Motor evoked potentials and central motor conduction: studies of transcranial magnetic stimulation with recording from the leg.

To determine central conduction times in the corticospinal pathways of humans using magnetic stimulation, we have developed a method for consistently recording conduction times between the motor cortex and the L4-5 level of the spinal cord. In 30 subjects, motor evoked potentials (MEPs) were recorded from the tibialis anterior muscle following contralateral motor cortex and peroneal nerve stimulation. In 18 of these subjects, the L4-5 intervertebral space was stimulated. The stimuli consisted of single, painless, short-duration magnetic pulses. In 12 subjects, measurements were made during voluntary ankle dorsiflexion, and during vibration of the TA tendon at rest. All subjects had measureable MEP latencies of 30.3 +/- 2.2 msec (mean +/- S.D.). The central motor conduction time (CMCT) was calculated using both a direct as well as an indirect method. The direct method in 18 subjects had a mean value of 16.2 +/- 1.7 msec, while the indirect method in all 30 subjects was 13.8 +/- 1.8 msec. No significant correlation of the CMCT was found with either age or height in these subjects. Ankle dorsiflexion significantly reduced the MEP latency and increased the amplitude, whereas vibration of the TA tendon significantly increased the amplitude alone. We conclude that MEPs may be consistently and painlessly measured in the lower extremity using magnetic stimulation in adults. Facilitation of the MEPs was produced more consistently by voluntary contraction than by vibratory stimulation of the tibialis anterior muscle tendon. Finally, CMCT was independent of both age and height in our study population.     

Magnetic stimulation study in patients with myotonic dystrophy

To further define motor nervous system alterations in myotonic dystrophy (MD), motor potentials to transcranial and cervical magnetic stimulation (MEPs) were recorded from the right abductor pollicis brevis muscle in 10 patients with MD and in 10 healthy controls. Cortical and cervical latencies, central motor conduction time (CMCT), stimulus threshold intensity and cortical MEP amplitudes expressed both as absolute values and as %M were analysed. MEP cervical latency, absolute or relative amplitude and excitability threshold did not significantly differ in patients and controls. The mean cortical motor latency and CMCT were significantly prolonged in MD patients with respect to normal subjects. Moreover, CMCTs were found to be significantly related to stimulus threshold intensity (P=0.03) and only marginally related to absolute cortical amplitude (P=0.06). These findings are indicative of a central motor delay, also related to decreased excitability of motor neurons, in patients with MD. No correlations were found between individual neurophysiological parameters and age, duration of disease and clinical impairment. Our results suggest that magnetic stimulation studies can detect subclinical dysfunctions of the central motor system in MD patients, as one of the multisystemic manifestations of the disease, rather independent of the primitive muscle damage.     

Prolonged central motor conduction time of lower limb muscle in spinocerebellar ataxia 6.

We investigated the function of corticospinal tract in spinocerebellar ataxia 6 (SCA6) by measuring the central motor conduction time (CMCT). Motor evoked potentials (MEP) of tibialis anterior (TA) muscle were elicited by magnetic stimulation to motor cortex and spinal cord in 9 SCA6 patients and 10 normal height- and age-matched subjects. CMCT in lower limb of SCA6 patients (18.1+/-1.9 ms) was significantly prolonged than that of the normal subjects (15.0+/-1.0 ms) ((p < 0.001). The prolonged CMCT was well correlated with the duration of disease (p = 0.005), but MEP amplitudes and stimulation intensities were not significantly different. These results indicate that the corticospinal tract function is also impaired and correlate with the disease duration in SCA6.     

Motor evoked potentials in multiple sclerosis patients without walking limitation: amplitude vs. conduction time abnormalities

We used Motor Evoked Potentials (MEPs), elicited by transcranial magnetic stimulation, for assessing a motor pathways dysfunction in a selected group of Multiple Sclerosis (MS) patients, without limitation in walking. We selected 32 Relapsing Remitting MS patients, in remission phase, with EDSS ≤ 3.5 and 20 healthy individuals with similar height and age distribution. We measured the following MEP parameters: motor thresholds; central motor conduction time (CMCT); amplitude and area, both expressed as MEP/CMAP ratio. Patients were divided into two groups according to the EDSS score: non-disabled group (ND; EDSS 0–1.5) and disabled group (D; EDSS 2–3.5). Mean average MEP values were significantly different in the patients compared with the controls. Even in MS patients with no or minor neurological signs (ND group), MEP parameters showed differences from controls and furthermore all MEP parameters were significantly different in the D group compared with the ND group. The 75% of the patients had an amplitude or area alteration; this percentage was significantly higher than the percentage of patients with a CMCT alteration (56.2%). In addition, CMCT increase was always associated with reduced amplitude and area, but amplitude and area alterations were present also in patients with normal CMCT. In early stages of MS, the higher percentage shown in alteration of MEP amplitudes and areas as opposed to CMCTs has not previously been highlighted in the literature. Independently of its pathogenesis (demyelination or axonal loss), the amplitude or area decrease should be considered in clinical trials and in follow-up studies, as a marker of the motor pathways dysfunction, at least as much as CMCT increase. Received in revised form: 3 April 2006     

