The origin of activity in the biceps brachii muscle during voluntary contractions of the contralateral elbow flexor muscles

During strong voluntary contractions, activity is not restricted to the target muscles. Other muscles, including contralateral muscles, often contract. We used transcranial magnetic stimulation (TMS) to analyse the origin of these unintended contralateral contractions (termed “associated” contractions). Subjects (n = 9) performed maximal voluntary contractions (MVCs) with their right elbow-flexor muscles followed by submaximal contractions with their left elbow flexors. Electromyographic activity (EMG) during the submaximal contractions was matched to the associated EMG in the left biceps brachii during the right MVC. During contractions, TMS was delivered to the motor cortex of the right or left hemisphere and excitatory motor evoked potentials (MEPs) and inhibitory (silent period) responses recorded from left biceps. Changes at a spinal level were investigated using cervicomedullary stimulation to activate corticospinal paths (n = 5). Stimulation of the right hemisphere produced silent periods of comparable duration in associated and voluntary contractions (218 vs 217 ms, respectively), whereas left hemisphere stimulation caused a depression of EMG but no EMG silence in either contraction. Despite matched EMG, MEPs elicited by right hemisphere stimulation were ∼1.5–2.5 times larger during associated compared to voluntary contractions (P < 0.005). Similar inhibition of the associated and matched voluntary activity during the silent period suggests that associated activity comes from the contralateral hemisphere and that motor areas in this (right) hemisphere are activated concomitantly with the motor areas in the left hemisphere. Comparison of the MEPs and subcortically evoked potentials implies that cortical excitability was greater in associated contractions than in the matched voluntary efforts.     

Activity patterns of the diaphragm during voluntary movements in awake cats

The diaphragm is an important inspiratory muscle, and is also known to participate in the postural function. However, the activity of the diaphragm during voluntary movements has not been fully investigated in awake animals. In order to investigate the diaphragmatic activity during voluntary movements such as extending or rotating their body, we analyzed the electromyogram (EMG) of the diaphragm and trunk muscles in the cat using a technique for simultaneous recordings of EMG signals and video images. Periodic respiratory discharges occurred in the left and right costal diaphragm when the cat kept still. However, once the cat moved, their periodicity and/or synchrony were sometimes buried by non-respiratory activity. Such non-periodic diaphragmatic activities during voluntary movements are considered as the combination of respiratory activity and non-respiratory activity. Most of the diaphragmatic activities started shortly after the initiation of standing-up movements and occurred after the onset of trunk muscle activities. Those activities were more active compared to the normal respiratory activity. During rotation movements, left and right diaphragmatic activities showed asymmetrical discharge patterns and higher discharges than those during the resting situation. This asymmetrical activity may be caused by taking different lengths of each side of the diaphragm and trunk muscles. During reaching movements, the diaphragmatic activity occurred prior to or with the onset of trunk muscle activities. It is likely that diaphragmatic activities during reaching movements and standing-up movements may have been controlled by some different control mechanisms of the central nervous system. This study will suggest that the diaphragmatic activity is regulated not only by the respiratory center but also by inputs from the center for voluntary movements and/or sensory reflex pathways under the awake condition.     

Effects of combined cortical and acoustic stimuli on muscle activity

Hitherto, it has proven difficult to investigate interactions between cerebral and brainstem motor systems in the human. We hypothesised that transcranial magnetic stimulation (TMS) centred over the dorsal premotor and primary motor cortices might elicit net facilitatory cortico-reticular effects that could interact at the level of the brainstem with a habituated startle to give a reticulospinal discharge and electromyographic (EMG) response with a longer latency than the direct corticospinal response. Conversely, any reticulo-cortical activity evoked by a habituated startle should influence the size of the direct response to cortical TMS. EMG was recorded from active left deltoid muscle in nine healthy volunteers. Acoustic stimulation was delivered binaurally through headphones and repeated until the startle response was habituated. When TMS was centred over the right dorsal premotor or primary motor cortices and delivered 50 ms after the habituated acoustic stimulus, the contralateral direct motor evoked potential was inhibited, compared with the response elicited by TMS alone. The contralateral silent period was shortened and associated with less of a decrease in EMG levels relative to TMS alone. Indeed, an actual increase in EMG over baseline levels occurred in the later half of the silent period in all subjects. We conclude that both cortico-reticular and reticular-cortical effects could be elicited in deltoid through the combination of acoustic stimulation and TMS at short interstimulus intervals. Effects were similar with TMS over premotor and primary motor cortex.     

