Excitability of Spinal Inhibitory Circuits in Patients with Spasticity

Interrogating interneurone function using threshold tracking of the H reflex in healthy subjects and patients with motor neurone disease

OBJECTIVE

The excitability of the lower motoneurone pool is traditionally tested using the H reflex and a constant-stimulus paradigm, which measures changes in the amplitude of the reflex response. This technique has limitations because reflex responses of different size must involve the recruitment or inhibition of different motoneurones. The threshold-tracking technique ensures that the changes in excitability occur for an identical population of motoneurones. We aimed to assess this technique and then apply it in patients with motor neurone disease (MND).

METHODS

The threshold-tracking approach was assessed in 17 healthy subjects and 11 patients with MND. The soleus H reflex was conditioned by deep peroneal nerve stimulation producing reciprocal Ia and so-called D1 and D2 inhibitions, which are believed to reflect presynaptic inhibition of soleus Ia afferents.

RESULTS

Threshold tracking was quicker than the constant-stimulus technique and reliable, properties that may be advantageous for clinical studies. D1 inhibition was significantly reduced in patients with MND.

CONCLUSIONS

Threshold tracking is useful and may be preferable under some conditions for studying the excitability of the motoneurone pool. The decreased D1 inhibition in the patients suggests that presynaptic inhibition may be reduced in MND.

SIGNIFICANCE

Reduced presynaptic inhibition could be evidence of an interneuronopathy in MND. It is possible that the hyperreflexia is a spinal pre-motoneuronal disorder, and not definitive evidence of corticospinal involvement in MND.

Association Between Plantarflexion Torque Variability In Quiet Stance And During Force And Position Tasks

This study examined the association between plantarflexion torque variability during quiet bipedal standing (QS) and during plantarflexion force- and position-matching tasks (FT and PT, respectively). In QS, participants stood still over a force plate, and the mean plantarflexion torque level exerted by each subject in QS (divided by 2 to give the torque due to a single leg) served as the target torque level for right leg FT and PT (performed with the participants seated with their right knee fully extended). During FT participants controlled the force level exerted by the foot against a rigid restraint, while during PT they controlled the angular position of the ankle when sustaining equivalent inertial loads. Standard deviation (SD) of plantarflexion torque was computed from torque signals acquired during periods with and without visual feedback. Significant correlations were found between plantarflexion torque variability in QS and FT (r = 0.8615, p < 0.0001 and r = 0.8838, p = 0.0003 for visual and no visual conditions, respectively) as well as between QS and PT (r = 0.8046, p = 0.003 and r = 0.7332, p = 0.0103 for visual and no visual conditions, respectively), regardless of vision availability. No significant differences were found between the correlations for Qs vs FT and QS vs PT (t(8) = 0.4778, p = 0.6455 and t(8) = 1.6819, p = 0.1310 for visual and no visual conditions, respectively), as assessed by "Hotelling-Williams" tests for equality among dependent correlations. The results indicate that simple measurements of plantarflexion torque fluctuations during FT and PT may be used to estimate balance ability. From a practical standpoint, it is suggested that rehabilitation protocols designed to regain/improve balance function may be based on the performance of FTs or PTs executed in a seated position.

Relationship between spasticity and spinal neural circuits in patients with chronic hemiparetic stroke

Spasticity is a common problem in patients with stroke that contributes to motor dysfunction. However, the pathophysiological mechanisms underlying spasticity are not fully understood. The purpose of the present study was to explain the relationship between features of spinal neural circuits assessed using electrophysiological techniques and the clinical manifestations of stroke. The participants were 71 patients with chronic hemiparetic stroke. To assess spinal neural circuits, Hmax/Mmax of the forearm flexor muscles and reciprocal inhibition (RI) between forearm extensor and flexor muscles with the H reflex conditioning-test paradigm were measured. The relationships between electrophysiological parameters and clinical variables (age, time from stroke onset, upper extremity functional scores, and spasticity) were then analyzed. It was found that the third phase of RI (RI-3) correlated with the modified Ashworth scores of the wrist and finger flexors. No other correlations were found between electrophysiological and clinical measures. These results suggest that RI-3 is associated with spasticity and may be helpful to understand the basis of post-stroke spasticity.

