The silent period in a muscle of the human hand

Test-retest reliability of cortico-spinal measurements in the rectus femoris at different contraction levels

Single-pulse Transcranial Magnetic Stimulation (TMS) and, very recently, lumbar stimulation (LS) have been used to measure cortico-spinal excitability from various interventions using maximal or submaximal contractions in the lower limbs. However, reliability studies have overlooked a wide range of contraction intensities for MEPs, and no reliability data is available for LEPs. This study investigated the reliability of motor evoked potentials and lumbar evoked potentials at different stimulation intensities and contraction levels in m.rectus femoris. Twenty-two participants performed non-fatiguing isometric knee extensions at 20 and 60% of maximum voluntary contraction (MVC). LS induced a lumbar-evoked potential (LEP) of 25 and 50% resting maximal compound action potential (M-max). TMS stimulator output was adjusted to 120, 140, and 160% of active motor threshold (aMT). In each contraction, a single MEP or LEP was delivered. Ten contractions were performed at each stimulator intensity and contraction level in random order. Moderate-to-good reliability was found when LEP was normalized to M-max/Root Mean Square in all conditions (ICC:0.74-0.85). Excellent reliability was found when MEP was normalized to Mmax for all conditions (ICC > 0.90) at 60% of MVC. Good reliability was found for the rest of the TMS conditions. Moderate-to-good reliability was found for silent period (SP) elicited by LS (ICC: 0.71-0.83). Good-to-excellent reliability was found for SP elicited by TMS (ICC > 0.82). MEPs and LEPs elicited in m.rectus femoris appear to be reliable to assess changes at different segments of the cortico-spinal tract during different contraction levels and stimulator output intensities. Furthermore, the TMS- and LS- elicited SP was a reliable tool considered to reflect inhibitory processes at spinal and cortical levels.

Anatomo-Functional Origins of the Cortical Silent Period: Spotlight on the Basal Ganglia

The so-called cortical silent period (CSP) refers to the temporary interruption of electromyographic signal from a muscle following a motor-evoked potential (MEP) triggered by transcranial magnetic stimulation (TMS) over the primary motor cortex (M1). The neurophysiological origins of the CSP are debated. Previous evidence suggests that both spinal and cortical mechanisms may account for the duration of the CSP. However, contextual factors such as cortical fatigue, experimental procedures, attentional load, as well as neuropathology can also influence the CSP duration. The present paper summarizes the most relevant evidence on the mechanisms underlying the duration of the CSP, with a particular focus on the central role of the basal ganglia in the "direct" (excitatory), "indirect" (inhibitory), and "hyperdirect" cortico-subcortical pathways to manage cortical motor inhibition. We propose new methods of interpretation of the CSP related, at least partially, to the inhibitory hyperdirect and indirect pathways in the basal ganglia. This view may help to explain the respective shortening and lengthening of the CSP in various neurological disorders. Shedding light on the complexity of the CSP's origins, the present review aims at constituting a reference for future work in fundamental research, technological development, and clinical settings.

Diabetes Mellitus Type Has Impact on Cutaneous Silent Period

Introduction:

Neurophysiological tests allow accurate assessment of the function of the peripheral nervous system. Detection of neurophysiological changes allows us to understand the neurological clinical symptoms and signs of patients with type 1 and type 2 diabetes and the possibility for their symptomatic treatment.

Aim:

Evaluate the effect of diabetes mellitus on the “cutaneous silent period” in detecting diabetic polyneuropathy.

Material and Methods:

The study included 150 subjects, 90 suffering from diabetes, divided into three groups of 30, depending on the disease duration, and a control group of 60 respondents not suffering from diabetes or other polyneuropathies. The control group are referred for EMG analysis on another basis (cervical radiculopathy, brachialgia, etc.). Group 1 consisted of 30 subjects with diabetes mellitus type 2 and duration of illness up to 5 years. Group 2 consisted of 30 subjects with type 2 diabetes mellitus 2 and illness duration from 5 to 10 years. Group 3 consisted of 30 patients with type 1 diabetes mellitus. The study groups consisted of patients referred for EMNG analysis to the EMG office of the Clinical Center of Sarajevo University, Neurology Clinic and the Neurophysiology Laboratory in Ljubljana, from July 1, 2011 to May 1, 2016. All patients were examined neurologically and electroneurographic analysis was performed.

Results:

A statistically significant difference was found in the incidence of pathologic CSP with respect to the study groups, χ2 = 26.153; p=0.001. Pathologic CSP was more common in group 1 and group 2 of subjects (56.17%) compared to group 3 and control subjects, where it occurred in 13.3% of the cases.

Conclusion:

The pathological cutaneous period of silence was more frequent in subjects of group 1 and group 2, that is, in subjects with DM type 2, compared to subjects with DM type 1.

Fascial organisation of motor synergies: a hypothesis

In the field of biomechanics and motor control understanding movement coordination is paramount. Motor synergies represent the coordination of neural and physical elements embedded in our bodies in order to optimize the solutions to motor problems. Although we are able to measure and quantify the movement made manifested, we do not have confidence in explaining the anatomical bases of its organisation at different levels. It is our contention that the flexible hierarchical organization of movement relies on the fascial structurers to create functional linkages at different levels, and this concept attunes with the neural control of synergies. At the base of movement organization there is a (somatic) equilibrium point that exists on the fascia where the neurologically- and mechanically-generated tensions dynamically balance out. This somatic equilibrium point is at the base of postural control, afferent flow of information to the nervous system about the state of the muscles, and of the coordinative pre-activation of muscular contraction sequences specific for a synergy. Implications are discussed and suggestions for research and clinical applications are made.

