Stretch Reflex Loop Research Articles

Assessing Neural Connectivity and Associated Time Delays of Muscle Responses to Continuous Position Perturbations.

Both linear and nonlinear electromyographic (EMG) connectivity has been reported during the expression of stretch reflexes, though it is not clear whether they are generated by the same neural pathways. To answer this question, we aim to distinguish linear and nonlinear connectivity, as well as their delays in muscle responses, resulting from continuous elbow joint perturbations. We recorded EMG from Biceps Brachii muscle when eight able-bodied participants were performing a steady elbow flexion torque while simultaneously receiving a continuous position perturbation. Using a recently developed phase coupling metric, we estimated linear and nonlinear connectivity as well as their associated delays between Biceps EMG responses and perturbations. We found that the time delay for linear connectivity (24.5 ± 5.4ms) is in the range of short-latency stretch reflex period (< 35ms), while that for nonlinear connectivity (53.8 ± 3.2ms) is in the range of long-latency stretch reflex period (40-70ms). These results suggest that the estimated linear connectivity between EMG and perturbations is very likely generated by the mono-synaptic spinal stretch reflex loop, while the nonlinear connectivity may be associated with multi-synaptic supraspinal stretch reflex loops. As such, this study provides new evidence of the nature of neural connectivity related to the stretch reflex.

Characteristics of Rest and Postural Tremors in Parkinson's Disease: An Analysis of Motor Unit Firing Synchrony and Patterns.

The neural mechanisms responsible for resting and postural tremor in Parkinson’s disease (PD) have been the object of considerable study, much of it focusing on supraspinal sites. Here, we adopted an alternative approach that emphasizes motor unit (MU) firing synchrony and patterns of discharge. To explore if these could account for known features of PD tremor, we recorded the instantaneous acceleration of the upper limb of 23 PD patients at rest or while they tried to hold a stable posture together with surface EMG and single MU discharges of upper limb muscles. Spectral, coherence and cross-correlation analyses of the recorded signals demonstrated alternating epoch-I and epoch-II intervals in PD patients both at rest and while they held a stable posture. Epoch-II intervals are characterized by the presence of 4–8 Hz overt tremor, enhanced MU synchrony and spike-doublets or triplets bearing a one-to-one relation to each tremor cycle. Epoch-I resembled physiological tremor in that it was characterized by 6–10 Hz non-overt tremor, lower MU synchrony and rhythmical MU firing at the intrinsic rate of the unit. The frequency of overt and non-overt tremor remained the same whether the patient was at rest or held a stable posture and the same was true of the remaining characteristics of epoch-I and epoch-II. The mean interval between spikes of a doublet/triplet varied between 30 and 50 ms and, for any given patient, remained roughly constant throughout measurements. This is the first time that enhanced MU synchrony and spike doublets/triplets characterized by relatively stable interspike intervals, are shown to accompany the overt tremor of PD patients. To account for our findings we propose that a two-state oscillatory spinal stretch reflex loop generates overt parkinsonian tremor in response to intermittent, descending, relatively high frequency oscillatory signals.

The Dynamics of Voluntary Force Production in Afferented Muscle Influence Involuntary Tremor.

Voluntary control of force is always marked by some degree of error and unsteadiness. Both neural and mechanical factors contribute to these fluctuations, but how they interact to produce them is poorly understood. In this study, we identify and characterize a previously undescribed neuromechanical interaction where the dynamics of voluntary force production suffice to generate involuntary tremor. Specifically, participants were asked to produce isometric force with the index finger and use visual feedback to track a sinusoidal target spanning 5–9% of each individual's maximal voluntary force level. Force fluctuations and EMG activity over the flexor digitorum superficialis (FDS) muscle were recorded and their frequency content was analyzed as a function of target phase. Force variability in either the 1–5 or 6–15 Hz frequency ranges tended to be largest at the peaks and valleys of the target sinusoid. In those same periods, FDS EMG activity was synchronized with force fluctuations. We then constructed a physiologically-realistic computer simulation in which a muscle-tendon complex was set inside of a feedback-driven control loop. Surprisingly, the model sufficed to produce phase-dependent modulation of tremor similar to that observed in humans. Further, the gain of afferent feedback from muscle spindles was critical for appropriately amplifying and shaping this tremor. We suggest that the experimentally-induced tremor may represent the response of a viscoelastic muscle-tendon system to dynamic drive, and therefore does not fall into known categories of tremor generation, such as tremorogenic descending drive, stretch-reflex loop oscillations, motor unit behavior, or mechanical resonance. Our findings motivate future efforts to understand tremor from a perspective that considers neuromechanical coupling within the context of closed-loop control. The strategy of combining experimental recordings with physiologically-sound simulations will enable thorough exploration of neural and mechanical contributions to force control in health and disease.

