Plasticity of Reflexes From the Foot During Locomotion After Denervating Ankle Extensors in Intact Cats

Forelimb movements contribute to hindlimb cutaneous reflexes during locomotion in cats

During quadrupedal locomotion, interactions between spinal and supraspinal circuits and somatosensory feedback coordinate forelimb and hindlimb movements. How this is achieved is not clear. To determine whether forelimb movements modulate hindlimb cutaneous reflexes involved in responding to an external perturbation, we stimulated the superficial peroneal nerve in six intact cats during quadrupedal locomotion and during hindlimb-only locomotion (with forelimbs standing on stationary platform) and in two cats with a low spinal transection (T12-T13) during hindlimb-only locomotion. We compared cutaneous reflexes evoked in six ipsilateral and four contralateral hindlimb muscles. Results showed similar occurrence and phase-dependent modulation of short-latency inhibitory and excitatory responses during quadrupedal and hindlimb-only locomotion in intact cats. However, the depth of modulation was reduced in the ipsilateral semitendinosus during hindlimb-only locomotion. Additionally, longer-latency responses occurred less frequently in extensor muscles bilaterally during hindlimb-only locomotion, whereas short-latency inhibitory and longer-latency excitatory responses occurred more frequently in the ipsilateral and contralateral sartorius anterior, respectively. After spinal transection, short-latency inhibitory and excitatory responses were similar to both intact conditions, whereas mid- or longer-latency excitatory responses were reduced or abolished. Our results in intact cats and the comparison with spinal-transected cats suggest that the absence of forelimb movements suppresses inputs from supraspinal structures and/or cervical cord that normally contribute to longer-latency reflex responses in hindlimb extensor muscles.NEW & NOTEWORTHY During quadrupedal locomotion, the coordination of forelimb and hindlimb movements involves central circuits and somatosensory feedback. To demonstrate how forelimb movement affects hindlimb cutaneous reflexes during locomotion, we stimulated the superficial peroneal nerve in intact cats during quadrupedal and hindlimb-only locomotion as well as in spinal-transected cats during hindlimb-only locomotion. We show that forelimb movement influences the modulation of hindlimb cutaneous reflexes, particularly the occurrence of long-latency reflex responses.

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.

Sensory Perturbations from Hindlimb Cutaneous Afferents Generate Coordinated Functional Responses in All Four Limbs during Locomotion in Intact Cats

Coordinating the four limbs is an important feature of terrestrial mammalian locomotion. When the foot dorsum contacts an obstacle, cutaneous mechanoreceptors send afferent signals to the spinal cord to elicit coordinated reflex responses in the four limbs to ensure dynamic balance and forward progression. To determine how the locomotor pattern of all four limbs changes in response to a sensory perturbation evoked by activating cutaneous afferents from one hindlimb, we electrically stimulated the superficial peroneal (SP) nerve with a relatively long train at four different phases (mid-stance, stance-to-swing transition, mid-swing, and swing-to-stance transition) of the hindlimb cycle in seven adult cats. The largest functional effects of the stimulation were found at mid-swing and at the stance-to-swing transition with several changes in the ipsilateral hindlimb, such as increased activity in muscles that flex the knee and hip joints, increased joint flexion and toe height, increased stride/step lengths and increased swing duration. We also observed several changes in support periods to shift support from the stimulated hindlimb to the other three limbs. The same stimulation applied at mid-stance and the swing-to-stance transition produced more subtle changes in the pattern. We observed no changes in stride and step lengths in the ipsilateral hindlimb with stimulation in these phases. We did observe some slightly greater flexions at the knee and ankle joints with stimulation at mid-stance and a reduction in double support periods and increase in triple support. Our results show that correcting or preventing stumbling involves functional contributions from all four limbs.

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Inhibition and Facilitation of the Spinal Locomotor Central Pattern Generator and Reflex Circuits by Somatosensory Feedback From the Lumbar and Perineal Regions After Spinal Cord Injury

Somatosensory feedback from peripheral receptors dynamically interacts with networks located in the spinal cord and brain to control mammalian locomotion. Although somatosensory feedback from the limbs plays a major role in regulating locomotor output, those from other regions, such as lumbar and perineal areas also shape locomotor activity. In mammals with a complete spinal cord injury, inputs from the lumbar region powerfully inhibit hindlimb locomotion, while those from the perineal region facilitate it. Our recent work in cats with a complete spinal cord injury shows that they also have opposite effects on cutaneous reflexes from the foot. Lumbar inputs increase the gain of reflexes while those from the perineal region decrease it. The purpose of this review is to discuss how somatosensory feedback from the lumbar and perineal regions modulate the spinal locomotor central pattern generator and reflex circuits after spinal cord injury and the possible mechanisms involved. We also discuss how spinal cord injury can lead to a loss of functional specificity through the abnormal activation of functions by somatosensory feedback, such as the concurrent activation of locomotion and micturition. Lastly, we discuss the potential functions of somatosensory feedback from the lumbar and perineal regions and their potential for promoting motor recovery after spinal cord injury.