The Error Power Spectrum as a Technique for Assessing the Performance of the Human Operator in a Simple Task

The measurement of performance of a human operator in a closed loop control system is considered; it is suggested that the power spectrum of the fluctuations of his tracking error (or error spectral density curve) gives a useful picture of performance and the equipment and technique for producing such a curve is described briefly.

As an example of the technique the power spectra obtained on several subjects performing a simple task with a pressure joystick are given and the implications of the shape of the curve are discussed.

Tremor's glove-an innovative electrical muscle stimulation therapy for intractable tremor in Parkinson's disease: A randomized sham-controlled trial

BACKGROUND

Medically refractory resting tremor is a debilitating symptom of Parkinson's disease (PD) patients. In our pilot study, modulation of peripheral reflex mechanism by electrical muscle stimulation (EMS) temporarily suppressed tremor.

OBJECTIVES

To investigate the efficacy of EMS, delivered using Tremor's glove, as a treatment of resting hand tremor.

PATIENTS AND METHODS

Thirty PD patients with medically refractory resting tremor were randomly allocated to a Tremor's glove group (n=15) or a sham glove group (n=15). Gloves were placed on the most tremulous hand for 30min per testing session. Demographics, clinical rating scales, and tremor parameters (RMS of angular velocity and angular displacement, peak magnitude, and frequency) were assessed before and during stimulation. Correlations with validated clinical rating scales were performed.

RESULTS

There were no statistically significant differences between groups in demographics, rating scales, or tremor parameters. During stimulation, significant reduction in RMS angular velocity (as percentage) in every axis and peak magnitude in axis (x-, y-) and UPDRS tremor score, were found with Tremor's glove compared to the sham groups (p<0.05, each). Significant moderate correlations were observed between a percentage reduction of RMS angular velocity in every axis and UPDRS tremor scores. Mean duration of tremor reduction after stimulation was 107.78±104.15s. No serious adverse events were observed.

CONCLUSION

In this study, EMS-based Tremor's glove was effective in suppressing resting hand tremor in PD patients. Tremor's glove is light-weight with a good safety profile, making it a future potential therapeutic option for PD patients with medically refractory tremor.

Input-Output Characteristics of Late Corticospinal Silent Period Induced by Transcranial Magnetic Stimulation

PURPOSE

Corticospinal silent period (SP) may be interrupted by a burst of muscle activity followed by a second (late) SP, generally assumed to be a continuation from the primary SP. Our objective was to characterize the input-output behavior of the late SP.

METHODS

Transcranial magnetic stimulation was applied on the cortical representation area of the right-hand muscles of 12 healthy subjects. Single-pulse transcranial magnetic stimulation was given with varying stimulation intensities normalized to the individual resting motor threshold (60% to 130% of the resting motor threshold) during voluntary muscle contraction. Electromyogram was recorded from first dorsal interosseous and abductor pollicis brevis muscles. Primary and late SPs were analyzed as absolute SPs, and input-output characteristics were assessed.

RESULTS

The late SP exhibited fundamentally different input-output characteristics from that of the primary SP. The late SP most likely presented itself at stimulation intensities of 90% to 100% of the resting motor threshold.

CONCLUSIONS

Different input-output characteristics of the late SP compared with the primary SP indicate that the late SP possess mechanisms different from the primary SP. The exact origin of the late SP remains unclear. Understanding the origins of the late SP could provide valuable insight on corticospinal inhibitory processes.

Corticospinal control strategies underlying voluntary and involuntary wrist movements

The difference between voluntary and involuntary motor actions has been recognized since ancient times, but the nature of this difference remains unclear. We compared corticospinal influences at wrist positions established before and after voluntary motion with those established before and after involuntary motion elicited by sudden removal of a load (the unloading reflex). To minimize the effect of motoneuronal excitability on the evaluation of corticospinal influences, motor potentials from transcranial magnetic stimulation of the wrist motor cortex area were evoked during an EMG silent period produced by brief muscle shortening. The motoneuronal excitability was thus equalized at different wrist positions. Results showed that the unloading reflex was generated in the presence of a corticospinal drive, rather than autonomously by the spinal cord. Although the tonic EMG levels were substantially different, the corticospinal influences remained the same at the pre- and post-unloading wrist positions. These influences however changed when subjects voluntarily moved the wrist to another position. Previous studies showed that the corticospinal system sets the referent position (R) at which neuromuscular posture-stabilizing mechanisms begin to act. In self-initiated actions, the corticospinal system shifts the R to relay these mechanisms to a new posture, thus converting them from mechanisms resisting to those assisting motion. This solves the classical posture-movement problem. In contrast, by maintaining the R value constant, the corticospinal system relies on these posture-stabilizing mechanisms to allow involuntary responses to occur after unloading. Thus, central control strategies underlying the two types of motor actions are fundamentally different.

