Motor-unit responses in human wrist flexor and extensor muscles to transcranial cortical stimuli

Further observations on the facilitation of muscle responses to cortical stimulation by voluntary contraction

The effect of voluntary contraction on the discharge of single motor units following electrical and magnetic stimulation of the motor cortex was examined using the post-stimulus time histogram (PSTH) technique. The latencies of responses in single motor units of the first dorsal interosseous muscle to cortical stimulation were 2-4 msec shorter when the muscle was contracting than when at rest in 9 of 10 units studied. These latency differences are comparable with those recorded by surface electromyography for compound muscle action potentials following cortical stimulation in relaxed and active muscles. The new findings are that the intensity of cortical stimulation required to discharge a resting motor unit to produce a single PSTH peak produced multiple PSTH peaks when the same unit was contracting. The timing of the PSTH peak of relaxed motor unit discharge corresponded to one of the later PSTH peaks (usually the second) when the motor unit was voluntarily activated. These findings are in keeping with our previous suggestions that the longer latency of responses in relaxed muscles is due to the time taken for temporal summation of multiple descending corticospinal volleys at the cortico-motoneurone synapse. Facilitation produced by voluntary contraction occurs at least in part at the level of the spinal cord by lowering motoneurone threshold to enable discharge on the initial descending volley. The higher threshold of relaxed muscles is related to the higher intensities of stimulation needed to recruit multiple descending volleys and discharge resting motoneurones.

Magnetic brain stimulation: a tool to explore the action of the motor cortex on single human spinal motoneurones

The human brain can be stimulated by a single intense magnetic pulse over the scalp. Currents induced within the cranium excite the motor cortex and cause limb muscles to contract. The discharge of single motor units, the firing of which is maintained by voluntary effort, can be modulated by magnetic stimuli. Peri-stimulus time histograms suggest that after a cortical stimulus spinal motoneurones are induced to fire by a sequence of EPSPs arising from a train of impulses transmitted monosynaptically over fast-conducting corticospinal fibres. In multiple sclerosis both dispersion of this descending volley and partial transmission failure can impair motoneurone excitation and may explain motor symptoms in these patients.

Spinal motor neuron excitability during the silent period after cortical stimulation

During tonic voluntary muscle contraction, a period of electromyographic silence follows the motor evoked potential produced by transcranial stimulation of the contralateral motor cortex. We studied the silent period in the wrist flexors of 3 normal volunteers and a deafferented patient during 20% of maximal contraction. To test the excitability of the spinal motor neuron pool during the period of silence, the H-reflex was evoked in the normal subjects at different intervals after cortical stimulation. The amplitude of the H-reflex in the silent period was expressed as a percentage of the amplitude during complete muscle relaxation. The H-reflex was profoundly depressed at the beginning of the silent period (13.5-27% of the control measurement), but showed a clear tendency to recover toward the end of the silent period despite continued absence of muscle activation (71-84% of the control). Moreover, the silent period in the deafferented patient was of longer duration than can be accounted for by segmental mechanisms. These findings imply, at least in the late part of the silent period, that a reduction in the excitability of the spinal motor neuron pool plays only a minor role in determining the phenomenon and that it is probably caused by lack of cortical drive.

Isoflurane-induced attenuation of motor evoked potentials caused by electrical motor cortex stimulation during surgery