Cutaneous inputs from perineal region facilitate spinal locomotor activity and modulate cutaneous reflexes from the foot in spinal cats

It is well known that mechanically stimulating the perineal region potently facilitates hindlimb locomotion and weight support in mammals with a spinal transection (spinal mammals). However, how perineal stimulation mediates this excitatory effect is poorly understood. We evaluated the effect of mechanically stimulating (vibration or pinch) the perineal region on ipsilateral (9-14 ms onset) and contralateral (14-18 ms onset) short-latency cutaneous reflex responses evoked by electrically stimulating the superficial peroneal or distal tibial nerve in seven adult spinal cats where hindlimb movement was restrained. Cutaneous reflexes were evoked before, during, and after mechanical stimulation of the perineal region. We found that vibration or pinch of the perineal region effectively triggered rhythmic activity, ipsilateral and contralateral to nerve stimulation. When electrically stimulating nerves, adding perineal stimulation modulated rhythmic activity by decreasing cycle and burst durations and by increasing the amplitude of flexors and extensors. Perineal stimulation also disrupted the timing of the ipsilateral rhythm, which had been entrained by nerve stimulation. Mechanically stimulating the perineal region decreased ipsilateral and contralateral short-latency reflex responses evoked by cutaneous inputs, a phenomenon we observed in muscles crossing different joints and located in different limbs. The results suggest that the excitatory effect of perineal stimulation on locomotion and weight support is mediated by increasing the excitability of central pattern-generating circuitry and not by increasing excitatory inputs from cutaneous afferents of the foot. Our results are consistent with a state-dependent modulation of reflexes by spinal interneuronal circuits.

Long-lasting changes in muscle activation and step cycle variables induced by repetitive sensory stimulation to discrete areas of the foot sole during walking

We examined whether repetitive electrical stimulation to discrete foot sole regions that are phase-locked to the step cycle modulates activity patterns of ankle muscles and induces neuronal adaptation during human walking. Nonnoxious repetitive foot sole stimulation (STIM; 67 pulses at 333 Hz) was given to the medial forefoot (f-M) or heel (HL) regions at 1) the stance-to-swing transition, 2) swing-to-stance transition, or 3) midstance, during every step cycle for 10 min. Stance, but not swing, durations were prolonged with f-M STIM delivered at stance-to-swing transition, and these changes remained for up to 20-30 min after the intervention. Electromyographic (EMG) burst durations and amplitudes in the ankle extensors were also prolonged and persisted for 20 min after the intervention. Interestingly, STIM to HL was ineffective at inducing modulation, suggesting stimulation location-specific adaptation. In contrast, STIM to HL (but not f-M), at the swing-to-stance phase transition, shortened the step cycle by premature termination of swing. Furthermore, the onset of EMG bursts in the ankle extensors appeared earlier than in the control condition. STIM delivered during the midstance phase was ineffective at modulating the step cycle, highlighting phase-dependent adaptation. These effects were absent when STIM was applied while mimicking static postures for each walking phase during standing. Our findings suggest that the combination of walking-related neuronal activity with repetitive sensory inputs from the foot can generate short-term adaptation that is phase-dependent and localized to the site of STIM.NEW & NOTEWORTHY Repetitive (∼10 min) long (200 ms) trains of sensory stimulation to discrete areas of the foot sole produce persistent changes in muscle activity and cycle timing during walking. Interactions between the delivery phase and stimulus location determine the expression of the adaptations. These observations bear striking similarities to those in decerebrate cat experiments and may be usefully translated to improving locomotor function after neurotrauma.

Mechanically stimulating the lumbar region inhibits locomotor-like activity and increases the gain of cutaneous reflexes from the paws in spinal cats

Mechanically stimulating the dorsal lumbar region inhibits locomotion and reduces weight support during standing in rabbits and cats. However, how this inhibitory effect from the lumbar skin is mediated is poorly understood. Here we evaluated the effect of mechanically stimulating (vibration or pinch) the dorsal lumbar region on short-latency (8- to 13-ms onset) cutaneous reflex responses, evoked by electrically stimulating the superficial peroneal or distal tibial nerves, in seven adult cats with a low thoracic spinal transection (spinal cats). Cutaneous reflexes were evoked before, during, and after mechanical stimulation of the dorsal lumbar region. We found that mechanically stimulating the lumbar region by vibration or manual pinch abolished alternating bursts of activity between flexors and extensors initiated by nerve stimulation. The activity of extensor muscles was abolished bilaterally, whereas the activity of some ipsilateral flexor muscles was sustained during vibration/pinch. Mechanically stimulating the lumbar region increased ipsilateral and contralateral short-latency excitatory responses evoked by cutaneous inputs, a phenomenon that was generalized to muscles crossing different joints and located in different limbs. Our results indicate that the inhibitory effect on locomotion and weight support is not mediated by reducing cutaneous reflex gain and instead points to an inhibition of central pattern-generating circuitry, particularly the extensor component. The results provide greater insight into interactions between different types of somatosensory inputs within spinal motor circuits.NEW & NOTEWORTHY Vibration or pinch of the lumbar region in spinal-transected cats abolished alternating bursts of activity between flexors and extensors initiated by nerve stimulation. Mechanically stimulating the lumbar region increased ipsilateral and contralateral short-latency excitatory responses evoked by cutaneous inputs in hindlimb muscles. Sensory inputs from mechanoreceptors of the lumbar region do not mediate their inhibitory effect on locomotion and weight support by reducing the gain of short-latency excitatory cutaneous reflexes from the foot.

