Ranu Jung

Activity-dependent plasticity in spinal cord injury.

The adult mammalian central nervous system (CNS) is capable of considerable plasticity, both in health and disease. After spinal neurotrauma, the degrees and extent of neuroplasticity and recovery depend on multiple factors, including the level and extent of injury, postinjury medical and surgical care, and rehabilitative interventions. Rehabilitation strategies focus less on repairing lost connections and more on influencing CNS plasticity for regaining function. Current evidence indicates that strategies for rehabilitation, including passive exercise, active exercise with some voluntary control, and use of neuroprostheses, can enhance sensorimotor recovery after spinal cord injury (SCI) by promoting adaptive structural and functional plasticity while mitigating maladaptive changes at multiple levels of the neuraxis. In this review, we will discuss CNS plasticity that occurs both spontaneously after SCI and in response to rehabilitative therapies.

Real-time interaction between a neuromorphic electronic circuit and the spinal cord

We present a novel demonstration of real-time dynamic interaction between an oscillatory spinal cord (isolated lamprey nervous system) and electronic hardware that mimics the spinal motor pattern generating circuitry. The spinal cord and the neuromorphic circuit were interfaced in unidirectional and bidirectional modes. Bidirectional coupling resulted in stable, persistent oscillations. This experimental platform offers a unique paradigm to examine the intrinsic dynamics of neural circuitry. The neuromorphic analog very large scale integration (aVLSI) design and real-time capabilities of this approach may provide a particularly powerful means of restoring complex neuromotor function using neuroprostheses.

Cardiorespiratory responses to glutamatergic antagonists in the caudal ventrolateral medulla of rats.

The role of caudal ventrolateral medullary (CVLM) depressor neurons in influencing arterial pressure and ventilation as well as the baroreflex control of arterial pressure was investigated, and the part played by excitatory N-methyl-D-aspartate (NMDA) and non-NMDA receptors in mediating the responses was determined. In urethane-anesthetized, spontaneously breathing rats unilateral microinjections into the caudal depressor area of the broad-band glutamatergic antagonist kynurenic acid (KYN, 5 nmol or 1.58 nmol), or NMDA antagonist 2-amino-5-phosphonovaleric acid (2-APV, 2.7 nmol), or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 0.257 nmol) caused a respiratory arrest within 4 min and the animals had to be artificially ventilated. Respiratory frequency increased on injecting KYN and CNQX while it did not change significantly with 2-APV. Apnea resulted from progressive decrease in tidal volume. During the apnea ventilation with 5% CO2 did not revive breathing. Mean arterial pressure (MAP) increased significantly with KYN and 2-APV injections but not with CNQX. The baroreflex decrease of MAP, elicited by left or right aortic depressor nerve stimulation, was significantly reduced or abolished after bilateral microinjections of all 3 antagonists. Ventilation as well as the baroreflex usually recovered after 1-1.5 h. Microinjections of the same doses of antagonists into the facial nucleus, as well as application of KYN (25 nmol) to the ventral medullary surface above the hypoglossal rootlets, had no significant effect. The results support previous findings that the CVLM neurons of the rat inhibit sympathetic neurons providing the vasomotor tone, and that an intact CVLM is obligatory for mediating the baroreflex decrease of arterial pressure. The results also indicate that: (1) the CVLM is essential for sustaining ventilation in the rat; (2) only NMDA receptors are involved in maintaining baseline blood pressure while both NMDA and non-NMDA receptors mediate the baroreceptor depressor reflex; and (3) both NMDA and non-NMDA receptor activation is necessary to sustain ventilation.

Neuromuscular stimulation therapy after incomplete spinal cord injury promotes recovery of interlimb coordination during locomotion.

The mechanisms underlying the effects of neuromuscular electrical stimulation (NMES) induced repetitive limb movement therapy after incomplete spinal cord injury (iSCI) are unknown. This study establishes the capability of using therapeutic NMES in rodents with iSCI and evaluates its ability to promote recovery of interlimb control during locomotion. Ten adult female Long Evans rats received thoracic spinal contusion injuries (T9; 156 +/- 9.52 Kdyne). 7 days post-recovery, 6/10 animals received NMES therapy for 15 min/day for 5 days, via electrodes implanted bilaterally into hip flexors and extensors. Six intact animals served as controls. Motor function was evaluated using the BBB locomotor scale for the first 6 days and on 14th day post-injury. 3D kinematic analysis of treadmill walking was performed on day 14 post-injury. Rodents receiving NMES therapy exhibited improved interlimb coordination in control of the hip joint, which was the specific NMES target. Symmetry indices improved significantly in the therapy group. Additionally, injured rodents receiving therapy more consistently displayed a high percentage of 1:1 coordinated steps, and more consistently achieved proper hindlimb touchdown timing. These results suggest that NMES techniques could provide an effective therapeutic tool for neuromotor treatment following iSCI.

