Modulation of flexion reflex induced by hip angle changes in human spinal cord injury

Flexion Reflex Can Interrupt and Reset the Swimming Rhythm

The spinal cord can generate the hip flexor nerve activity underlying leg withdrawal (flexion reflex) and the rhythmic, alternating hip flexor and extensor activities underlying locomotion and scratching, even in the absence of brain inputs and movement-related sensory feedback. It has been hypothesized that a common set of spinal interneurons mediates flexion reflex and the flexion components of locomotion and scratching. Leg cutaneous stimuli that evoke flexion reflex can alter the timing of (i.e., reset) cat walking and turtle scratching rhythms; in addition, reflex responses to leg cutaneous stimuli can be modified during cat and human walking and turtle scratching. Both of these effects depend on the phase (flexion or extension) of the rhythm in which the stimuli occur. However, similar interactions between leg flexion reflex and swimming have not been reported. We show here that a tap to the foot interrupted and reset the rhythm of forward swimming in spinal, immobilized turtles if the tap occurred during the swim hip extensor phase. In addition, the hip flexor nerve response to an electrical foot stimulus was reduced or eliminated during the swim hip extensor phase. These two phase-dependent effects of flexion reflex on the swim rhythm and vice versa together demonstrate that the flexion reflex spinal circuit shares key components with or has strong interactions with the swimming spinal network, as has been shown previously for cat walking and turtle scratching. Therefore, leg flexion reflex circuits likely share key spinal interneurons with locomotion and scratching networks across limbed vertebrates generally.

SIGNIFICANCE STATEMENT

The spinal cord can generate leg withdrawal (flexion reflex), locomotion, and scratching in limbed vertebrates. It has been hypothesized that there is a common set of spinal cord neurons that produce hip flexion during flexion reflex, locomotion, and scratching based on evidence from studies of cat and human walking and turtle scratching. We show here that flexion reflex and swimming also share key spinal cord components based on evidence from turtles. Foot stimulation can reset the timing of the swimming rhythm and the response to each foot stimulation can itself be altered by the swim rhythm. Collectively, these studies suggest that spinal cord neuronal networks underlying flexion reflex, multiple forms of locomotion, and scratching share key components.

Locomotor training alters the behavior of flexor reflexes during walking in human spinal cord injury

In humans, a chronic spinal cord injury (SCI) impairs the excitability of pathways mediating early flexor reflexes and increases the excitability of late, long-lasting flexor reflexes. We hypothesized that in individuals with SCI, locomotor training will alter the behavior of these spinally mediated reflexes. Nine individuals who had either chronic clinically motor complete or incomplete SCI received an average of 44 locomotor training sessions. Flexor reflexes, elicited via sural nerve stimulation of the right or left leg, were recorded from the ipsilateral tibialis anterior (TA) muscle before and after body weight support (BWS)-assisted treadmill training. The modulation pattern of the ipsilateral TA responses following innocuous stimulation of the right foot was also recorded in 10 healthy subjects while they stepped at 25% BWS to investigate whether body unloading during walking affects the behavior of these responses. Healthy subjects did not receive treadmill training. We observed a phase-dependent modulation of early TA flexor reflexes in healthy subjects with reduced body weight during walking. The early TA flexor reflexes were increased at heel contact, progressively decreased during the stance phase, and then increased throughout the swing phase. In individuals with SCI, locomotor training induced the reappearance of early TA flexor reflexes and changed the amplitude of late TA flexor reflexes during walking. Both early and late TA flexor reflexes were modulated in a phase-dependent pattern after training. These new findings support the adaptive capability of the injured nervous system to return to a prelesion excitability and integration state.