A Spinal Mechanism Related to Left-Right Symmetry Reduces Cutaneous Reflex Modulation Independently of Speed During Split-Belt Locomotion

Task- and phase-dependent reflex modulation during locomotion is well established, but we do not know the signals driving this modulation. To determine whether signals related to left-right symmetry of the locomotor pattern modulate cutaneous reflexes, we stimulated the superficial peroneal nerve in five intact female cats and in four spinal-transected cats (spinal cats, two males and two females) during split-belt locomotion at different left-right speeds. We compared cutaneous reflexes evoked in three ipsilateral and two contralateral hindlimb muscles during split-belt locomotion with those evoked during tied-belt (equal left-right speeds) locomotion at matched speeds of the slow and fast limbs. Our results showed similar phase-dependent modulation of cutaneous reflexes during tied-belt and split-belt locomotion in intact and spinal cats. During tied-belt locomotion in intact cats, an increase in speed significantly increased reflex modulation from minimum to maximum values, whereas in spinal cats, we observed a significant decrease. However, in all muscles of intact and spinal cats, split-belt locomotion significantly reduced reflex modulation compared with tied-belt locomotion independently of which limb was stepping on the slow or fast belt. Additionally, reflex modulation correlated more with spatial left-right symmetry, as opposed to a temporal one, in intact and spinal cats. Our results indicate that signals related to left-right symmetry reduce cutaneous reflex modulation independently of speed via a spinal mechanism. We propose that asymmetric sensory feedback from the left and right legs alters the state of the spinal network, thereby reducing cutaneous reflexes to prevent inputs from destabilizing a potentially unstable pattern.SIGNIFICANCE STATEMENT When we contact an obstacle during walking, receptors in the skin send signals to the CNS to alter the trajectory of the leg to maintain balance. This response, or reflex, is different when the leg is in the air and when it is contacting the ground. The reflex also differs when we walk at different speeds. Here, we investigated this reflex when the left and right legs were walking at different speeds on a split-belt treadmill in cats. We show that the reflex is smaller during split-belt locomotion compared with when both legs are walking at equal speeds. We propose that the spinal locomotor network controlling walking reduces the reflex response to optimize balance when gait is unstable.

Adaptation after vastus lateralis denervation in rats demonstrates neural regulation of joint stresses and strains

In order to produce movements, muscles must act through joints. The translation from muscle force to limb movement is mediated by internal joint structures that permit movement in some directions but constrain it in others. Although muscle forces acting against constrained directions will not affect limb movements, such forces can cause excess stresses and strains in joint structures, leading to pain or injury. In this study, we hypothesized that the central nervous system (CNS) chooses muscle activations to avoid excessive joint stresses and strains. We evaluated this hypothesis by examining adaptation strategies after selective paralysis of a muscle acting at the rat’s knee. We show that the CNS compromises between restoration of task performance and regulation of joint stresses and strains. These results have significant implications to our understanding of the neural control of movements, suggesting that common theories emphasizing task performance are insufficient to explain muscle activations during behaviors.

Although most of us will never achieve the grace and dexterity of professional ballerina Misty Copeland, we each make sophisticated, complex movements every day. Even simple movements often involve coordinating many muscles throughout the body. Moreover, because we have so many muscles, there are often multiple ways that we could use them to make the same movement. So which ones do we use, and why?

Many studies into muscle control focus on how the muscles activate to perform a task like kicking a soccer ball. But muscles do more than just move the limbs; they also act on joints. Contracting a muscle exerts strain on bones and the ligaments that hold joints together. If these strains become excessive, they may cause pain and injury, and over a longer time may lead to arthritis. It would therefore make sense if the nervous system factored in the need to protect joints when turning on muscles.

The quadriceps are a group of muscles that stretch along the front of the thigh bone and help to straighten the knee. To investigate whether the nervous system selects muscle activations to avoid joint injuries, Alessando et al. studied rats that had one particular quadriceps muscle paralyzed. The easiest way for the rats to adapt to this paralysis would be to increase the activation of a muscle that performs the same role as the paralyzed one, but places more stress on the knee joint. Instead, Alessando et al. found that the rats increase the activation of a muscle that minimizes the stress placed on the knee, even though this made it more difficult for the rats to recover their ability to use the leg in certain tasks.

The results presented by Alessando et al. may have important implications for physical therapy. Clinicians usually work to restore limb movements so that a task is performed in a way that is similar to how it was done before the injury. But sometimes repairing the damage can change the mechanical properties of the joint – for example, reconstructive surgery may replace a damaged ligament with a graft that has a different strength or stiffness. In those cases, performing movements in the same way as before the surgery could place abnormal stress on the joint. However, much more research is needed before recommendations can be made for how to rehabilitate rats after injury, let alone humans.

