Reduction In Motor-evoked Potentials Research Articles

Stereoscopic Monitoring Technique for Motor Area Tumors

The balance between comprehensive intraoperative neurophysiological monitoring (IONM) for both upper and lower limbs while ensuring the reliability of motor evoked potentials (MEPs) is paramount in motor area surgery. It is commonly difficult to obtain good simultaneous stimulation of both upper and lower limbs. A series of factors can bias MEP accuracy, and inappropriate stimulation intensity can result in unreliable monitoring. The presented IONM technique is based on the concurrent use of both transcranial and cortical strip electrodes to facilitate simultaneous monitoring of both upper and lower limbs at optimized stimulation intensities to increase IONM accuracy during motor area surgery. Ten nonconsecutive motor area tumors were studied. Good visualization of both limbs was observed in the series at a low amperage (1.2 mA from thestrip electrode and 165.3 mA from the transcranial electrode). Our analysis confirms concordance between the IONM data and postoperative outcomes. An MEP reduction >20% and >50% correlated with postoperative modified Rankin scale score changes without false-negative IONM findings. The technique was demonstrated to be accurate in providing a good simultaneous neurophysiological evaluation of both upper and lower limbs with an optimized and stimulation amplitude. The technique results in a low encumbrance of electrodes in the surgical field. Our results have confirmed the "proof of concept," its reliability and feasibility.

A Diffusion Tensor Imaging-Based Prognostic Classification for Surgery of Intrinsic Lesions Involving the Motor Pathways.

The critical role of different adjuncts in improving the neurological outcome in intrinsic brain lesions affecting eloquent areas is demonstrated by their more diffuse utilization. Neurosurgeons often rely on preoperative and intraoperative diffusion tensor imaging tractography to improve the operative strategy and prognosis. We aimed to identify and validate a diffusion tensor imaging-based classification considering the relationship between the brain lesion and the corticospinal tract to predict a >50% reduction of motor evoked potentials (MEPs) during surgical excision of lesions involving the motor pathways. We included patients consecutively enrolled at our institution between April 2020 and September 2022 with 3 patterns of increasing complexity according to the relationship between the lesion and the corticospinal tract as identified on preoperative diffusion tensor imaging. Outcome measures were >50% reduction in intraoperative MEPs and neurological outcome defined as unchanged, improved, or worsened. The study included 83 patients. A statistically significant linear trend between higher rates of reduction of MEPs and higher classification grades was observed (P= 0.001), with sensitivity 0.60, specificity 0.88, accuracy 0.83, and area under the curve 0.75. Higher grades were associated with worse neurological outcomes (P= 0.02). The classification proved reliable in anticipating reduction in intraoperative MEPs and in predicting neurological outcome. Using this classification in patients undergoing surgery for lesions involving the motor pathways could help in counseling the patient, surgical planning, enhancing teamwork of operating room personnel, and improving the patient's prognosis.

흉추 종양 수술에서 복직근을 이용한 신경계감시의 가능성: 증례 보고

Intraoperative neurophysiologic monitoring (IONM) has been widely used to prevent nerve damage during spinal tumor surgery. However, when the tumor is located in the thoracic vertebrae, it is impossible to monitor lower motor neurons such as nerve roots by conventional methods. To solve this problem, the rectus abdominis muscle, which is innervated by the thoracic nerve root, can be used. We report a case using the rectus abdominis motor evoked potentials (MEP) and free-running electromyography in a patient with schwannoma located at the T12 thoracic vertebrae. During surgery, a reduction in MEP was seen upon removal of the tumor, but not less than 50%. Neurotonic discharge was seen upon removal of the tumor, but was not sustained. After the operation, the patient did not report any neurological symptoms. This case supports that MEP and free-running electromyography of the rectus abdominis muscle can be feasible for monitoring in thoracic spine surgery.

Comparison of nerve conduction study and transcranial magnetic stimulation for early diagnosis and prognosis prediction of idiopathic facial palsy.