Long-Term Plasticity in Reflex Excitability Induced by Five Weeks of Arm and Leg Cycling Training after Stroke

Neural connections remain partially viable after stroke, and access to these residual connections provides a substrate for training-induced plasticity. The objective of this project was to test if reflex excitability could be modified with arm and leg (A &amp; L) cycling training. Nineteen individuals with chronic stroke (more than six months postlesion) performed 30 min of A &amp; L cycling training three times a week for five weeks. Changes in reflex excitability were inferred from modulation of cutaneous and stretch reflexes. A multiple baseline (three pretests) within-subject control design was used. Plasticity in reflex excitability was determined as an increase in the conditioning effect of arm cycling on soleus stretch reflex amplitude on the more affected side, by the index of modulation, and by the modulation ratio between sides for cutaneous reflexes. In general, A &amp; L cycling training induces plasticity and modifies reflex excitability after stroke.

Modulation of spinal inhibitory reflexes depends on the frequency of transcutaneous electrical nerve stimulation in spastic stroke survivors

Neurophysiological studies in healthy subjects suggest that increased spinal inhibitory reflexes from the tibialis anterior (TA) muscle to the soleus (SOL) muscle might contribute to decreased spasticity. While 50 Hz is an effective frequency for transcutaneous electrical nerve stimulation (TENS) in healthy subjects, in stroke survivors, the effects of TENS on spinal reflex circuits and its appropriate frequency are not well known. We examined the effects of different frequencies of TENS on spinal inhibitory reflexes from the TA to SOL muscle in stroke survivors. Twenty chronic stroke survivors with ankle plantar flexor spasticity received 50-, 100-, or 200-Hz TENS over the deep peroneal nerve (DPN) of the affected lower limb for 30 min. Before and immediately after TENS, reciprocal Ia inhibition (RI) and presynaptic inhibition of the SOL alpha motor neuron (D1 inhibition) were assessed by adjusting the unconditioned H-reflex amplitude. Furthermore, during TENS, the time courses of spinal excitability and spinal inhibitory reflexes were assessed via the H-reflex, RI, and D1 inhibition. None of the TENS protocols affected mean RI, whereas D1 inhibition improved significantly following 200-Hz TENS. In a time-series comparison during TENS, repeated stimulation did not produce significant changes in the H-reflex, RI, or D1 inhibition regardless of frequency. These results suggest that the frequency-dependent effect of TENS on spinal reflexes only becomes apparent when RI and D1 inhibition are measured by adjusting the amplitude of the unconditioned H-reflex. However, 200-Hz TENS led to plasticity of synaptic transmission from the antagonist to spastic muscles in stroke survivors.

D1 and D2 Inhibitions of the Soleus H-Reflex Are Differentially Modulated during Plantarflexion Force and Position Tasks

Presynaptic inhibition (PSI) has been shown to modulate several neuronal pathways of functional relevance by selectively gating the connections between sensory inputs and spinal motoneurons, thereby regulating the contribution of the stretch reflex circuitry to the ongoing motor activity. In this study, we investigated whether a differential regulation of Ia afferent inflow by PSI may be associated with the performance of two types of plantarflexion sensoriomotor tasks. The subjects (in a seated position) controlled either: 1) the force level exerted by the foot against a rigid restraint (force task, FT); or 2) the angular position of the ankle when sustaining inertial loads (position task, PT) that required the same level of muscle activation observed in FT. Subjects were instructed to maintain their force/position at target levels set at ~10% of maximum isometric voluntary contraction for FT and 90° for PT, while visual feedback of the corresponding force/position signals were provided. Unconditioned H-reflexes (i.e. control reflexes) and H-reflexes conditioned by electrical pulses applied to the common peroneal nerve with conditioning-to-test intervals of 21 ms and 100 ms (corresponding to D1 and D2 inhibitions, respectively) were evoked in a random fashion. A significant main effect for the type of the motor task (FT vs PT) (p = 0.005, η²_p = 0.603) indicated that PTs were undertaken with lower levels of Ia PSI converging onto the soleus motoneuron pool. Additionally, a significant interaction between the type of inhibition (D1 vs D2) and the type of motor task (FT vs PT) (p = 0.038, η²_p = 0.395) indicated that D1 inhibition was associated with a significant reduction in PSI levels from TF to TP (p = 0.001, η²_p = 0.731), whereas no significant difference between the tasks was observed for D2 inhibition (p = 0.078, η²_p = 0.305). These results suggest that D1 and D2 inhibitions of the soleus H-reflex are differentially modulated during the performance of plantarflexion FT and PT. The reduced level of ongoing PSI during PT suggests that, in comparison to FT, there is a larger reliance on inputs from muscle spindles primary afferents when the neuromuscular system is required to maintain position-controlled plantarflexion contractions.