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.

The modulation of locomotor speed is maintained following partial denervation of ankle extensors in spinal cats

Speed modulation requires spatiotemporal adjustments and altered neural drive to different muscles. The loss of certain muscles produces changes in the locomotor pattern and functional compensation. However, how the loss of specific muscles affects speed modulation has not been specifically investigated. Here, we denervated the lateral gastrocnemius and soleus muscles unilaterally in seven cats that had recovered hindlimb locomotion following complete spinal transection (spinal cats). Hindlimb locomotion was tested at 10 speeds, from 0.1 to 1.0 m/s, before, 1-2 days, and 1-8 wk after denervation. Six of seven cats performed hindlimb locomotion 1-2 days postdenervation at all speeds, with the exception of two out of those six cats that did not perform stable stepping at 0.9 and 1.0 m/s. All seven cats performed hindlimb locomotion 1-8 wk postdenervation at all speeds. In some cats, at 1-2 days postdenervation, the ipsilateral hindlimb performed more steps than the contralateral hindlimb, particularly at slow speeds. This 2:1 coordination disappeared over time. In three cats, the linear increase in the amplitude of the electromyography of the ipsilateral medial gastrocnemius was reduced with increasing speed early after denervation before recovering later on. Overall, the results indicate that spinal circuits interacting with sensory feedback from the hindlimbs compensate for the partial loss of ankle extensors, retaining the ability to modulate locomotor speed. NEW & NOTEWORTHY We investigated speed modulation after denervating 2 ankle extensors unilaterally at 10 treadmill speeds in spinal-transected cats. Although we observed new forms of left-right coordination and changes in muscle activity of a remaining synergist, modulation of spatiotemporal variables with increasing speed was largely maintained after denervation. The results indicate that spinal locomotor centers interacting with sensory feedback compensate for the loss of ankle extensors, allowing speed modulation.

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.

Nonlinear Modulation of Cutaneous Reflexes with Increasing Speed of Locomotion in Spinal Cats

Cutaneous reflexes are important for responding rapidly to perturbations, correcting limb trajectory, and strengthening support. During locomotion, they are modulated by phase to generate functionally appropriate responses. The goal of the present study was to determine whether cutaneous reflexes and their phase-dependent modulation are altered with increasing speed and if this is accomplished at the spinal level. Four adult cats that recovered stable hindlimb locomotion after spinal transection were implanted with electrodes to record hindlimb muscle activity chronically and to stimulate the superficial peroneal nerve electrically to evoke cutaneous reflexes. The speed-dependent modulation of cutaneous reflexes was assessed by evoking and characterizing ipsilateral and contralateral responses in semitendinosus, vastus lateralis, and lateral gastrocnemius muscles at four treadmill speeds: 0.2, 0.4, 0.6, and 0.8 m/s. The amplitudes of ipsilateral and contralateral responses were largest at intermediate speeds of 0.4 and 0.6 m/s, followed by the slowest and fastest speeds of 0.2 and 0.8 m/s, respectively. The phase-dependent modulation of reflexes was maintained across speeds, with ipsilateral and contralateral responses peaking during the stance-to-swing transition and swing phase of the ipsilateral limb or midstance of the contralateral limb. Reflex modulation across speeds also correlated with the spatial symmetry of the locomotor pattern, but not with temporal symmetry. That the cutaneous reflex amplitude in all muscles was similarly modulated with increasing speed independently of the background level of muscle activity is consistent with a generalized premotoneuronal spinal control mechanism that could help to stabilize the locomotor pattern when changing speed.SIGNIFICANCE STATEMENT When walking, receptors located in the skin respond to mechanical pressure and send signals to the CNS to correct the trajectory of the limb and to reinforce weight support. These signals produce different responses, or reflexes, if they occur when the foot is contacting the ground or in the air. This is known as phase-dependent modulation of reflexes. However, when walking at faster speeds, we do not know if and how these reflexes are changed. In the present study, we show that reflexes from the skin are modulated with speed and that this is controlled at the level of the spinal cord. This modulation could be important in preventing sensory signals from destabilizing the walking pattern.