Corticospinal disinhibition during dual action

When attempting to perform two tasks simultaneously, the human motor as well as the cognitive system shows interference. Such interference often causes altered activation of the cortical area representing each task compared to the single task condition. We investigated changes in corticospinal inhibition during dual action by transcranial magnetic stimulation (TMS). Single-pulse TMS was applied to the left motor cortex, triggered by right leg movement (tibialis anterior muscle) while the right abductor digiti minimi (ADM) muscle was moderately activated (10–20% of the maximal voluntary contraction). The background electromyography (EMG) activity of ADM was measured before and during the leg movement. The silent period (SP) and amplitude of motor evoked potential (MEP) following magnetic stimulation in active ADM were compared for the conditions with and without leg movement. The mean area of the rectified EMG activity of ADM did not alter, while the SP was significantly shortened during leg movement compared to that without leg movement. MEP amplitude was comparable between the two conditions. These results suggest that corticospinal inhibition tested by the SP duration is reduced during the movement of another body part, presumably in order to help maintain muscle force by compensating interference-related alteration in motor cortical activation.     

Is the long-latency stretch reflex in human masseter transcortical?

A long-latency stretch reflex (LLSR) has been described in the human masseter muscle, but its pathway remains uncertain. To investigate this, the excitability of corticomotoneuronal (CM) cells projecting to masseter motoneurons during the LLSR was assessed with transcranial magnetic stimulation (TMS). A facilitated response to TMS would be evidence of a LLSR pathway that traverses the motor cortex. Surface electromyogram electrodes were placed over the left or right masseter, and subjects (n=10) bit on bars with their incisor teeth at 10% of maximal electromyographic activity (EMG). Servo-controlled displacements were imposed on the lower jaw to evoke a short- and long-latency stretch reflex in masseter. TMS intensity was just suprathreshold for a response in contralateral masseter. Trials consisted of: (1) stretch alone, (2) TMS alone, and (3) TMS with a preceding conditioning stretch at varied conditioning-testing (C-T) intervals chosen to combine TMS with the short-latency stretch reflex (3 ms, 5 ms) and the LLSR (23–41 ms). Masseter EMG was rectified and averaged. With TMS alone, mean (± SE) MEP area above baseline was 56±9%. The area of masseter MEPs above baseline in the C-T trials was calculated from each EMG average following subtraction of the response to stretch alone. Conditioning muscle stretch had no significant effect on masseter MEPs evoked by TMS with any C-T interval (ANOVA; P=0.90). In addition, subjects were unable to modify the SLSR or LLSR by voluntary command. It is concluded that the long-latency stretch reflex in the masseter does not involve the motor cortex and is not influenced by "motor set". Electronic Publication     

The excitability of human cortical inhibitory circuits responsible for the muscle silent period after transcranial brain stimulation

