Facilitation of motor evoked potentials by somatosensory afferent stimulation.

The effect of an electrically induced peripheral afferent volley upon electrical and magnetic motor evoked potentials (MEPs) from muscles of the upper and lower extremities was studied in 16 healthy volunteers. A standard conditioning-test (C-T) paradigm was employed whereby the test stimulus (transcranial electric or magnetic) was applied at random time intervals, from 10 msec prior to 90 msec after the conditioning stimulus (peripheral nerve stimulus). MEP amplitude facilitation was observed for the majority of the upper extremity muscles tested at two distinct periods, one occurring at short, and the other at long C-T intervals. This bimodal trend of MEP facilitation was found to be equally as prominent in the lower extremity muscles tested. The period of short C-T interval facilitation is consistent with modifications in the spinal excitability of the segmental motoneuron pool. On the other hand, the period of long C-T interval facilitation is suggested to be due to alterations in excitability of the motor cortex as a result of the arrival of the orthodromic sensory volley. Although most pronounced in muscles innervated by the nerve to which the conditioning stimulus was applied, this bimodal facilitatory effect was also observed in adjacent muscles not innervated by the stimulated nerve. Qualitatively, the conditioned MEPs from the upper and lower extremities responded similarly to both electrical and magnetic trans-cranial stimulation. In addition, our study demonstrates that the C-T paradigm has potential for use in the assessment of spinal and cortical sensorimotor integration by providing quantitative information which cannot be obtained through isolated assessment of sensory and/or motor pathways.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of hand muscle motoneurones by peripheral nerve stimulation in the relaxed human subject. Antidromic versus orthodromic input

In active muscle, a supramaximal conditioning stimulus to peripheral nerve produces a classic silent period in the EMG. The present experiments examined the effect of this type of conditioning stimulus on motoneurone excitability in relaxed muscle.EMG responses evoked by transcranial magnetic stimulation of the brain were recorded from the first dorsal interosseus muscle (FDI) in 10 healthy subjects and 5 patients with sensory neuropathy. These responses (motor evoked potentials) were conditioned by supramaximal peripheral nerve stimuli given 0–150 msec beforehand. In the normal subjects, the classic silent period in the FDI lasted about 100 msec. The same conditioning stimulus only abolished motor evoked potentials when the conditioning-test interval was so short that the antidromic peripheral nerve volley collided with the orthodromic volley set up by magnetic brain stimulation. At longer conditioning-test intervals, although remarkably inhibited (65% mean suppression between 10 and 40 msec), the test motor potential was never completely abolished and gradually recovered by 100 msec.Inhibition of cortically evoked motor potentials did not depend upon activity set up by the conditioning stimulus in peripheral nerve sensory fibres. The patients with complete peripheral sensory neuropathy had the same extent and time-course of inhibition as the normal subjects. We conclude that in relaxed subjects the inhibitory effect of peripheral conditioning results almost exclusively from the motoneuronal inhibitory mechanisms consequent to antidromic invasion.     

Afferent excitation of human motor cortex as revealed by enhancement of direct cortico-spinal actions on motoneurones

Changes in motor cortex excitability induced by somatosensory afferences were evaluated in 5 subjects by testing how the short-latency cortico-spinal effects evoked by transcranial magnetic stimulation in flexor carpi radialis (FCR) motoneurones were influenced by volleys in median nerve afferent fibres. Transcranial magnetic stimulation induced two facilitatory peaks on FCR H reflex, the first at a conditioning-test interval of about −3 msec and the second at 0 msec, separated by a phase of inhibition. If an electric shock to the median nerve at the wrist, 0.8-1 × motor threshold (MT) for thenar muscles, preceded the cortical stimulus by 18–25 msec, an increase in size of both facilitatory peaks was observed. The increase was partly due to a direct action of the median nerve volley on motoneurones. When this contribution was subtracted, two peaks of additional facilitation resulted as the effect of combined conditioning. Additional facilitation was present even during the short-lasting phase ascribed to monosynaptic cortico-spinal excitation of motoneurones, i.e., the first millisecond of the earliest facilitatory peak. This result indicates that cortical responsiveness to magnetic stimulation had been enhanced by the peripheral stimulus. The time course of the excitability changes in motor cortex was compared with the cortical somatosensory evoked potentials (SEPs) induced by the same peripheral stimulus. Additional facilitation was present immediately after the N20 peak of SEPs and lasted 8–10 msec. Additional facilitation had the same threshold as N20 (0.6 × MT) and grew in parallel with it when grading the afferent stimulus up to 1 MT.     

Audio-spinal influence in man studied by the H-reflex and its possible role on rhythmic movements synchronized to sound.

An investigation was made of the time course of audio-spinal influences in man using the H-reflex technique and non-startling sounds. It was found that in all subjects the sound potentiated the H-reflex at a central latency of 80 msec, the peak facilitation (185%) being attained at 110--130 msec. The mean duration of this facilitation was 200 msec ranging from 120 to 460 msec. No inhibition was seen to follow the excitatory period. An habituation study showed a significant drop in peak facilitation after exposure to ten conditioning stimuli but a constant increase of the H-reflex above control level even after 60 presentations. The time course of this audiospinal facilitation was superposed over the EMG events during hopping to a simplified musical stimulus. In this situation, landing occurred some 50 msec prior to the ON beat or strong beat of the music. With this mode of synchronization, the timing of the ON and OFF beats of the musical stimulus would be suitable to potentiate the EMG events related respectively to the peak upwards acceleration determining the take-off and to the landing. It is inferred that during synchronized stereotyped movements to repetitive auditory stimuli, the motor events are timed to make best use of a potential audio-spinal facilitation.     