Characteristics and Organization of Discharge Properties in Rat Hindlimb Motoneurons

The discharge properties of hindlimb motoneurons in ketamine-xylazine anesthetized rats were measured to assess contributions of persistent intrinsic currents to these characteristics and to determine their distribution in motoneuron pools. Most motoneurons (30/37) responded to ramp current injections with adapting patterns of discharge and the frequency-current (f-I) relations of nearly all motoneurons included a steep subprimary range of discharge. Despite the prevalence of adapting f-I relations, responses included indications that persistent inward currents (PICs) were activated, including increased membrane noise and prepotentials before discharge, as well as counterclockwise hysteresis and secondary ranges in f-I relations. Examination of spike thresholds and afterhyperpolarization (AHP) trajectories during repetitive discharge revealed systematic changes in threshold and trajectory within the subprimary, primary, and secondary f-I ranges. These changes in the primary and secondary ranges were qualitatively similar to those described previously for cat motoneurons. Within the subprimary range, AHP trajectories often included shallow approaches to threshold following recruitment and slope of the AHP ramp consistently increased until the subprimary range was reached. We suggest that PICs activated near recruitment contributed to these slope changes and formation of the subprimary range. Discharge characteristics were strongly correlated with motoneuron size, using input conductance as an indicator of size. Discharge adaptation, recruitment current, and frequency increased with input conductance, whereas both subprimary and primary f-I gains decreased. These results are discussed with respect to potential mechanisms and their functional implications.

Interaction between the caudal brainstem and the lamprey central pattern generator for locomotion

Because of its remarkable simplicity and the robustness of the isolated preparation, the lamprey has been used as a model system to study locomotion and its central pattern generator. The function of the spinal cord is relatively well understood in this context, but the role of the brain or even the caudal brainstem remains less so. We here present a study of the interaction between the caudal brainstem and the spinal pattern generator for locomotion. We show that the interaction is highly complex, with both feedforward input from the brainstem to spinal cord and feedback input from the spinal cord to brainstem playing a significant role in the motor output during locomotion. The brainstem, when diffusely stimulated pharmacologically, can initiate fictive locomotion, or it can disrupt or alter the ongoing D-glutamate initiated motor output. The nature of the disruptions vary greatly, and can induce generalized irregularity, while the alterations can include accelerating or decelerating of the bursting. All behaviors are displayed with spectrograms of the motor nerve discharge. We also show that the unstimulated brainstem can disrupt as well as slow the bursting, but in a complex fashion. Finally, a slow episodic behavior initiated from the caudal brainstem is also described. This can be elicited either by D-glutamate to the brainstem or by ascending activity from the spinal cord pattern generator. Thus, we demonstrate that the interaction between the brainstem and the spinal cord during the production of locomotion is highly complex. The locomotion that is exhibited by the combined brainstem-spinal cord preparation is extremely variable. This is in striking contrast to the variability of the locomotor output pharmacologically induced in the spinal cord alone. The latter preparation exhibits remarkable regularity, or upon occasion, irregularity, but not the routine irregularity or the systemic up and down changes in frequency seen with the brainstem present. However, the pattern of frequency changes induced by the brainstem is not predictable, and remains to be understood.

Persistent Currents and Discharge Patterns in Rat Hindlimb Motoneurons

We report here the first direct measurements of persistent inward currents (PICs) in rat hindlimb motoneurons, obtained from ketamine-xylazine anesthetized rats during slow voltage ramps performed by single-electrode somatic voltage clamp. Most motoneurons expressed PICs and current-voltage (I-V) relations often contained a negative-slope region (NSR; 13/19 cells). PICs activated at -52.7 ± 3.89 mV, 9 mV negative to spike threshold. NSR onset was -44.2 ± 4.1 mV. PIC amplitudes were assessed by maximum inward currents measured relative to extrapolated leak current and to NSR-onset current. PIC conductance at potentials just positive to activation was assessed by the relative change in slope conductance (g(in)/g(leak)). PIC amplitudes varied widely; some exceeded 5 and 10 nA relative to current at NSR onset or leak current, respectively. PIC amplitudes did not vary significantly with input conductance, but PIC amplitudes normalized by recruitment current decreased with increasing input conductance. Similarly, g(in)/g(leak) decreased with increasing input conductance. Currents near resting potential on descending limbs of I-V relations were often outward, relative to ascending-limb currents. This residual outward current was correlated with increases in leak conductance on the descending limb and with input conductance. Excluding responses with accommodation, residual outward currents matched differences between recruitment and derecruitment currents, suggesting a role for residual outward current in frequency adaptation. Comparison of potentials for PIC activation and NSR onset with interspike trajectories during discharge demonstrated correspondence between PIC activation and frequency-current (f-I) range boundaries. Contributions of persistent inward and outward currents to motoneuron discharge characteristics are discussed.