Speech production as state feedback control

Spoken language exists because of a remarkable neural process. Inside a speaker's brain, an intended message gives rise to neural signals activating the muscles of the vocal tract. The process is remarkable because these muscles are activated in just the right way that the vocal tract produces sounds a listener understands as the intended message. What is the best approach to understanding the neural substrate of this crucial motor control process? One of the key recent modeling developments in neuroscience has been the use of state feedback control (SFC) theory to explain the role of the CNS in motor control. SFC postulates that the CNS controls motor output by (1) estimating the current dynamic state of the thing (e.g., arm) being controlled, and (2) generating controls based on this estimated state. SFC has successfully predicted a great range of non-speech motor phenomena, but as yet has not received attention in the speech motor control community. Here, we review some of the key characteristics of speech motor control and what they say about the role of the CNS in the process. We then discuss prior efforts to model the role of CNS in speech motor control, and argue that these models have inherent limitations – limitations that are overcome by an SFC model of speech motor control which we describe. We conclude by discussing a plausible neural substrate of our model.

Myotatic reflex: its input-output relation

The dynamic properties of the myotatic reflex and of its components were determined by a systems-analysis approach. The gain and phase relations between an applied stretch, whiclh initiates the reflex, and the output of the primary mulscle spindles, which impinge upon (alpha)-motoneurons, are not further changed by the properties of the motoneurons. The dynamic relation between motoneuron activity and the resultant muscle tension balances these changes in gain and phase; the result is a flat gain and nearly zero phase difference between stretch and tension produced by the myotatic reflex. Moreover, the distribution of activity in multiple channels extends the range in which the overall reflex is linear.

Sensory feedback to the cerebral cortex during voluntary movement in man

This article describes a series of experiments directed toward the following questions: (1) Do signals from musculotendinous receptors reach consciousness? (2) Does feed-forward information of muscular force and expected extent of voluntary movement exist? To answer these questions data from voluntary compression of springs and strain-gauge have been analysed in healthy young subjects and in patients with unilateral focal lesions of the cerebral hemispheres.

By successive elimination of information from other sources it was possible to verify that receptors in muscles and tendons do signal movement magnitude and muscular tension to the cerebral cortex, and that this information does indeed reach consciousness. There also exists a feed-forward mechanism signalling parameters of voluntary contraction. However, it is unclear whether peripheral, subcortical, or intracortical loops are directly involved. Perception of signals of muscular tension is abolished by lesions of the contralateral cortex near the central sulcus. It is possible that there exist separate cortical projection areas for kinaesthetic signals from muscles and from joints.

Methodology for combined TMS and EEG

The combination of transcranial magnetic stimulation (TMS) with simultaneous electroencephalography (EEG) provides us the possibility to non-invasively probe the brain's excitability, time-resolved connectivity and instantaneous state. Early attempts to combine TMS and EEG suffered from the huge electromagnetic artifacts seen in EEG as a result of the electric field induced by the stimulus pulses. To deal with this problem, TMS-compatible EEG systems have been developed. However, even with amplifiers that are either immune to or recover quickly from the pulse, great challenges remain. Artifacts may arise from the movement of electrodes, from muscles activated by the pulse, from eye movements, from electrode polarization, or from brain responses evoked by the coil click. With careful precautions, many of these problems can be avoided. The remaining artifacts can be usually reduced by filtering, but control experiments are often needed to make sure that the measured signals actually originate in the brain. Several studies have shown the power of TMS-EEG by giving us valuable information about the excitability or connectivity of the brain.

Positive force feedback in human walking

The objective of this study was to determine if load receptors contribute to the afferent-mediated enhancement of ankle extensor muscle activity during the late stance phase of the step cycle. Plantar flexion perturbations were presented in late stance while able-bodied human subjects walked on a treadmill that was declined by 4%, inclined by 4% or held level. The plantar flexion perturbation produced a transient, but marked, presumably spinally mediated decrease in soleus EMG that varied directly with the treadmill inclination. Similarly, the magnitude of the control step soleus EMG and Achilles' tendon force also varied directly with the treadmill inclination. In contrast, the ankle angular displacement and velocity were inversely related to the treadmill inclination. These results suggest that Golgi tendon organ feedback, via the group Ib pathway, is reduced when the muscle-tendon complex is unloaded by a rapid plantar flexion perturbation in late stance phase. The changes in the unload response with treadmill inclination suggest that the late stance phase soleus activity may be enhanced by force feedback.

Central Resetting of Neuromuscular Steady States May Underlie Rhythmical Arm Movements

Changing the steady-state configuration of the body or its segments may be an important function of central pattern generators for locomotion and other rhythmical movements. Thereby, muscle activation, forces, and movement may emerge following a natural tendency of the neuromuscular system to achieve the current steady-state configuration. To verify that transitions between different steady states occur during rhythmical movements, we asked standing subjects to swing one or both arms synchronously or reciprocally at ∼0.8 Hz from the shoulder joints. In randomly selected cycles, one arm was transiently arrested by an electromagnetic device. Swinging resumed after some delay and phase resetting. During bilateral swinging, the nonperturbed arm often stopped before resuming swinging at a position that was close to either the extreme forward or the extreme backward arm position observed before the perturbation. Oscillations usually resumed when both arms arrived at similar extreme positions when a synchronous bilateral pattern was initially produced or at the opposite positions if the initial pattern was reciprocal. Results suggest that a central generator controls both arms as a coherent unit by producing transitions between its steady state (equilibrium) positions. By controlling these positions, the system may define the spatial boundaries of movement. At these positions, the system may halt the oscillations, resume them at a new phase (as observed in the present study), or initiate a new motor action. Our findings are relevant to locomotion and suggest that walking may also be generated by transitions between several equilibrium configurations of the body, possibly accomplished by modulation and gating of proprioceptive reflexes.