Dysfunction of spinal motor conduction during surgical procedures may not be reflected by changes in somatosensory evoked potential waveforms. A method of monitoring that allows direct and continuous assessment of motor function within the central nervous system during surgery would be useful. This paper describes one such method utilizing noninvasive electric cortical stimulation to evoke muscle activity (the motor evoked potential, or MEP) during surgery. The effect of isoflurane (superimposed on a baseline of N2O/narcotic anesthesia) on MEP's in response to cortical stimulation is specifically examined. Eight patients undergoing elective neurosurgical operations were included in the study. All patients received a background of general anesthesia and partial nondepolarizing neuromuscular blockade. The motor cortex was stimulated electrically via self-adhesive scalp electrodes. Electromyographic responses from multiple muscles were measured with subdermal electroencephalograph-type needle electrodes. Motor responses to stimulation were continually recorded on magnetic tape for off-line analysis. Once closing of the surgical incision was begun, a series of four to five stimuli of constant magnitude were applied to obtain "baseline" MEP responses. Patients were then ventilated with isoflurane for up to 8 minutes, during which time stimuli were continued every 15 to 20 seconds. Comparison was made of MEP responses for trials before, 1 minute after, and 5 minutes after the addition of isoflurane. All patients demonstrated reproducible motor responses to cortical stimulation during surgery. Addition of isoflurane [isoflurane)exp, less than or equal to 0.5%) to pre-existing anesthesia caused marked attenuation of MEP amplitudes in all patients within 5 minutes of its application, without affecting neuromuscular transmission as judged by direct peripheral nerve stimulation. It is concluded that: 1) monitoring motor system integrity and function with electric transcranial cortical stimulation during surgery is feasible when utilizing an N2O/narcotic anesthetic protocol; and 2) the quality of data obtained will likely suffer with the addition of isoflurane.

Variability of cortically evoked motor responses in multiple sclerosis

The calculated central motor conduction time (CMCT), onset latency variability (expressed as the mean consecutive difference; MCD) and amplitude (expressed as percentage of maximum peripheral M wave size) of electromyographic (EMG) responses in the first dorsal interosseous (FDI) muscle following magnetic motor cortex stimulation were investigated in 20 normal subjects and 21 patients with multiple sclerosis (MS). EMG responses were present in all patients studied. CMCT was prolonged (greater than 8.1 msec; the mean CMCT for normals plus 3 S.D.) in 19 out of 42 muscles (12 patients). Onset latency variability was increased (greater than 1.1 msec; mean plus 3 S.D. for normals) in 20 out of 42 muscles (14 patients). Maximal response amplitudes varied between 5% and 67% and were not significantly different from the normal group (range 16-64%). In 3 patients, increased onset latency variability was the only neurophysiological abnormality. Prolonged CMCT was the sole abnormal finding in only 1 patient. Abnormally large onset latency variability was associated with the clinical finding of both impaired fine finger movements and increased finger jerks. Abnormal CMCT was associated with increased finger jerks only. This study confirms the findings of prolonged CMCT in multiple sclerosis. The additional finding of abnormal variability in response latencies which correlates with the clinical signs suggests that this variability may also be a useful measure of pyramidal tract function.

Non-invasive evaluation of central motor tract excitability changes following peripheral nerve stimulation in healthy humans

The interval between muscle stretch and the onset of the long latency electromyographic responses (LLRs) has been theoretically fragmented into an afferent time (AT), taken at the peak of wave N20 of somatosensory evoked potentials and an efferent time (ET), calculated by means of magnetic transcranial stimulation (TCS), the two being separated by a cortical interval (CI). If this were the case, the afferent input should progressively 'energize' the sensorimotor cortex during the CI and change the excitability of cortico-spinal tracts. To investigate this, motor evoked potentials (MEPs) from thumb flexor muscles were recorded, whilst a conditioning stimulation of median or ulnar nerve randomly preceded (10-48 msec intervals) magnetic brain TCS. Nerve stimulation was adjusted to motor threshold and amplitudes of conditioned and test MEPs at different nerve-TCS interstimulus intervals were evaluated. Conditioned MEPs were significantly attenuated with nerve-TCS intervals between 16 and 20 msec for elbow and 20 and 22 msec for wrist stimulation. This was followed by MEP potentiation with nerve-TCS intervals corresponding to the sum of AT + CI (mean 23.2 msec, range 21.7-24.8). The onset latency of facilitated conditioned MEPs was about 1 msec briefer than that of test MEPs, but invariably longer than the latency of MEPs facilitated by a voluntary contraction. This protocol did not demonstrate amplitude facilitation of the segmental H reflex, corroborating the idea that the facilitated part of the conditioning nerve-TCS curve receives a transcortical loop contribution.

