Peripheral Conduction Time Research Articles

Abnormal Corticospinal Excitability in Patients with Disorders of Consciousness

BackgroundTranscranial magnetic stimulation (TMS) has been frequently used to explore changes in the human motor cortex in different conditions, while the extent of motor cortex reorganization in patients in vegetative state (VS) (now known as unresponsive wakefulness syndrome, UWS) and minimally conscious (MCS) states due to severe brain damage remains largely unknown. Objective/hypothesisIt was hypothesized that cortical motor excitability would be decreased and would correlate to the level of consciousness in patients with disorders of consciousness. MethodsCorticospinal excitability was assessed in 47 patients (24 VS/UWS and 23 MCS) and 14 healthy controls. The test parameters included maximal peak-to-peak M-wave (Mmax), F-wave persistence, peripheral and central motor conduction times, sensory (SEP) and motor evoked (MEP) potential latencies and amplitudes, resting motor threshold (RMT), stimulus/response curves, and short latency afferent inhibition (SAI). TMS measurements were correlated to the level of consciousness (assessed using the Coma Recovery Scale-Revised). ResultsOn average, the patient group had lower Mmax, lower MEP and SEP amplitudes, higher RMTs, narrower stimulus/response curves, and reduced SAI compared to the healthy controls (P < 0.05). The SAI alterations were correlated to the level of consciousness (P < 0.05). ConclusionsThe findings demonstrated the impairment of the cortical inhibitory circuits in patients with disorders of consciousness. Moreover, the significant relationship was found between cortical inhibition and clinical consciousness dysfunction.

Connections between expiratory bulbospinal neurons and expiratory motoneurons in thoracic and upper lumbar segments of the spinal cord

Cross-correlation of neural discharges was used to investigate the connections between expiratory bulbospinal neurons (EBSNs) in the caudal medulla and expiratory motoneurons innervating thoracic and abdominal muscles in anesthetized cats. Peaks were seen in the cross-correlation histograms for around half of the EBSN-nerve pairs for the following: at T8, the nerve branches innervating internal intercostal muscle and external abdominal oblique muscle and a more distal branch of the internal intercostal nerve; and at L1, a nerve branch innervating internal abdominal oblique muscle and a more distal branch of the ventral ramus. Fewer peaks were seen for the L1 nerve innervating external abdominal oblique, but a paucity of presumed α-motoneuron discharges could explain the rarity of the peaks in this instance. Taking into account individual EBSN conduction times to T8 and to L1, as well as peripheral conduction times, nearly all of the peaks were interpreted as representing monosynaptic connections. Individual EBSNs showed connections at both T8 and L1, but without any discernible pattern. The overall strength of the monosynaptic connection from EBSNs at L1 was found to be very similar to that at T8, which was previously argued to be substantial and responsible for the temporal patterns of expiratory motoneuron discharges. However, we argue that other inputs are required to create the stereotyped spatial patterns of discharges in the thoracic and abdominal musculature.

Neural conduction and excitability following a simple warm up

ObjectiveThis study examined the effect of a generic, active warm up on neural and muscular conduction time. DesignSingle group, pre-post design. MethodsCentral and peripheral neuromuscular conduction time was quantified in the abductor pollicis brevis (APB) and gastrocnemius muscles of 18 healthy participants (mean age 25.9±5.8 years, 12 males) using transcranial magnetic stimulation (TMS) and M-wave techniques, prior to and immediately following an active warm up consisting of 5min running at 65% of maximum heart rate. Neural conduction time, for both TMS and M-wave, was quantified as the time between stimulus artefact and deflection of the wave form, whilst muscle conduction time for TMS and M-wave, was quantified from the stimulus artefact to the absolute peak twitch response. ResultsFollowing the warm up protocol, a significant reduction in muscle conduction time was found in both TMS and M-wave of 0.43ms (P=0.02) and 0.30ms (P=0.001) for the APB; and 0.29ms (P<0.001) and 0.87ms (P=0.003) for the gastrocnemius, respectively. No change was found in neural conduction using either TMS or M-wave techniques. ConclusionsThese findings support previous data which demonstrate an improvement in muscular conduction time and subsequent improvement in athletic performance post warm up. The data also make evident that changes in muscular conduction time are a global response to warm up and are not directly related to muscular activity. In contrast, neural conduction time did not change and should not be confused with changes in muscular conduction time in the literature.