Ankle extensor proprioceptors contribute to the enhancement of the soleus EMG during the stance phase of human walking

A rapid plantar flexion perturbation applied to the ankle during the stance phase of the step cycle during human walking unloads the ankle extensors and produces a marked decline in the soleus EMG. This demonstrates that sensory activity contributes importantly to the enhancement of the ankle extensor muscle activation during human walking. On average, the EMG begins to decline approximately 52 ms after the perturbation. In contrast, a rapid dorsi flex ion perturbation produces a group Ia mediated short-latency stretch reflex burst with an onset latency of approximately 36 ms. The transmission of sensory traffic from the foot and ankle was suppressed in 10 subjects by an anaesthetic nerve block produced with local injections of lidocaine hydrochloride. The anaesthetic block had no effect on the stance phase soleus EMG, the latencies of the EMG responses, or the magnitude of the EMG decline following the plantar flexion perturbation. Therefore, it is more likely that proprioceptive afferents, rather than cutaneous afferents, contribute to the background soleus EMG during the late stance phase of the step cycle. The large difference in onset latencies between the short-latency reflex and unload responses suggests that the largest of the active group Ia afferents might not contribute strongly to the background soleus EMG, although it remains to be determined which of the proprioceptive pathways provide the more important contributions.Key words: afferent feedback, gait, locomotion, stretch reflex.

Sustained muscle contractions maintained by autonomous neuronal activity within the human spinal cord

It is well known that muscle contraction can be easily evoked in the human soleus muscle by applying single-pulse electrical stimulation to the tibial nerve at the popliteal fossa. We herein reveal the unexpected phenomenon of muscle contractions that can be observed when train stimulation is used instead. We found, in 11 human subjects, that transient electrical train stimulation (1-ms pulses, 50 Hz, 2 s) was able to induce sustained muscle contractions in the soleus muscle that outlasted the stimulation period for greater than 1 min. Subjects were unaware of their own muscle activity, suggesting that this is an involuntary muscle contraction. In fact, the excitability of the primary motor cortex (M1) with the sustained muscle contractions evaluated by transcranial magnetic stimulation was lower than the excitability with voluntary muscle contractions even when both muscle contraction levels were matched. This finding indicates that M1 was less involved in maintaining the muscle contractions. Furthermore, the muscle contractions did not come from spontaneous activity of muscle fibers or from reverberating activity within closed neuronal circuits involving motoneurons. These conclusions were made based on the respective evidence: 1) the electromyographic activity was inhibited by stimulation of the common peroneal nerve that has inhibitory connections to the soleus motoneuron pool and 2) it was not abolished after stopping the reverberation (if any) for approximately 100 ms by inducing the silent period that followed an H-reflex. These findings indicate that the sustained muscle contractions induced in this study are most likely to be maintained by autonomous activity of motoneurons and/or interneurons within the human spinal cord.

Peripheral silent periods in essential tremor

BACKGROUND

Pathophysiology of essential tremor (ET) is controversial. In the present study, peripherally induced silent period (SP) in ET patients is studied.

AIMS AND OBJECTIVES

To study if the peripherally induced SP was different in ET patients as compared to age matched healthy controls.

MATERIAL AND METHODS

24 patients of ET diagnosed according to diagnostic criteria of Louis et al. [Neurology 50 (1998) 1351] (mean age 45.37+/-14.86 years) and an equal number of healthy controls (mean age 36.21+/-15.72 years) were recruited for the study. Peripherally induced SP was recorded according to the methods already described. Student's t-test and Wilcoxon sign rank test were used for statistical analysis.

RESULTS

The peripheral SP was 50.29+/-50.15 and 93.04+/-35.93 ms (p=0.0014) in ET patients and controls respectively.

CONCLUSION

Our study shows that peripheral silent period is shorter in patients of ET as compared to normal individuals.

Excitability of the human trigeminal motoneuronal pool and interactions with other brainstem reflex pathways

We studied the properties of motoneurones and Ia-motoneuronal connections in the human trigeminal system, and their functional interactions with other brainstem reflex pathways mediated by non-muscular (Abeta) afferents. With surface EMG recordings we tested the recovery cycles of the heteronymous H-reflex in the temporalis muscle and the homonymous silent period in the masseter muscle both elicited by stimulation of the masseteric nerve at the infratemporal fossa in nine healthy subjects. In four subjects single motor-unit responses were recorded from the temporalis muscle. In six subjects we also tested the effect of the stimulus to the mental nerve on the temporalis H-reflex and, conversely, the effect of Ia input (stimulus to the masseteric nerve) on the R1 component of the blink reflex in the orbicularis oculi muscle. The recovery cycle of the H-reflex showed a suppression peaking at the 5-20 ms interval; conversely the time course of the masseteric silent period was facilitated at comparable intervals. The inhibition of the test H-reflex was inversely related to the level of background voluntary contraction. Single motor units were unable to fire consistently in response to the test stimulus at intervals shorter than 50 ms. Mental nerve stimulation strongly depressed the H-reflex. The time course of this inhibition coincided with the EMG inhibition elicited by mental nerve stimulation during voluntary contraction. The trigeminal Ia input facilitated the R1 component of the blink reflex when the supraorbital test stimulation preceded the masseteric conditioning stimulation by 2 ms. We conclude that the time course of the recovery cycle of the heteronymous H-reflex in the temporalis muscle reflects the after-hyperpolarization potential (AHP) of trigeminal motoneurones, and that the Ia trigeminal input is integrated with other brainstem reflexes.