The silent period after transcranial magnetic brain stimulation mainly reflects the activity of inhibitory circuits in the human motor cortex. To assess the excitability of the cortical inhibitory mechanisms responsible for the silent period after transcranial stimulation, we studied, in 15 healthy human subjects, the recovery cycle of the silent period evoked by transcranial and mixed nerve stimulation delivered with a paired stimulation technique. The recovery cycle is defined as the time course of the changes in the size or duration of a conditioned test response when pairs of stimuli (conditioning and test) are used at different conditioning-test intervals. The recovery cycle of the duration of the silent period in the first dorsal interosseous (FDI) muscle during maximum voluntary contraction after transcranial magnetic stimulation was studied by delivering paired magnetic shocks (a conditioning shock and a test shock) at 120% motor-threshold intensity. Conditioning-test intervals ranged from 20-550 ms. The recovery cycle of the silent period in the FDI muscle during maximum voluntary contraction after nerve stimulation was evaluated by paired, supramaximum bipolar electrical stimulation of the ulnar nerve at the wrist (conditioning-test intervals ranging from 20 to 550 ms). Electromyographic activity was recorded by a pair of surface-disk electrodes over the FDI muscle. The recovery cycle of the silent period after transcranial magnetic stimulation delivered through the large round coil showed two phases of facilitation (lengthening of the silent period), one at 20-40 ms and the other at 180-350 ms conditioning-test intervals, with an interposed phase of inhibition (shortening of the silent period) at 80-160 ms. The conditioning magnetic shock left the size of the test motor-evoked potentials statistically unchanged during maximum voluntary contraction. Paired transcranial stimulation with a figure-of-eight coil increased the duration of the test silent period only at short conditioning-test intervals. Conditioning nerve stimulation left the silent period produced by test nerve stimulation unchanged. In conclusion, after a single transcranial magnetic shock, inhibitory circuits in the human motor cortex undergo distinctive short-term changes in their excitability, probably involving different mechanisms.     

Discharge pattern of tonically activated motor units during unloading

In order to analyse the EMG pattern during unloading of brachial biceps muscle, the interference EMG and single motor unit activity were investigated. The measurements were done on seven healthy subjects with two types of unloading techniques: a) active unloading, when the subjects resisted against an external load (10, 20, 30 and 40 N) which is suddenly released, and b) passive unloading, performed by low inertia torque motors with independently adjustable background extension and suddenly applied flexion torques. Following active unloading the silent period duration, the amplitude of the rebound and its segmentation into consecutive bursts is changing with initial load, whereas the silent period latency remains constant. Following passive unloading the acceleration influences predominantly the amplitude of the rebound, without changing its latency and silent period duration. The initial voluntary activity influences both silent period duration and rebound parameters (latency, amplitude and duration).     

Comparison of motor effects following subcortical electrical stimulation through electrodes in the globus pallidus internus and cortical transcranial magnetic stimulation

Current concepts of transcranial magnetic stimulation (TMS) over the primary motor cortex are still under debate as to whether inhibitory motor effects are exclusively of cortical origin. To further elucidate a potential subcortical influence on motor effects, we combined TMS and unilateral subcortical electrical stimulation (SES) of the corticospinal tract. SES was performed through implanted depth electrodes in eight patients treated with deep brain stimulation (DBS) for severe dystonia. Chronaxie, conduction velocity (CV) of the stimulated fibres and poststimulus time histograms of single motor unit recordings were calculated to provide evidence of an activation of large diameter myelinated fibres by SES. Excitatory and inhibitory motor effects recorded bilaterally from the first dorsal interosseus muscle were measured after SES and focal TMS of the motor cortex. This allowed us to compare motor effects of subcortical (direct) and cortical (mainly indirect) activation of corticospinal neurons. SES activated a fast conducting monosynaptic pathway to the alpha motoneuron. Motor responses elicited by SES had significantly shorter onset latency and shorter duration of the contralateral silent period compared to TMS induced motor effects. Spinal excitability as assessed by H-reflex was significantly reduced during the silent period after SES. No ipsilateral motor effects could be elicited by SES while TMS was followed by an ipsilateral inhibition. The results suggest that SES activated the corticospinal neurons at the level of the internal capsule. Comparison of SES and TMS induced motor effects reveals that the first part of the TMS induced contralateral silent period should be of spinal origin while its later part is due to cortical inhibitory mechanisms. Furthermore, the present results suggest that the ipsilateral inhibition is predominantly mediated via transcallosal pathways.This paper is dedicated to Bernd-Ulrich Meyer, who died in a plane accident     

Changes in presumed motor cortical activity during fatiguing muscle contraction in humans