After stroke bidirectional modulation of soleus stretch reflex amplitude emerges during rhythmic arm cycling

OBJECTIVES

after stroke a typical presentation is exaggerated stretch reflexes (SRs) on the more affected (MA) side. The present study evaluated the contribution of presynaptic inhibition (PSI) induced by arm cycling and homosynaptic depression (HD) to the modulation of hyperreflexia at the ankle after stroke. Possible asymmetry of these effects between the MA and less affected (LA) legs was also assessed.

METHODS

soleus SR was conditioned by: arm cycling at 1 Hz (to increase Ia PSI); or, a preceding conditioning tendon tap applied 1 s before the test stimulus (to induce HD). The extent of conditioning effects was compared between the MA and the LA legs.

RESULTS

for both MA and LA legs, rhythmic arm movement induced a bidirectional effect in different participants, either increasing or decreasing SR amplitude (p < 0.05). HD had a significant effect in both legs (p < 0.05), however, the effect of both a previous muscle stretch and arm cycling was not different between the MA and the LA legs.

CONCLUSION

our data reveal a bidirectional reflex modulation induced by arm cycling that produced facilitation in some and suppression in other participants after stroke. Relative SR amplitude modulation did not differ between the LA and MA legs. We speculate that alterations in SR amplitude modulation after stroke may reflect specific changes in both presynaptic afferent transmission mechanisms and fusimotor control.

SIGNIFICANCE

the present findings open new perspectives on the characterization of pathophysiology of stroke during the performance of functionally relevant motor tasks.

Depression and recovery of reflex amplitude during electrical stimulation after spinal cord injury

OBJECTIVE

The aim of this study was to quantify, for the first time, H-reflexes evoked during prolonged trains of wide-pulse neuromuscular electrical stimulation (WP-NMES) in individuals with chronic spinal cord injury (SCI). We hypothesised that after the first H-reflex, reflex amplitudes would be depressed (due to post-activation depression), but would recover and this recovery would be enhanced after a "burst" of 100 Hz WP-NMES.

METHODS

Soleus M-waves and H-reflexes evoked during WP-NMES (1 ms pulse width) of the tibial nerve were quantified in nine individuals with SCI. WP-NMES was delivered in two patterns: "constant-frequency" (15 or 20 Hz for 12 s) and "burst-like" (15-100-15 Hz or 20-100-20 Hz; 4 s each phase) at an intensity that evoked an M-wave between 10% and 15% of the maximal M-wave (M(max)).

RESULTS

During constant frequency stimulation, after the initial depression from the first to the second H-reflex (1st: 57% M(max); 2nd: 25% M(max)), H-reflexes did not recover significantly and were 37% M(max) at the end of the stimulus train. During the burst-like pattern, after the initial depression (1st: 62% M(max); 2nd: 30%), reflexes recovered completely by the end of the stimulation (to 55% M(max)) as they were not significantly different from the first H-reflex. M-waves were initially depressed (1st: 12% M(max); 2nd: 7% M(max)) then did not change throughout the stimulation and were not significantly different between stimulation patterns. An analysis of covariance indicated that the depression in M-wave amplitude did not account for the depression in H-reflex amplitude.

CONCLUSIONS

Relatively large H-reflexes were recorded during both patterns of NMES. The brief burst of 100 Hz stimulation restored H-reflexes to their initial amplitudes, effectively reversing the effects of post-activation depression.

SIGNIFICANCE

For individuals with chronic SCI, generating contractions through central pathways may help reduce muscle atrophy and produce contractions that are more fatigue-resistant for rehabilitation, exercise programs, or to perform activities of daily living.

The effects of wide pulse neuromuscular electrical stimulation on elbow flexion torque in individuals with chronic hemiparetic stroke

OBJECTIVE

Neuromuscular electrical stimulation that incorporates wide pulse widths (1ms) and high frequencies (100Hz; wide pulse-NMES (WP-NMES)) augments contractions through an increased reflexive recruitment of motoneurons in individuals without neurological impairments and those with spinal cord injury. The current study was designed to investigate whether WP-NMES also augments contractions after stroke. We hypothesized that WP-NMES would generate larger contractions in the paretic arm compared to the non-paretic arm due to increased reflex excitability for paretic muscles after stroke.