Interaction of Stretch Reflex Loop with Descending B-Oscillations and the Generation of Tremor in Parkinson’s Disease: A Study of Motor Unit Firing Synchrony and Patterns (P5.369)

Interaction of Stretch Reflex Loop with Descending B-Oscillations and the Generation of Tremor in Parkinson’s Disease: A Study of Motor Unit Firing Synchrony and Patterns (P5.369)

Neurophysiological changes induced by the botulinum toxin type A injection in children with cerebral palsy

In the last few years botulinum toxin type A (BTX-A) has been widely used in the management of spasticity in children with cerebral palsy in order to reduce hypertonicity and improve functional outcomes enhancing motor skill development. The botulinum toxin injection seems to interact with intrafusal and extrafusal fibers producing a reduction of hypertone both through synaptic blockade and inhibition of stretch reflex loop and these changes may influence not only the spinal cord but also the central nervous system (CNS). The purpose of our study was to determine the neurophysiological changes induced by the BTX-A through an evaluation of cortical somatosensory Evoked Potential (SEP) and Soleus H wave, that is the index of excitability of stretch reflex loop. Eighteen children with Cerebral Palsy (CP), aged between 5 and 12, were recruited at Children’s Hospital “Bambino Gesù” of Rome. All children were evaluated with appropriate clinical scales before and 1 month after the BTX-A injection. Neurophysiological measurements were performed before, and 1 month after botulinum toxin injection through lower limb SEPs, M-wave and Soleus H wave recording. After the injection the results showed a statistically significant improvement both of clinical scales and the neurophysiological variables. These findings suggest that spasticity itself can be considered as a factor affecting the cortical SEPs. And even though it seems that BTX-A does not have any direct central effect on sensory pathways we suppose an indirect mechanism on modulation of afferent fibers Ia due to the modification induced by BTX-A to central loop reflex.

Acute whole body vibration training increases vertical jump and flexibility performance in elite female field hockey players

Objective: To quantify the acute effect of whole body vibration (WBV) training on arm countermovement vertical jump (ACMVJ), grip strength, and flexibility performance.Methods: Eighteen female elite field hockey players each...

Parallel Neuronal Mechanisms Underlying Physiological Force Tremor in Steady Muscle Contractions of Humans

We present results from a study of the 6-to 12-Hz force tremor in relation to motor unit (MU) firing synchrony. Our experimental observations from 32 subjects, 321 contractions, and 427 recorded MUs reveal that tremor is accompanied by corresponding, in-phase MU rhythms that are additional to the ones at the MU intrinsic firing rates. This rhythmical synchrony is widespread and has a uniform strength that ranges from near zero to very large (MU/MU coherence > 0.50) in different contractions. Both the synchrony and the tremor are suppressed during ischemia, and this strongly suggests an involvement of spindle feedback in their generation. Furthermore, in the presence of substantial synchrony, the tremor enhancement, relative to the minimal tremor of ischemia, reflects the strength of the synchrony. Theoretical considerations based on these observations indicate that the muscle force signal is expected to show 1) frequency components in the band of the firing rates of the last-recruited, large MUs, and 2) because of the synchronized MU rhythms, an additional, distinct component with a size reflecting the strength of synchrony. Furthermore, synchronized MU rhythms, with frequencies in the 6- to 12-Hz range, are expected to arise from self-oscillations in the monosynaptic stretch reflex loop, due primarily to the associated muscle delay (several tens of milliseconds). Our results therefore reveal the parallel action of two tremor mechanisms, one of which involves MU synchrony probably caused by loop action. Clearly, the results on the synchrony and its impact also apply to other possible generators of tremor synchrony, including supraspinal ones.