Aim: Changes in sensory information from active muscles accompany fatiguing exercise and the force-generating capacity deteriorates. The central motor commands therefore must adjust depending on the task performed. Muscle potentials evoked by transcranial magnetic stimulation (TMS) change during the course of fatiguing muscle activity, which demonstrates activity changes in cortical or spinal networks during fatiguing exercise. Here, we investigate cortical mechanisms that are actively involved in driving the contracting muscles. Methods: During a sustained submaximal contraction (30% of maximal voluntary contraction) of the elbow flexor muscles we applied TMS over the motor cortex. At an intensity below motor threshold, TMS reduced the ongoing muscle activity in biceps brachii. This reduction appears as a suppression at short latency of the stimulus-triggered average of rectified electromyographic (EMG) activity. The magnitude of the suppression was evaluated relative to the mean EMG activity during the 50 ms prior to the cortical stimulus. Results: During the first 2 min of the fatiguing muscle contraction the suppression was 10 ± 0.9% of the ongoing EMG activity. At 2 min prior to task failure the suppression had reached 16 ± 2.1%. In control experiments without fatigue we did not find a similar increase in suppression with increasing levels of ongoing EMG activity. Conclusion: Using a form of TMS which reduces cortical output to motor neurones (and disfacilitates them), this study suggests that neuromuscular fatigue increases this disfacilitatory effect. This finding is consistent with an increase in the excitability of inhibitory circuits controlling corticospinal output.     

Differential effects of low-intensity motor cortical stimulation on the inspiratory activity in scalene muscles during voluntary and involuntary breathing

To assess the cortical contribution to breathing, low-intensity transcranial magnetic stimulation (TMS) was delivered over the motor cortex in 10 subjects during: (i) voluntary static inspiratory efforts, (ii) hypocapnic voluntary ventilation (end-tidal CO(2), 2.7±0.4% mean±SD), and (iii) hypercapnic involuntary ventilation (end-tidal CO(2), 6.0±0.7%). Electromyographic activity (EMG) was recorded from the scalene muscles (obligatory inspiratory muscles) and was significantly suppressed by TMS at short latency (17.2±1.7ms). The scalene EMG was reduced to 76±8% and 76±7% in voluntary breathing and the static inspiratory effort, respectively, but only to 91±10% during the involuntary ventilation, significantly less than during the two voluntary tasks (p<0.005). Thus, with differences in chemical drive to breathe, TMS shows differences in the cortical contribution to inspiratory activity in scalene muscles. Voluntary breathing showed larger suppression than involuntary breathing, when the suppression was marginal. The results strongly suggest that drive from fast-conducting corticospinal neurones contributes to inspiratory activity in scalenes during voluntary breathing but is not required during involuntary breathing.     

Motor control of the diaphragm in anesthetized rabbits

Diaphragmatic regions are recruited in a specialized manner either as part of a central motor program during non-respiratory maneuvers, e.g. vomiting, or because of reflex responses, e.g. esophageal distension. Some studies in cats and dogs suggest that crural and costal diaphragm may be differentially activated also in response to respiratory stimuli from chemoreceptors or lung and chest wall mechanoreceptors. To verify whether this could occur also in other species, the EMG activity from the sternal, costoventral, costodorsal, and crural diaphragm was recorded in 42 anesthetized rabbits in response to various respiratory maneuvers, such as chemical stimulation, mechanical loading, lung volume and postural changes before and after vagotomy, or a non-respiratory maneuver such as esophageal distension. Regional activity was evaluated from timing of the raw EMG signal, and amplitude and shape of the moving average EMG. In all animals esophageal distension caused greater inhibition of the crural than sternal and costal diaphragm, whereas under all the other conditions differential diaphragmatic activation never occurred. These results indicate that in response to respiratory stimuli the rabbit diaphragm behaves as a single unit under the command of the central respiratory control system.     