Reflex wind-up in early chronic spinal injury: plasticity of motor outputs

In this study we evaluate temporal summation (wind-up) of reflexes in select distal and proximal hindlimb muscles in response to repeated stimuli of the distal tibial or superficial peroneal nerves in cats 1 mo after complete spinal transection. This report is a continuation of our previous paper on reflex wind-up in the intact and acutely spinalized cat. To evaluate reflex wind-up in both studies, we recorded electromyographic signals from the following left hindlimb muscles: lateral gastrocnemius (LG), tibialis anterior (TA), semitendinosus (ST), and sartorius (Srt), in response to 10 electrical pulses to the tibial or superficial peroneal nerves. Two distinct components of the reflex responses were considered, a short-latency compound action potential (CAP) and a longer duration bout of sustained activity (SA). These two response types were shown to be differentially modified by acute spinal injury in our previous work (Frigon A, Johnson MD, Heckman CJ. J Physiol 590: 973-989, 2012). We show that these responses exhibit continued plasticity during the 1-mo recovery period following acute spinalization. During this early chronic phase, wind-up of SA responses returned to preinjury levels in one muscle, the ST, but remained depressed in all other muscles tested. In contrast, CAP response amplitudes, which were initially potentiated following acute transection, returned to preinjury levels in all muscles except for Srt, which continued to show marked increase. These findings illustrate that spinal elements exhibit considerable plasticity during the recovery process following spinal injury and highlight the importance of considering SA and CAP responses as distinct phenomena with unique underlying neural mechanisms.NEW & NOTEWORTHY This research is the first to assess temporal summation, also called wind-up, of muscle reflexes during the 1-mo recovery period following spinal injury. Our results show that two types of muscle reflex activity are differentially modulated 1 mo after spinal cord injury (SCI) and that spinal reflexes are altered in a muscle-specific manner during this critical period. This postinjury plasticity likely plays an important role in spasticity experienced by individuals with SCI.

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 "beneficial" effects of locomotor training after various types of spinal lesions in cats and rats

This chapter reviews a number of experiments on the recovery of locomotion after various types of spinal lesions and locomotor training mainly in cats. We first recall the major evidence on the recovery of hindlimb locomotion in completely spinalized cats at the T13 level and the role played by the spinal locomotor network, also known as the central pattern generator, as well as the beneficial effects of locomotor training on this recovery. Having established that hindlimb locomotion can recover, we raise the issue as to whether spinal plastic changes could also contribute to the recovery after partial spinal lesions such as unilateral hemisections. We found that after such hemisection at T10, cats could recover quadrupedal locomotion and that deficits could be improved by training. We further showed that, after a complete spinalization a few segments below the first hemisection (at T13, i.e., the level of previous studies on spinalization), cats could readily walk with the hindlimbs within hours of completely severing the remaining spinal tracts and not days as is usually the case with only a single complete spinalization. This suggests that neuroplastic changes occurred below the first hemisection so that the cat was already primed to walk after the spinalization subsequent to the hemispinalization 3 weeks before. Of interest is the fact that some characteristic kinematic features in trained or untrained hemispinalized cats could remain after complete spinalization, suggesting that spinal changes induced by training could also be durable. Other studies on reflexes and on the pattern of "fictive" locomotion recorded after curarization corroborate this view. More recent work deals with training cats in more demanding situations such as ladder treadmill (vs. flat treadmill) to evaluate how the locomotor training regimen can influence the spinal cord. Finally, we report our recent studies in rats using compressive lesions or surgical complete spinalization and find that some principles of locomotor recovery in cats also apply to rats when adequate locomotor training is provided.

Synchronous and asynchronous electrically evoked motor activities during wind-up stimulation are differentially modulated following an acute spinal transection

In this study, we used a novel technique to study reflex wind-up when the spinal cord is intact and following an acute spinal transection. Specifically, we evaluated reflex responses evoked by a series of 10 electrical pulses to the tibial or superficial peroneal nerves in 9 decerebrate adult cats, before and after an acute spinal transection. Electromyograms were recorded in four hindlimb muscles (lateral gastrocnemius, tibialis anterior, semitendinosus, and sartorius) to evaluate reflex amplitude, duration, and the temporal summation of reflex responses, so-called wind-up. We identified two distinct reflex responses evoked by electrical stimulation of the tibial or superficial peroneal nerves on the basis of their pattern of change following acute spinal transection, a short-latency (∼10 ms) compound action potential (CAP) that was followed by a burst of sustained activity (SA). Wind-up of CAP and SA amplitudes was clearly present when the spinal cord was intact but was drastically reduced after acute spinalization in some muscles. Moreover, CAP and SA reflex responses were differentially modified by the acute spinalization. When the effects of acute spinal transection were significant, CAP responses were increased after acute spinalization, whereas SA responses were reduced, suggesting that the two signals are regulated by different neuronal mechanisms. The present results provide the first assessment of reflex wind-up before and after an acute spinal transection in the same animals and indicate that different reflex components must be considered separately when evaluating changes in neuronal excitability following SCI.

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.

Altered activation patterns by triceps surae stretch reflex pathways in acute and chronic spinal cord injury

Spinal reflexes are modified by spinal cord injury (SCI) due the loss of excitatory inputs from supraspinal structures and changes within the spinal cord. The stretch reflex is one of the simplest pathways of the central nervous system and was used presently to evaluate how inputs from primary and secondary muscle spindles interact with spinal circuits before and after spinal transection (i.e., spinalization) in 12 adult decerebrate cats. Seven cats were spinalized and allowed to recover for 1 mo (i.e., chronic spinal state), whereas 5 cats were evaluated before (i.e., intact state) and after acute spinalization (i.e., acute spinal state). Stretch reflexes were evoked by stretching the left triceps surae (TS) muscles. The force evoked by TS muscles was recorded along with the activity of several hindlimb muscles. Stretch reflexes were abolished in the acute spinal state due to an inability to activate TS muscles, such as soleus (Sol) and lateral gastrocnemius (LG). In chronic spinal cats, reflex force had partly recovered but Sol and LG activity remained considerably depressed, despite the fact that injecting clonidine could recruit these muscles during locomotor-like activity. In contrast, other muscles not recruited in the intact state, most notably semitendinosus and sartorius, were strongly activated by stretching TS muscles in chronic spinal cats. Therefore, stretch reflex pathways from TS muscles to multiple hindlimb muscles undergo functional reorganization following spinalization, both acute and chronic. Altered activation patterns by stretch reflex pathways could explain some sensorimotor deficits observed during locomotion and postural corrections after SCI.