Parallel facilitatory reflex pathways from the foot and hip to flexors and extensors in the injured human spinal cord

Spinal integration of sensory signals associated with hip position, muscle loading, and cutaneous sensation of the foot contributes to movement regulation. The exact interactive effects of these sensory signals under controlled dynamic conditions are unknown. The purpose of the present study was to establish the effects of combined plantar cutaneous afferent excitation and hip movement on the Hoffmann (H) and flexion reflexes in people with a spinal cord injury (SCI). The flexion and H-reflexes were elicited through stimulation of the right sural (at non-nociceptive levels) and posterior tibial nerves respectively. Reflex responses were recorded from the ipsilateral tibialis anterior (TA) (flexion reflex) and soleus (H-reflex) muscles. The plantar cutaneous afferents were stimulated at three times the perceptual threshold (200 Hz, 24-ms pulse train) at conditioning-test intervals that ranged from 3 to 90 ms. Sinusoidal movements were imposed to the right hip joint at 0.2 Hz with subjects supine. Control and conditioned reflexes were recorded as the hip moved in flexion and extension. Leg muscle activity and sagittal-plane joint torques were recorded. We found that excitation of plantar cutaneous afferents facilitated the soleus H-reflex and the long latency flexion reflex during hip extension. In contrast, the short latency flexion reflex was depressed by plantar cutaneous stimulation during hip flexion. Oscillatory joint forces were present during the transition phase of the hip movement from flexion to extension when stimuli were delivered during hip flexion. Hip-mediated input interacts with feedback from the foot sole to facilitate extensor and flexor reflex activity during the extension phase of movement. The interactive effects of these sensory signals may be a feature of impaired gait, but when they are appropriately excited, they may contribute to locomotion recovery in these patients.

A model of cerebrocerebello-spinomuscular interaction in the sagittal control of human walking

A computationally developed model of human upright balance control (Jo and Massaquoi on Biol cybern 91:188-202, 2004) has been enhanced to describe biped walking in the sagittal plane. The model incorporates (a) non-linear muscle mechanics having activation level -dependent impedance, (b) scheduled cerebrocerebellar interaction for control of center of mass position and trunk pitch angle, (c) rectangular pulse-like feedforward commands from a brainstem/ spinal pattern generator, and (d) segmental reflex modulation of muscular synergies to refine inter-joint coordination. The model can stand when muscles around the ankle are coactivated. When trigger signals activate, the model transitions from standing still to walking at 1.5 m/s. Simulated natural walking displays none of seven pathological gait features. The model can simulate different walking speeds by tuning the amplitude and frequency in spinal pattern generator. The walking is stable against forward and backward pushes of up to 70 and 75 N, respectively, and with sudden changes in trunk mass of up to 18%. The sensitivity of the model to changes in neural parameters and the predicted behavioral results of simulated neural system lesions are examined. The deficit gait simulations may be useful to support the functional and anatomical correspondences of the model. The model demonstrates that basic human-like walking can be achieved by a hierarchical structure of stabilized-long loop feedback and synergy-mediated feedforward controls. In particular, internal models of body dynamics are not required.

Hip-phase-dependent flexion reflex modulation and expression of spasms in patients with spinal cord injury

The flexion reflex in human spinal cord injury (SCI) is believed to incorporate interneuronal circuits that consist elements of the stepping generator while ample evidence suggest that hip proprioceptive input is a controlling signal of locomotor output. In this study, we examined the expression of the non-nociceptive flexion reflex in response to imposed sinusoidal passive movements of the ipsilateral hip in human SCI. The flexion reflex was elicited by low-intensity stimulation (300 Hz, 30 ms pulse train) of the right sural nerve at the lateral malleolus, and recorded from the tibialis anterior (TA) muscle. Sinusoidal hip movements were imposed to the right hip joint at 0.2 Hz by a Biodex system while subjects were supine. The effects of leg movement on five leg muscles along with hip, knee, and ankle joint torques were established simultaneously with the modulation pattern of the flexion reflex during hip oscillations. Phase-dependent modulation of the flexion reflex was present during hip movement, with the reflex to be significantly facilitated during hip extension and suppressed during hip flexion. The phase-dependent flexion reflex modulation coincided with no changes in TA pre- and post-stimulus background ongoing activity during hip extension and flexion. Reflexive muscle and joint torque responses, induced by the hip movement and substantiated by excitation of flexion reflex afferents, were entrained to specific phases of hip movement. Joint torque responses were consistent with multi-joint spasmodic muscle activity, which was present mostly during the transition phase of the hip from flexion to extension and from mid- to peak extension. Our findings provide further evidence on the interaction of hip proprioceptors with spinal interneuronal circuits engaged in locomotor pathways, and such interaction should be considered in rehabilitation protocols employed to restore sensorimotor function in people with SCI.