Latency of motor evoked potentials to focal transcranial stimulation varies as a function of scalp positions stimulated

We recorded motor evoked potentials (MEP) to transcranial magnetic stimulation from abductor pollicis brevis (APB), flexor carpi radialis (FCR), biceps brachii and deltoid muscles at rest and during slight voluntary activation. An 8-shaped coil connected to a Cadwell MES-10 magnetic stimulator was positioned over different scalp positions 1 cm apart. At least 24 stimuli were delivered at each location. Latencies of MEPs were compared with those obtained by electrical and magnetic stimulation during muscle activation. Progressively longer MEP latencies were obtained in 5 groups depending on the type and position of stimulation. The shortest latencies were obtained with (1) electrical stimulation during muscle contraction and (2) non-focal magnetic stimulation during muscle contraction; magnetic stimulation at rest produced longer latencies with stimulation of (3) an optimal position, (4) a suboptimal position, and (5) a non-optimal position. Mean latency differences between successive groups were 1.9, 2.0, 1.6, and 2.6 msec for APB. Similar latency differences were found for the other arm muscles. The results are compatible with the hypothesis that the different latencies evoked by stimulation at different scalp locations are determined by the summation at spinal motoneurons of excitatory postsynaptic potentials generated by successive numbers of I waves.

Influence of posture and voluntary background contraction upon compound muscle action potentials from anterior tibial and soleus muscle following transcranial magnetic stimulation

Compound muscle action potentials (CMAPs) were recorded from anterior tibial (TA) and soleus (SOL) muscles following transcranial magnetic stimulation of motor cortex in 10 healthy subjects: (1) while standing upright without support, (2) while sitting, and (3) while lying supine. The results of this study demonstrate a significant influence of posture upon the amplitudes of CMAPs. Postural facilitation presented itself, firstly, in terms of a higher incidence of bilateral activation of distal leg muscles during stance and, secondly, as significant enhancement of the amplitude of CMAPs while standing as compared to lying supine. The onset latency, however, did not disclose a significant shortening during stance. To assess the effects of preinnervation subjects voluntarily adjusted the level of TA activity to 5%, 10% and 20% of maximum isometric force respectively before cortex stimulation. Voluntary background contraction resulted in a significant increase of amplitude of CMAPs but, in contrast to postural facilitation, concomitant with a significant decrease in onset latency. These results point to a somewhat different mechanism of facilitation during stance as compared to voluntary preinnervation. But it cannot be decided whether cortical mechanisms, different descending systems, the spinal circuitry or a combination of these factors is responsible for the observed effects.

Motor potentials evoked by paired cortical stimuli

We recorded the motor evoked potentials (MEPs) from the abductor pollicis brevis muscle, after supramaximal electrical transcranial stimulation, and studied the effect of paired transcranial shocks with varying interstimulus time intervals, in 10 normal subjects, 4 patients with median nerve neuropathy and 2 patients with motoneurone disease. In relaxed muscles the amplitude of the MEP evoked by a single shock averaged 30% of the M wave. With intervals from 1 to 2.5 msec 2 shocks evoked one MEP far larger in size than the control MEP (70% of the M wave). With intervals of 10 msec and longer, the 2 shocks evoked 2 independent MEPs; the size of the MEP following the second shock (test) was inversely correlated with the size of the control MEP: the more the control MEP approached the size of the M wave, the smaller the test MEP. Single motor unit records showed that, in the normal subjects and patients with peripheral neuropathy, the same motor unit was activated either by the first or the second shock, whereas in the patients with motoneurone disease it fired twice. In active muscles, the control MEP averaged 70% of the M wave. With intervals of 10 msec and longer the test MEP was markedly suppressed; with 100 msec intervals it fully recovered. In relaxed muscles, by delivering a double shock at a 1.5 msec interval, thus evoking a large MEP, followed by a second double-shock, the test MEP was completely suppressed for a period of 20 msec; it began to recover at 50 msec intervals and fully recovered after 150 msec.(ABSTRACT TRUNCATED AT 250 WORDS)

Examination of the descending pathway to the external anal sphincter and pelvic floor muscles by transcranial cortical stimulation