Stretch reflex gain in cat triceps surae muscles with compliant loads.

The triceps surae (TS) stretch reflex was measured in decerebrate cats during crossed extensor stimulation (tonic contractions) and after spinalization during rhythmic locomotor activity. The TS reflex force in response to a short pulse stretch measured during tonic contractions at low level of background activity was greater than when more background activity was present at the time of application of stretch. In contrast, the reflex force measured during rhythmic contractions was very small at low level of background force (flexion phase) and increased at moderate and high levels of background activity (extension phase). Thus, even in reduced preparations, a task modulation of the stretch reflex occurs. Throughout the experimental procedure, the torque motor used to stretch the muscles behaved like a spring of a preset compliance (from isometric to very compliant). A reflex model was used to simulate the responses obtained experimentally. The gain of the stretch reflex loop was estimated for each load condition and both behavioural tasks. The reflex loop gain was significantly larger as the compliance of the external load increased for both tonic and rhythmic contractions, although to a lesser extent in the phasically activated muscles. During rhythmic locomotor contractions the gain was less than 1, assuring stability of the system. In contrast, during tonic contractions against a compliant load the gain exceeded 1, consistent with the instability (oscillations, clonus) seen at times under these load conditions. However, the high gain and instability was only transient, since repeated stretch reduced the gain. Thus, non-linearities in the system assured vigorous responses at the onset of perturbations, but then weaker responses to ongoing perturbations to reduce the chance of feedback instability (clonus).

Long-lasting reductions of spasticity induced by skin electrical stimulation.

We studied the effects of electrical stimulation of the skin on upper extremity spasticity in nine hemiparetic stroke subjects. The effects were quantified by comparing reflex torque responses elicited during ramp and hold angular perturbations of the elbow recorded before and after low-intensity skin stimulation. Electrical stimulation was applied to skin over the biceps muscle for a period of ten minutes at a 20 Hz frequency, pulse duration 0.1 ms, with an intensity level below motor threshold but above sensory threshold. In seven of the nine subjects, stimulation of skin over spastic muscle reduced peak torque responses in both flexors and extensors for at least 30 min. In these seven subjects there were significant increases in mean threshold angle for the onset of reflex torque so that a greater angular rotation was required to initiate the stretch reflex response. This shift occurred without change in reflex impedance. The origins of these long-term changes in reflex torque are unclear, but may reflect synaptic plasticity of spinal circuitry outside the stretch reflex loop.

Stability and parameter studies of a stretch reflex loop model.

A model of the arm muscle stretch reflex loop is considered. Investigations are carried out on a system consisting of models of the forearm, the muscle spindle in a feedback structure, and the arm muscle. Specifically, the forearm is modeled as an inverted pendulum, the experimentally derived fourth-order muscle spindle model is taken from the literature, and two linear muscle models are employed. The stability of the feedback structure is investigated as a function of various free parameters in the component models. Frequency response profiles are analyzed for the muscle/muscle spindle combination, and are compared to experimentally derived data available in the literature. Finally, digital computer simulations are exhibited for a system corresponding to a quick release experiment, and for a system in which a periodic force is applied to the forearm.

Computer simulation of Parkinsonian tremor

A computer simulation of tremor using a mathematical model is presented, which integrates the traditional two theories; ‘peripheral feedback theory’ for physiological tremor and central oscillator theory' for Parkinsonian tremor. The model includes a basic stretch-reflex loop, which is modulated by the central nervous system through the gamma fibres. Fatigue of intrafusal fibres, which results from chronic stimulation by the gamma system, is assumed to be the main cause of Parkinsonian tremor. A computer simulation using the model shows that both physiological tremor and Parkinsonian tremor can be caused by the varying contractivity of the intrafusal fibres.