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

Following injury to the nervous system, there is a range of possible functional outcomes that can only be partly explained by the extent of injury. Moreover, treatments effective in certain individuals might not work in others. Why such variability from one individual to another, in terms of functional outcomes and responsiveness to a given treatment following a similar injury? The answer to that question is not simple, and to begin to answer we must first consider that individuals of the same species can be quite variable in terms of neuronal circuit parameters involved in performing a given task. Interindividual variability can be subtle but the term "variability" in this chapter will be used to denote marked differences between individuals at the systems level (e.g., spinal reflexes, bursts of muscle activity, kinematics) during the same motor behavior, with an emphasis on locomotion. Injury to any level of the nervous system, in turn, can further compound this variability by altering spared neuronal connections. The aim of the present chapter is to (1) review studies that have investigated interindividual variability, (2) review studies that have described variable adaptive mechanisms following spinal and/or peripheral nerve lesions during locomotion, and (3) discuss the implications of intersubject variability for locomotor adaptation.

Feedback and feedforward locomotor adaptations to ankle-foot load in people with incomplete spinal cord injury

Humans with spinal cord injury (SCI) modulate locomotor output in response to limb load. Understanding the neural control mechanisms responsible for locomotor adaptation could provide a framework for selecting effective interventions. We quantified feedback and feedforward locomotor adaptations to limb load modulations in people with incomplete SCI. While subjects airstepped (stepping performed with kinematic assistance and 100% bodyweight support), a powered-orthosis created a dorisflexor torque during the "stance phase" of select steps producing highly controlled ankle-load perturbations. When given repetitive, stance phase ankle-load, the increase in hip extension work, 0.27 J/kg above baseline (no ankle-load airstepping), was greater than the response to ankle-load applied during a single step, 0.14 J/kg (P = 0.029). This finding suggests that, at the hip, subjects produced both feedforward and feedback locomotor modulations. We estimate that, at the hip, the locomotor response to repetitive ankle-load was modulated almost equally by ongoing feedback and feedforward adaptations. The majority of subjects also showed after-effects in hip kinetic patterns that lasted 3 min in response to repetitive loading, providing additional evidence of feedforward locomotor adaptations. The magnitude of the after-effect was proportional to the response to repetitive ankle-foot load (R(2) = 0.92). In contrast, increases in soleus EMG amplitude were not different during repetitive and single-step ankle-load exposure, suggesting that ankle locomotor modulations were predominately feedback-based. Although subjects made both feedback and feedforward locomotor adaptations to changes in ankle-load, between-subject variations suggest that walking function may be related to the ability to make feedforward adaptations.

Locomotor changes in length and EMG activity of feline medial gastrocnemius muscle following paralysis of two synergists

The mechanism of the compensatory increase in electromyographic activity (EMG) of a cat ankle extensor during walking shortly after paralysis of its synergists is not fully understood. It is possible that due to greater ankle flexion in stance in this situation, muscle spindles are stretched to a greater extent and, thus, contribute to the EMG enhancement. However, also changes in force feedback and central drive may play a role. The aim of the present study was to investigate the short-term (1- to 2-week post-op) effects of lateral gastrocnemius (LG) and soleus (SO) denervation on muscle fascicle and muscle–tendon unit (MTU) length changes, as well as EMG activity of the intact medial gastrocnemius (MG) muscle in stance during overground walking on level (0%), downslope (−50%, presumably enhancing stretch of ankle extensors in stance) and upslope (+50%, enhancing load on ankle extensors) surfaces. Fascicle length was measured directly using sonomicrometry, and MTU length was calculated from joint kinematics. For each slope condition, LG-SO denervation resulted in an increase in MTU stretch and peak stretch velocity of the intact MG in early stance. MG muscle fascicle stretch and peak stretch velocity were also higher than before denervation in downslope walking. Denervation significantly decreased the magnitude of MG fascicle shortening and peak shortening velocity during early stance in level and upslope walking. MG EMG magnitude in the swing and stance phases was substantially greater after denervation, with a relatively greater increase during stance of level and upslope walking. These results suggest that the fascicle length patterns of MG muscle are significantly altered when two of its synergists are in a state of paralysis. Further, the compensatory increase in MG EMG is likely mediated by enhanced MG length feedback during downslope walking, enhanced feedback from load-sensitive receptors during upslope walking and enhanced central drive in all walking conditions.