P18.3 Peripheral and central (CCT) conduction times to proximal and distal muscles of lower limbs: methodological aspects and clinical application in MS patients

P18.3 Peripheral and central (CCT) conduction times to proximal and distal muscles of lower limbs: methodological aspects and clinical application in MS patients

Measurement of motor root conduction time at the early stage of Guillain–Barre syndrome

As they are mainly performed at distal nerve parts, routine electrophysiological examinations can fail to detect the abnormalities at the early stage of Guillain-Barre syndrome (GBS) because of predominant involvement of proximal nerve segments. Measurements focused on proximal conduction can provide additional findings. We investigated the diagnostic significance of motor root conduction time (MRCT) at the early stage of GBS. Study was performed in 30 healthy volunteers and within the first week of 12 patients with GBS. MRCT was calculated as the latency differences between motor nerve conduction time (MNCT) obtained by cervical magnetic stimulation and total peripheral motor conduction time obtained by electrical stimulation of ulnar nerve. Also MNCT/MRCT ratio was calculated in each subject. There were statistically significant differences between groups for MRCT (P < 0.0001) and MNCT/MRCT ratio (P < 0.0001). Although the F-wave latency at ulnar nerve was abnormal only in 33%, MRCT was significantly prolonged in 83% of patients. In three patients, prolongation of MRCT was the only abnormality at the first electrodiagnostic examination. Motor root conduction time as a non-invasive method can provide additional and confirmative information at the early stage of GBS in which routine nerve conduction studies may fail to detect the focal demyelination.

Somatosensory evoked potentials in epileptic children treated with carbamazepine or valproate in monotherapy – a preliminary study

Data from the literature suggest that long-term therapy with various antiepileptic drugs can be responsible for the functional disturbances within the nervous system e.g. peripheral neuropathy and encephalopathy. Useful and non-invasive instruments for evaluation of even subclinical nerve conduction abnormalities in somatosensory tracts are somatosensory evoked potentials (SEPs). The aim of this study was to assess the potentially drug-induced abnormalities in the SEP parameters in epileptic children, treated chronically in monotherapy with one of the two most often used antiepileptic drugs: valproate (VPA) or carbamazepine (CBZ). SEP from left median nerve stimulation were recorded in twenty children with idiopathic/cryptogenic epilepsy treated in monotherapy with CBZ (9 patients) or VPA (11 patients). The mean age of the patients was 13.4 ± 2.9 years (range 7-17 years). The plasma concentrations of the drugs were consistently within therapeutic range. The mean duration of treatment was eight months. The control group consisted of twenty-four age-matched children with tension type headache. The latencies of the components: N9, N13, N20, P25 and the peripheral conduction time (PCT) and central conduction time (CCT) were analyzed. No significant differences in all analyzed SEP parameters between the epileptic and control children were found. Our results indicate that during the first 8 months from the beginning of antiepileptic treatment in children, monotherapy with VPA or CBZ does not induce nerve conduction disturbances within both the peripheral- and the central part of the somatosensory tracts, detected in SEP examination.

