Chapter 7--interindividual variability and its implications for locomotor adaptation following peripheral nerve and/or spinal cord injury

Changes in intra- and interlimb reflexes from forelimb cutaneous afferents after staggered thoracic lateral hemisections during locomotion in cats

In quadrupeds, such as cats, cutaneous afferents from the forepaw dorsum signal external perturbations and send inputs to spinal circuits to co-ordinate the activity in muscles of all four limbs. How these cutaneous reflex pathways from forelimb afferents are reorganized after an incomplete spinal cord injury is not clear. Using a staggered thoracic lateral hemisections paradigm, we investigated changes in intralimb and interlimb reflex pathways by electrically stimulating the left and right superficial radial nerves in seven adult cats and recording reflex responses in five forelimb and ten hindlimb muscles. After the first (right T5-T6) and second (left T10-T11) hemisections, forelimb-hindlimb co-ordination was altered and weakened. After the second hemisection, cats required balance assistance to perform quadrupedal locomotion. Short-, mid- and long-latency homonymous and crossed reflex responses in forelimb muscles and their phase modulation remained largely unaffected after staggered hemisections. The occurrence of homolateral and diagonal mid- and long-latency responses in hindlimb muscles evoked with left and right superficial radial nerve stimulation was significantly reduced at the first time point after the first hemisection, but partially recovered at the second time point with left superficial radial nerve stimulation. These responses were lost or reduced after the second hemisection. When present, all reflex responses, including homolateral and diagonal, maintained their phase-dependent modulation. Therefore, our results show a considerable loss in cutaneous reflex transmission from cervical to lumbar levels after incomplete spinal cord injury, albeit with preservation of phase modulation, probably affecting functional responses to external perturbations. KEY POINTS: Cutaneous afferent inputs co-ordinate muscle activity in the four limbs during locomotion when the forepaw dorsum contacts an obstacle. Thoracic spinal cord injury disrupts communication between spinal locomotor centres located at cervical and lumbar levels, impairing balance and limb co-ordination. We investigated cutaneous reflexes from forelimb afferents during quadrupedal locomotion by electrically stimulating the superficial radial nerve bilaterally, before and after staggered lateral thoracic hemisections in cats. We showed a loss/reduction of mid- and long-latency homolateral and diagonal reflex responses in hindlimb muscles early after the first hemisection that partially recovered with left superficial radial nerve stimulation, before being reduced after the second hemisection. Targeting cutaneous reflex pathways from forelimb afferents projecting to the four limbs could help develop therapeutic approaches aimed at restoring transmission in ascending and descending spinal pathways.

Brief High-Velocity Motor Skill Training Increases Step Frequency and Improves Length/Frequency Coordination in Slow Walkers With Chronic Motor-Incomplete Spinal Cord Injury

OBJECTIVE

To quantify spatiotemporal coordination during overground walking among persons with motor-incomplete spinal cord injury (PwMISCI) by calculating the step length (SL)/step frequency (SF) ratio (ie, the Walk Ratio [WR]) and to examine the effects of motor skill training (MST) on the relationship between changes in these parameters and walking speed (WS).

DESIGN

Between-day exploratory analysis.

SETTING

Research laboratory in a rehabilitation hospital PARTICIPANTS: PwMISCI (N=26).

INTERVENTIONS

3-day high-velocity MST.

MAIN OUTCOME MEASURES

Overground WS, SL, SF, and WR measured during the 10-Meter Walk Test.

RESULTS

Among the full sample, MST was associated with increases in WS, SL, SF, and a decrease in the WR. Relative change in WS and SF was higher among slow (ΔWS=↑46%, ΔSF=↑28%) vs fast (ΔWS=↑16%, ΔSF=↑8%) walkers. Change in the WR differed between groups (slow: ΔWR=↓10%; fast: ΔWR=0%). Twenty-six percent of the variability observed in ΔWR among slow walkers could be explained by ΔSF, while ΔSL did not contribute to ΔWR. Among fast walkers, ΔSL accounted for more than twice the observed ΔWR (43%) compared to ΔSF (15%).