脊髓型颈椎病病人经颅磁电刺激运动诱发电位的对比研究

目的探讨磁电刺激运动诱发电位（ＭＥＰ）在脊髓型颈椎病（ＣＳＭ）的应用价值，并对其临床相关性进行分析。方法采用经颅磁、电刺激对３０例脊髓型颈椎病病人以及年龄性别等相配匹的３０名健康成人分别于外展小指肌、肱二头肌及下肢展短肌表面进行ＭＥＰ的检测。结果全部病人的ＭＥＰ都出现异常，表现为潜伏期、中枢传导时间（ＣＭＣＴ）延长，时限增宽，波辐降低或不能引出。磁刺激ＭＥＰ的ＣＭＣＴ和皮层刺激潜伏期与脊髓型颈椎病临床日本整形外科协会（ＪＯＡ）评分间有密切相关性，能较好地反映ＣＳＭ病人的病情。结论ＭＥＰ在检测ＣＳＭ病人运动功能方面具有定量评价作用。与电刺激相比，磁刺激ＭＥＰ能更好地反映ＣＳＭ病人的病情。     

Effect of therapy on motor cortical excitability in Parkinson's disease

OBJECTIVE: To assess the impact of the disease stage and therapy on motor cortical excitability in Parkinson's disease (PD). METHODS: Twenty newly diagnosed and medication-free, early stage patients, 20 late stage patients under antiparkinsonian therapy and 20 normal healthy controls were included. Motor threshold (MT), amplitudes of motor evoked potential (MEP), motor evoked potential amplitude/compound muscle action potential amplitude (MEP/CMAP) ratio, central motor conduction time (CMCT) and cortical silent period (CSP) were measured by stimulation of the motor cortex using a 13.5 cm circular coil and recordings from abductor digiti minimi muscle. Following the first study protocol, early stage patients were given therapy and the same protocol was repeated three months later. RESULTS: Motor threshold was lower; and the MEP/CMAP ratio was higher in early and late stage patients than normals. In early stage patients after proper therapy, the MTs became higher than before therapy, but still remained lower than normals. In late stage patients, the CMCTs were shorter than the early stage patients before therapy and normals, but there was no difference between the early stage patients and normals. In early stage patients after therapy, the CMCT became longer than before therapy and this difference was significant in both late stage patients and normals. Although more prominent in late stage patients, the CSP duration in both PD groups was found shorter than normals. In early stage patients, after therapy, the CSP durations became significantly longer compared with before therapy. CONCLUSION: These findings suggest that the motor cortical excitability increases in PD because of the impairment of the corticomotoneuronal inhibitory system.     

Motor and cortical sensory evoked potentials by magnetic stimulation

Motor evoked potentials (MEP) by magnetic stimulation on the scalp and the spinous processes of the 7th cervical (C 7) and 5th lumbar (L 5) vertebrae were studied in 20 normal subjects and 10 patients with the pyramidal tract lesions. The magnetic stimulator composed of two flat helical coils with mean inner diameters of 12.0 and 2.2 cm. The evoked muscle action potentials were recorded from the thenar muscle in the hand and abductor hallucis muscle in the leg. The mean peak latencies of MEP recorded from the thenar muscle were 22.1 +/- 1.7 and 12.8 +/- 0.9 msec at the stimulations on the scalp and C 7, respectively. The central motor conduction time (CMCT) between the cortex and C 7 was 9.1 +/- 1.1 msec. On the other hand, the peak latencies of MEP were 41.0 +/- 3.2 and 21.6 +/- 2.3 msec at the stimulations on the scalp and L 5, respectively. CMCT between the cortex and L 5 was 19.3 +/- 2.3 msec. The patients with pyramidal tract involvements showed delayed peak latencies or absent MEP. The cortical somatosensory evoked potentials (SEP) by the noninvasive magnetic stimulation on the levels of Th 10, Th 12 and L 5 spines, gluteus and ankle were studied in 20 normal subjects and 7 patients with neurological diseases. Cortical components P 2 and N 2 were recorded clearly in all normal subjects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Central motor conduction time (CMCT) in hereditary motor sensory neuropathy type I (HMSN type I)