Twitch interpolation in human muscles: mechanisms and implications for measurement of voluntary activation

An electrical stimulus delivered to a muscle nerve during a maximal voluntary contraction usually produces a twitchlike increment in force. The amplitude of this "interpolated twitch" is widely used to measure voluntary "activation" of muscles. In the present study, a computer model of the human adductor pollicis motoneuron pool was used to investigate factors that affect the interpolated twitch. Antidromic occlusion of naturally occurring orthodromic potentials was modeled, but reflex effects of the stimulus were not. In simulations, antidromic collisions occurred with probabilities of between approximately 16% (in early recruited motoneurons) and nearly 100% (in late recruited motoneurons). The model closely predicted experimental data on the amplitude and time course of the rising phase of interpolated twitches over the full range of voluntary forces, except that the amplitude of interpolated twitches was slightly overestimated at intermediate contraction intensities. Small interpolated twitches (4.7% of the resting twitch) were evident in simulated maximal voluntary contractions, but were nearly completely occluded when mean peak firing rate was increased to approximately 60 Hz. Simulated interpolated twitches did not show the marked force drop that follows the peak of the twitch, and when antidromic collisions were excluded from the model interpolated twitch amplitude was slightly increased and time-to-peak force was prolonged. These findings suggest that both antidromic and reflex effects reduce the amplitude of the interpolated twitch and contribute to the force drop that follows the twitch. The amplitude of the interpolated twitch was related to "excitation" of the motoneuron pool in a nonlinear way, so that at near-maximal contraction intensities (>90% maximal voluntary force) increases in excitation produced only small changes in interpolated twitch amplitude. Thus twitch interpolation may not provide a sensitive measure of motoneuronal excitation at near-maximal forces. Increases in the amplitude of interpolated twitches such as have been observed in fatigue and various pathologies may reflect large reductions in excitation of the motoneuron pool.

The mixed nerve silent period is prolonged during a submaximal contraction sustained to failure

The purpose of this study was to determine the effect of a sustained contraction of vastus lateralis on the silent period (SP) in the surface electromyogram (EMG) following direct neural stimulation. Five men and 5 women performed isometric knee extension at 30% maximal voluntary contraction (MVC) to the limit of endurance. During the contraction, EMG increased, and superimposed twitch amplitude and time to peak tension decreased, but the SP duration did not change. After 10 min of recovery, MVC had returned to its initial value, and the potentiated twitch amplitude was 70% of initial value, but the SP was now 11% shorter. Based on these results, we hypothesize that during a sustained contraction of 30% MVC, the increase in central drive may have been offset by inhibitory input from the periphery, but after 10 min of recovery the SP was shortened because of increased central drive. This aspect of the SP's behavior should be taken into account whenever it is employed as a diagnostic tool.

Recurrent inhibition in humans

Methods have been developed to investigate recurrent inhibition (RI) in humans. A conditioning reflex discharge is used to evoke in motoneurones (MNs) supplying homonymous and synergistic muscles, an inhibition the characteristics of which are consistent with RI: it appears and increases with the conditioning motor discharge, has a short latency and a long duration, and is enhanced by an agonist of acetylcholine. As in the cat, homonymous RI exists in all explored motor nuclei of the limbs except those of the digits and the pattern of distribution of heteronymous RI closely matches that of monosynaptic Ia excitation. However, striking inter-species differences exist concerning the distribution of heteronymous RI since it is much more widely extended in the human lower limb than in the cat hindlimb, whereas it is more restricted in the upper limb than in the cat forelimb. Changes in transmission in the recurrent pathway have been investigated during various voluntary or postural contractions involving different (homonymous, synergistic, antagonistic) muscles and it has been found that the activation of Renshaw cells (RCs) by the voluntary motor discharge via recurrent collaterals was powerfully controlled by descending tracts: for example, during homonymous contraction, RI evoked by a given conditioning reflex discharge is much smaller during strong than during weak contraction, which suggests that the descending control of RCs might contribute to the regulation of muscle force. The finding that RC inhibition is more marked during phasic than during tonic contraction of similar force of the homonymous muscle is discussed in relation with the projections of RCs to Ia interneurones mediating reciprocal inhibition. Only in patients with progressive paraparesis is there evidence for decreased RI at rest which may contribute to the exaggeration of the passively-induced stretch reflex underlying spasticity. However, despite the seemingly normal RI at rest in most patients, the control of RCs during voluntary movements is disturbed in these patients, which probably contributes to their motor disability.

Increased cortical inhibition induced by apomorphine in patients with Parkinson's disease

The electromyographic silent period following the motor potential evoked by cortical magnetic stimulation is decreased in parkinsonian patients. In this study we investigated whether the decrease in the silent period is connected only with parkinsonian symptoms. We evaluated the effect of apomorphine (a potent and rapid dopamine-agonist) on the changes in the peripheral and central silent period in 29 patients with Parkinson's disease and in two patients affected by multisystem atrophy (MSA). Apomorphine injection was found to induce a significant improvement in the central silent period in parkinsonian patients but not in the MSA patients, suggesting a relation between the clinical parkinsonian symptoms (akinesia and rigidity) and the silent period duration. The central silent period changes after apomorphine injection could be used as an adjunctive, safe and effective diagnostic tool to assess dopamine responsiveness of parkinsonian syndromes.