CONCLUSIONS

On the whole, WR values among PwMISCI are higher than previous reports in other neurologic populations; however, values among fast walkers were comparable to noninjured adults. Slow walkers demonstrated greater variability in the WR, with higher values associated with slower WS. Following MST, increases in WS coincided with a decrease in the WR among slow walkers, mediated primarily through an effect on SF. This finding may point to a specific mechanism by which MST facilitates improvements in WS among PwMISCI with greater mobility deficits.

Changes in intra- and interlimb reflexes from hindlimb cutaneous afferents after staggered thoracic lateral hemisections during locomotion in cats

When the foot dorsum contacts an obstacle during locomotion, cutaneous afferents signal central circuits to coordinate muscle activity in the four limbs. Spinal cord injury disrupts these interactions, impairing balance and interlimb coordination. We evoked cutaneous reflexes by electrically stimulating left and right superficial peroneal nerves before and after two thoracic lateral hemisections placed on opposite sides of the cord at 9- to 13-week interval in seven adult cats (4 males and 3 females). We recorded reflex responses in ten hindlimb and five forelimb muscles bilaterally. After the first (right T5-T6) and second (left T10-T11) hemisections, coordination of the fore- and hindlimbs was altered and/or became less consistent. After the second hemisection, cats required balance assistance to perform quadrupedal locomotion. Short-latency reflex responses in homonymous and crossed hindlimb muscles largely remained unaffected after staggered hemisections. However, mid- and long-latency homonymous and crossed responses in both hindlimbs occurred less frequently after staggered hemisections. In forelimb muscles, homolateral and diagonal mid- and long-latency response occurrence significantly decreased after the first and second hemisections. In all four limbs, however, when present, short-, mid- and long-latency responses maintained their phase-dependent modulation. We also observed reduced durations of short-latency inhibitory homonymous responses in left hindlimb extensors early after the first hemisection and delayed short-latency responses in the right ipsilesional hindlimb after the first hemisection. Therefore, changes in cutaneous reflex responses correlated with impaired balance/stability and interlimb coordination during locomotion after spinal cord injury. Restoring reflex transmission could be used as a biomarker to facilitate locomotor recovery. KEY POINTS: Cutaneous afferent inputs coordinate muscle activity in the four limbs during locomotion when the foot dorsum contacts an obstacle. Thoracic spinal cord injury disrupts communication between spinal locomotor centres located at cervical and lumbar levels, impairing balance and limb coordination. We investigated cutaneous reflexes during quadrupedal locomotion by electrically stimulating the superficial peroneal nerve bilaterally, before and after staggered lateral thoracic hemisections of the spinal cord in cats. We showed a loss/reduction of mid- and long-latency responses in all four limbs after staggered hemisections, which correlated with altered coordination of the fore- and hindlimbs and impaired balance. Targeting cutaneous reflex pathways projecting to the four limbs could help develop therapeutic approaches aimed at restoring transmission in ascending and descending spinal pathways.

Changes in intra- and interlimb reflexes from hindlimb cutaneous afferents after staggered thoracic lateral hemisections during locomotion in cats

When the foot dorsum contacts an obstacle during locomotion, cutaneous afferents signal central circuits to coordinate muscle activity in the four limbs. Spinal cord injury disrupts these interactions, impairing balance and interlimb coordination. We evoked cutaneous reflexes by electrically stimulating left and right superficial peroneal nerves before and after two thoracic lateral hemisections placed on opposite sides of the cord at 9-13 weeks interval in seven adult cats (4 males and 3 females). We recorded reflex responses in ten hindlimb and five forelimb muscles bilaterally. After the first (right T5-T6) and second (left T10-T11) hemisections, coordination of the fore- and hindlimbs was altered and/or became less consistent. After the second hemisection, cats required balance assistance to perform quadrupedal locomotion. Short-latency reflex responses in homonymous and crossed hindlimb muscles largely remained unaffected after staggered hemisections. However, mid- and long-latency homonymous and crossed responses in both hindlimbs occurred less frequently after staggered hemisections. In forelimb muscles, homolateral and diagonal mid- and long-latency response occurrence significantly decreased after the first and second hemisections. In all four limbs, however, when present, short-, mid- and long-latency responses maintained their phase-dependent modulation. We also observed reduced durations of short-latency inhibitory homonymous responses in left hindlimb extensors early after the first hemisection and delayed short-latency responses in the right ipsilesional hindlimb after the first hemisection. Therefore, changes in cutaneous reflex responses correlated with impaired balance/stability and interlimb coordination during locomotion after spinal cord injury. Restoring reflex transmission could be used as a biomarker to facilitate locomotor recovery.