Multiple electrophysiologic studies have been conducted in the evaluation of facial neuropathy. In our study, the diagnostic and prognostic values of nerve conduction studies (NCSs) and transcranial magnetic stimulation (TMS) were compared for idiopathic unilateral facial neuropathy. We recruited patients with newly diagnosed idiopathic unilateral facial neuropathy and performed a blink reflex test, facial NCSs, and TMS. The amplitude of facial compound muscle action potential (CMAP) and motor evoked potential (MEP) between the affected and unaffected sides of the face was compared. A total of 30 patients were enrolled in the final analysis. TMS yielded a better detection rate, and MEP reduction rate was significantly higher than CMAP reduction rate, early in the course of the disease. Poor prognosis was positively associated with the CMAP reduction rate. The cutoff value of the CMAP reduction rate in the prediction of poor prognosis was established as 0.42. Facial TMS could detect idiopathic unilateral facial neuropathy with a high sensitivity when used as an early diagnostic tool. Facial NCS could predict prognosis, and the CMAP reduction rate was significantly associated with poor short-term prognosis.

Early motor reactivity to observed human body postures is affected by body expression, not gender

The early response to emotional stimuli involves a transient suppression of motor reactivity to favor monitoring of emotionally relevant information. Using transcranial magnetic stimulation (TMS), we have previously shown that viewing emotional body postures induces an early and transient reduction in motor excitability. Yet, it remains unclear whether early motor responses to emotional bodies are automatic or influenced by top-down factors such as task- or gender-related effects. To address these issue, we administered TMS over the right motor cortex (M1) during observation of still pictures of fearful expressions, happy expressions, neutral movements and neutral static body postures, and recorded motor-evoked potentials (MEPs) at an early phase of processing (i.e., at 100–125 ms from stimulus onset). To test gender-related effects, we presented male and female models to male and female participants. To test task-related effects, we asked participants to categorize the different body postures into either four (4AFC: fearful, happy, neutral movements, or static postures) or two distinct categories (2AFC: emotional or neutral postures). Results showed a reduction of MEPs for fearful and happy body postures relative to neutral movements and static postures. This motor suppression was not influenced by the gender of the actor, the gender of the observer, or the task performed. These findings indicate that early motor responses to observed human body postures are affected by the type of expression displayed by the observed model more than by task- or gender-related effects, suggesting these responses may be relatively automatic.

Fear of movement is associated with corticomotor depression in response to acute experimental muscle pain.

Acute musculoskeletal pain is associated with reductions in corticomotor output that persists even after pain resolves. Factors that contribute to corticomotor depression following acute pain are unknown. This study examined whether psychological factors, including pain catastrophising, kinesiophobia, and implicit theories of pain, were associated with corticomotor depression following acute experimental muscle pain. Forty-two healthy individuals participated. Participants completed three questionnaires: Pain Catastrophising Scale, Tampa Scale of Kinesiophobia, and Implicit Theories of Pain Scale. Acute pain was induced into the right extensor carpi radialis brevis (ECRB) muscle by injection of hypertonic saline. Corticomotor depression was assessed as a reduction in motor-evoked potentials measured from ECRB muscle in response to transcranial magnetic stimulation before, immediately after, and at 10, 20, and 30min following pain resolution. Corticomotor depression was present at each time point relative to baseline (p < 0.001). Higher levels of kinesiophobia were associated with less corticomotor depression 10-min post pain resolution (r = 0.32, p = 0.03), but not at any other time point (p > 0.11). When corticomotor depression was compared between individuals with 'high' and 'low' kinesiophobia, a similar relationship was observed: Individuals with high compared to low kinesiophobia displayed less corticomotor depression immediately after (p = 0.02) and 10min post pain (p = 0.02), but not at 20 or 30min (p = 0.05 for both). No relationship was observed with any other psychological variable (p > 0.15). These data provide preliminary support for a relationship between pain-related fear of movement and corticomotor depression in response to acute pain. These findings may have implications for clinical musculoskeletal pain disorders.

Triple motor mapping: transcranial, bipolar, and monopolar mapping for supratentorial glioma resection adjacent to motor pathways.