Adaptive Control of Movement for Neuromuscular Stimulation-Assisted Therapy in a Rodent Model

Neuromotor therapy after spinal cord or brain injury often attempts to utilize activity-dependent plasticity to promote functional recovery. Neuromuscular electrical stimulation that activates paralyzed or paretic muscles may enhance passive assistance therapy by activating more muscle mass and enriching the sensory pattern with appropriately timed muscle spindle activation. To enable studies of activity-dependent plasticity, a rodent model for stimulation-assisted locomotor therapy was developed previously. To be effective, however, such a system must allow lengthy sessions of repetitive movements. In this study, we implemented an adaptive pattern generator/pattern shaper (PG/PS) control system for a rodent model of neuromotor therapy and evaluated its ability to generate accurate and repeatable hip movements in lengthy sessions by adjusting the activation patterns of an agonist/antagonist muscle pair. In 100-cycle movement trials, the PG/PS control system provided excellent movement tracking (< 10% error), but stimulation levels steadily increased to account for muscle fatigue. In trials using an intermittent movement paradigm (100 sets of five-cycle bouts interspersed by 20-s rest periods), excellent performance (< 8% error) was also observed with less stimulation, thus indicating reduced muscle fatigue. These results demonstrate the ability of the PG/PS control system to utilize an agonist/antagonist muscle pair to control movement at a joint in a rodent model. The demonstration of repeatable movements over lengthy intermittent sessions suggests that it may be well suited to provide efficient neuromotor therapy.

NEUROMUSCULAR ELECTRICAL STIMULATION OF THE HINDLIMB MUSCLES FOR MOVEMENT THERAPY IN A RODENT MODEL

Neuromuscular electrical stimulation (NMES) can provide functional movements in people after central nervous system injury. The neuroplastic effects of long-term NMES-induced repetitive limb movement are not well understood. A rodent model of neurotrauma in which NMES can be implemented may be effective for such investigations. We present a rodent model for NMES of the flexor and extensor muscles of the hip, knee, and ankle hindlimb muscles. Custom fabricated intramuscular stimulating electrodes for rodents were implanted near identified motor points of targeted muscles in ten adult, female Long Evans rats. The effects of altering NMES pulse stimulation parameters were characterized using strength duration curves, isometric joint torque recruitment curves and joint angle measures. The data indicate that short pulse widths have the advantage of producing graded torque recruitment curves when current is used as the control parameter. A stimulus frequency of 75 Hz or more produces fused contractions. The data demonstrate ability to accurately implant the electrodes and obtain selective, graded, repeatable, strong muscle contractions. Knee and ankle angular excursions comparable to those obtained in normal treadmill walking in the same rodent species can be obtained by stimulating the target muscles. Joint torques (normalized to body weight) obtained were larger than those reported in the literature for small tailed therian mammals and for peak isometric ankle plantarflexion in a different rodent species. This model system could be used for investigations of NMES assisted hindlimb movement therapy.

Repetetive hindlimb movement using intermittent adaptive neuromuscular electrical stimulation in an incomplete spinal cord injury rodent model

The long-term objective of this work is to understand the mechanisms by which electrical stimulation based movement therapies may harness neural plasticity to accelerate and enhance sensorimotor recovery after incomplete spinal cord injury (iSCI). An adaptive neuromuscular electrical stimulation (aNMES) paradigm was implemented in adult Long Evans rats with thoracic contusion injury (T8 vertebral level, 155+/-2 Kdyne). In lengthy sessions with lightly anesthetized animals, hip flexor and extensor muscles were stimulated using an aNMES control system in order to generate desired hip movements. The aNMES control system, which used a pattern generator/pattern shaper structure, adjusted pulse amplitude to modulate muscle force in order to control hip movement. An intermittent stimulation paradigm was used (5-cycles/set; 20-second rest between sets; 100 sets). In each cycle, hip rotation caused the foot plantar surface to contact a stationary brush for appropriately timed cutaneous input. Sessions were repeated over several days while the animals recovered from injury. Results indicated that aNMES automatically and reliably tracked the desired hip trajectory with low error and maintained range of motion with only gradual increase in stimulation during the long sessions. Intermittent aNMES thus accounted for the numerous factors that can influence the response to NMES: electrode stability, excitability of spinal neural circuitry, non-linear muscle recruitment, fatigue, spinal reflexes due to cutaneous input, and the endogenous recovery of the animals. This novel aNMES application in the iSCI rodent model can thus be used in chronic stimulation studies to investigate the mechanisms of neuroplasticity targeted by NMES-based repetitive movement therapy.