Motor unit recruitment and firing rates interaction in the control of human muscles

Muscle contractions are modulated by the number of motor units recruited and their respective firing rates. The work described in this report documents an interplay between recruitment and firing rates of motor units. The recruitment of a new motor unit appears to have a disfacilitatory influence on the firing rates of previously activated motor units. It is speculated that this effect is likely to be mediated, at least partially, via the stretch reflex loop and possibly by the recurrent inhibition of the Renshaw circuit. Such a mechanism would be functionally useful in providing smooth control of muscle output via peripheral circuitry (consisting of proprioceptive reflexes and recurrent inhibition), thus lessening the amount of detailed supervision of the α-motoneuron pool required by the central nervous system.

The influence of the gamma system on cross-correlated activity of Ia muscle spindles and its relation to information transmission

In the decerebrate cat during static or slowly varying (<0.3 Hz) muscle stretches, the activity of muscle spindle (MS) pairs was poorly correlated and such correlation was not changed after ventral root sectioning. With slightly faster sinusoidal stretches (0.3–20 Hz) activity in pairs of MS was also poorly correlated. However, sectioning of the ventral roots produced preferential firing frequencies in the same muscle spindle pairs and increased their degree of correlation. The increase in correlation between MS activity detected after suppressing the gamma bias appeared to arise from extrinsic 60 Hz power line vibration in the range of micrometers. Nevertheless, activation of the gamma system could suppress such phase locking. When frequencies above 20 Hz were used, the gamma system could not decorrelate the MS pair activity since the two units became locked to their common input signal. It is suggested that decorrelation of MS activity by gamma influence may improve the fidelity of the information transmitted by the Ia MS ensemble by filtering distortion harmonics, as well as damping tremor oscillations in the stretch reflex loop.

Frequency-response analysis of vestibular-induced neck reflex in cat. II. Functional significance of cervical afferents and polysynaptic descending pathways.

1. Three major inputs to cervical motoneurons were analyzed with frequency-response methods in decerebrate and unanesthetized cats: the afferent system including the gamma-loop, the vestibulospinal tract, and some polysynaptic tract that transmits vestibular influences in the vestibulocollic reflex. 2. The dorsal roots C1-C4 were cut in order to open the feedback loop through the gamma-fiber spindle-afferent system. No effects were observed on the gain and the phase lag of motor-unit response. However, the DC components of the response were consistently decreased by deafferentation both in motor-unit and compound EMG responses. Firing of muscle spindle afferents was modulated during oscillation of the turntable. The majority of responses of spindle afferents was in phase with the simultaneously recorded extrafusal motor activity. These results indicate the existence of alpha-gamma coactivation in the vestibulocollic reflex. 3. Intracellular recording from cervical motoneurons showed that monosynaptic EPSP's were induced by dorsal root stimulation. By far the larger EPSP's, however, were observed with latencies of 10-15 ms, showing the existence of more powerful polysynaptic routes. It is postulated that the cervical afferent system controls the gain of the reflex by changing the number of motor units participating in the vestibulocollic reflex through some polysynaptic pathways, and that the afferent system scarcely changes, the gain or phase lag of individual motor units through a short stretch reflex loop. 4. The medial vestibulospinal tract in the MLF was interrupted, but no effects were detected on the dynamic characteristics of the frequency response. This shows the existence of other effective descending pathways which function as an integrator. 5. Remarkable phase advances were induced by intravenous infusion of Nembutal. This suggests that the Nembutal decreased the activity of the polysynaptic pathways and that the neural integrator, composed of polysynaptic networks, was prevented from fulfilling its ordinary function.

Reflex responses of gamma motoneurones to vibration of the muscle they innervate.

