modularly 'inserted' into the ongoing motor plan. Our data indicate that observed ISMS responses in frog emerge from limited recruitment of interneurons and associated single motor units within and across motor pools. Some interneurons and single motor units are selectively facilitated by either ISMS or wipe, while others are recruited by both stimuli. To further investigate the role of single units within modular motor responses, we adapted the Stochastic Dynamic Operator (SDO) framework to describe the spike-triggered changes in signal dynamics. SDOs behave as state-dependent change-of-state descriptions of signal behavior and may be applied to discern state-dependent spike effects. SDOs can be functionally clustered to identify interneurons and motor units contributing similarly to observed or latent factors (e.g., modules or synergy drives), and gain-scheduled SDOs can reconstruct stochastic signals, including EMG. SDOs may also be extended to describe other stochastic variables, including phase. Our results point towards the neural mechanisms and foundations that support motor modularity in the spinal frog motor repertoire and its responses to perturbations and inputs.

Motor modularity and perturbation analyzed using stochastic dynamic operators

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Motor modularity and perturbation analyzed using stochastic dynamic operators

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