Effects of H-reflex up-conditioning on GABAergic terminals on rat soleus motoneurons

Operant down-conditioning of the soleus H-reflex in people after stroke

Through operant conditioning, spinal reflex behaviors can be changed. Previous studies in rats indicate that the sensorimotor cortex and corticospinal tract are essential in inducing and maintaining reflex changes induced through conditioning. In people with incomplete spinal cord injury (SCI), an operant down-conditioning protocol decreased the soleus H-reflex size and improved walking speed and symmetry, suggesting that a partially preserved spinal cord can support conditioning-induced plasticity and benefit from it. This study examined whether down-conditioning can decrease the soleus H-reflex in people with supraspinal injury (i.e., cortical or subcortical stroke). Operant down-conditioning was applied to the soleus H-reflex in a cohort of 12 stroke people with chronic spastic hemiparesis (>12 months from stroke onset of symptoms). Each participant completed 6 baseline and 30 conditioning sessions over 12 weeks. In each baseline session, 225 control H-reflexes were elicited without any feedback on H-reflex size. In each conditioning session, 225 conditioned H-reflexes were elicited while the participant was asked to decrease H-reflex size and was given visual feedback as to whether the resulting H-reflex was smaller than a criterion value. In six of 12 participants, the conditioned H-reflex became significantly smaller by 30% on average, whereas in other 6 participants, it did not. The difference between the subgroups was largely attributable to the difference in across-session control reflex change. Ten-meter walking speed was increased by various extent (+0.04 to +0.35, +0.14 m/s on average) among the six participants whose H-reflex decreased, whereas the change was 0.00 m/s on average for the rest of participants. Although less than what was seen in participants with SCI, the fact that conditioning succeeded in 50% of stroke participants supports the feasibility of reflex down-conditioning in people after stroke. At the same time, the difference in across-session control reflex change and conditioning success rate may reflect a critical role of supraspinal activity in producing long-term plasticity in the spinal cord, as previous animal studies suggested.

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.

Effects of Repeated 20-Hz Electrical Stimulation on Functional Recovery Following Peripheral Nerve Injury

One hour of 20-Hz continuous electrical stimulation (ES) applied at the time of injury promotes the regeneration of axons in cut peripheral nerves. A more robust enhancement of peripheral axon regeneration is achieved by 2 weeks of daily treadmill exercise. We investigated whether repeated applications of brief ES (mES) would be more effective in promoting regeneration than a single application. Sciatic nerves of C57B6 mice were cut and repaired by end-to-end anastomosis. At that time and every third day for 2 weeks, the repaired nerve was stimulated for 1 hour at 20 Hz. In controls, injured mice were either untreated or treated with ES only once. Direct muscle responses recorded from reinnervated muscles in awake animals were observed earlier both in mice treated with ES and mES than untreated controls. Their amplitudes increased progressively over the post transection study period, but the rate of this progression was increased significantly only in animals treated once with ES. Monosynaptic H reflexes recovered to pretransection levels in both untreated and singly treated mice but in the animals treated repeatedly, they were maintained at more than twice that of the same reflexes recorded prior to injury. In anatomical analyses, both excitatory and inhibitory synaptic contacts with the cell bodies of injured motoneurons, including those expressing the vesicular glutamate transporter 1 (VGLUT1), were sustained in mice treated repeatedly but not in singly treated or untreated mice. Repeated ES does not enhance the rate of restoration of functional muscle reinnervation and results in the retention of exaggerated reflexes.

Operant conditioning of the motor-evoked potential and locomotion in people with and without chronic incomplete spinal cord injury