A sensory signal related to left-right symmetry modulates intra- and interlimb cutaneous reflexes during locomotion in intact cats

Introduction

During locomotion, cutaneous reflexes play an essential role in rapidly responding to an external perturbation, for example, to prevent a fall when the foot contacts an obstacle. In cats and humans, cutaneous reflexes involve all four limbs and are task- and phase modulated to generate functionally appropriate whole-body responses.

Methods

To assess task-dependent modulation of cutaneous interlimb reflexes, we electrically stimulated the superficial radial or superficial peroneal nerves in adult cats and recorded muscle activity in the four limbs during tied-belt (equal left-right speeds) and split-belt (different left-right speeds) locomotion.

Results

We show that the pattern of intra- and interlimb cutaneous reflexes in fore- and hindlimbs muscles and their phase-dependent modulation were conserved during tied-belt and split-belt locomotion. Short-latency cutaneous reflex responses to muscles of the stimulated limb were more likely to be evoked and phase-modulated when compared to muscles in the other limbs. In some muscles, the degree of reflex modulation was significantly reduced during split-belt locomotion compared to tied-belt conditions. Split-belt locomotion increased the step-by-step variability of left-right symmetry, particularly spatially.

Discussion

These results suggest that sensory signals related to left-right symmetry reduce cutaneous reflex modulation, potentially to avoid destabilizing an unstable pattern.

Reorganization of motor modules for standing reactive balance recovery following pyridoxine-induced large-fiber peripheral sensory neuropathy in cats

Task-level goals such as maintaining standing balance are achieved through coordinated muscle activity. Consistent and individualized groupings of synchronously activated muscles can be estimated from muscle recordings in terms of motor modules or muscle synergies, independent of their temporal activation. The structure of motor modules can change with motor training, neurological disorders, and rehabilitation, but the central and peripheral mechanisms underlying motor module structure remain unclear. To assess the role of peripheral somatosensory input on motor module structure, we evaluated changes in the structure of motor modules for reactive balance recovery following pyridoxine-induced large-fiber peripheral somatosensory neuropathy in previously collected data in four adult cats. Somatosensory fiber loss, quantified by postmortem histology, varied from mild to severe across cats. Reactive balance recovery was assessed using multidirectional translational support-surface perturbations over days to weeks throughout initial impairment and subsequent recovery of balance ability. Motor modules within each cat were quantified by non-negative matrix factorization and compared in structure over time. All cats exhibited changes in the structure of motor modules for reactive balance recovery after somatosensory loss, providing evidence that somatosensory inputs influence motor module structure. The impact of the somatosensory disturbance on the structure of motor modules in well-trained adult cats indicates that somatosensory mechanisms contribute to motor module structure, and therefore may contribute to some of the pathological changes in motor module structure in neurological disorders. These results further suggest that somatosensory nerves could be targeted during rehabilitation to influence pathological motor modules for rehabilitation.NEW & NOTEWORTHY Stable motor modules for reactive balance recovery in well-trained adult cats were disrupted following pyridoxine-induced peripheral somatosensory neuropathy, suggesting somatosensory inputs contribute to motor module structure. Furthermore, the motor module structure continued to change as the animals regained the ability to maintain standing balance, but the modules generally did not recover pre-pyridoxine patterns. These results suggest changes in somatosensory input and subsequent learning may contribute to changes in motor module structure in pathological conditions.