METHODS

The biceps brachii muscles were stimulated bilaterally in 10 individuals with chronic hemiparetic stroke. Four stimulation patterns were delivered to explore the effects of pulse width and frequency on contraction amplitude: 20-100-20Hz (4s each phase, 1ms pulse width); 20-100-20Hz (4s each phase, 0.1ms); 20Hz for 12s (1ms); and 100Hz for 12s (1ms). Elbow flexion torque and electromyography were recorded.

RESULTS

Stimulation that incorporated 1ms pulses evoked more torque in the paretic arm than the non-paretic arm. When 0.1ms pulses were used there was no difference in torque between arms. For both arms, torque declined significantly during the constant frequency 100Hz stimulation and did not change during the constant frequency 20Hz stimulation.

CONCLUSIONS

The larger contractions generated by WP-NMES are likely due to increased reflexive recruitment of motoneurons, resulting from increased reflex excitability on the paretic side.

SIGNIFICANCE

NMES that elicits larger contractions may allow for development of more effective stroke rehabilitation paradigms and functional neural prostheses.

Avaliação da inibição recíproca em humanos durante contrações isométricas dos músculos tibial anterior e sóleo

Os objetivos do presente trabalho foram: (1) desenvolver um método para estimar o grau de inibição recíproca (IR) entre músculos antagonistas em humanos (sóleo e tibial anterior) e (2) comparar os níveis de IR no repouso, na dorsiflexão (DF) e na flexão plantar (FP). Participaram nove sujeitos saudáveis com idade entre 20 e 30 anos, quatro homens e cinco mulheres. Os sujeitos permaneceram sentados numa cadeira com o pé direito apoiado e fixo num pedal acoplado a um torquímetro; as medições foram feitas no repouso e durante contração isométrica dos músculos dorsiflexores e flexores plantares do tornozelo. A onda H do músculo sóleo foi captada por eletrodos de superfície. O reflexo H (RH) "teste" do músculo sóleo foi medido aplicando-se um estímulo na fossa poplítea (nervo tibial). O reflexo H "condicionado" foi obtido pelo pareamento de dois estímulos: o primeiro aplicado sobre a cabeça da fíbula e o segundo, na fossa poplítea, após 1 a 3 ms.. As amplitudes pico-a-pico dos RH teste e condicionado foram utilizadas para o cálculo da IR. Os valores de IR foram: 16,41%±8,68 no repouso; 21,94%±5,39 na DF e 3,12%±11,84 na FP. Foi constatada menor inibição recíproca na FP quando comparada às demais condições (p<0,05), mas não houve aumento da IR detectável, pela metodologia aplicada, durante a dorsiflexão em comparação ao repouso. Os resultados sugerem que a IR sofre modulação durante a atividade voluntária.

Spinal reflexes, mechanisms and concepts: From Eccles to Lundberg and beyond

This review focuses on investigations by Sir John Eccles and co-workers in Canberra, AUS in the 1950s, in which they used intracellular recordings to unravel the organization of neuronal networks in the cat spinal cord. Five classical spinal reflexes are emphasized: recurrent inhibition of motoneurons via motor axon collaterals and Renshaw cells, pathways from muscle spindles and Golgi tendon organs, presynaptic inhibition, and the flexor reflex. To set the scene for these major achievements I first provide a brief account of the understanding of the spinal cord in "reflex" and "voluntary" motor activities from the beginning of the 20th century. Next, subsequent work is reviewed on the convergence on spinal interneurons from segmental sensory afferents and descending motor pathways, much of which was performed and inspired by Anders Lundberg's group in Gothenburg, SWE. This work was the keystone for new hypotheses on the role of spinal circuits in normal motor control. Such hypotheses were later tested under more natural conditions; either by recording directly from interneurons in reduced animal preparations or by use of indirect non-invasive techniques in humans performing normal movements. Some of this latter work is also reviewed. These developments would not have been possible without the preceding work on spinal reflexes by Eccles and Lundberg. Finally, there is discussion of how Eccles' work on spinal reflexes remains central (1) as new techniques are introduced on direct recording from interneurons in behaving animals; (2) in experiments on plastic neuronal changes in relation to motor learning and neurorehabilitation; (3) in experiments on transgenic animals uncovering aspects of human pathophysiology; and (4) in evaluating the function of genetically identified classes of neurons in studies on the development of the spinal cord.