Maximal safe resection of gliomas near motor pathways is facilitated by intraoperative mapping. The authors and other groups have described the use of bipolar or monopolar direct stimulation to identify functional tissue, as well as transcranial or transcortical motor evoked potentials (MEPs) to monitor motor pathways. Here, the authors describe their initial experience using all 3 modalities to identify, monitor, and preserve cortical and subcortical motor systems during glioma surgery. Intraoperative mapping data were extracted from a prospective registry of glioma resections near motor pathways. Additional demographic, clinical, pathological, and imaging data were extracted from the electronic medical record. All patients with new or worsened postoperative motor deficits were followed for at least 6 months. Between January 2018 and August 2019, 59 operations were performed in 58 patients. Overall, patients in 6 cases (10.2%) had new or worse immediate postoperative deficits. Patients with temporary deficits all had at least Medical Research Council grade 4/5 power. Only 2 patients (3.4%) had permanently worsened deficits after 6 months, both of which were associated with diffusion restriction consistent with ischemia within the corticospinal tract. One patient's deficit improved to 4/5 and the other to 4/5 proximally and 3/5 distally in the lower limb, allowing ambulation following rehabilitation. Subcortical motor pathways were identified in 51 cases (86.4%) with monopolar high-frequency stimulation, but only in 6 patients using bipolar stimulation. Transcranial or cortical MEPs were diminished in only 6 cases, 3 of which had new or worsened deficits, with 1 permanent deficit. Insula location (p = 0.001) and reduction in MEPs (p = 0.01) were the only univariate predictors of new or worsened postoperative deficits. Insula location was the only predictor of permanent deficits (p = 0.046). The median extent of resection was 98.0%. Asleep triple motor mapping is safe and resulted in a low rate of deficits without compromising the extent of resection.

Corticomotor excitability reduction induced by experimental pain remains unaffected by performing a working memory task as compared to staying at rest.

Experimental pain inhibits primary motor cortex (M1) excitability. Attenuating pain-related inhibition of M1 excitability may be useful during rehabilitation in individuals with pain. One strategy to attenuate M1 excitability is to influence prefrontal and premotor cortex activity. Working memory tasks, e.g. the two-back task (TBT), engage prefrontal and premotor cortices and may influence M1 excitability. We hypothesized that performing the TBT during pain would influence pain-related changes in M1 excitability. Participants (n = 28) received rigorous training in the TBT before baseline testing. Experimental pain was induced by injecting hypertonic saline into the first dorsal interosseous (FDI) muscle. Participants rated pain intensity on a 0-10 numerical rating scale (NRS) every second min until pain-resolved (PR) during the performance of the TBT (n = 14) or during REST (n = 14). In the TBT, letters were presented pseudo-randomly, and accuracy and reaction time to identified letters corresponding to letters shown two times back were recorded. M1 excitability was assessed using transcranial magnetic stimulation. Motor-evoked potentials (MEPs) were recorded at baseline, and at PR, PR + 10, PR + 20, and PR + 30min. Four minutes after hypertonic saline injection, the pain NRS scores were higher in the TBT group than the REST group (p = 0.009). No time × group interaction was found for MEPs (p = 0.73), but a main effect of time (p < 0.0005) revealed a reduction of MEPs at PR up until PR + 30 (p < 0.008). The TBT accuracy improved at PR + 30 in both groups (p = 0.019). In conclusion, the pain-induced reduction in corticomotor excitability was unaffected by performing a working memory task, despite greater pain in the TBT group.

Temporal Profile and Limb-specificity of Phasic Pain-Evoked Changes in Motor Excitability