Quantitative assessment of myelopathy patients using motor evoked potentials produced by transcranial magnetic stimulation

Motor evoked potentials (MEPs) study using transcranial magnetic stimulation (TMS) may give a functional assessment of corticospinal conduction. But there are no large studies on MEPs using TMS in myelopathy patients. The purpose of this study is to confirm the usefulness of MEPs for the assessment of the myelopathy and to investigate the use of MEPs using TMS as a screening tool for myelopathy. We measured the MEPs of 831 patients with symptoms and signs suggestive of myelopathy using TMS. The MEPs from the abductor digiti minimi (ADM) and abductor hallucis (AH) muscles were evoked by transcranial magnetic brain stimulation. Central motor conduction time (CMCT) is calculated by subtracting the peripheral conduction time from the MEP latency. Later, 349 patients had surgery for myelopathy (operative group) and 482 patients were treated conservatively (nonoperative group). CMCTs in the operative group and nonoperative group were assessed. MEPs were prolonged in 711 patients (86%) and CMCTs were prolonged in 493 patients (59%) compared with the control patients. CMCTs from the ADM and AH in the operative group were significantly more prolonged than that in the nonoperative group. All patients in the operative group showed prolongation of MEPs or CMCTs or multiphase of the MEP wave. MEP abnormalities are useful for an electrophysiological evaluation of myelopathy patients. Moreover, MEPs may be effective parameters in spinal pathology for deciding the operative treatment.

Age- and sex-related differences in brainstem auditory evoked potentials in secondary school students living in Northern European Russia

Brainstem auditory evoked potentials (BAEPs) were studied in 46 1st- to 11th-year students (22 boys and 24 girls) of a rural secondary school in Arkhangel’sk oblast. The objective of this work was to study age- and sex-related differences in BAEP characteristics in children and adolescents, living in the North and assess the BAEP characteristics as compared to reference values. In all three age groups of students, interpeak intervals I–III, III–V, and I–V characterizing the peripheral and central conduction times were shorter in girls than in boys. Interpeak interval III–V tended to increase with age only in boys (at puberty), with a significant increase in the latencies of waves I, III, and V. The BAEP characteristics in the subjects examined included a shorter peak latency and a greater amplitude of wave I (except senior students), relatively prolonged interpeak interval I–III, and more pronounced sex-related differences in BAEPs, especially at puberty. These findings show that it is necessary to revise regional reference values for BAEPs, differentiated by sex and age, including at puberty.

Diagnosis of Nonorganic Monoplegia With Single-Pulse Transcranial Magnetic Stimulation

Sir: Single-pulse transcranial magnetic stimulation (spTMS) is used in clinical practice to evaluate the integrity of the corticospinal tracts.1 Here, we describe how spTMS assisted in proving that a patient with plegia of an upper limb and ipsilateral ptosis was malingering. Case report.A 35-year-old woman was admitted to the neurology department in August 2007 due to plegia of the upper limb and ptosis on the right, which had been present for the previous 4 years but had worsened suddenly in the last 24 hours. Neurologic examination disclosed monoplegia (Medical Research Council power grade 0 on the right upper limb). There were no signs of spasticity or sensory deficits. Plantar responses were flexor. Right ptosis was present, but it was atypical, giving the impression of a voluntary contraction of the mimic muscles of the left, rather than that of a pyramidal lesion. Magnetic resonance imaging (MRI) of the brain and cervical spinal cord, nerve conduction studies, electromyogram, and lumbar puncture had been performed 3 times previously. The results were unremarkable. Repeat brain and cervical MRI were again normal. We therefore re-evaluated the patient, focusing on potential nonorganic causes of her clinical picture (hysteria or malingering). Indeed, the patient's husband reported that the symptoms had worsened several times, always in periods of family distress. Given this, we proceeded to spTMS evaluation. Single-pulse TMS was delivered with a Magstim Rapid-2 stimulator (Magstim Company Ltd., Whitland, United Kingdom). The hand motor cortex was targeted with a 70-mm figure-of-8 coil. Stimuli were delivered at 100% stimulator output. Motor-evoked potentials (MEPs) were recorded from the abductor digiti minimii (ADM) muscles bilaterally. The central motor conduction time (CMCT) was calculated as CMCT 14;= (MEP latency) 14;– 14;(peripheral conduction time).1 The CMCT was within normal limits (right ADM 14;= 14;6.8 ms, left ADM 14;= 14;6.7 ms). During the examination, the patient was distressed by the fact that she was not accustomed to the procedure and did not know its diagnostic potential. She was also surprised by the movement of her right upper limb in response to the magnetic pulses, and she persistently questioned us as to the meaning of our findings. On the basis of the normal CMCT, we reassured her that things were much better than she thought so far, that her symptoms would improve, and that she could accelerate her recovery by cooperating with the attending physicians. During the following day, the patient's right upper limb was still plegic, yet the ptosis was disappearing during conversation and reappearing when she was relaxed. Furthermore, she announced to her husband that the spTMS evaluation indicated a grave prognosis, contrary to what we had told her. On the basis of this evidence, we diagnosed malingering (according to DSM-IV-TR criteria) and referred the patient to the psychiatry department for further evaluation. Single-pulse TMS is a relatively new technique that is not used frequently in everyday practice; thus, patients are unaccustomed to it. As a result, it can prove useful in the management of malingerers simulating corticospinal lesions by providing neurophysiologic evidence on the integrity of the corticospinal tracts and by producing effects that surprise the patients. Spyros N. Deftereos, M.D., Ph.D. Gregory N. Panagopoulos, M.D. Department of Neurology Dimitra D. Georgonikou, M.D. Department of Psychiatry Elyssaios C. Karageorgiou, M.D. Panagiota N. Kefalou, M.D. Clementine E. Karageorgiou, M.D., Ph.D. Department of Neurology, Athens General Hospital “G. Gennimatas”, Athens, Greece