The motor system of 13 cases with hereditary motor sensory neuropathy (HMSN) type I were analysed by clinical neurophysiological method. The motor conduction velocity (MCV) and F wave latency in lower motor neurone were markedly delayed. The latency of the muscle evoked potential (MEP) by cortical magnetic stimulation were also markedly delayed. The central motor conduction times (CMCT) were calculated by two methods. CMCT-mag was calculated by subtraction of the MEP by cervical magnetic stimulation from the MEP by cortical magnetic stimulation. CMCT-f was calculated by subtraction of the [(F wave latency -1 + distal latency)/2] from the MEP by cortical magnetic stimulation. There were positive correlation between CMCT-f and CMCT-mag. CMCT of HMSN type I were divided to two groups. CMCT of the first group was markedly delayed. CMCT of the second group was mildly delayed or normal. The former group showed marked weakness in distal muscles clinically. The latter group showed mild or moderate weakness in distal muscles clinically. All these patients did not show any pyramidal tract signs, which could be covered by severe lower motor neurone involvements. The classification of HMSN type I by gene was well known, genetical analysis might be important to these groups in HMSN type I.     

Intracortical excitability in patients with relapsing–remitting and secondary progressive multiple sclerosis

We designed this study to investigate possible correlations between variables measuring primary motor cortex excitability detected by single and paired-pulse transcranial magnetic stimulation (TMS) and the severity of clinical manifestations in patients with multiple sclerosis (MS). Thirty patients with MS in remission, 16 with relapsing–remitting (RR), 14 with secondary progressive disease (SP) and 17 healthy subjects participated in the study. In each subject, the central motor conduction time (CMCT) was calculated, and single-pulse and paired-pulse TMS at 3 and 10 ms interstimulus intervals was delivered over the primary motor cortex of the dominant hemisphere to measure the amplitude of motor-evoked potentials (MEPs), motor threshold (MTh), intracortical inhibition (ICI) and facilitation (ICF). Correlations were determined between the patients’ TMS findings and magnetic resonance imaging (MRI) (lesion load) and clinical features (expanded disability status scale, EDSS score). EDSS scores were significantly higher in SPMS than in RRMS patients. The MTh was significantly higher, and the MEP was significantly smaller in SPMS patients than in RRMS patients and control subjects. All patients had longer CMCTs than healthy subjects. In all patients, paired-pulse TMS elicited an inhibited test MEP at the 3-ms ISI and a facilitated test MEP at the 10 ms ISI. Post hoc analysis showed that ICI was significantly lower in SPMS patients than in those with RRMS and healthy subjects. EDSS scores correlated significantly with TMS measures (MEP, ICI, CMCT and MTh), but not with MRI lesion load. It was found that intracortical excitability as measured with TMS differs according to the clinical course of MS; it remains normal in patients with low EDSS scores and is altered in patients with high EDSS scores.     

Facilitation and reciprocal inhibition by imagining thumb abduction.

It is well known that motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) of the motor cortex are facilitated by voluntary muscle contraction. We evaluated the effects of imagination of movements on MEP latencies of agonist and antagonist muscles in the hand using TMS. Twenty-two healthy volunteers were studied. TMS delivered at rest and while imagining tonic abduction of the right thumb. MEPs were recorded in response to magnetic stimulation over the scalp and cervical spine (C7-T1), and central motor conduction times (CMCT) were calculated. MEPs were recorded from right abductor pollicis brevis muscle (APB) and adductor pollicis muscle (AP) simultaneously. Imagination of abduction resulted in a shortened latency of MEPs in the APB muscle, and a prolonged latency in the AP muscle. But the imagination caused no significant change in the latency of MEPs elicited by stimulation over the cervical spine. The changes of the CMCT may account for these latency changes with imagination of movement. These findings indicate that imagination of thumb abduction facilitates motoneurons of agonist muscle and has an inhibitory effect on those of antagonist muscle (reciprocal inhibition).     

Preservation of central motor conduction in patients with spinal muscular atrophy type II

We evaluated the central (motor cortex to C8 motoneuron) and peripheral (C8 motoneuron to the muscle) motor conduction in 14 limbs of 7 patients with the intermediate form of spinal muscular atrophy (SMA II). The central motor conduction time (CMCT) was calculated using motor evoked potentials (MEPs) by transcranial magnetic stimulation and the results of a conventional F wave study. Peripheral conduction abnormality was found in 6 median nerves (43%) and 10 ulnar nerves (71%). Even in these patients with peripheral conduction abnormalities, the CMCT was consistently normal whenever the MEP was recorded. These results indicate that the motor conduction of the corticospinal fibers remains normal in SMA II.     