Specific Vagus Nerve Lesion Have Distinctive Physiologic Mechanisms of Dysphagia

Swallowing is complex at anatomical, functional, and neurological levels. The connections among these levels are poorly understood, yet they underpin mechanisms of swallowing pathology. The complexity of swallowing physiology means that multiple failure points may exist that lead to the same clinical diagnosis (e.g., aspiration). The superior laryngeal nerve (SLN) and the recurrent laryngeal nerve (RLN) are branches of the vagus that innervate different structures involved in swallowing. Although they have distinct sensory fields, lesion of either nerve is associated clinically with increased aspiration. We tested the hypothesis that despite increased aspiration in both case, oropharyngeal kinematic changes and their relationship to aspiration would be different in RLN and SLN lesioned infant pigs. We compared movements of the tongue and epiglottis in swallows before and after either RLN or SLN lesion. We rated swallows for airway protection. Posterior tongue ratio of safe swallows changed in RLN (p = 0.01) but not SLN lesioned animals. Unsafe swallows post lesion had different posterior tongue ratios in RLN and SLN lesioned animals. Duration of epiglottal inversion shortened after lesion in SLN animals (p = 0.02) but remained unchanged in RLN animals. Thus, although SLN and RLN lesion lead to the same clinical outcome (increased aspiration), the mechanisms of failure of airway protection are different, which suggests that effective therapies may be different with each injury. Understanding the specific pathophysiology of swallowing associated with specific neural insults will help develop targeted, disease appropriate treatments.

Progressive adaptation of whole-limb kinematics after peripheral nerve injury

The ability to recover purposeful movement soon after debilitating neuromuscular injury is essential to animal survival. Various neural and mechanical mechanisms exist to preserve whole-limb kinematics despite exhibiting long-term deficits of individual joints following peripheral nerve injury. However, it is unclear whether functionally relevant whole-limb movement is acutely conserved following injury. Therefore, the objective of this longitudinal study of the injury response from four individual cats was to test the hypothesis that whole-limb length is conserved following localized nerve injury of ankle extensors in cats with intact nervous systems. The primary finding of our study was that whole-limb kinematics during walking was not immediately preserved following peripheral nerve injuries that paralyzed subsets of ankle extensor muscles. Instead, whole-limb kinematics recovered gradually over multiple weeks, despite having the mechanical capacity of injury-spared muscles across all joints to achieve immediate functional recovery. The time taken to achieve complete recovery of whole-limb kinematics is consistent with an underlying process that relies on neuromuscular adaptation. Importantly, the gradual recovery of ankle joint kinematics remained incomplete, discontinuing once whole-limb kinematics had fully recovered. These findings support the hypothesis that a whole-limb representation of healthy limb function guides a locomotor compensation strategy after neuromuscular injury that arrests progressive changes in the joint kinematics once whole-limb kinematics is regained.

Intralimb and Interlimb Cutaneous Reflexes during Locomotion in the Intact Cat

When the foot contacts an obstacle during locomotion, cutaneous inputs activate spinal circuits to ensure dynamic balance and forward progression. In quadrupeds, this requires coordinated reflex responses between the four limbs. Here, we investigated the patterns and phasic modulation of cutaneous reflexes in forelimb and hindlimb muscles evoked by inputs from all four limbs. Five female cats were implanted to record muscle activity and to stimulate the superficial peroneal and superficial radial nerves during locomotion. Stimulating these nerves evoked short-, mid-, and longer-latency excitatory and/or inhibitory responses in all four limbs that were phase-dependent. The largest responses were generally observed during the peak activity of the muscle. Cutaneous reflexes during mid-swing were consistent with flexion of the homonymous limb and accompanied by modification of the stance phases of the other three limbs, by coactivating flexors and extensors and/or by delaying push-off. Cutaneous reflexes during mid-stance were consistent with stabilizing the homonymous limb by delaying and then facilitating its push-off and modifying the support phases of the homolateral and diagonal limbs, characterized by coactivating flexors and extensors, reinforcing extensor activity and/or delaying push-off. The shortest latencies of homolateral and diagonal responses were consistent with fast-conducting disynaptic or trisynaptic pathways. Descending homolateral and diagonal pathways from the forelimbs to the hindlimbs had a higher probability of eliciting responses compared with ascending pathways from the hindlimbs to the forelimbs. Thus, in quadrupeds, intralimb and interlimb reflexes activated by cutaneous inputs ensure dynamic coordination of the four limbs, producing a whole-body response.SIGNIFICANCE STATEMENT The skin contains receptors that, when activated, send inputs to spinal circuits, signaling a perturbation. Rapid responses, or reflexes, in muscles of the contacted limb and opposite homologous limb help maintain balance and forward progression. Here, we investigated reflexes during quadrupedal locomotion in the cat by electrically stimulating cutaneous nerves in each of the four limbs. Functionally, responses appear to modify the trajectory or stabilize the movement of the stimulated limb while modifying the support phase of the other limbs. Reflexes between limbs are mediated by fast-conducting pathways that involve excitatory and inhibitory circuits controlling each limb. The comparatively stronger descending pathways from cervical to lumbar circuits controlling the forelimbs and hindlimbs, respectively, could serve a protective function.