Subcortical somatosensory evoked potentials after posterior tibial nerve stimulation in children

We report our normative data of somatosensory evoked potentials (SEP) after posterior tibial nerve (PTN) stimulation from a group of 89 children and 18 adults, 0.4–29.2 years of age. We recorded near-field potentials from the peripheral nerve, the cauda equina, the lumbar spinal cord and the somatosensory cortex. Far-field potentials were recorded from the scalp electrodes with a reference at the ipsilateral ear. N8 (peripheral nerve) and P40 (cortex) were present in all children but one. N20 (cauda equina) and N22 (lumbar spinal cord) were recorded in 94 and 106 subjects, respectively. P30 and N33 (both waveforms probably generated in the brainstem) were recorded in 103 and 101 subjects, respectively. Latencies increased with age, while central conduction times including the cortical component, decreased with age (up to about age 10 years). The amplitudes of all components were very variable in each age group. We report our normative data of the interpeak latencies N8–N22 (peripheral conduction time), N22–P30 (spinal conduction time), N22–P40 (central conduction time) and P30–P40 (intracranial conduction time). These interpeak latencies should be useful to assess particular parts of the pathway. The subcortical PTN-SEPs might be of particular interest in young or retarded children and during intraoperative monitoring, when the cortical peaks are influenced by sedation and sleep, or by anesthesia.

Reorganization of the cortico-spinal pathway in patients with chronic complete thoracic spinal cord injury: A study of motor evoked potentials

To evaluate the change in motor evoked potential parameters following transcranial magnetic stimulation in patients with chronic complete thoracic cord injury. A cross-sectional study. Eighteen patients with chronic complete thoracic cord injury and 18 age- and sex-matched healthy controls were included in the study. The mean post-injury duration was 13.0 (standard deviation (SD) 6.0) years. The latency, amplitude, central conduction time and peripheral conduction time of motor evoked potentials from bilateral abductor pollicis brevis and first dorsal interosseous muscles following transcranial magnetic stimulation were measured and compared between the patients and healthy controls. The predicting variables for central conduction time, including age, sex, height, illness duration and job activity, were analysed using a simple correlation and stepwise multiple regression model. The patients with complete thoracic cord injury had longer central conduction time recording of the dominant hand in both abductor pollicis brevis and first dorsal interosseous muscles. The difference in latency approached significance between the patients and controls. There was no statistical difference in amplitude between them. Regression analysis demonstrated that patients who were older, less physically active and with longer illness duration showed prolonged central conduction time. The central conduction time in the dominant hand of chronic complete thoracic cord injury is prolonged. This study revealed motor reorganization of the central nervous system in complete thoracic cord injury. Decreased physical activity and prolonged illness may cause these changes.