Foot drop is very common among people with chronic incomplete spinal cord injury (SCI) and likely stems from SCI that disturbs the corticospinal activation of the ankle dorsiflexor tibialis anterior (TA). Thus, if one can recover or increase the corticospinal excitability reduced by SCI, motor function recovery may be facilitated. Here, we hypothesized that in people suffering from weak dorsiflexion due to chronic incomplete SCI, increasing the TA motor-evoked potential (MEP) through operant up-conditioning can improve dorsiflexion during locomotion, while in people without any injuries, it would have little impact on already normal locomotion. Before and after 24 MEP conditioning or control sessions, locomotor electromyography (EMG) and kinematics were measured. This study reports the results of these locomotor assessments. In participants without SCI, locomotor EMG activity, soleus Hoffmann reflex modulation, and joint kinematics did not change, indicating that MEP up-conditioning or repeated single-pulse transcranial magnetic stimulation (i.e., control protocol) does not influence normal locomotion. In participants with SCI, MEP up-conditioning increased TA activity during the swing-to-swing stance transition phases and ankle joint motion during locomotion in the conditioned leg and increased walking speed consistently. In addition, the swing-phase TA activity and ankle joint motion also improved in the contralateral leg. The results are consistent with our hypothesis. Together with the previous operant conditioning studies in humans and rats, the present study suggests that operant conditioning can be a useful therapeutic tool for enhancing motor function recovery in people with SCI and other central nervous system disorders. NEW & NOTEWORTHY This study examined the functional impact of operant conditioning of motor-evoked potential (MEP) to transcranial magnetic stimulation that aimed to increase corticospinal excitability for the ankle dorsiflexor tibialis anterior (TA). In people with chronic incomplete spinal cord injury (SCI), MEP up-conditioning increased TA activity and improved dorsiflexion during locomotion, while in people without injuries, it had little impact on already normal locomotion. MEP conditioning may potentially be used to enhance motor function recovery after SCI.

Mechanisms and functional implications of motoneuron adaptations to increased physical activity

Motoneurons demonstrate adaptations in their physiological properties to alterations in chronic activity levels. The most consistent change that appears to result from endurance-type exercise training is the reduced excitatory current required to initiate and maintain rhythmic firing. While the precise mechanisms through which these neurons adapt to activity are currently unknown, evidence exists that adaptation may involve alterations in the expression of genes that code for membrane receptors, which can influence the responses of neurons to transmitters during activation. The influence of these adaptations may also extend to the resting condition, where ambient levels of neuroactive substances may influence ion conductances at rest, and thus result in the activation or inhibition of specific ion conductances that underlie the measurements of increased excitability that have been reported for motoneurons in the anesthetised state. We have applied motoneuron excitability and muscle unit contractile changes with endurance training to a mathematical computerized model of motor unit recruitment (Heckman and Binder 1991; J. Neurophysiol. 65(4):952–967). The results from the modelling exercise demonstrate increased task efficiency at relative levels of effort during a submaximal contraction. The physiological impact that nerve and muscle adaptations have on the neuromuscular system during standardized tasks seem to fit with reported changes in motor unit behaviour in trained human subjects.

Operant conditioning of the tibialis anterior motor evoked potential in people with and without chronic incomplete spinal cord injury

The activity of corticospinal pathways is important in movement control, and its plasticity is essential for motor skill learning and re-learning after central nervous system (CNS) injuries. Therefore, enhancing the corticospinal function may improve motor function recovery after CNS injuries. Operant conditioning of stimulus-induced muscle responses (e.g., reflexes) is known to induce the targeted plasticity in a targeted pathway. Thus, an operant conditioning protocol to target the corticospinal pathways may be able to enhance the corticospinal function. To test this possibility, we investigated whether operant conditioning of the tibialis anterior (TA) motor evoked potential (MEP) to transcranial magnetic stimulation can enhance corticospinal excitability in people with and without chronic incomplete spinal cord injury (SCI). The protocol consisted of 6 baseline and 24 up-conditioning/control sessions over 10 wk. In all sessions, TA MEPs were elicited at 10% above active MEP threshold while the sitting participant provided a fixed preset level of TA background electromyographic activity. During baseline sessions, MEPs were simply measured. During conditioning trials of the conditioning sessions, the participant was encouraged to increase MEP and was given immediate feedback indicating whether MEP size was above a criterion. In 5/8 participants without SCI and 9/10 with SCI, over 24 up-conditioning sessions, MEP size increased significantly to ~150% of the baseline value, whereas the silent period (SP) duration decreased by ~20%. In a control group of participants without SCI, neither MEP nor SP changed. These results indicate that MEP up-conditioning can facilitate corticospinal excitation, which is essential for enhancing motor function recovery after SCI. NEW & NOTEWORTHY We investigated whether operant conditioning of the motor evoked potential (MEP) to transcranial magnetic stimulation can systematically increase corticospinal excitability for the ankle dorsiflexor tibialis anterior (TA) in people with and without chronic incomplete spinal cord injury. We found that up-conditioning can increase the TA MEP while reducing the accompanying silent period (SP) duration. These findings suggest that MEP up-conditioning produces the facilitation of corticospinal excitation as targeted, whereas it suppresses inhibitory mechanisms reflected in SP.