Asymmetric Changes in Cutaneous Reflexes After a Partial Spinal Lesion and Retention Following Spinalization During Locomotion in the Cat

Locomotion involves dynamic interactions between the spinal cord, supraspinal signals, and peripheral sensory inputs. After incomplete spinal cord injury (SCI), interactions are disrupted, and remnant structures must optimize function to maximize locomotion. We investigated if cutaneous reflexes are altered following a unilateral partial spinal lesion and whether changes are retained within spinal circuits after complete spinal transection (i.e., spinalization). Four cats were chronically implanted with recording and stimulating electrodes. Cutaneous reflexes were evoked with cuff electrodes placed around left and right superficial peroneal nerves. Control data, consisting of hindlimb kinematics and electromyography (bursts of muscular activity and cutaneous reflexes), were recorded during treadmill locomotion. After stable control data were achieved (53–67 days), a partial spinal lesion was made at the 10th or 11th thoracic segment (T10–T11) on the left side. Cats were trained to walk after the partial lesion, and following a recovery period (64–80 days), a spinalization was made at T13. After the partial lesion, changes in short-latency excitatory (P1) homologous responses between hindlimbs, evoked during swing, were largely asymmetric in direction relative to control values, whereas changes in longer-latency excitatory (P2) and crossed responses were largely symmetric in direction. After spinalization, cats could display hindlimb locomotion within 1 day. Early after spinalization, reflex changes persisted a few days, but over time homologous P1 responses increased symmetrically toward or above control levels. Therefore changes in cutaneous reflexes after the partial lesion and retention following spinalization indicate an important spinal plasticity after incomplete SCI.

Contribution of hindpaw cutaneous inputs to the control of lateral stability during walking in the cat

To delineate the role of cutaneous feedback from the paws in the regulation of balance during walking, we compared the corrective responses of cats to lateral support surface translation before and after cutaneous denervation of the hindpaws. In addition, we compared characteristics of undisturbed walking before and after denervation. Electromyographic and kinematic data were collected from three cats trained to walk across a walkway, the central portion of which could be unexpectedly translated laterally in either direction. Following denervation, all of the cats changed their step width, lowered their pelvis, and spent more time with the hindlegs in double-support when walking across the walkway. When displaced by lateral support surface translations, the denervated cats made larger lateral steps and required more than a single step to regain balance. However, none of the cats fell following the denervation. The appearance and latency of the responses evoked in the hindleg muscles by the perturbations were unaffected by the denervation. However, the amplitude of these responses was affected by the loss of cutaneous inputs. Responses evoked at paw contact were significantly reduced in most muscles in the absence of cutaneous input, whereas responses evoked at end of stance revealed significant increases in gluteus medius activity with little influence on the activity of other muscles. Therefore the loss of cutaneous inputs leads to instability during gait. Although cutaneous feedback from the hindpaws is not essential for triggering corrective responses to support surface disturbances, it appears that cutaneous inputs are important for scaling the responses initiated by other cues.

Partial denervation of ankle extensors prior to spinalization in cats impacts the expression of locomotion and the phasic modulation of reflexes

Following peripheral nerve sections some locomotor deficits appear which are gradually compensated for by spinal and supraspinal mechanisms. The present work is aimed at identifying contributions of both types of mechanisms. We performed a denervation of the left lateral gastrocnemius-soleus (LGS) muscles in three cats which was followed by a spinalization at the 13th thoracic segment. Three other cats were not denervated prior to spinalization (i.e. intact) and served as controls. Over the years, in our laboratory, there have been no instances in which cats did not express spinal locomotion with treadmill training and/or clonidine administration. After spinalization, cats were trained on a treadmill to express spinal locomotion. Reflexes, evoked by stimulating the left tibial nerve at the ankle, the electromyography of several hindlimb muscles, and kinematics were recorded during locomotion before and after denervation, during recovery, and following complete spinalization. Denervating the left LGS before spinalization induced considerable variability in the expression of spinal locomotion from one cat to another, which was not observed in the three controls. Variability ranged from a greater ankle yield in the denervated limb in one cat to inability to recover locomotion after spinalization in another. In the two denervated cats that recovered locomotion after spinalization, some reflex changes differed from "normal" spinal cats (i.e. intact at the time of spinalization), suggesting that reorganization of spinal circuits after spinalization is dissimilar to what normally takes place if denervation is performed before spinalization. First, we conclude that the transient locomotor deficits initially incurred following the LGS denervation in cats with an intact spinal cord reappear after complete spinalization indicating that supraspinal mechanisms were involved in maintaining the adapted locomotion. Second, the reappearance of locomotor deficits and/or impairment in expressing spinal locomotion suggests that spinal mechanisms were profoundly altered to compensate for the initial denervation partly because the reflex modulation is abnormal. If the same denervation is performed after spinalization only transient deficits are observed and spinal locomotion is not compromised.