Plantar cutaneous input modulates differently spinal reflexes in subjects with intact and injured spinal cord

STUDY DESIGN

Spinal reflex excitability study in sensory-motor incomplete spinal cord-injured (SCI) and spinal intact subjects.

OBJECTIVES

To investigate the effects of plantar cutaneous afferent excitation on the soleus H-reflex and flexion reflex in both subject groups while seated.

SETTING

Rehabilitation Institute of Chicago and City University of New York, USA.

METHODS

The flexion reflex in SCI subjects was elicited by non-nociceptive stimulation of the sural nerve. In normal subjects, it was also elicited via innocuous medial arch foot stimulation. In both cases, reflex responses were recorded from the ipsilateral tibialis anterior muscle. Soleus H-reflexes were elicited and recorded via conventional methods. Both reflexes were conditioned by plantar cutaneous afferent stimulation at conditioning test intervals ranging from 3 to 90 ms.

RESULTS

Excitation of plantar cutaneous afferents resulted in facilitation of the soleus H-reflex and late flexion reflex in SCI subjects. In normal subjects, the soleus H-reflex was depressed while the late flexion reflex was absent. The early flexion reflex was irregularly observed in SCI patients, while in normal subjects a bimodal reflex modulation pattern was observed.

CONCLUSION

The effects of plantar cutaneous afferents change following a lesion to the spinal cord leading to exaggerated activity in both flexors and extensors. This suggests impaired modulation of the spinal inhibitory mechanisms involved in the reflex modulation. Our findings should be considered in programs aimed to restore sensorimotor function and promote recovery in these patients.

SPONSORSHIP

NIH, NICHD, Grant no. 1R03 HD 043951-01 and PSC CUNY Research Award no. 67051-0036.

VGLUT1 and GLYT2 labeling of sacrocaudal motoneurons in the spinal cord injured spastic rat

Spasticity of the midline (axial) musculature may hinder (1) performing transfers, (2) efficient extremity and head movements, and (3) efficient respiration. Currently, gaps exist in our knowledge of the pathophysiology involved in spasticity development within the axial musculature. The goals of this study were (1) to study the effects of S(2) transection on the number and distribution of glutamatergic inputs, arising from primary afferents, and glycinergic inputs to sacrocaudal motoneurons; and (2) to correlate changes in these synaptic inputs with the development of spasticity within the tail musculature, which are the caudal counterparts to the trunk axial musculature. Animals with S(2) spinal transection were tested behaviorally using our established system. At 1, 2, 4, and 12 weeks post-injury, sacrocaudal motoneurons were retrogradely labeled with cholera toxin beta-subunit (CTB), and temporal changes in vesicular glutamate transporter 1 (VGLUT1) and glycine transporter 2 (GlyT2) inputs to CTB-labeled motoneurons were visualized using antibodies specific for each synaptic type and confocal microscopy. These time points correspond to each of 4 stages of spasticity development. There was no significant change in either VGLUT1 or GlyT2 labeling of sacrocaudal motoneurons at any of the time points examined. Spinal cord injury-induced spasticity, in the tail musculature, does not appear to involve either an increase in monosynaptic glutamatergic inputs from myelinated afferents or a decrease in glycinergic inputs to sacrocaudal motoneurons.

Pre- and post-alpha motoneuronal control of the soleus H-reflex during sinusoidal hip movements in human spinal cord injury