Left-right coordination from simple to extreme conditions during split-belt locomotion in the chronic spinal adult cat

KEY POINTS

Coordination between the left and right sides is essential for dynamic stability during locomotion. The immature or neonatal mammalian spinal cord can adjust to differences in speed between the left and right sides during split-belt locomotion by taking more steps on the fast side. We show that the adult mammalian spinal cord can also adjust its output so that the fast side can take more steps. During split-belt locomotion, only certain parts of the cycle are modified to adjust left-right coordination, primarily those associated with swing onset. When the fast limb takes more steps than the slow limb, strong left-right interactions persist. Therefore, the adult mammalian spinal cord has a remarkable adaptive capacity for left-right coordination, from simple to extreme conditions.

ABSTRACT

Although left-right coordination is essential for locomotion, its control is poorly understood, particularly in adult mammals. To investigate the spinal control of left-right coordination, a spinal transection was performed in six adult cats that were then trained to recover hindlimb locomotion. Spinal cats performed tied-belt locomotion from 0.1 to 1.0 m s^-1 and split-belt locomotion with low to high (1:1.25-10) slow/fast speed ratios. With the left hindlimb stepping at 0.1 m s^-1 and the right hindlimb stepping from 0.2 to 1.0 m s^-1 , 1:1, 1:2, 1:3, 1:4 and 1:5 left-right step relationships could appear. The appearance of 1:2+ relationships was not linearly dependent on the difference in speed between the slow and fast belts. The last step taken by the fast hindlimb displayed longer cycle, stance and swing durations and increased extensor activity, as the slow limb transitioned to swing. During split-belt locomotion with 1:1, 1:2 and 1:3 relationships, the timing of stance onset of the fast limb relative to the slow limb and placement of both limbs at contact were invariant with increasing slow/fast speed ratios. In contrast, the timing of stance onset of the slow limb relative to the fast limb and the placement of both limbs at swing onset were modulated with slow/fast speed ratios. Thus, left-right coordination is adjusted by modifying specific parts of the cycle. Results highlight the remarkable adaptive capacity of the adult mammalian spinal cord, providing insight into spinal mechanisms and sensory signals regulating left-right coordination.

Adaptive muscle plasticity of a remaining agonist following denervation of its close synergists in a model of complete spinal cord injury