The inferior olive is essential for long-term maintenance of a simple motor skill

The inferior olive (IO) is essential for operant down-conditioning of the rat soleus H-reflex, a simple motor skill. To evaluate the role of the IO in long-term maintenance of this skill, the H-reflex was down-conditioned over 50 days, the IO was chemically ablated, and down-conditioning continued for up to 102 more days. H-reflex size just before IO ablation averaged 62(±2 SE)% of its initial value (P < 0.001 vs. initial). After IO ablation, H-reflex size rose to 75-80% over ∼10 days, remained there for ∼30 days, rose over 10 days to above its initial value, and averaged 140(±14)% for the final 10 days of study (P < 0.01 vs. initial). This two-stage loss of down-conditioning maintenance correlated with IO neuronal loss (r = 0.75, P < 0.01) and was similar to the loss of down-conditioning that follows ablation of the cerebellar output nuclei dentate and interpositus. In control (i.e., unconditioned) rats, IO ablation has no long-term effect on H-reflex size. These results indicate that the IO is essential for long-term maintenance of a down-conditioned H-reflex. With previous data, they support the hypothesis that IO and cortical inputs to cerebellum combine to produce cerebellar plasticity that produces sensorimotor cortex plasticity that produces spinal cord plasticity that produces the smaller H-reflex. H-reflex down-conditioning appears to depend on a hierarchy of plasticity that may be guided by the IO and begin in the cerebellum. Similar hierarchies may underlie other motor learning.

7-Nitroindazole enhances c-Fos expression in spinal neurons in rats realizing operant movements

The expression of c-Fos and NADPH-diaphorase reactivity (NADPH-dr) in the cervical spinal cord was studied in adult male Wistar rats that realized operant reflexes after inhibition of neuronal nitric oxide synthase. Fos-immunoreactive neurons were visualized immunohistochemically in the C6/C7 spinal segments in the control, realized operant movements animals, and/or 7-nitroindazole (7-NI) injected rats. The mean numbers of immunoreactive interneurons and motoneurons (per section) were significantly greater in the Nucleus proprius (+240%) and motor nuclei (+600%) in rats of the 7-NI-pretreated and operant reflex realized group than in the isolated operant reflex realized group. Our study showed intensive staining of NADPH-dr axon terminals on the somata and initial parts of dendrites of motoneurons in experimental rats when the disodium salt of malic acid was added to the staining solution. Suppression of NO release is associated with potentiation of neuronal activation induced by descending supraspinal and proprioceptive signaling within the spinal cord.

Operant conditioning of spinal reflexes: from basic science to clinical therapy

New appreciation of the adaptive capabilities of the nervous system, recent recognition that most spinal cord injuries are incomplete, and progress in enabling regeneration are generating growing interest in novel rehabilitation therapies. Here we review the 35-year evolution of one promising new approach, operant conditioning of spinal reflexes. This work began in the late 1970’s as basic science; its purpose was to develop and exploit a uniquely accessible model for studying the acquisition and maintenance of a simple behavior in the mammalian central nervous system (CNS). The model was developed first in monkeys and then in rats, mice, and humans. Studies with it showed that the ostensibly simple behavior (i.e., a larger or smaller reflex) rests on a complex hierarchy of brain and spinal cord plasticity; and current investigations are delineating this plasticity and its interactions with the plasticity that supports other behaviors. In the last decade, the possible therapeutic uses of reflex conditioning have come under study, first in rats and then in humans. The initial results are very exciting, and they are spurring further studies. At the same time, the original basic science purpose and the new clinical purpose are enabling and illuminating each other in unexpected ways. The long course and current state of this work illustrate the practical importance of basic research and the valuable synergy that can develop between basic science questions and clinical needs.

Cortical stimulation causes long-term changes in H-reflexes and spinal motoneuron GABA receptors

The cortex gradually modifies the spinal cord during development, throughout later life, and in response to trauma or disease. The mechanisms of this essential function are not well understood. In this study, weak electrical stimulation of rat sensorimotor cortex increased the soleus H-reflex, increased the numbers and sizes of GABAergic spinal interneurons and GABAergic terminals on soleus motoneurons, and decreased GABA(A) and GABA(B) receptor labeling in these motoneurons. Several months after the stimulation ended the interneuron and terminal increases had disappeared, but the H-reflex increase and the receptor decreases remained. The changes in GABAergic terminals and GABA(B) receptors accurately predicted the changes in H-reflex size. The results reveal a new long-term dimension to cortical-spinal interactions and raise new therapeutic possibilities.