Older adults can maximally activate the biceps brachii muscle by voluntary command

Because some of the decline in strength with age may be explained by an impairment of muscle activation, the purpose of this study was to determine the activation level achieved in biceps brachii by older adults during a maximum voluntary contraction (MVC). This capability was assessed with two superimposition techniques: one calculated the activation level that was achieved during an MVC, and the other provided an estimate of the expected MVC force based on extrapolation with submaximal forces. The activation level in biceps brachii was incomplete (< 100%) for the young (n = 16) and elderly (n = 16) subjects, with the elderly subjects exhibiting the greater deficit. In contrast, there was no difference between the measured and expected MVC forces for either group of subjects, whether the extrapolation involved a third-order polynomial or linearization of the data. Because of the lower signal-to-noise ratio associated with the measurement of activation level and the greater number of measurements that contributed to the estimate of the expected MVC force, we conclude that the older adults were able to achieve complete activation of the biceps brachii muscle during an MVC.

Influence of stimulus cross talk on results of the twitch-interpolation technique at the biceps brachii muscle

Biceps brachii muscles of five healthy volunteers were tested with a high-resolution twitch-interpolation technique. Parameters of the electrical surface stimulation were varied. It was found that a supramaximal stimulus strength activates both biceps and triceps brachii motor units simultaneously severely affecting twitch-interpolation results. Crosstalk contamination of twitches, however, can be avoided, if submaximal stimuli are used yielding twitch-interpolation results for the biceps-brachii that are similar to those of the quadriceps muscle.

Crossed and direct effects of digital nerves stimulation on motor evoked potential: a study with magnetic brain stimulation

We studied the influence of contralateral and ipsilateral cutaneous digital nerve stimulation on motor evoked potentials (MEPs) elicited in hand muscles by transcranial magnetic stimulation (TMS). We tested the effect of different magnetic stimulus intensities on MEPs recorded from the thenar eminence (TE) muscles of the right hand while an electrical conditioning stimulus was delivered to the second finger of the same hand with an intensity four times above the sensory threshold. Amplitude decrement of conditioned MEPs as a function of magnetic stimulus intensity was observed. The lowest TMS stimulus intensity produced the largest decrease in conditioned MEPs. Moreover, we investigated the effects of ipsilateral and contralateral electrical digital stimulation on MEPs elicited in the right TE and biceps muscle using an intensity 10% above the threshold. Marked MEP inhibition in TE muscles following both ipsilateral and contralateral digital stimulation is the main finding of this study. The decrease in conditioned MEP amplitude to ipsilateral stimulation reached a level of 50% of unconditioned MEP amplitude with the circular coil and 30% with the focal coil. The amplitude of conditioned MEPs to contralateral digital stimulation showed a decrease of 60% with the circular coil and more than 50% with the focal coil. The onset of the inhibitory effect of contralateral stimulation using the focal coil occurred at a mean of 15 ms later than that of ipsilateral stimulation. No MEP inhibition was observed when recording from proximal muscles. Ipsilateral and contralateral digital stimulation had no effect on F wave at appropriate interstimulus intervals, where the main MEP suppression was noted. We stress the importance of selecting an appropriate test stimulus intensity to evaluate MEP inhibition by digital nerves stimulation. Spinal and cortical sites of sensorimotor integration are adduced to explain the direct and crossed MEP inhibition following digital nerves stimulation.

Magnetic transcranial stimulation: clinical interest of the silent period in acute and chronic stages of stroke

There is little information on the silent period during facilitation of the target muscle at the acute stage of stroke and the ultimate clinical status. We studied 69 subjects with transcranial magnetic stimulation: 20 matched controls and 49 hemiparetic patients investigated 7 and 90 days after the stroke (D7, D90). We measured the silent period duration (SPD) in the first dorsal interosseous muscle at 10 and 100% of maximal voluntary isometric contraction (VIC). The SPD index (the ratio of SPD at VIC 100% by SPD at VIC 10%) at D7 was matched with the clinical outcome at D90. Two patterns of responses could be determined at D7. In the normal subjects and in 27 out of 32 patients who eventually recovered satisfactory function at D90, the SPDs were stable during facilitation (SPD index 100%). On the contrary, in 10 out of the 17 patients with a poor functional outcome, the mean SPD decreased when VIC was increased (SPD index 80%); besides, their muscle tone was significantly increased at D90. Similar patterns were still present in the patients at D90: the mean SPD indexes were not significantly different from D7. We conclude that in the early stage of stroke, a low SPD index appears to be correlated with the eventual occurrence of spasticity.

Effects of diazepam, baclofen and thiopental on the silent period evoked by transcranial magnetic stimulation in humans

The cortical silent period evoked by magnetic transcranial stimulation and the peripheral silent period were studied in healthy subjects after intravenous injection of diazepam, baclofen or thiopental. None of the drugs tested changed the peripheral silent period. But, unexpectedly, diazepam significantly shortened the cortical silent period, the inhibitory effect lasting about 30 min. In experiments using paired transcranial stimuli, the conditioning shock inhibited the test response to a similar extent with and without diazepam. Although baclofen did not change the cortical silent period, it reduced the size of the H reflex in the forearm muscles. Thiopental also left the duration of the cortical silent period unchanged. These findings show that the cortical silent period can be modified pharmacologically. Diazepam possibly shortens the silent period by modulating GABA A receptors at a subcortical site.