A clinical study of corticospinal and peripheral conductions to proximal lower limb muscles

Objective To investigate the clinical utility of the newly developed “quadriceps combined technique” (QCT), which provides a global evaluation of the central and peripheral conduction to the proximal muscles of lower limbs, in a variety of central and peripheral neurological disorders. Methods Using surface recordings from the vastus medialis of the quadriceps muscle, we analyzed amplitudes and latencies of M response, patellar T reflex and motor evoked potentials (MEPs) after transcranial magnetic stimulation. We studied 180 patients with disorders impairing proximal strength of one or both lower limbs and compared them with 100 controls reported previously. Results The best parameters to detect central motor disorders were the central motor conduction time, MEP/M amplitude, T/MEP amplitude and latency ratios, whereas peripheral motor conduction time (PMCT) was best to assess peripheral disorders. The best parameter to identify proximal peripheral disorder was PMCTprox, whereas for distal peripheral disorders M amplitude and T/MEP amplitude ratio were most discriminative. Conclusions We report a simple, rapidly performed and well-tolerated method that improves proximal lower limbs evaluation, helps distinguishing pathologic from physiological brisk reflexes and provides clues for etiologic diagnosis. Significance The QCT is a sensitive and specific tool to investigate central and peripheral neurological disorders.

Significant correlation between corticospinal tract conduction block and prolongation of central motor conduction time in compressive cervical myelopathy

We examined the relationship between the CMCT and features of spinal cord evoked potentials (SCEPs) among 25 patients with compressive cervical myelopathy to elucidate the mechanism underlying the prolonged central motor conduction time (CMCT) in patients with compressive cervical myelopathy. CMCT values were calculated by measuring motor evoked potentials from the abductor digiti minimi muscles (ADMs) and abductor hallucis muscles (AHs) following transcranial magnetic stimulation and peripheral conduction times determined in the ulnar and tibial nerves. SCEPs following transcranial electrical stimulation were recorded intraoperatively from C2-3, C6-7 and T11-12. The shorter/longer CMCTs between the patients' right and left ADMs and AHs were 9.5 ± 3.2/11.5 ± 3.8 and 16.2 ± 2.8/18.8 ± 3.3 ms, respectively (mean ± SD). The percentage ratio of the amplitude of the D-wave at C6-7 or T11-12 to that at C2-3 was 19.4 ± 14.2 or 3.2 ± 3.1%, respectively. The CMCT value was significantly correlated with the attenuation of SCEP amplitude, but not with SCEP latency both at C6-7 and T11-12, suggesting that CMCT prolongation is primarily due to corticospinal conduction block rather than conduction delay. Spinal motor neurons might need more time to fire in patients with compressive cervical myelopathy when corticospinal potentials, but not conduction, are attenuated, thereby resulting in prolonged CMCT.

Electrophysiological evaluation of motor pathways to proximal lower limb muscles: A combined method and reference values

Objective In contrast with their important functional and clinical role, motor pathways to proximal muscles of lower limbs are rarely investigated in clinical neurophysiology. We describe a method to evaluate central and peripheral pathways to these muscles and report reference values. Methods Recording of both quadriceps was performed in 100 subjects. We analyzed the maximal M response after electrical stimulation of the femoral nerve, the patellar T reflex and the motor evoked potential (MEP) after transcranial magnetic stimulation. We defined the central motor conduction time as the difference between the MEP latency and the peripheral motor conduction time, estimated as the half of the T latency minus 0.5 ms. Results The mean MEP latency is 20.6 ms (SD 1.99 ms), central motor conduction time 10.1 ms (SD 1.29 ms), MEP/M amplitude ratio 60.0% (SD 15.75%). Normal limits according to height and age are provided for each parameter and for interside asymmetry. Conclusions This method to investigate the central and peripheral motor pathways supplying the L2–L4 myotomes is simple, painless and rapidly performed. The T reflex provides additional information on the proximal sensory pathways. Significance This method will be useful in many clinical conditions.