H-reflex up-conditioning encourages recovery of EMG activity and H-reflexes after sciatic nerve transection and repair in rats

Operant conditioning of the spinal stretch reflex or its electrical analog, the H-reflex, produces spinal cord plasticity and can thereby affect motoneuron responses to primary afferent input. To explore whether this conditioning can affect the functional outcome after peripheral nerve injury, we assessed the effect of up-conditioning soleus (SOL) H-reflex on SOL and tibialis anterior (TA) function after sciatic nerve transection and repair. Sprague Dawley rats were implanted with EMG electrodes in SOL and TA and stimulating cuffs on the posterior tibial nerve. After control data collection, the sciatic nerve was transected and repaired and the rat was exposed for 120 d to continued control data collection (TC rats) or SOL H-reflex up-conditioning (TU rats). At the end of data collection, motoneurons that had reinnervated SOL and TA were labeled retrogradely. Putative primary afferent terminals [i.e., terminals containing vesicular glutamate transporter-1 (VGLUT1)] on SOL motoneurons were studied immunohistochemically. SOL (and probably TA) background EMG activity recovered faster in TU rats than in TC rats, and the final recovered SOL H-reflex was significantly larger in TU than in TC rats. TU and TC rats had significantly fewer labeled motoneurons and higher proportions of double-labeled motoneurons than untransected rats. VGLUT1 terminals were significantly more numerous on SOL motoneurons of TU than TC rats. Combined with the larger H-reflexes in TU rats, this anatomical finding supports the hypothesis that SOL H-reflex up-conditioning strengthened primary afferent reinnervation of SOL motoneurons. These results suggest that H-reflex up-conditioning may improve functional recovery after nerve injury and repair.

Activity-dependent plasticity of spinal locomotion: implications for sensory processing

The lumbosacral spinal cord of mammals contains the neural circuitry capable of generating full weight-bearing locomotion of the hind limbs without any supraspinal input. One or more interventions, for example, pharmacological, epidural stimulation, and/or locomotor training, however, are necessary to gain access to and modulate the properties of this circuitry and to facilitate recovery of full weight-bearing locomotion after spinal cord injury.

Pain and learning in a spinal system: contradictory outcomes from common origins

The long-standing belief that the spinal cord serves merely as a conduit for information traveling to and from the brain is changing. Over the past decade, research has shown that the spinal cord is sensitive to response-outcome contingencies, demonstrating that spinal circuits have the capacity to modify behavior in response to differential environmental cues. If spinally transected rats are administered shock contingent on leg extension (controllable shock), they will maintain a flexion response that minimizes shock exposure. If, however, this contingency is broken, and shock is administered irrespective of limb position (uncontrollable shock), subjects cannot acquire the same flexion response. Interestingly, each of these treatments has a lasting effect on behavior; controllable shock enables future learning, while uncontrollable shock produces a long-lasting learning deficit. Here we suggest that the mechanisms underlying learning and the deficit may have evolved from machinery responsible for the spinal processing of noxious information. Experiments have shown that learning and the deficit require receptors and signaling cascades shown to be involved in central sensitization, including activation of NMDA and neurokinin receptors, as well as CaMKII. Further supporting this link between pain and learning, research has also shown that uncontrollable stimulation results in allodynia. Moreover, systemic inflammation and neonatal hindpaw injury each facilitate pain responding and undermine the ability of the spinal cord to support learning. These results suggest that the plasticity associated with learning and pain must be placed in a balance in order for adaptive outcomes to be observed.

H-reflex down-conditioning greatly increases the number of identifiable GABAergic interneurons in rat ventral horn

H-reflex down-conditioning increases GABAergic terminals on spinal cord motoneurons. To explore the origins of these terminals, we studied the numbers and distributions of spinal cord GABAergic interneurons. The number of identifiable GABAergic interneurons in the ventral horn was 78% greater in rats in which down-conditioning was successful than in naive rats or rats in which down-conditioning failed. No increase occurred in other spinal lamina or on the contralateral side. This finding supports the hypothesis that the corticospinal tract influence that induces the motoneuron plasticity underlying down-conditioning reaches the motoneuron through GABAergic interneurons in the ventral horn.