In 26 neurologically normal patients and 9 healthy volunteers EMG responses after transcranial cortical stimulation (TCCS) were recorded from the external anal sphincter (EAS), the anterior tibial muscle (TA), the bulbocavernosus muscle (BC) and the rectus abdominis muscle (RA). Electrical TCCS was used in 29 subjects and magnetic TCCS in 6 subjects. Response patterns in the different muscles in relation to the strength of the stimulus were analyzed. It was found that the response patterns related to the strength of stimulation differed totally between the TA and the EAS. When the stimulus strength was increased stepwise, a response with a latency of 31.9 +/- 2.5 msec was first recorded in the TA, followed at higher strength by a secondary response with a latency of approximately 100 msec. In contrast, a response with a latency of 105.5 +/- 23.9 msec was first recorded in the EAS. The latency of this response gradually shortened with increasing stimulus strength until a response with a constant latency of 36.1 +/- 6.1 was obtained. In some subjects the response pattern in the BC was similar to that in the TA, and in others it was similar to that in the EAS. Responses in the TA, RA and EAS were all facilitated during voluntary contraction of the EAS. Both responses in the TA and in the EAS were facilitated by voluntary contraction of the TA. During voluntary contraction of the TA an inhibitory period was always recorded, while in the EAS no inhibitory periods were observed during either contraction or relaxation. The hypothesis that the fastest cortico-motoneuronal pathway to the EAS is polysynaptic is proposed.

Excitation of the corticospinal tract by electromagnetic and electrical stimulation of the scalp in the macaque monkey

1. The responses evoked by non-invasive electromagnetic and surface anodal electrical stimulation of the scalp (scalp stimulation) have been studied in the monkey. Conventional recording and stimulating electrodes, placed in the corticospinal pathway in the hand area of the left motor cortex, left medullary pyramid and the right spinal dorsolateral funiculus (DLF), allowed comparison of the actions of non-invasive stimuli and conventional electrical stimulation. 2. Responses to electromagnetic stimulation (with the coil tangential to the skull) were studied in four anaesthetized monkeys. In each case short-latency descending volleys were recorded in the contralateral DLF at threshold. In two animals later responses were also seen at higher stimulus intensities. Both early and late responses were of corticospinal origin since they could be completely collided by appropriately timed stimulation of the pyramidal tract. The latency of the early response in the DLF indicated that it resulted from direct activation of corticospinal neurones: its latency was the same as the latency of the antidromic action potentials evoked in the motor cortex from the recording site in the DLF. 3. Scalp stimulation, which was also investigated in three of the monkeys, evoked short-latency volleys at threshold and at higher stimulus intensities these were followed by later waves. The short-latency volleys could be collided from the pyramid and, at threshold, had latencies compatible with direct activation of corticospinal neurones. The longer latency volleys were also identified as corticospinal in origin. 4. The latency of the early volley evoked by electromagnetic stimulation remained constant with increasing stimulus intensities. In contrast, with scalp stimulation above threshold the latency of the early volleys decreased considerably, indicating remote activation of the corticospinal pathway below the level of the motor cortex. In two monkeys both collision and latency data suggest activation of the corticospinal pathway as far caudal as the medulla. 5. The majority of fast corticospinal fibres could be excited by scalp stimulation with intensities of 20% of maximum stimulator output. Electromagnetic stimulation at maximum stimulator output elicited a volley of between 70 and 90% of the size of the maximal volley evoked from the pyramidal electrodes. 6. Electromagnetic stimulation was also investigated in one awake monkey during the performance of a precision grip task. Short-latency EMG responses were evoked in hand and forearm muscles. The onsets of these responses were approximately 0.8 ms longer than the responses evoked by electrical stimulation of the pyramid.(ABSTRACT TRUNCATED AT 400 WORDS)