Demonstration of a second rapidly conducting cortico-diaphragmatic pathway in humans

Functional imaging studies in normal humans have shown that the supplementary motor area (SMA) and the primary motor cortex (PMC) are coactivated during various breathing tasks. It is not known whether a direct pathway from the SMA to the diaphragm exists, and if so what properties it has. Using transcranial magnetic stimulation (TMS) a site at the vertex, representing the diaphragm primary motor cortex, has been identified. TMS mapping revealed a second area 3 cm anterior to the vertex overlying the SMA, which had a rapidly conducting pathway to the diaphragm (mean latency 16.7 ± 2.4 ms). In comparison to the vertex, the anterior position was characterized by a higher diaphragm motor threshold, a greater proportional increase in motor-evoked potential (MEP) amplitude with voluntary facilitation and a shorter silent period. Stimulus–response curves did not differ significantly between the vertex and anterior positions. Using paired TMS, we also compared intracortical inhibition/facilitation (ICI/ICF) curves. In comparison to the vertex, the MEP elicited from the anterior position was not inhibited at short interstimulus intervals (1–5 ms) and was more facilitated at long interstimulus intervals (9–20 ms). The patterns of response were identical for the costal and crural diaphragms. We conclude that the two coil positions represent discrete areas that are likely to be the PMC and SMA, with the latter wielding a more excitatory effect on the diaphragm.     

Movement-related cortical activation with voluntary pinch task: simultaneous monitoring of near-infrared spectroscopy signals and movement-related cortical potentials

This study was designed to evaluate hemodynamic and electrophysiological motor cortex responses to voluntary finger pinching in humans, with simultaneous recording of near-infrared spectroscopy (NIRS) signals and movement-related cortical potentials (MRCP). Six healthy, right-handed subjects performed 100 trials of voluntary right-thumb index-finger pinching with about a 10-second interval at their own pace. Throughout the session, 48 regions over the bilateral motor cortex were assessed by NIRS, while MRCP and electromyogram (EMG) were simultaneously monitored. MRCP started 1536±58 ms before EMG onset and peaked 127±24 ms after EMG onset. NIRS data showed bilateral prefrontal cortex at 0.5±0.1 s before EMG onset and bilateral dorsal premotor cortex activations at 0.6±0.1 s before EMG onset. The hand area of the sensorimotor cortex was activated left-dominantly, seen obviously peaked at 3.7±0.2 s after EMG onset. The comparison between MRCP and NIRS results raised the possibility that the vascular response to neural activity occurs within 4 s with a voluntary pinch task. These results indicate that our technique allows detailed study of the motor control. Our method is a promising strategy for event-related motor control and neurovascular coupling studies.     

Electrical stimulation of the human common peroneal nerve elicits lasting facilitation of cortical motor-evoked potentials

Motor-evoked potentials (MEP) in the tibialis anterior (TA) muscle were shown to be facilitated by repetitive electrical stimulation of the common peroneal (CP) nerve at intensities above motor threshold. The TA electromyogram (EMG) and ankle flexion force were recorded in response to transcranial magnetic stimulation (TMS) of the leg area of the motor cortex to evaluate the excitability of cortico-spinal-muscular pathways. Repetitive stimulation of the CP nerve at 25 Hz for 30 min increased the MEP by 50.3 ± 13.6% (mean ± S.E.) at a TMS intensity that initially gave a half-maximum MEP (MEP_h). In contrast the maximum MEP (MEP_max) did not change. Ankle flexion force (103 ± 21.9%) and silent period duration (75.3 ± 12.9%) also increased. These results suggest an increase in corticospinal excitability, rather than total connectivity due to repetitive CP stimulation. Facilitation was evident after as little as 10 min of stimulation and persisted without significant decrement for at least 30 min after stimulation. The long duration of silent period following CP stimulation (99.2 ± 14.8 ms) suggests that this form of stimulation may have effects on the motor cortex. To exclude the possibility that MEP_h facilitation was primarily due to sensory fibre activation, we performed several control experiments. Preferentially activating Ia muscle afferents by vibration in the absence of motor activity had no significant effect. Cutaneous afferent activation via stimulation of the superficial peroneal nerve increased the amplitude of responses at MEP_max rather than MEP_h. Concurrent tendon vibration and superficial peroneal nerve stimulation failed to facilitate TA MEP responses. In summary, repetitive electrical stimulation of the CP nerve elicits lasting changes in corticospinal excitability, possibly as a result of co-activating motor and sensory fibres.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Modulation of associative human motor cortical plasticity by attention