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.     

Motor-evoked potentials in response to fatiguing grip exercise in multiple sclerosis patients.

OBJECTIVE: This study examined central and peripheral effects of fatiguing exercise (3 min maximal grip) in healthy controls (n=10) and multiple sclerosis (MS) subjects with weakness, MS-W (n=16) and normal motor function, MS-NM (n=16) in the studied extremity.Method: Transcranial magnetic stimulation (TMS) was used to assess resting and facilitated motor-evoked potentials (MEPs) of abductor pollicus brevis (APB) and flexor carpi radialis (FCR) muscles before and after fatiguing exercise. Exercise-induced depletion and recovery of phosphocreatine (PCr) were measured using (31)P magnetic resonance spectroscopy ((31)PMRS) in FCR. RESULTS AND CONCLUSION: MS subjects demonstrated significantly lower peak force and a faster decline in force than controls. Contralateral muscle activation (hand grip) before the fatigue protocol resulted in significantly increased MEP amplitudes in all groups. Contralateral hand grip following fatiguing exercise resulted in significantly higher MEP amplitudes in controls and MS-NM subjects, but not MS-W subjects. Fatiguing exercise resulted in prolonged central motor conduction time (CMCT) in MS subjects, but not controls. No group differences in PCr depletion or resynthesis were observed. All groups demonstrated significant post-exercise depression (PED) of MEP amplitude that persisted beyond the time course of PCr recovery, indicating fatigue was central in origin. MS subjects were less able than controls to increase cortical excitability using contralateral muscle activation following fatiguing exercise, possibly indicating impaired conduction in the corpus callosum.     

Postexercise facilitation of motor evoked potentials following transcranial magnetic stimulation: a study in normal subjects

The size of the motor evoked potential (MEP) elicited by transcranial magnetic stimulation increases soon after a nonexhaustive voluntary contraction of the target muscle (postexercise facilitation). Our aim was to determine whether the duration or intensity of voluntary muscle contraction influenced postexercise facilitation in normal subjects. We recorded the MEP from the thenar muscles following contractions of different durations (5, 15, and 30 s) and intensities (10%, 25%, and 50% of maximal voluntary contraction). We found that every combination of the tested intensities and durations of physical effort could induce postexercise MEP facilitation. Although the degree of postexercise MEP facilitation was comparable across the different durations and intensities, the maximal facilitation was observed with the shortest and strongest muscle contraction. Our study thus defines the optimal setting to study postexercise facilitation for clinical purposes.     

Central motor conduction after magnetic stimulation in diabetes

Central motor conduction times (CMCTs), obtained by means of magnetic stimulation of the motor cortex and spinal roots, were studied in 138 patients affected by diabetes mellitus but with no signs or symptoms of central nervous system (CNS) involvement. CMCTs were significantly increased in diabetic patients (p < 0.001, t-test) with respect to normal controls, with values exceeding upper confidence limits (mean +/- 2.5 SD of controls) in about 30% of patients. There was no correlation between CMCT delay and type of diabetes (insulin-dependent or non-insulin-dependent), patient age, disease duration, degree of metabolic control compensation, presence or absence of retinopathy or nephropathy, and presence or absence of peripheral or autonomic neuropathy. CNS involvement in diabetes mellitus is discussed. Particular emphasis is given to the sensitivity and reliability of CMCTs obtained by means of magnetic stimulation as a tool in the early diagnosis of CNS functional alterations in diabetes mellitus.     

Transcranial magnetic stimulation: role in the evaluation of disability in multiple sclerosis

BACKGROUND: In patients with multiple sclerosis (MS), transcranial magnetic stimulation (TMS) has shown significant prolongation of central motor conduction time (CMCT). Abnormal CMCT may reflect sub-clinical involvement of motor pathways and correlate with clinical motor disability. OBJECTIVE: To determine the diagnostic yield of TMS in MS and the possible correlation of TMS abnormalities with clinical disability. MATERIALS AND METHODS: Thirty patients with clinically definite MS presenting in acute relapse or with progressive disease course and 30 healthy controls were evaluated. TMS parameters evaluated included threshold intensity, motor evoked potentials (MEP) amplitudes and latencies and CMCT. Reassessment studies were done after three months. STATISTICAL ANALYSIS: Student t-test, Mann-Whitney U test and Spearman's rank correlation test were used to assess the relationships. RESULTS: Patients with MS had significantly higher threshold intensities, prolonged CMCT and reduced MEP amplitudes as compared to controls. Abnormalities in at least one parameter were observed in 86.7% of patients. When inter-side asymmetries in MEP latency and/or in CMCT were considered, the diagnostic yield increased to 96.7%. The diagnostic yield was 74.7% for visual evoked potentials, 13.3% for brainstem auditory evoked response and 10% for cerebrospinal fluid oligoclonal band. One MS patient without pyramidal or cerebellar dysfunction had prolonged CMCT. CMCT abnormalities correlated significantly with the degree of pyramidal signs, limb ataxia, intention tremor, dysdiadokokinesia and overall cerebellar score. In patients who had clinical improvement, follow-up studies showed improvement in CMCT parameters. CONCLUSION: TMS is a highly sensitive technique to evaluate cortico-spinal conduction abnormalities in MS that may have no clinical correlate and in monitoring the course of the disease. The effects of cerebellar dysfunction on TMS results need further evaluation.     

