Training locomotor networks

Abstract

For a complete adult spinal rat to regain some weight-bearing stepping capability, it appears that a sequence of specific proprioceptive inputs that are similar, but not identical, from step to step must be generated over repetitive step cycles. Furthermore, these cycles must include the activation of specific neural circuits that are intrinsic to the lumbosacral spinal cord segments. For these sensorimotor pathways to be effective in generating stepping, the spinal circuitry must be modulated to an appropriate excitability level. This level of modulation is sustained from supraspinal input in intact, but not spinal, rats. In a series of experiments with complete spinal rats, we have shown that an appropriate level of excitability of the spinal circuitry can be achieved using widely different means. For example, this modulation level can be acquired pharmacologically, via epidural electrical stimulation over specific lumbosacral spinal cord segments, and/or by use-dependent mechanisms such as step or stand training. Evidence as to how each of these treatments can "tune" the spinal circuitry to a "physiological state" that enables it to respond appropriately to proprioceptive input will be presented. We have found that each of these interventions can enable the proprioceptive input to actually control extensive details that define the dynamics of stepping over a range of speeds, loads, and directions. A series of experiments will be described that illustrate sensory control of stepping and standing after a spinal cord injury and the necessity for the "physiological state" of the spinal circuitry to be modulated within a critical window of excitability for this control to be manifested. The present findings have important consequences not only for our understanding of how the motor pattern for stepping is formed, but also for the design of rehabilitation intervention to restore lumbosacral circuit function in humans following a spinal cord injury.

Recovery of hindlimb locomotion after incomplete spinal cord injury in the cat involves spontaneous compensatory changes within the spinal locomotor circuitry

“…Moreover, the repetitive sensory inputs provided by the treadmill have been shown to facilitate the recovery of locomotor patterns (Barbeau and Rossignol 1987;de Leon et al 1998;Edgerton et al 2008;Harkema 2001;Lovely et al 1986;Van de Crommert et al 1998). In the present study, our untrained cats were free to move in their cage during the hemispinal period and were not forced to use their two hindlimbs concomitantly, as was the case with cats trained daily to walk in a straight path on treadmill.…”

Step Training Reinforces Specific Spinal Locomotor Circuitry in Adult Spinal Rats

“…Daily locomotor training of chronic spinal animals tends to normalize many properties of the spinal cord after an injury (for review, see Edgerton et al, 2008). Functionally, it appears that the pathways that are more efficient in performing the task successfully can be activated with greater consistency after a training period.…”

Supraspinal Control Predicts Locomotor Function and Forecasts Responsiveness to Training after Spinal Cord Injury

“…69,70 Beyond the ''learning'' in simple segmental reflexes described earlier, the more complex spinal circuits associated with locomotor central pattern generators also learn in response to training. 5,80,81 In particular, there is increased inhibition of polysynaptic spinal reflexes with improvements in walking, suggesting better inhibitory control of spinal circuits. 82 Adaptation and plasticity in these circuits is thought to contribute significantly to the improvements in walking function observed after locomotor training in persons with SCI.…”