The role of attention in generating motor memories remains controversial principally because it is difficult to separate the effects of attention from changes in kinematics of motor performance. We attempted to disentangle attention from performance effects by varying attention while plasticity was induced in human primary motor cortex by external stimulation in the absence of voluntary movement. A paired associative stimulation (PAS) protocol was employed consisting of repetitive application of single afferent electric stimuli, delivered to the right median nerve, paired with single-pulse transcranial magnetic stimulation (TMS) over the optimal site for activation of the right abductor pollicis brevis muscle (APB) to generate near-synchronous events in the left primary motor cortex. In experiment 1, the spatial location of attention was varied. PAS failed to induce plasticity when the subject's attention was directed to their left hand, away from the right target hand the cortical representation of which was being stimulated by PAS. In experiment 2, the grade of attention to the target hand was manipulated. PAS-induced plasticity was maximal when the subject viewed their target hand, and its magnitude was slightly reduced when the subject could only feel their hand. Conversely, plasticity was completely blocked when the subject's attention was diverted from the target hand by a competing cognitive task. A similar modulation by attention was observed for PAS-induced changes in the duration of the silent period evoked by TMS in voluntarily contracted muscle. Associative plasticity in the human motor cortex depends decisively on attention.     

The effect of a contralateral contraction on maximal voluntary activation and central fatigue in elbow flexor muscles

A long-duration, submaximal contraction of a hand muscle increases central fatigue during a subsequent contraction in the other hand. However, this 'cross-over' of central fatigue between limbs is small and the location within the central nervous system at which this effect occurs is unknown. We investigated this 'cross-over' by measurement of the force and EMG responses to transcranial magnetic stimulation of the motor cortex (TMS). To produce central fatigue, we used sustained maximal voluntary contractions (MVCs). In the first study, subjects (n=10) performed four 1-min sustained MVCs of the elbow flexors, alternating between the left and right arms (two MVCs per arm). The sustained MVCs were performed consecutively with no rest periods. In the second study, the same subjects made two sustained 1-min MVCs with the same arm with a 1-min rest between efforts. During each sustained MVC, a series of TMS and brachial plexus stimuli were delivered. Surface EMG was recorded from biceps brachii and brachioradialis muscles bilaterally. Voluntary activation was estimated during each MVC using measurement of the force increments to TMS. On average during each sustained MVC, voluntary activation declined by 7–12% (absolute change, P<0.001) and voluntary force declined by 35–45% MVC (P<0.001), whereas the cortical motor-evoked potential increased (P<0.001) and the subsequent silent period lengthened (P<0.001). The average voluntary activation and voluntary force were similar during two sustained MVCs performed by the same arm, when separated by 1 min of rest. However, when the 1-min rest interval was replaced with a sustained contraction performed by the other arm, the average voluntary activation was 2.9% worse in the second contraction (absolute change, P<0.05), while it did not alter voluntary force production or the EMG responses to TMS. Therefore, in maximal exercise of 4 min duration, the 'cross-over' of central fatigue between limbs is small in the elbow flexors and has a minor functional effect. Our data suggest that voluntary drive from the motor cortex is slightly less able to drive the muscle maximally after a fatiguing voluntary contraction on the contralateral side. Electronic Publication     

Stability of corticospinal excitability and grip force in intrinsic hand muscles in man over a 24-h period