1. High frequency vibration was applied to the tendon of the non-contracting triceps surae muscle while recording the background discharges of single gamma fibres only small nerve bundles were cut, leaving most of the nerve supply to the triceps intact. 2. 22% out of a total of sixty-three gamma efferents were tonically inhibited by vibration. The inhibition appeared between 25 and 50mum peak-to-peak amplitude of vibration and increased to a plateau for amplitudes of about 100mum. The dependence of the tonic vibration reflex of alpha-efferents on the amplitude of vibration was found to be similar. Increasing the frequency of vibration from 150 to 300 Hz increased the degree of inhibition. 3. 33% of the fusimotor neurones investigated responded to muscle vibration with an increase in discharge rate. The threshold amplitudes of this reflex ranged from 20 to 50mum. Some features of the reflex, in particular the parallel post-vibratory facilitation found in alpha and gamma efferents, pointed to a polysynaptic pathway organized in an alpha-gamma linkage. 4. All gamma efferents inhibited by vibration showed inhibitory responses to antidromic stimulation of the parent ventral root, and most of them were inhibited by ramp stretch of the triceps. The gamma motoneurones facilitated by vibration, however, were excited by muscle stretch and were less susceptible to antidromic inhibition, some lacking it completely. 5. Cutting the nerves to triceps abolished the inhibitory as well as the excitatory responses of gamma efferents to muscle vibration. Both fusimotor reflexes were preserved after spinal section and subsequent administration of L-DOPA. 6. It is concluded that both of the fusimotor reflex effects of vibration are caused by excitation of primary spindle endings within the triceps. The inhibition of fusimotor neurones is thought to be mediated by Renshaw cells activated during vibration. The significance of positive feed-back on to gamma motoneurones as a result of autogenetic facilitation by Ia afferents is discussed in connexion with stability in the stretch reflex loop.

Entrainment of motor-unit discharges as a neuronal mechanism of synchronization.

The neuronal mechanism which gives rise to the synchronization of motor-unit discharges has been inferred from an analysis of the interspike intervals of individual motor-unit discharges recorded from the soleus muscle. The motor units were divided into two groups on the basis of characteristic changes in their spike trains. The first group maintained a stationary discharge pattern throughout the process of synchronization with a firing rate of approximately 10 spikes/s. Small unidentified units simultaneously recorded gradually grouped around the individual spikes of the first group motor unit and with this process, high-frequency force oscillation appeared phase-locked with each of grouped discharges. The mean period of force oscillation was almost identical to the mean discharge interval. Therefore, the first group motor unit was considered as a pacemaker of this force-oscillation. The second group motor unit underwent from its initially stationary process to a transitional process characterized by spike dropouts from an otherwise regular spike train. When both groups of motor units were recorded by the same electrode, it was found that the firing rate of the second group motor unit discharges gradually approached that of the first group, and the spikes of the first and the second group motor units occurred near or at the same time. The number of double intervals decreased in a highly predictable fashion with an increase in a firing rate. It was furthermore observed that the spikes of a given motor unit whose discharges interval-to-period ratio is smaller at the beginning of transitional process was entrained to the first group motor-unit discharges with a faster time course than the unit whose discharge interval-to-period ratio is larger. The synchronizing process was described from the relations between shorter discharge interval-to-period ratios and the longer-to-shorter interval ratios obtained at several stages from the beginning of transitional process to the final synchronization. Their relations were best drawn by the second-order regression lines. The faster time course of synchronization was reflected in the larger value of coefficient a in the equation. The results of this and previous study (23) further provided evidence to justify that interaction of motor-unit discharges is responsible for the synchronization. Although the neuronal limiting device of the firing-rate control to approximately 10 spikes/s still remains unsolved, the possibility was considered that a disinhibitory neuronal network first acts to synchronize independently firing motoneurons and leads to the oscillation of the stretch reflex loop. This closed-loop system was considered as a site for the stored motor program and the use of disinhibitory neuronal network was discussed in relation to the Harmon's model of neuromimes.

Control and Regulation of the Human Motor System

Current theory considers motor control to be effected over two pathways, one via the alpha motoneurons to the muscle and the other via the gamma motoneurons to the muscle spindles. This mechanism, described as the "alpha-gamma linkage," permits the control of the willed muscle output with the simultaneous maintenance of postural equilibrium by regulation of the muscle's length sensors. The stretch reflex thus remains a functional component of voluntary motor activity. We describe an additional mechanism which can play a role in this control task. This mechanism is control of the gain of the stretch reflex loop. Experimental evidence for the existence of this mechanism is given, and its significance discussed. Not only are the control system dynamics strongly affected, but an additional feature is that the computations required of the central nervous system are simplified.