Late muscular responses to transcranial cortical stimulation in man

Transcranial cortical stimulation was performed in healthy human subjects. Electrical and magnetic stimuli were applied over the motor areas of the hand and the leg. Surface EMGs were recorded from the antagonistic extensor and flexor carpi radialis and the anterior tibial and triceps surae muscles. Except for the well known early (primary) responses several secondary responses have been recorded. (1) A response with a latency of approximately 100 msec and with higher threshold than the primary response. It was found in approximately half of the muscles recorded from except for the anterior tibial where it was comparatively rare. It could appear both contra- and ipsilaterally and increased in amplitude with preactivation of the target muscle. It is probably not of startle origin but its appearance may be influenced by an approximately simultaneous startle effect from the scalp stimulus. (2) In some subjects a response with a latency of 50-60 msec was recorded. Sometimes it was easily separated from the primary response and from S 100 but sometimes the differentiation was difficult. (3) It was found that the known inhibitory period which follows the primary response was contralateral and increased in duration with increasing stimulus strength. The increase in duration may be due to inhibitory feedback from receptors activated by the simultaneously increasing primary muscle twitch. (4) In relaxed muscle we also observed a contralateral response with a latency exceeding 150 msec. The latency increased with increasing stimulus strength. This increase seems to be related to discontinuance of the inhibitory period.

Motor-evoked potentials in masseter muscle by electrical and magnetic stimulation in intact alert man

The electromyographic responses of the masseter after different types of transcranial stimulation were recorded with surface and needle electrodes. Magnetic stimulation at 4 cm lateral to the vertex on the biauricular line elicited MEPs in the contralateral masseter (latency 6.9 ms) due to activation of motor cortex or adjacent elements along the cortico-nuclear pathway. The ipsilateral responses to the same stimuli and to more lateral ones had shorter latencies and were ascribed to direct stimulation of the trigeminal nerve, probably its intracisternal portion. This was also the probable origin of the ipsilateral MEPs after both anodic and cathodic bipolar electrical stimulation at 7 and 11 cm lateral to the vertex on the biauricular line.

Focal stimulation of human cerebral cortex with the magnetic coil: a comparison with electrical stimulation

Percutaneous stimulation of human motor cortex electrically (focal anode) and with magnetic coils (MCs) of various designs is compared. The theoretical prediction was confirmed that positioning the standard round MC laterally and orientating it more towards the vertical induces an electric field appropriate for directly exciting corticospinal neurons (cf., the conventional tangential orientation at the vertex). Thus, during voluntary contraction, minimal latency compound motor action potentials (CMAPs) in contralateral arm were elicited both by focal anodic and appropriately orientated MC stimulation. Conduction time from motor cortex to motoneuron was estimated by subtracting peripheral conduction time and monosynaptic delay at the motoneuron from the overall CMAP latency, yielding an estimated corticospinal conduction velocity as high as 66 m/sec. Discontinuous latency variations observed in population CMAPs or individual motor units approximated mono- or polysynaptic cortical synaptic delays and, therefore, are attributed to the intervals between direct and early, or late indirect corticospinal discharges. A TV computer system was used to track movements of individual digits and the hand following MC stimulation. An appropriately orientated MC readily elicited movements predominantly of a single digit, implying focal activation of motor cortex. A double square and a small pointed MC proved especially convenient for eliciting reproducibly single digit movements. Stronger stimulation revealed a topographical gradient in the responses of the different digits. Responses to a given MC stimulus a little above threshold were variable in amplitude, which could not be explained by the relationship of stimulus to phase of the cardiac or respiratory cycle. Overall, our findings indicate the importance of appropriately orientating a standard round MC and using a specially designed MC to obtain the various types of motor response to stimulation of cerebral cortex.

Physiological properties of the motor units of the wrist extensor muscles in man

The physiological properties of 355 motor units (MUs) recorded in the extensor carpi radialis muscles were studied in 34 healthy human subjects during isometric contractions. MU selective twitches were educed from the whole muscle force using the spike-triggered averaging method. The twitch contraction times and twitch forces were measured. From these data it was attempted to estimate the distribution of fast and slow MUs in the muscles studied. Mu recruitment thresholds were systematically measured during stereotyped slow ramp contractions (force increase = 0.25 N. s-1). Degrees of correlation between contraction times, twitch forces and recruitment thresholds were pair analysed by computing simple regression curves and correlation coefficients. The degrees of correlation were compared between 245 Mus recorded in 34 subjects and 66 MUs recorded in a single subject. Analysis of the instantaneous discharge frequency of 132 MUs showed the existence of a remarkable degree of correlation (correlation coefficient, r = -0.75) between the "frequency rise times" (discharge onset to maximal frequency) and the MU twitch contraction times; i.e., the "frequency rise times" increase when the twitch contraction time decrease. The possibility that muscle contraction may be differentially modulated on the basis of this discharge property of the M Us is discussed. The results are compared to previous data and the limitations of the spike-triggered averaging method applied to long muscles in man are extensively discussed.