A fundamental function of nociception is to trigger defensive motor responses to threatening events. Here, we explored the effects of phasic pain on the motor excitability of ipsilateral and contralateral arms. We reasoned that the occurrence of a short-lasting nociceptive stimulus should result in a specific modulation of motor excitability for muscles involved in the withdrawal of the stimulated limb. This was assessed using transcranial magnetic stimulation (TMS) of the left and right primary motor cortex to elicit motor-evoked potentials (MEPs) in three flexor and two extensor muscles of both arms. To assess the time-course of nociception-motor interactions, TMS pulses were triggered 50–2000 ms after delivering short-lasting nociceptive laser stimuli to the left or right hand. We made three main observations. First, nociceptive stimuli induced an early-latency (100 ms) enhancement of MEPs in flexor muscles of the stimulated hand. Considering its latency, this modulation is likely consequent to nociceptive-motor interactions at spinal level. This early and lateralized enhancement was followed by a later (150–400 ms) MEP reduction in extensor muscles of the stimulated hand and flexor muscles of both hands, predominant at the stimulated hand. Finally, we observed a long-lasting (600–2000 ms) MEP enhancement in muscles of the non-stimulated hand. These later effects of the nociceptive stimulus could reflect nociception-motor interactions occurring at cortical level.

Incidence of peripheral nerve injury during shoulder arthroplasty when motor evoked potentials are monitored.

To report the incidence of clinically detectable nerve injuries when utilizing transcranial electrical motor evoked potentials (MEPs) during shoulder arthroplasty. A retrospective review of patients undergoing shoulder arthroplasty with continuous IONM was performed. The criteria for nerve alerts was an 80% amplitude reduction in MEPs. The primary outcome measure was post-operative clinically detectable nerve deficit. An additional retrospective analysis on a subset of cases using an all-or-none (100% amplitude reduction) criterion applied to the deltoid was performed. Two hundred eighty four arthroplasty cases were included. There were no permanent post-operative nerve injuries and two transient nerve injuries (0.7%). MEP alerts occurred in 102 cases (36.2%). Nineteen (6.7%) cases did not have signals return above alert threshold at closure. These cases were significantly associated with post-operative nerve injury (p = 0.03). There were no false negatives, making sensitivity 100% and specificity was 93.9%. In the subset of cases in which an all-or-none criterion was retrospectively applied to just the deltoid, MEP alerts occurred in just 17.9% of cases; specificity improved to 98.0%. We conclude that utilization of the real-time diagnostic MEP data during shoulder arthroplasty aids surgeons in decision making regarding impending peripheral nerve injuries.

Empathy or Ownership? Evidence from Corticospinal Excitability Modulation during Pain Observation.

Recent studies show that motor responses similar to those present in one's own pain (freezing effect) occur as a result of observation of pain in others. This finding has been interpreted as the physiological basis of empathy. Alternatively, it can represent the physiological counterpart of an embodiment phenomenon related to the sense of body ownership. We compared the empathy and the ownership hypotheses by manipulating the perspective of the observed hand model receiving pain so that it could be a first-person perspective, the one in which embodiment occurs, or a third-person perspective, the one in which we usually perceive the others. Motor-evoked potentials (MEPs) by TMS over M1 were recorded from first dorsal interosseous muscle, whereas participants observed video clips showing (a) a needle penetrating or (b) a Q-tip touching a hand model, presented either in first-person or in third-person perspective. We found that a pain-specific inhibition of MEP amplitude (a significantly greater MEP reduction in the "pain" compared with the "touch" conditions) only pertains to the first-person perspective, and it is related to the strength of the self-reported embodiment. We interpreted this corticospinal modulation according to an "affective" conception of body ownership, suggesting that the body I feel as my own is the body I care more about.

Bidirectional variability in motor cortex excitability modulation following 1mA transcranial direct current stimulation in healthy participants.

Due to the high interindividual response variability following transcranial direct current stimulation (tDCS), it is apparent that further research of the long‐lasting effects of the stimulation technique is required. We aimed to investigate interindividual variability following anodal tDCS and cathodal tDCS in a large‐scale prospective cross‐over study. Motor cortex physiology measurements were obtained using transcranial magnetic stimulation (TMS) in 59 healthy participants comparing motor‐evoked potential (MEP) magnitudes following two tDCS paradigms: 1 mA anodal tDCS for 13 min and 1 mA cathodal tDCS for 9 min. Analysis compared MEP changes over time for both polarities. Additionally, we applied hierarchical cluster analysis to assess the dynamics of poststimulation changes. Overall, anodal tDCS resulted in a significant increase in corticospinal excitability lasting for 40 min poststimulation, whereas cathodal tDCS did not alter corticospinal excitability. Cluster analysis revealed for cathodal tDCS both a cluster showing significant stable MEP reduction and a second cluster displaying MEP increase over time. Two diametrical clusters were also found for anodal tDCS. Regardless of polarity, individuals with MEP increase following stimulation showed steeper cortical recruitment curves compared to the clusters with decreased MEP magnitudes. The observed findings confirm a bidirectional modulation of corticospinal excitability following 1 mA tDCS in separate subgroups and the relationship to cortical recruitment.