Complete spinal cord injury (SCI) alters the contractile properties of skeletal muscle, and although exercise can induce positive changes, it is unclear whether the remaining motor system can produce adaptive muscle plasticity in response to a subsequent peripheral nerve injury. To address this, the nerve supplying the lateral gastrocnemius (LG) and soleus muscles was sectioned unilaterally in four cats that had recovered hindlimb locomotion after spinal transection. In these spinal cats, kinematics and electromyography (EMG) were collected before and for 8 wk after denervation. Muscle histology was performed on LG and medial gastrocnemius (MG) bilaterally in four spinal and four intact cats. In spinal cats, cycle duration for the hindlimb ipsilateral or contralateral to the denervation could be significantly increased or decreased compared with predenervation values. Stance duration was generally increased and decreased for the contralateral and ipsilateral hindlimbs, respectively. The EMG amplitude of MG was significantly increased bilaterally after denervation and remained elevated 8 wk after denervation. In spinal cats the ipsilateral LG was significantly smaller than the contralateral LG, whereas the ipsilateral MG weighed significantly more than the contralateral MG. Histological characterizations revealed significantly larger fiber areas for type IIa fibers of the ipsilateral MG in three of four spinal cats. Microvascular density in the ipsilateral MG was significantly higher than in the contralateral MG. In intact cats, no differences were found for muscle weight, fiber area, or microvascular density between homologous muscles. Therefore, the remaining motor system after complete SCI retains the ability to produce adaptive muscle plasticity.

The spinal control of locomotion and step-to-step variability in left-right symmetry from slow to moderate speeds

When speed changes during locomotion, both temporal and spatial parameters of the pattern must adjust. Moreover, at slow speeds the step-to-step pattern becomes increasingly variable. The objectives of the present study were to assess if the spinal locomotor network adjusts both temporal and spatial parameters from slow to moderate stepping speeds and to determine if it contributes to step-to-step variability in left-right symmetry observed at slow speeds. To determine the role of the spinal locomotor network, the spinal cord of 6 adult cats was transected (spinalized) at low thoracic levels and the cats were trained to recover hindlimb locomotion. Cats were implanted with electrodes to chronically record electromyography (EMG) in several hindlimb muscles. Experiments began once a stable hindlimb locomotor pattern emerged. During experiments, EMG and bilateral video recordings were made during treadmill locomotion from 0.1 to 0.4 m/s in 0.05 m/s increments. Cycle and stance durations significantly decreased with increasing speed, whereas swing duration remained unaffected. Extensor burst duration significantly decreased with increasing speed, whereas sartorius burst duration remained unchanged. Stride length, step length, and the relative distance of the paw at stance offset significantly increased with increasing speed, whereas the relative distance at stance onset and both the temporal and spatial phasing between hindlimbs were unaffected. Both temporal and spatial step-to-step left-right asymmetry decreased with increasing speed. Therefore, the spinal cord is capable of adjusting both temporal and spatial parameters during treadmill locomotion, and it is responsible, at least in part, for the step-to-step variability in left-right symmetry observed at slow speeds.

Gait analysis at multiple speeds reveals differential functional and structural outcomes in response to graded spinal cord injury

Open-field behavioral scoring is widely used to assess spinal cord injury (SCI) outcomes, but has limited usefulness in describing subtle changes important for posture and locomotion. Additional quantitative methods are needed to increase the resolution of locomotor outcome assessment. This study used gait analysis at multiple speeds (GAMS) across a range of mild-to-severe intensities of thoracic SCI in the rat. Overall, Basso, Beattie, and Bresnahan (BBB) scores and subscores were assessed, and detailed automated gait analysis was performed at three fixed walking speeds (3.5, 6.0, and 8.5 cm/sec). Variability in hindpaw brake, propel, and stance times were analyzed further by integrating across the stance phase of stepping cycles. Myelin staining of spinal cord sections was used to quantify white matter loss at the injury site. Varied SCI intensity produced graded deficits in BBB score, BBB subscores, and spinal cord white matter and total volume loss. GAMS measures of posture revealed decreased paw area, increased limb extension, altered stance width, and decreased values for integrated brake, propel, and stance. Measures of coordination revealed increased stride frequency concomitant with decreased stride length, resulting in deviation from consistent forelimb/hindlimb coordination. Alterations in posture and coordination were correlated to impact severity. GAMS results correlated highly with functional and histological measures and revealed differential relationships between sets of GAMS dynamics and cord total volume loss versus epicenter myelin loss. Automated gait analysis at multiple speeds is therefore a useful tool for quantifying nuanced changes in gait as an extension of histological and observational methods in assessing SCI outcomes.