Locomotor and Reflex Adaptation After Partial Denervation of Ankle Extensors in Chronic Spinal Cats

This work investigates the capacity of the spinal cord to generate locomotion after a complete spinal section and its ability to adapt its locomotor pattern after a peripheral nerve lesion. To study this intrinsic adaptive capacity, the left lateral gastrocnemius-soleus (LGS) nerve was sectioned in three cats that expressed a stable locomotion following a complete spinal transection. The electromyograph (EMG) of multiple hindlimb muscles and reflexes, evoked by stimulating the left tibial (Tib) nerve at the ankle, were recorded before and after denervation during treadmill locomotion. Following denervation, the mean amplitude of EMG bursts of multiple hindlimb muscles increased during locomotion, similar to what is found after an identical denervation in otherwise intact cats. Reflex changes were noted in ipsilateral flexors, such as semitendinosus and tibialis anterior, but not in the ipsilateral knee extensor vastus lateralis following denervation. The present results demonstrate that the spinal cord possesses the circuitry necessary to mediate increased EMG activity in multiple hindlimb muscles and also to produce changes in reflex pathways after a muscle denervation. The similarity of changes following LGS denervation in cats with an intact and transected spinal cord suggests that spinal mechanisms play a major role in the locomotor adaptation.

Adaptive changes of the locomotor pattern and cutaneous reflexes during locomotion studied in the same cats before and after spinalization

Descending supraspinal inputs exert powerful influences on spinal reflex pathways in the legs. Removing these inputs by completely transecting the spinal cord changes the state (i.e. the configuration of the spinal circuitry) of the locomotor network and undoubtedly generates a reorganization of reflex pathways. To study changes in reflex pathways after a complete spinalization, we recorded spinal reflexes during locomotion before and after a complete transection of the spinal cord at the 13th thoracic segment in cats. We chronically implanted electrodes in three cats, to record electromyography (EMG) in several hindlimb muscles and around the left tibial (Tib) nerve at the ankle to elicit reflexes during locomotion before and after spinalization in the same cat. Control values of kinematics, EMGs and reflexes were obtained during intact locomotion for 33-60 days before spinalization. After spinalization, cats were trained 3-5 times a week on a motorized treadmill. Recordings resumed once a stable spinal locomotion was achieved (26-43 days), with consistent plantar foot placement and full hindquarter weight support without perineal stimulation. Changes in Tib nerve reflex responses after spinalization in the same cat during locomotion were found in all muscles studied and were often confined to specific phases of the step cycle. The most remarkable change was the appearance of short-latency excitatory responses in some ipsilateral ankle extensors during stance. Short-latency excitatory responses in the ipsilateral tibialis anterior were increased during stance, whereas in other flexors such as semitendinosus and sartorius, increases were mostly confined to swing. Longer-latency excitatory responses in ipsilateral flexors were absent or reduced. Responses evoked in limb muscles contralateral to stimulation were generally increased throughout the step cycle. These reflex changes after spinalization provide important clues regarding the functional reorganization of reflex pathways during spinal locomotion.

Short-Latency Crossed Inhibitory Responses in Extensor Muscles During Locomotion in the Cat

During locomotion, contacting an obstacle generates a coordinated response involving flexion of the stimulated leg and activation of extensors contralaterally to ensure adequate support and forward progression. Activation of motoneurons innervating contralateral muscles (i.e., crossed extensor reflex) has always been described as an excitation, but the present paper shows that excitatory responses during locomotion are almost always preceded by a short period of inhibition. Data from seven cats chronically implanted with bipolar electrodes to record electromyography (EMG) of several hindlimb muscles bilaterally were used. A stimulating cuff electrode placed around the left tibial and left superficial peroneal nerves at the level of the ankle in five and two cats, respectively, evoked cutaneous reflexes during locomotion. During locomotion, short-latency (∼13 ms) inhibitory responses were frequently observed in extensors of the right leg (i.e., contralateral to the stimulation), such as gluteus medius and triceps surae muscles, which were followed by excitatory responses (∼25 ms). Burst durations of the left sartorius (Srt), a hip flexor, and ankle extensors of the right leg increased concomitantly in the mid- to late-flexion phases of locomotion with nerve stimulation. Moreover, the onset and offset of Srt and ankle extensor bursts bilaterally were altered in specific phases of the step cycle. Short-latency crossed inhibition in ankle extensors appears to be an integral component of cutaneous reflex pathways in intact cats during locomotion, which could be important in synchronizing EMG bursts in muscles of both legs.

Plasticity of locomotor sensorimotor interactions after peripheral and/or spinal lesions

The present paper reviews aspects of locomotor sensorimotor interactions by focussing on work performed in spinal cats. We provide a brief overview of spinal locomotion and describe the effects of various types of sensory deprivations (e.g. rhizotomies, and lesions of muscle and cutaneous nerves) to highlight the spinal neuroplasticity necessary for adapting to sensory loss. Recent work on plastic interactions between reflex pathways that could be responsible for such plasticity, in particular changes in proprioceptive and cutaneous pathways that occur during locomotor training of spinal cats, is discussed. Finally, we describe how stimulation of some sensory inputs via various limb manipulations or intraspinal electrical stimulation can affect the expression of spinal locomotion. We conclude that sensory inputs are critical not only for locomotion but also that changes in the efficacy of sensory transmission and in the interactions between sensory pathways could participate in the normalization of locomotion after spinal and/or peripheral lesions.