Is There Any Benefit of Neuromonitoring During Descending and Thoracoabdominal Aortic Aneurysm Repair?

Paraplegia remains the most feared and a devastating complication after descending and thoracoabdominal aneurysm operative repair (DTA and TAAAR). Neuromonitoring, particularly use of motor-evoked potentials (MEPs), for this surgery has gained popularity. However, ambiguity remains regarding its use and benefit. We systematically reviewed the literature to assess the benefit and applicability of neuromonitoring in DTA and TAAAR. Electronic searches were performed on 4 major databases from inception until February 2014 to identify relevant studies. Eligibility decisions, method quality, data extraction, and analysis were performed according to predefined clinical criteria and end points. Among the studies matching our inclusion criteria, 1297 patients had MEP monitoring during DTA and TAAAR. In-hospital mortality was low (6.9% ± 3.6). Immediate neurological deficit was low (3.5% ± 2.6). In one third of patients (30.4% ± 14.2), the MEPs dropped below threshold, which were 30.4% and 29.4% with threshold levels of 75% and 50%, respectively. A range of surgical techniques were applied after reduction in MEPs. Most patients whose MEPs dropped and remained below threshold had immediate permanent neurological deficit (92.0% ± 23.6). Somatosensory-evoked potentials were reported in one third of papers with little association between loss of somatosensory-evoked potentials and permanent neurological deficit (16.7% ± 28.9%). We demonstrate that MEPs are useful at predicting paraplegia in patients who lose their MEPs and do not regain them intraoperatively. To date, there is no consensus regarding the applicability and use of MEPs. Current evidence does not mandate or support MEP use.

Inhibition of the primary motor cortex can alter one's “sense of effort”: Effects of low-frequency rTMS

Studies using force-matching tasks have suggested that when we feel a “sense of effort,” cortical regions may act to increase motor commands, and thus recruit additional motor units, in order to compensate for the exerted force. We hypothesized that suppressing activity in the primary motor cortex (M1), which is the source of the motor commands, would initiate the same process, and induce the same sense of effort. In a force-matching task, grip force was applied to ‘right’ hand and 10 healthy participants were asked to try to exert the same amount by using ‘left’ hand, with no visual feedback. On some trials, low-frequency, repetitive transcranial magnetic stimulation (lf-rTMS) was used to suppress the M1 and the primary somatosensory cortex (SI) in the left hemisphere, separately. Results showed that participants tended to overestimate the level of exerted force by up to 24%. In contrast, sham stimulation of the M1 and lf-rTMS over the SI did not significantly affect participants’ estimations. Further, the M1 suppression resulted in a 42% reduction in motor-evoked potentials. Thus, the M1 suppression can affect our sense of effort, suggesting that compensatory neural mechanisms that increase the MI activity may play an important role in producing senses of effort.

Reliability of intraoperative neurophysiological monitoring using motor evoked potentials during resection of metastases in motor-eloquent brain regions