Facilitation of magnetic motor evoked potentials during the mixed nerve silent period

We studied motor neuron excitability during the mixed nerve silent period (MNSP) in a hand muscle using magnetic motor evoked potentials (MEPs) and F-waves. MEPs elicited between the V1 and V2 potentials of the MNSP were much larger than control MEPs elicited at rest, and were even comparable in size to control MEPs elicited during voluntary contraction. This facilitation of MEPs occurred without shortening of MEP latency, suggesting a supraspinal mechanism. MEPs were facilitated during the MNSP when elicited with a figure-8-shaped coil in a posterior-anterior orientation, but not when MEPs of the same size were elicited with the coil held in a lateral-medial orientation. F-waves elicited during the MNSP were variable between subjects, and not consistently different from control F-waves elicited at rest. Our findings may reflect increased cortical motor excitability during the MNSP, possibly related to activation of muscle afferents by mixed nerve stimulation.

Inhibition of hand muscle motoneurones by peripheral nerve stimulation in the relaxed human subject. Antidromic versus orthodromic input

In active muscle, a supramaximal conditioning stimulus to peripheral nerve produces a classic silent period in the EMG. The present experiments examined the effect of this type of conditioning stimulus on motoneurone excitability in relaxed muscle. EMG responses evoked by transcranial magnetic stimulation of the brain were recorded from the first dorsal interosseus muscle (FDI) in 10 healthy subjects and 5 patients with sensory neuropathy. These responses (motor evoked potentials) were conditioned by supramaximal peripheral nerve stimuli given 0-150 msec beforehand. In the normal subjects, the classic silent period in the FDI lasted about 100 msec. The same conditioning stimulus only abolished motor evoked potentials when the conditioning-test interval was so short that the antidromic peripheral nerve volley collided with the orthodromic volley set up by magnetic brain stimulation. At longer conditioning-test intervals, although remarkably inhibited (65% mean suppression between 10 and 40 msec), the test motor potential was never completely abolished and gradually recovered by 100 msec. Inhibition of cortically evoked motor potentials did not depend upon activity set up by the conditioning stimulus in peripheral nerve sensory fibres. The patients with complete peripheral sensory neuropathy had the same extent and time-course of inhibition as the normal subjects. We conclude that in relaxed subjects the inhibitory effect of peripheral conditioning results almost exclusively from the motoneuronal inhibitory mechanisms consequent to antidromic invasion.

Modulation of the human nociceptive reflex by cyclic movements

During static conditions the nociceptive reflex is known to vary as a function of, for example, the stimulus position, stimulus intensity, and muscle contraction. The aim of the present human study was to investigate whether the reflex and the corresponding perception of pain are modulated by cyclic movements of the limb involved. Reflexes, evoked by nociceptive electric stimulation of the sural nerve, were recorded from the biceps femoris and the rectus femoris muscles in eight volunteers. Four different experiments were performed to compare the nociceptive reflex and pain score elicited during active isometric/dynamic flexion/extension of the knee joint. The amplitudes of the reflexes were largest for the dynamic conditions. The reflexes, evoked during dynamic extension and isometric contraction of the rectus femoris muscle, had the shortest latencies but the recordings from the biceps femoris muscle were larger than from the rectus femoris muscle. Knee joint angle recordings showed that the largest angle variations occurred for the dynamic conditions and were only marginally disturbed for the isometric conditions. A given stimulus intensity evoked the highest pain intensity during isometric contractions. This indicates that there would seem to be no causal relationship between the size of the nociceptive reflex and the pain intensity.

Conduction abnormalities detected by silent period testing

Cutaneous and mixed nerve silent periods (SPs) were analyzed in 2 patients with pure sensory neuropathy and in 5 healthy controls. In patients, sensory nerve action potentials (SNAPs) and somatosensory evoked potentials (SEPs) were absent, but digital and mixed nerve stimulation evoked complete SPs in voluntarily contracting thenar muscles. Marked differences in SP latencies were found between the patients, despite shared diagnoses and virtually identical nerve conduction studies. When compared with controls, SPs were normal in the first patient and markedly delayed in the second. Such findings suggest that SPs can identify conduction abnormalities not detected by routine studies. The pathologic correlate to these abnormalities remains uncertain, although electrophysiologic data suggest that the smaller, slower conducting delta fibers carry the impulses that generate these SPs.

The cortical silent period and amyotrophic lateral sclerosis

The cortical silent period (C-SP) was elicited by transcranial magnetic stimulation in 25 normal subjects and 19 patients with amyotrophic lateral sclerosis (ALS). The inhibitory (S-X) period was highly stimulus intensity (SI)-dependent (mean r2 = 0.89 for both normals and patients with ALS). The range of the C-SP (difference between maximum and minimum S-X intervals) was age-dependent for normals (r2 = 0.701, P < 0.001) but not patients with ALS. Means, maximums and ranges for the C-SP were not significantly different between normal and ALS groups and thresholds to cortical stimulation were also comparable. There was a significant, linear, relation between the maximum C-SP and disease duration of ALS (P = 0.002). The maximum C-SP was shorter early in the disease. It is hypothesized that the reduced inhibition early in the course of ALS might reflect glutamate-induced corticomotoneuronal excitotoxicity.

