Epidural stimulation induced modulation of spinal locomotor networks in adult spinal rats.

The importance of the in vivo dynamic nature of the circuitries within the spinal cord that generate locomotion is becoming increasingly evident. We examined the characteristics of hindlimb EMG activity evoked in response to epidural stimulation at the S1 spinal cord segment in complete midthoracic spinal cord-transected rats at different stages of postlesion recovery. A progressive and phase-dependent modulation of monosynaptic (middle) and long-latency (late) stimulation-evoked EMG responses was observed throughout the step cycle. During the first 3 weeks after injury, the amplitude of the middle response was potentiated during the EMG bursts, whereas after 4 weeks, both the middle and late responses were phase-dependently modulated. The middle- and late-response magnitudes were closely linked to the amplitude and duration of the EMG bursts during locomotion facilitated by epidural stimulation. The optimum stimulation frequency that maintained consistent activity of the long-latency responses ranged from 40 to 60 Hz, whereas the short-latency responses were consistent from 5 to 130 Hz. These data demonstrate that both middle and late evoked potentials within a motor pool are strictly gated during in vivo bipedal stepping as a function of the general excitability of the motor pool and, thus, as a function of the phase of the step cycle. These data demonstrate that spinal cord epidural stimulation can facilitate locomotion in a time-dependent manner after lesion. The long-latency responses to epidural stimulation are correlated with the recovery of weight-bearing bipedal locomotion and may reflect activation of interneuronal central pattern-generating circuits.

Facilitation of stepping with epidural stimulation in spinal rats: role of sensory input.

We investigated the role of afferent information during recovery of coordinated rhythmic activity of the hindlimbs in rats with a complete spinal cord section (approximately T8) and unilateral deafferentation (T12-S2) to answer the following questions: (1) Can bilateral stepping be generated with only afferent projections intact on one side? (2) Can the sensory input from the non-deafferented side compensate for the loss of the afferent input from the deafferented side through the crossed connections within the lumbosacral spinal cord? (3) Which afferent projections to the spinal cord from the non-deafferented side predominantly mediate the effect of epidural stimulation to facilitate stepping? Recovery of stepping ability was tested under the facilitating influence of epidural stimulation at the S1 spinal segment, or epidural stimulation plus quipazine, a 5-HT agonist. All chronic spinal rats were able to generate stepping-like patterns on a moving treadmill on the non-deafferented, but not deafferented, side from 3 to 7 weeks after surgery when facilitated by epidural stimulation. Adaptation to the loss of unilateral afferent input was evident at 7 weeks after surgery, when some movements occurred on the deafferented side. Spinal-cord-evoked potentials were observed on both sides, although middle (monosynaptic) and late (long latency) responses were more prominent on the non-deafferented side. The afferent information arising from the non-deafferented side, however, eventually could mediate limited restoration of hindlimb movements on the deafferented side. These data suggest that facilitation of stepping with epidural stimulation is mediated primarily through ipsilateral afferents that project to the locomotor networks.

Training locomotor networks.

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.

Implications of assist-as-needed robotic step training after a complete spinal cord injury on intrinsic strategies of motor learning.

Robotic training paradigms that enforce a fixed kinematic control might be suboptimal for rehabilitative training because they abolish variability, an intrinsic property of neuromuscular control (Jezernik et al., 2003). In the present study we introduce "assist-as-needed" (AAN) robotic training paradigms for rehabilitation of spinal cord injury subjects. To test the efficacy of these robotic control strategies to teach spinal mice to step, we divided 27 adult female Swiss-Webster mice randomly into three groups. Each group was trained robotically by using one of three control strategies: a fixed training trajectory (Fixed group), an AAN training paradigm without interlimb coordination (Band group), and an AAN training paradigm with bilateral hindlimb coordination (Window group). Beginning at 14 d after a complete midthoracic spinal cord transection, the mice were trained daily (10 min/d, 5 d/week) to step on a treadmill 10 min after the administration of quipazine (0.5 mg/kg), a serotonin agonist, for a period of 6 weeks. During weekly performance evaluations, the mice trained with the AAN window paradigm generally showed the highest level of recovery as measured by the number, consistency, and periodicity of steps during the testing sessions. In all three measurements there were no significant differences between the Band and the Fixed training groups. These results indicate that the window training approach, which includes loose alternating interlimb coordination, is more effective than a fixed trajectory paradigm with rigid alternating interlimb coordination or an AAN paradigm without any interlimb constraints in promoting robust postinjury stepping behavior.

Spinal cord-transected mice learn to step in response to quipazine treatment and robotic training.

In the present study, concurrent treatment with robotic step training and a serotonin agonist, quipazine, generated significant recovery of locomotor function in complete spinal cord-transected mice (T7-T9) that otherwise could not step. The extent of recovery achieved when these treatments were combined exceeded that obtained when either treatment was applied independently. We quantitatively analyzed the stepping characteristics of spinal mice after alternatively administering no training, manual training, robotic training, quipazine treatment, or a combination of robotic training with quipazine treatment, to examine the mechanisms by which training and quipazine treatment promote functional recovery. Using fast Fourier transform and principal components analysis, significant improvements in the step rhythm, step shape consistency, and number of weight-bearing steps were observed in robotically trained compared with manually trained or nontrained mice. In contrast, manual training had no effect on stepping performance, yielding no improvement compared with nontrained mice. Daily bolus quipazine treatment acutely improved the step shape consistency and number of steps executed by both robotically trained and nontrained mice, but these improvements did not persist after quipazine was withdrawn. At the dosage used (0.5 mg/kg body weight), quipazine appeared to facilitate, rather than directly generate, stepping, by enabling the spinal cord neural circuitry to process specific patterns of sensory information associated with weight-bearing stepping. Via this mechanism, quipazine treatment enhanced kinematically appropriate robotic training. When administered intermittently during an extended period of robotic training, quipazine revealed training-induced stepping improvements that were masked in the absence of the pharmacological treatment.

Recovery of control of posture and locomotion after a spinal cord injury: solutions staring us in the face.

Over the past 20 years, tremendous advances have been made in the field of spinal cord injury research. Yet, consumed with individual pieces of the puzzle, we have failed as a community to grasp the magnitude of the sum of our findings. Our current knowledge should allow us to improve the lives of patients suffering from spinal cord injury. Advances in multiple areas have provided tools for pursuing effective combination of strategies for recovering stepping and standing after a severe spinal cord injury. Muscle physiology research has provided insight into how to maintain functional muscle properties after a spinal cord injury. Understanding the role of the spinal networks in processing sensory information that is important for the generation of motor functions has focused research on developing treatments that sharpen the sensitivity of the locomotor circuitry and that carefully manage the presentation of proprioceptive and cutaneous stimuli to favor recovery. Pharmacological facilitation or inhibition of neurotransmitter systems, spinal cord stimulation, and rehabilitative motor training, which all function by modulating the physiological state of the spinal circuitry, have emerged as promising approaches. Early technological developments, such as robotic training systems and high-density electrode arrays for stimulating the spinal cord, can significantly enhance the precision and minimize the invasiveness of treatment after an injury. Strategies that seek out the complementary effects of combination treatments and that efficiently integrate relevant technical advances in bioengineering represent an untapped potential and are likely to have an immediate impact. Herein, we review key findings in each of these areas of research and present a unified vision for moving forward. Much work remains, but we already have the capability, and more importantly, the responsibility, to help spinal cord injury patients now.