Resection of gliomas in or adjacent to the motor system is widely performed using intraoperative neuromonitoring (IOM). For resection of cerebral metastases in motor-eloquent regions, however, data are sparse and IOM in such cases is not yet widely described. Since recent studies have shown that cerebral metastases infiltrate surrounding brain tissue, this study was undertaken to assess the value and influence of IOM during resection of supratentorial metastases in motor-eloquent regions. Between 2006 and 2011, the authors resected 206 consecutive supratentorial metastases, including 56 in eloquent motor areas with monitoring of monopolar direct cortically stimulated motor evoked potentials (MEPs). The authors evaluated the relationship between the monitoring data and the course of surgery, clinical data, and postoperative imaging. Motor evoked potential monitoring was successful in 53 cases (93%). Reduction of MEP amplitude correlated better with postoperative outcomes when the threshold for significant amplitude reduction was set at 80% (only > 80% reduction was considered significant decline) than when it was set at 50% (> 50% amplitude reduction was considered significant decline). Evidence of residual tumor was seen on MR images in 28% of the cases with significant MEP reduction. No residual tumor was seen in any case of stable MEP monitoring. Moreover, preoperative motor deficit, recursive partitioning analysis Class 3, and preoperative radiotherapy were independent risk factors for a new surgery-related motor weakness (occurring in 64% of patients with and 11% of patients without radiotherapy, p > 0.01). Continuous MEP monitoring provides reliable monitoring of the motor system and also influences the course of operation in resection of cerebral metastases. However, in establishing warning criteria, only an amplitude decline > 80% of the baseline should be considered significant.

Differential modulation of motor cortex excitability in BDNF Met allele carriers following experimentally induced and use‐dependent plasticity

The purpose of this study was to investigate how healthy young subjects with one of three variants of the brain-derived neurotrophic factor (BDNF) gene modulate motor cortex excitability following experimentally induced and use-dependent plasticity interventions. Electromyographic recordings were obtained from the right first dorsal interosseous (FDI) muscle of 12 Val/Val, ten Val/Met and seven Met/Met genotypes (aged 18-39 years). Transcranial magnetic stimulation of the left hemisphere was used to assess changes in FDI motor-evoked potentials (MEPs) following three separate interventions involving paired associative stimulation, a simple ballistic task and complex visuomotor tracking task using the index finger. Val/Val subjects increased FDI MEPs following all interventions (≥ 25%, P < 0.01), whereas the Met allele carriers only showed increased MEPs after the simple motor task (≥ 26%, P < 0.01). In contrast to the simple motor task, there was no significant change in MEPs for the Val/Met subjects (7%, P = 0.50) and a reduction in MEPs for the Met/Met group (-38%, P < 0.01) following the complex motor task. Despite these differences in use-dependent plasticity, the performance of both motor tasks was not different between BDNF genotypes. We conclude that modulation of motor cortex excitability is strongly influenced by the BDNF polymorphism, with the greatest differences observed for the complex motor task. We also found unique motor cortex plasticity in the rarest form of the BDNF polymorphism (Met/Met subjects), which may have implications for functional recovery after disease or injury to the nervous system in these individuals.

Psychopathy and the mirror neuron system: Preliminary findings from a non-psychiatric sample

Recent advances in social neuroscience suggest a link between empathy and the mirror neuron system (MNS). Impaired empathy is one of the core diagnostic features of psychopathic personality disorder. In the present study, we investigated whether psychopathic personality traits in a non-psychiatric sample were related to MNS function. Healthy participants viewed short videos known to activate the sensorimotor MNS for pain (a needle penetrating a human hand) while transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEP) were recorded as a measure of motor cortex excitability. Individual psychopathic personality traits were assessed using the Psychopathic Personality Inventory (PPI) and correlated with the MEP findings. Consistent with previous data, observation of the painful stimulus was associated with a significant reduction in the amplitude of the TMS-induced MEP. Interestingly, the level of corticospinal excitability modulation was positively correlated with individual scores on the coldheartedness subscale of the PPI, such that individuals with the greatest MEP reduction were the ones scoring highest on the coldheartedness measure. These data suggest the existence of a functional link between ‘motor empathy’ and psychopathy.