The maximum voluntary muscle force can vary throughout the day; typically being low in the morning and high in the evening. The nature of this possible variation has been investigated with respect to corticospinal excitability. Six healthy subjects were studied. Maximum voluntary contraction (MVC) in the thenar muscles was measured. In addition, we monitored several indices of corticospinal excitability using electromyographic (EMG) recording and transcranial magnetic stimulation (TMS) of the motor cortex. Motor evoked potentials (MEPs) were recorded while relaxed and at 10% MVC when the silent period was assessed as an index of corticospinal inhibition. Readings were taken every 3 h for 24 h. MVC of the thenar muscles did not change significantly over the 24 h. The mean areas, latencies and durations of MEPs did not show significant changes over the 24-h test period with the muscle relaxed or contracted; however, MEP area did vary between sessions at all stimulus intensities suggesting non-time-of-day-dependent changes in corticospinal excitability. Furthermore, the extent and duration of the silent period seen after the MEP in the contracted muscle did not change significantly over the 24 h of the experiment at any stimulus intensity. These results provide evidence that the MVC force of the thenar muscles and their responses to TMS are stable throughout the course of the day and suggest that, in hand muscles, corticospinal excitability may not be subject to circadian variation.     

Role of the diaphragm in trunk rotation in humans

The objectives of the present study were to test the hypothesis that the costal diaphragm contracts during ipsilateral rotation of the trunk and that such trunk rotation increases the motor output of the muscle during inspiration. Monopolar electrodes were inserted in the right costal hemidiaphragm in six subjects, and electromyographic (EMG) recordings were made during isometric rotation efforts of the trunk to the right ("ipsilateral rotation") and to the left ("contralateral rotation"). EMG activity was simultaneously recorded from the parasternal intercostal muscles on the right side. The parasternal intercostals were consistently active during ipsilateral rotation but silent during contralateral rotation. In contrast, the diaphragm was silent in the majority of rotations in either direction, and whenever diaphragm activity was recorded, it involved very few motor units. In addition, whereas parasternal inspiratory activity substantially increased during ipsilateral rotation and decreased during contralateral rotation, inspiratory activity in the diaphragm was essentially unaltered and the discharge frequency of single motor units in the muscle remained at 13-14 Hz in the different postures. It is concluded that 1) the diaphragm makes no significant contribution to trunk rotation and 2) even though the diaphragm and parasternal intercostals contract in a coordinated manner during resting breathing, the inspiratory output of the two muscles is affected differently by voluntary drive during trunk rotation.     

Excitability of the ipsilateral motor cortex during phasic voluntary hand movement

We investigated the influence of self-paced, phasic voluntary hand movement on the excitability of the ipsilateral motor cortex. Single- and paired-pulse transcranial magnetic stimulation (TMS) was applied to the right motor cortex triggered by EMG onset of self-paced movements of individual right hand fingers at intervals ranging from 13 to 2,000 ms. Motor evoked potentials (MEPs) were evaluated in several left arm muscles. Significant suppression of MEP amplitudes was observed when TMS was applied between 35 and 70 ms after EMG onset. This inhibition was diffuse, affecting "adjacent" muscles (those near the homologous muscle in the same extremity) as well as homologous muscles, but more inhibition was observed in adjacent and distal muscles than homologous and proximal muscles. Significant inhibition of ipsilateral motor cortex was produced by index finger movements (both the extensor indicis proprius and the first dorsal interosseus), but not by little finger movement (the abductor digiti minimi). Paired-pulse TMS (at 2- and 10-ms interstimulus intervals) showed a significant increase in intracortical facilitation (ICF) selectively in the homologous muscle when triggered by self-paced movement of the opposite hand, but no change was observed in intracortical inhibition. When stimulation was triggered by self-paced movements, the silent period of the homologous muscle was significantly shortened, but the F-wave and compound muscle action potential were unchanged. Our findings demonstrate that voluntary hand movement exerts an inhibitory influence on a diffuse area of the ipsilateral motor cortex. This inhibitory influence is both time and movement dependent. The inhibitory influence is nonselective, while the facilitatory influence (enhancing ICF) appears to act selectively on the homologous muscles. These effects are most likely mediated by a transcallosal pathway. Electronic Publication