The aim of this study was to establish the contribution of hip-mediated sensory feedback to spinal interneuronal circuits during dynamic conditions in people with incomplete spinal cord injury (SCI). Specifically, we investigated the effects of synergistic and antagonistic group I afferents on the soleus H-reflex during imposed sinusoidal hip movements. The soleus H-reflex was conditioned by stimulating the common peroneal nerve (CPN) at short (2, 3, and 4 ms) and long (80, 100, and 120 ms) conditioning test (C-T) intervals to assess the reciprocal and pre-synaptic inhibition of the soleus H-reflex, respectively. The soleus H-reflex was also conditioned by medial gastrocnemius (MG) nerve stimulation at C-T intervals ranging from 4 to 7 ms to assess changes in autogenic Ib inhibition during hip movement. Sinusoidal hip movements were imposed to the right hip joint at 0.2 Hz by the Biodex system while subjects were supine. The effects of sinusoidal hip movement on five leg muscles along with hip, knee, and ankle joint torques were also established during sensorimotor conditioning of the reflex. Phase-dependent modulation of antagonistic and synergistic muscle afferents was present during hip movement, with the reciprocal, pre-synaptic, and Ib inhibition to be significantly reduced during hip extension and reinforced during hip flexion. Reflexive muscle and joint torque responses--induced by the hip movement--were entrained to specific phases of hip movement. This study provides evidence that hip-mediated input acts as a controlling signal of pre- and post-alpha motoneuronal control of the soleus H-reflex. The expression of these spinal interneuronal circuits during imposed sinusoidal hip movements is discussed with respect to motor recovery in humans after SCI.

Location specificity of plantar cutaneous reflexes involving lower limb muscles in humans

It is known that cutaneous reflexes in human hand muscles show strong location-specificity dependent on the digit stimulated. We hypothesized that in lower leg muscles the cutaneous reflex following tactile sensation of the plantar surface of the foot is also organized in a location-specific manner. The purpose of the present study was to test this hypothesis. Middle latency reflexes (approximately 70-110 ms, MLR) following non-noxious electrical stimulation to different locations on the plantar foot were recorded from 16 neurologically intact volunteers (15 males, 1 female). Electrical stimulation was given to the fore-medial (f-M), fore-lateral (f-L) and heel (HL) regions of the plantar surface of the right foot while the subjects performed isometric dorsiflexion (tibialis anterior, TA), plantarflexion (soleus, Sol and medial gastrocnemius, MG), eversion (peroneus longus, PL) and knee extension (vastus lateralis, VL) while sitting and standing. In the Sol and MG, an excitatory response was observed following HL stimulation, which was switched to an inhibitory response following f-M or f-L stimulation (P < 0.001). A reciprocal pattern in contrast to Sol was observed in the TA. In the PL, MLR exhibited significant excitation following both f-L and HL stimulation, which, however, was switched to an inhibitory response following f-M stimulation (P < 0.001). Moderate inhibition of the MLR was seen in the VL for all stimulated positions. Systematic stimulation along the lateral side of the plantar foot demonstrated that the reflex reversal occurred around the middle of the plantar foot in the Sol and TA. In all muscles tested, the slope of the regression line between the magnitude of the MLR and background electromyographic activity significantly decreased during standing compared with sitting except for the PL following f-L simulation. These results suggest that reflex effects from cutaneous nerves in the plantar foot onto the motoneurons innervating the lower leg muscles are organized in a highly topographic manner in humans. The organization of these reflexes may play an important role in the alteration of limb loading and/or ground contact in response to tactile sensation of the plantar foot while sitting and standing.

Functional plasticity following spinal cord lesions

Spinal cord injury results in marked modification and reorganization of several reflex pathways caudal to the injury. The sudden loss or disruption of descending input engenders substantial changes at the level of primary afferents, interneurons, and motoneurons thus dramatically influencing sensorimotor interactions in the spinal cord. As a general rule reflexes are initially depressed following spinal cord injury due to severe reductions in motoneuron excitability but recover and in some instances become exaggerated. It is thought that modified inhibitory connections and/or altered transmission in some of these reflex pathways after spinal injury as well as the recovery and enhancement of membrane properties in motoneurons underlie several symptoms such as spasticity and may explain some characteristics of spinal locomotion observed in spinally transected animals. Indeed, after partial or complete spinal lesions at the last thoracic vertebra cats recover locomotion when the hindlimbs are placed on a treadmill. Although some deficits in spinal locomotion are related to lesion of specific descending motor pathways, other characteristics can also be explained by changes in the excitability of reflex pathways mentioned above. Consequently it may be the case that to reestablish a stable walking pattern that modified afferent inflow to the spinal cord incurred after injury must be normalized to enable a more normal re-expression of locomotor rhythm generating networks. Indeed, recent evidence demonstrates that step training, which has extensively been shown to facilitate and ameliorate locomotor recovery in spinal animals, directly influences transmission in simple reflex pathways after complete spinal lesions.