Single fibre motor evoked potentials to brain, spinal roots and nerve stimulation. Comparisons of the 'central' and 'peripheral' response jitter to magnetic and electric stimuli

Single fibre motor evoked potentials to magnetic and electric non-invasive stimulation of brain, spinal cord and peripheral nerve were recorded in 8 healthy volunteers. The 'central motor jitter' and the 'peripheral motor jitter' were respectively calculated and a comparison between the magnetic and electric modalities was made. The highest degree of latency variability was observed for both magnetic and electric central motor jitter, whilst the peripheral motor jitter to nerve stimulation was as low as the neuromuscular one (range 16-60 microsecond). The magnetic 'central motor jitter' (range 94-1024 microsecond) was much larger than the electric one (range 55-280 microsecond), which was in the order of jitter calculated on H-reflex studies; moreover, the former was organized in a bi- or trimodal distribution. On the contrary, no significant differences were observed between the two modalities when the jitter to nerve stimulation was taken into account. Possible contributions of corticocortical circuitries containing several synaptic interruptions during magnetic as opposed to electric transcranial stimulation, is discussed.

A neurophysiological study of patients undergoing radical prostatectomy

24 men suffering from localized prostatic cancer undergoing radical retropubic nerve-sparing prostatectomy were investigated by the following electrophysiological methods: Bulbocavernosus reflexes elicited from the penile skin or the posterior urethra, sensory thresholds in the posterior urethra, cerebral evoked potentials after stimulation of the pudendal nerve or the posterior urethra. 15 men were examined 4-33 months postoperatively only, 5 men were examined only preoperatively and 4 men were examined both pre- and postoperatively. 10 men suffering from minor problems due to benign prostatic hyperplasia served as controls. In patients with localized cancer of the prostate, the findings did not differ from those in the control group. In the operated group the findings were pathological in a large proportion of the patients, indicating injuries both to nervous pathways running through the pelvic nerve plexus and in the pudendal nerve. The conclusions were: Localized cancer of the prostate has minimal or no risk at all of impaired functioning in the pelvic nervous pathways. Radical retropubic prostatectomy may in some cases be undertaken without any objective evidence of injury to these nervous pathways, but is often followed by findings indicating such injury. The dorsal nerve of the penis may be affected by the operation. Transcranial stimulation of the motor cortex is a useful method in the evaluation of prolonged or absent bulbocavernosus reflexes.

Responses in human intercostal and truncal muscles to motor cortical and spinal stimulation

Percutaneous electrical stimulation of the human motor cortex (up to 750 V) has been used to study the cortical projections to intercostal and truncal muscles. The latencies of electromyographic (EMG) responses were measured to motor cortical stimuli and also to spinal stimulation of the appropriate nerve roots. Following single anodal stimuli at (or near) the vertex, short-latency responses were recorded in pectoralis major (mean 9.6 msec), latissimus dorsi (9.7 msec), paravertebral muscles (11.1 msec), 3rd/4th parasternal intercostals (11.1 msec), and 6th/7th intercostal muscles (12.3 msec). Responses in each muscle group were potentiated by background voluntary respiratory and truncal manoeuvres which activated the muscles. The mean estimated central conduction times from motor cortex to spinal segmental level were 4.8 msec for pectoralis major, 5.8 msec for parasternal intercostals and 6.2 msec for 6th/7th intercostal muscles. The central conduction times and properties of the cortically evoked responses are consistent with a rapidly conducting, oligosynaptic pathway from the human motor cortex to accessory respiratory muscles and to truncal muscles.