Corticospinal tract conduction block results in the prolongation of central motor conduction time in compressive cervical myelopathy

Objective The objective of this study was to analyze corticospinal function in patients with compressive cervical myelopathy and to elucidate the mechanism underlying its prolonged central motor conduction time (CMCT). Methods Motor evoked potentials following transcranial magnetic stimulation (TMS) and peripheral conduction time in the ulnar and tibial nerves following electrical stimulation were measured from the abductor digiti minimi (ADM) and abductor hallucis (AH) muscles in 24 patients with compressive cervical myelopathy and used to calculate CMCT. Spinal cord evoked potentials (SCEPs) following transcranial electric stimulation were recorded intraoperatively from the C2-3 to C6-7 intervertebral levels. Correlations between prolonged CMCT and SCEP values were then estimated. Results The shorter/longer CMCT between the patients' right and left ADM and AH were 8.5±2.9/11.5±3.3 and 16.2±3.1/18.4±3.3 ms, respectively (mean±SD). The SCEPs amplitude at C6-7, compared to C2-3, was 25.7±21.0%. The attenuation of SCEP amplitude, but not latency, at C6-7 correlated significantly with CMCT prolongation. Conclusions Our data support the view that CMCT prolongation is primarily due to corticospinal conduction block, rather than conduction delay. Significance Insight was provided into the mechanism of corticospinal dysfunction in compressive cervical myelopathy.

Manipulation of peripheral neural feedback loops alters human corticomuscular coherence

Sensorimotor EEG shows approximately 20 Hz coherence with contralateral EMG. This could involve efferent and/or afferent components of the sensorimotor loop. We investigated the pathways responsible for coherence genesis by manipulating nervous conduction delays using cooling. Coherence between left sensorimotor EEG and right EMG from three hand and two forearm muscles was assessed in healthy subjects during the hold phase of a precision grip task. The right arm was then cooled to 10 degrees C for approximately 90 min, increasing peripheral motor conduction time (PMCT) by approximately 35% (assessed by F-wave latency). EEG and EMG recordings were repeated, and coherence recalculated. Control recordings revealed a heterogeneous subject population. In 6/15 subjects (Group A), the corticomuscular coherence phase increased linearly with frequency, as expected if oscillations were propagated along efferent pathways from cortex to muscle. The mean corticomuscular conduction delay for intrinsic hand muscles calculated from the phase-frequency regression slope was 10.4 ms; this is smaller than the delay expected for conduction over fast corticospinal pathways. In 8/15 subjects (Group B), the phase showed no dependence with frequency. One subject showed both Group A and Group B patterns over different frequency ranges. Following cooling, averaged corticomuscular coherence was decreased in Group A subjects, but unchanged for Group B, even though both groups showed comparable slowing of nervous conduction. The delay calculated from the slope of the phase-frequency regression was increased following cooling. However, the size of this increase was around twice the rise in PMCT measured using the F-wave (regression slope 2.33, 95% confidence limits 1.30-3.36). Both afferent and efferent peripheral nerves will be slowed by similar amounts following cooling. The change in delay calculated from the coherence phase therefore better matches the rise in total sensorimotor feedback loop time caused by cooling, rather than just the change in the efferent limb. A model of corticomuscular coherence which assumes that only efferent pathways contribute cannot be reconciled to these results. The data rather suggest that afferent feedback pathways may also play a role in the genesis of corticomuscular coherence.