Modulation of postural tremors at the wrist by supramaximal electrical median nerve shocks in essential tremor, Parkinson's disease and normal subjects mimicking tremor

The response of postural wrist tremors to supramaximal median nerve stimulation was examined in patients with hereditary essential tremor (n = 10) and Parkinson's disease (n = 9), and in normal subjects mimicking wrist tremor (n = 8). The average frequency of on-going tremor was the same in all three groups. Supramaximal peripheral nerve shocks inhibited and then synchronised the rhythmic electromyographic (EMG) activity of all types of tremor. The duration of inhibition ranged from 90 to 210ms, varying inversely with the frequency of on-going tremor. There was no significant difference in mean duration of inhibition or in the timing of the first peak after stimulation on the average rectified EMG records between the three groups. The degree to which supramaximal peripheral nerve shocks could modulate the timing of rhythmic EMG bursts in the forearm flexor muscles was also quantified by deriving a resetting index. No significant difference in mean resetting index of the three groups was found. These results suggest that such studies cannot be used to differentiate between the common causes of postural wrist tremors.

Silent period evoked by transcranial stimulation of the human cortex and cervicomedullary junction

1. The silent period evoked in the first dorsal interosseous (FDI) muscle after electrical and magnetic transcranial stimulation (TCS), electrical stimulation of the cervicomedullary junction and ulnar nerve stimulation was studied in ten healthy subjects. 2. With maximum-intensity shocks, the average duration of the silent period was 200 ms after electrical TCS, 300 ms after magnetic TCS, 43 ms after stimulation at the cervicomedullary junction and 100 ms after peripheral nerve stimulation. 3. The duration of the silent period, the amplitude of the motor-evoked potential, and the twitch force produced in the muscle were compared at increasing intensities of magnetic TCS. When the stimulus strength was increased from 30 to 70% of the stimulator output, the duration of the silent period lengthened as the amplitude of the motor potential and force of the muscle twitch increased. At 70 to 100% of the output, the amplitude of the motor potential and force of the muscle twitch saturated, whereas the duration of the silent period continued to increase. 4. Proximal arm muscle twitches induced by direct electrical stimulation of the biceps and extensor wrist muscles produced no inhibition of voluntary activity in the contracting FDI muscle. 5. The level of background activation had no effect on the duration of the silent period recorded in the FDI muscle after magnetic TCS. 6. Corticomotoneurone excitability after TCS was studied by means of a single magnetic conditioning shock and a test stimulus consisting either of one single magnetic shock or single and double electrical shocks (interstimulus interval 1.8 ms) in the relaxed muscle. A conditioning magnetic shock completely suppressed the response evoked by a second magnetic shock, reduced the size of the response evoked by a single electrical shock but did not affect the response evoked by double electrical shocks. Inhibition of the test magnetic shock was also present during muscle contraction. 7. Our findings indicate that the first 50 ms of the silent period after TCS are produced mainly by spinal mechanisms such as after-hyperpolarization and recurrent inhibition of the spinal motoneurones. If descending inhibitory fibres contribute, their contribution is small. Changes in proprioceptive input probably have a minor influence. From 50 ms onwards the silent period is produced mainly by cortical inhibitory mechanisms.

Relationship between stimulus strength and the cutaneous silent period

During sustained muscle contraction, an interval of reduced activity follows an electrical cutaneous stimulus, called the cutaneous silent period (CSP). To evoke a CSP, a single stimulus must be painful. We used single sural nerve stimuli to evoke a CSP in ipsilateral soleus muscle, and studied the relationships between stimulus strength, sensory action potential (SAP) morphology, and subjective experience. Near nerve electrodes were employed to record the sural SAP in order to record activity in slower conducting fibers in addition to A alpha fibers. In 6 normal subjects, the stimulus strength required to evoke a CSP ranged from 8 to 10 times threshold intensity. Pain threshold was slightly below that necessary to evoke the CSP. SAP shape changed with stimulus strength; main component amplitude occasionally increased as strength increased beyond 10 times threshold, and slowly conducting late components became more prominent. At stimulus intensities or at less than CSP threshold, components were seen conducting from 15-20 m/s that were not observed at lower intensities. We suggest that activation of sensory axons with conduction velocities in the range of A delta fibers are necessary to evoke the CSP, and that their potentials can be discerned in the SAP.

The muscle silent period following transcranial magnetic cortical stimulation

Transcranial magnetic stimulation (TMS) of the motor cortex during tonic muscle contraction produces a motor evoked potential followed by a silent period in the electromyogram. We sought to characterize the TMS induced silent period and to compare it to the silent period induced by supramaximal peripheral nerve stimulation. TMS was delivered to the motor cortex using a 9 cm diameter circular coil and the surface electromyogram was recorded from the contralateral abductor pollicis brevis muscle in six normal subjects. Increasing TMS stimulus intensity from 10 to 50% above threshold resulted in an increase in the duration of the silent period from a mean of 50 ms to 185 ms. Increasing the level of tonic muscle contraction from 5% of maximum to maximum resulted in a decrease in silent period duration from a mean of 155 ms to 133 ms. In contrast, the duration of the silent period following supramaximal median nerve stimulation showed greater shortening under similar conditions, from a mean of 160 ms at 5% of maximum contraction to 99 ms at 75% of maximum contraction. The TMS induced silent period was present during a TMS induced increase in the reaction time for a ballistic movement, the onset of movement being delayed until the end of the silent period. Peripheral nerve stimulation did not produce a delay in movement onset. The present findings favour a cortical origin for the TMS induced silent period, probably on the basis of intracortical inhibition, rather than peripheral inhibition of spinal motoneurones which is considered to be the basis for the silent period following peripheral nerve stimulation.