Crossed reflex reversal during human locomotion

During human walking, precise coordination between the two legs is required in order to react promptly to any sudden hazard that could threaten stability. The networks involved in this coordination are not yet completely known, but a direct spinal connection between soleus (SOL) muscles has recently been revealed. For this response to be functional, as previously suggested, we hypothesize that it will be accompanied by a reaction in synergistic muscles, such as gastrocnemius lateralis (GL), and that a reversal of the response would occur when an opposite reaction is required. In the present study, surface EMGs of contralateral SOL and GL were analyzed after tibial nerve (TN), sural nerve (SuN), and medial plantar nerve (MpN) stimulation during two tasks in which opposite reactions are functionally expected: normal walking (NW), just before ipsilateral heel strike, and hybrid walking (HW) (legs walking in opposite directions), at ipsilateral push off and contralateral touchdown. Early crossed facilitations were observed in the contralateral GL after TN stimulation during NW, and a reversal of such responses occurred during HW. These results underline the functional significance of short-latency crossed responses and represent the first evidence for short-latency reflex reversal in the contralateral limb for humans. Muscle afferents seem to mediate the response during NW, while during HW cutaneous afferents are likely involved. It is thus possible that different afferents mediate the crossed response during different tasks.

Cutaneous inputs from the back abolish locomotor-like activity and reduce spastic-like activity in the adult cat following complete spinal cord injury

Spasticity is a condition that can include increased muscle tone, clonus, spasms, and hyperreflexia. In this study, we report the effect of manually stimulating the dorsal lumbosacral skin on spontaneous locomotor-like activity and on a variety of reflex responses in 5 decerebrate chronic spinal cats treated with clonidine. Cats were spinalized 1 month before the terminal experiment. Stretch reflexes were evoked by stretching the left triceps surae muscles. Crossed reflexes were elicited by electrically stimulating the right tibial or superficial peroneal nerves. Wind-up of reflex responses was evoked by electrically stimulating the left tibial or superficial peroneal nerves. We found that pinching the skin of the back abolished spontaneous locomotor-like activity. We also found that back pinch abolished the rhythmic activity observed during reflex testing without eliminating the reflex responses. Some of the rhythmic episodes of activity observed during reflex testing were consistent with clonus with an oscillation frequency greater than 3 Hz. Pinching the skin of the back effectively abolished rhythmic activity occurring spontaneously or evoked during reflex testing, irrespective of oscillation frequency. The results are consistent with the hypothesis that locomotion and clonus are produced by common central pattern-generators. Stimulating the skin of the back could prove helpful in managing undesired rhythmic activity in spinal cord-injured humans.

Differential modulation of crossed and uncrossed reflex pathways by clonidine in adult cats following complete spinal cord injury

Clonidine, an α-noradrenergic agonist, facilitates hindlimb locomotor recovery after complete spinal transection (i.e. spinalization) in adult cats. However, the mechanisms involved in clonidine-induced functional recovery are poorly understood. Sensory feedback from the legs is critical for hindlimb locomotor recovery in spinalized mammals and clonidine could alter how spinal neurons respond to peripheral inputs in adult spinalized cats. To test this hypothesis we evaluated the effect of clonidine on the responses of hindlimb muscles, primarily in the left hindlimb, evoked by stretching the left triceps surae muscles and by stimulating the right tibial and superficial peroneal nerves in eight adult decerebrate cats that were spinalized 1 month before the terminal experiment. Cats were not trained following spinalization. Clonidine had no consistent effect on responses of ipsilateral muscles evoked by triceps surae muscle stretch. However, clonidine consistently potentiated the amplitude and duration of crossed extensor responses. Moreover, following clonidine injection, stretch and tibial nerve stimulation triggered episodes of locomotor-like activity in approximately one-third of trials. Differential effects of clonidine on crossed reflexes and on ipsilateral responses to muscle stretch indicate an action at a pre-motoneuronal site. We conclude that clonidine facilitates hindlimb locomotor recovery following spinalization in untrained cats by enhancing the excitability of central pattern generating spinal neurons that also participate in crossed extensor reflex transmission.