Effects of transcutaneous electrical nerve stimulation on motor cortex excitability in writer's cramp: Neurophysiological and clinical correlations

We recently reported that transcutaneous electrical nerve stimulation (TENS) applied over forearm flexor muscles, a paradigm producing in physiological conditions transient changes in corticomotoneuronal excitability of forearm muscles, may improve motor symptoms in writer's cramp (WC). In the present study, we explored the possibility that one or repeated sessions of TENS might have beneficial effects on handwriting in WC by remodulating cortical excitability of forearm agonist and antagonist muscles. Motor evoked potentials (MEPs) after transcranial magnetic stimulation of the left motor cortex were recorded from the right flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles, before and after 1 and 15 sessions of TENS applied over flexor muscles in patients and in a control group. One session of TENS induced a significant smaller reduction of MEPs from the FCR and a smaller increase of the MEPs from the ECR in patients than in normal subjects. In WC, repeated sessions of TENS had the effect to decrease MEP amplitude in the FCR and to increase it in the ECR. This modulation was paralleled by a handwriting improvement. In conclusion, repeated TENS sessions may have the effect to re-modulate excitability of the motor cortex in WC and this modulation might partially play a role in temporarily improving the handwriting.

Output of Human Motoneuron Pools to Corticospinal Inputs During Voluntary Contractions

This study investigated transmission of corticospinal output through motoneurons over a wide range of voluntary contraction strengths in humans. During voluntary contraction of biceps brachii, motor evoked potentials (MEPs) to transcranial magnetic stimulation of the motor cortex grow up to about 50% maximal force and then decrease. To determine whether the decrease reflects events at a cortical or spinal level, responses to stimulation of the cortex and corticospinal tract (cervicomedullary motor evoked potentials, CMEPs) as well as maximal M-waves (M(max)) were recorded during strong contractions at 50 to 100% maximum. In biceps and brachioradialis, MEPs and CMEPs (normalized to M(max)) evoked by strong stimuli decreased during strong elbow flexions. Responses were largest during contractions at 75% maximum and both potentials decreased by about 25% M(max) during maximal efforts (P < 0.001). Reductions were smaller with weaker stimuli, but again similar for MEPs and CMEPs. Thus the reduction in MEPs during strong voluntary contractions can be accounted for by reduced responsiveness of the motoneuron pool to stimulation. During strong contractions of the first dorsal interosseous, a muscle that increases voluntary force largely by frequency modulation, MEPs declined more than in either elbow flexor muscle (35% M(max), P < 0.001). This suggests that motoneuron firing rates are important determinants of evoked output from the motoneuron pool. However, motor cortical output does not appear to be limited at high contraction strengths.

Sensorimotor integration to cutaneous afferents in humans: the effect of the size of the receptive field

Transcranial magnetic stimulation (TMS) can be used to study sensorimotor integration in humans non-invasively. Motor excitability has been found to be inhibited when afferent stimuli are given to a peripheral nerve and precede TMS at interstimulus intervals (ISIs) of 20-50 ms. This phenomenon has been referred to as short-latency afferent inhibition (SAI). To better understand the functional meaning of these phenomena, we examined the effect of the size of the receptive field on SAI to cutaneous afferents in upper-limb sensorimotor areas in humans. We examined the effect of the stimulation of the isolated right first (D1), second (D2) and third finger (D3), the right second and third finger together (D23) and the right first three fingers together (D123) on the amplitude of motor evoked potentials (MEPs) to TMS in hand and forearm muscles. We examined the right abductor pollicis brevis (APB), first dorsal interosseous (FDI), extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles. Digital stimulation preceded TMS at ISIs of 20-50 ms. The effect of D2 stimulation was MEP inhibition (SAI), which was more marked and consistent in APB and FDI muscles than in ECR and FCR muscles. Similarly, D1 and D3 stimulation caused MEP reduction, while no MEP enhancement could be found to single finger stimulation. In contrast, D123 stimulation induced less effective SAI in upper-limb muscles. MEP potentiation was recorded in some muscles to D123 stimulation. A significant difference between D2 and D123 stimulation was found in APB (ISIs = 30-50 ms) and FDI (ISIs = 40-50 ms) muscles, but not in forearm muscles. The effect to D23stimulation on MEP amplitude was intermediate between those to D2 and D123 stimulation. Our data suggest that motor excitability to cutaneous afferents may be influenced by the size of the receptive fields, this effect being the result of increasing convergence between hand afferents in the somatosensory system. These phenomena appear to be topographically arranged across the representation of upper-limb muscles. These findings may help to understand the functional significance of SAI in normal physiology and pathophysiology.