Role of motor evoked potentials in diagnosis of cauda equina and lumbosacral cord lesions

To determine the diagnostic value of motor evoked potentials (MEPs) in the diagnosis of lumbosacral cord disorders. MEPs in 37 patients with sensory and motor deficits in the lower limbs were studied. MRI demonstrated spinal cord involvement in 10 patients and cauda equina lesions in 27 patients. A double determination of central motor conduction time (CMCT), calculated as the difference between the latencies of responses evoked by cortical and paravertebral magnetic stimulation and as the difference between the latency of cortical MEP and the total peripheral conduction time calculated from the F-wave latency, enabled discrimination between a delay along the proximal root and a delay along the corticospinal tract. An abnormality of the CMCT calculated with both techniques is indicative of central motor pathway damage, whereas an abnormality of the CMCT calculated from the latency of responses evoked by paravertebral magnetic stimulation associated with a normal CMCT calculated from the F-wave latency suggests a cauda equina lesion. Neurophysiologic findings strongly correlated with the lesion site documented by MRI (cauda equina or lumbosacral cord). All patients with MR evidence of cord involvement had an abnormality of CMCT calculated with both methods, suggesting a lesion of central motor pathways. Clinical examination often failed to document a spinal cord lesion, suggesting pure peripheral involvement in 5 of the 10 patients with MR evidence of cord lesion. Motor evoked potential recording is an accurate and easily applicable test for the diagnosis of lumbosacral spinal cord lesions.

The effect of electrical stimulation of the corticospinal tract on motor units of the human biceps brachii.

In healthy human subjects, descending motor pathways including the corticospinal tract were stimulated electrically at the level of the cervicomedullary junction to determine the effects on the discharge of motoneurones innervating the biceps brachii. Post-stimulus time histograms (PSTHs) were constructed for 15 single motor units following electrical stimulation of the corticospinal tract and for 11 units following electrical stimulation of large diameter afferents at the brachial plexus. Responses were assessed during weak voluntary contraction. Both types of stimulation produced a single peak at short latency in the PSTH (mean 8.5 and 8.7 ms, respectively) and of short duration (< 1.4 ms). In separate studies, we compared the latency of the responses to electrical stimulation of the corticospinal tract in the relaxed muscle with that in the contracting muscle. The latency was the same in the two conditions when the intensity of the stimulation was adjusted so that responses of the same size could be compared. Estimates of the descending conduction velocity and measurements of presumed peripheral conduction time suggest that there is less than 0.5 ms for spinal events (including synaptic delays). We propose that in response to electrical stimulation of the descending tract fibres, biceps motoneurones receive a large excitatory input with minimal dispersion and it presumably contains a dominant monosynaptic component.

Brain responses to signals ascending through C-fibers

The necessary and sufficient condition to record brain responses to signals ascending through C-fibers seems to be avoidance of concomitant activation of Adelta-fibers. Several explanations are offered in the literature. One more is added, based on the phenomenon of post-event desynchronisation. Four methods, having in common the ability to selectively activate C-fiber afferents, are currently used to obtain ultralate-laser evoked potentials (LEPs) in a reliable way and with the appropriate latency. Exception made for the difference in latency, mainly due to the difference in peripheral conduction time, morphology and dynamic topography of the ultralate-LEPs (C-fibers), are similar to the late-LEPs (Adelta-fibers). These results suggest that both pathways have common cortical generators.

Electrophysiological evaluation of conduction in the most proximal motor root segment

Root conduction time (RCT), defined as the time difference between M-wave latency by cervical magnetic stimulation (CMS) and the total peripheral motor conduction time calculated from the shortest F-wave latency, was investigated in patients with inflammatory demyelinating neuropathies (IDP) and amyotrophic lateral sclerosis (ALS). The minimal threshold for CMS also was studied. In the IDP patients, conduction in the proximal motor root segment was considered abnormal in 52% by the RCT and in 47% by the minimal threshold for CMS, whereas both were normal in 85% of the ALS patients. These findings suggest that the RCT and minimal threshold for CMS might be additional parameters for evaluating motor nerve conduction in IDP.