Mechanism of prolonged central motor conduction time in compressive cervical myelopathy.

OBJECTIVE: To investigate the mechanism of prolonged central motor conduction time (CMCT) in compressive cervical myelopathy, we compared the calculated CMCT following transcranial magnetic stimulation (TCM) and evoked spinal cord potentials (ESCPs) following transcranial electric stimulation (TCE). METHOD: Motor evoked potentials following TCM were recorded from abductor digiti minimi and abductor hallucis brevis muscles in 16 patients with compressive cervical myelopathy. CMCT was calculated by subtracting peripheral conduction time using peripheral nerve stimulation from MEP latency. ESCPs following TCE were recorded intraoperatively from posterior epidural space. RESULTS: CMCT was prolonged and significant attenuation of the ESCP amplitude following TCE was observed in all patients with cervical myelopathy. In 8 of 16 patients CMCT was significantly prolonged but ESCPs were recorded at the C6-7 level with normal negative peak latency. CONCLUSIONS: Prolonged CMCT may occur with only a minor amount of conduction slowing in the corticospinal tract in compressive cervical myelopathy. Impaired temporal summation of multiple descending potentials following TCM produced delays of motor neuron firing that contribute to the mechanism of prolonged CMCT.     

SEQUENCE TYPING CONFIRMS THAT CAMPYLOBACTER JEJUNI STRAINS ASSOCIATED WITH GUILLAIN-BARRÉ AND MILLER-FISHER SYNDROMES ARE OF DIVERSE GENETIC LINEAGE, SEROTYPE, AND FLAGELLA TYPE

To determine the involvement of the corticospinal tract in Guillain-Barré syndrome (GBS), we examined central motor conduction in patients with GBS-like symptoms and hyperreflexia using a magnetic stimulation technique. The subjects were 3 patients who exhibited ascending muscle weakness 2–4 weeks after preceding infections. Deep tendon reflexes were exaggerated in all four limbs of the 3 patients. The results of cerebrospinal fluid examinations revealed protein elevation without pleocytosis. The serum anti-GM(1) antibody titer was elevated in 2 patients. The results of nerve conduction study revealed axonal motor neuropathy and normal F-wave conduction. Central motor conduction time (CMCT) in patients with hyperreflexia was significantly delayed compared to that in patients with GBS and areflexia (p < 0.001), and the delayed CMCTs were significantly improved in the recovery periods (p < 0.001). Although hyperreflexia is a controversial symptom in patients with GBS, these findings indicate that there is functional corticospinal tract involvement in patients with a GBS variant.     

Development of the corticospinal system and hand motor function: central conduction times and motor performance tests

Maturation of the corticospinal (CS) tract and hand motor function provide paradigms for central nervous system development. In this study, involving 112 participants (aged from 0.2 to 30 years), we evaluated central motor conduction times (CMCT) obtained with transcranial magnetic stimulation (TMS) during preinnervation conditions of facilitation and relaxation. Auditory reaction time, velocity of a ballistic movement of the arm, finger tapping, diadochokinesis, and fine motor visuomanual tracking were also examined. The maturation profiles for every parameter were calculated. CMCTs for the different preinnervation conditions reached adult values at different times and this could be explained by maturation of excitability at the cortical and spinal level. A stable phase for CMCTs and reaction time was reached during childhood. Parameters which measured motor speed and skill indicated that the development of these continued into adulthood. The maturation of the fast CS tract seems to be completed before the acquisition of the related motor performance has been accomplished. In conclusion, we could demonstrate that data from several neurophysiological methods can be combined and used to study the maturation of the function of the nervous system. This approach could allow appraisal of pathological conditions that show parallels with omissions or lack of developmental progress.     

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.