Modification of nicotinic ganglionic transmission by muscarinic slow postsynaptic potentials in the in vitro rabbit superior cervical ganglion

The influence of slow muscarinic postsynaptic potentials, i.e., the s-IPSP and s-EPSP, on synaptic transmission mediated through nicotinic receptors was studied in the superior cervical ganglion of the rabbit. Postganglionic spikes and synaptic potentials were elicited by delivery of conditioning and test stimulus pulses to afferent fibers. When paired stimulus volleys were separated by brief intervals (20-100 msec) or long intervals (1,000-8,000 msec), the population spike elicited by the test stimulus was larger in amplitude than that elicited by the conditioning volley. When paired stimulus volleys were separated by 250-500 msec, the amplitude of the population spike elicited by the test volley was smaller than that elicited by the conditioning stimulus. Gallamine, which selectively blocks the s-IPSP, reduced the suppression of the test spike which occurred when stimulus IPIs ranged between 250-500 msec. Pirenzepine, which selectively blocks the s-EPSP, reduced the late facilitation of test postganglionic spikes which occurred with stimulus IPIs greater than 1,000 msec. The non-selective muscarinic antagonist QNB, produced changes in postganglionic spike amplitude that were similar to the combined effects of gallamine and pirenzepine. The evidence indicates that the s-IPSP and s-EPSP modified the excitability state of the ganglionic neurons and subsequent synaptic transmission that was mediated through nicotinic receptors.     

Human motor evoked responses to paired transcranial magnetic stimuli.

We studied the changes in motor pathway excitability induced by transcranial magnetic stimulation of the motor cortex, using paired stimuli (conditioning and test stimulus) and varying interstimulus interval (ISI). The effects induced depended on the stimulus intensity. At a low intensity, there was inhibition of the response to the test stimulus at ISIs of 5-40 msec, followed by facilitation at ISIs of 50-90 msec. At a high intensity, there was facilitation at ISIs of 25-50 msec, followed by inhibition at ISIs of 60-150 msec and, occasionally, by another phase of facilitation at ISIs of more than 200 msec. Only tentative explanations are currently possible for these effects: the inhibition observed at low intensities and short ISIs may be due to activation of cortical inhibitory mechanisms. The facilitation that follows may arise from the coincidence of various factors that transiently increase the excitability in alpha motoneurons. The early facilitation observed at high intensities seems to be a consequence of a rise in cortical excitability induced by the conditioning stimulus, causing an increase in the number or size, or both, of descending volleys from the test stimulus. The profound inhibition that follows probably results from a combination of both segmental and suprasegmental inhibitory mechanisms.     

Direct demonstration of the effect of lorazepam on the excitability of the human motor cortex.

OBJECTIVES: The present study explored the effects of lorazepam, a benzodiazepine with agonist action at the GABA(A) receptor, on human motor cortex excitability as tested using transcranial magnetic stimulation. METHODS: We recorded directly the descending volley evoked by single and paired transcranial magnetic stimulation from the spinal cord of a conscious subject with a cervical epidural electrode before and after a single oral dose of lorazepam. We evaluated the effects of lorazepam on the descending volleys evoked by a single magnetic stimulation and paired cortical stimulation using the intracortical inhibition paradigm (subthreshold conditioning stimulus) and the short latency intracortical facilitation paradigm (suprathreshold conditioning stimulus). RESULTS: Using a single magnetic stimulus lorazepam decreased the amplitude of the later I waves in the descending volley; this was accompanied by a decrease in the amplitude of the evoked EMG response. Using the intracortical inhibition paradigm lorazepam increased the amount of corticocortical inhibition, particularly at 4 and 5 ms interstimulus intervals. There was no effect on the amount of facilitation observed in the short latency intracortical facilitation paradigm. CONCLUSIONS: The present findings provide direct evidence that lorazepam increases the excitability of inhibitory circuits in the human motor cortex.     

Inhibitory and facilitatory effects from the peroneal nerve onto the soleus H-reflex in normal and spinal man

The effects of conditioning stimulation of a mixed nerve in the leg, the common peroneal nerve (CPN), on the ipsilateral soleus H-reflex were compared with the effects of stimulating its cutaneous branch, the superficial peroneal nerve (SPN), in two groups of subjects--normals and patients with spinal spasticity subsequent to a clinically complete transection of the spinal cord. Condition-test delays of 20 msec to 2 sec, measured from the end of the 20 msec train (3 pulses at 100 Hz), were investigated. In normal subjects, CPN stimulation at 1.4 X MT profoundly depressed the soleus H-reflex. There was an initial depression (peak 40-90 msec) followed by a slow recovery which was incomplete at condition-test delays of 2 sec. One-half of the subjects showed a late facilitation, or disinhibition, peaking at 170-190 msec. The inhibitory effects were attributed to activation of low threshold, groups I and II, muscle afferents because stimulation of the SPN, at 1.5 X threshold for a compound action potential recorded from the CPN, had only facilitatory effects on the soleus H-reflex. Facilitation occurred at condition-test delays of 30-190 msec. The cutaneous stimulation was presumed to activate the largest, A beta, cutaneous afferents as it elicited a weak paraesthesia on the dorsum of the foot. The results suggested that cutaneous afferents may have contributed to the late facilitation seen with CPN conditioning stimulation. In spinal cord-lesioned subjects, CPN stimulation depressed the soleus H-reflex but the decrease was less and the recovery was faster and more complete than in normals. The magnitude of the initial depression at 20-100 msec varied with the severity of the spasticity, subjects with mild spasticity showing less of a depression. Weak cutaneous conditioning stimulation either had no effect or produced a slight depression of the soleus H-reflex, providing clear evidence that transmission in the pathways mediating the facilitatory effects of cutaneous afferents onto extensor motoneuronal pools is depressed in spinal spasticity. This may shift the balance of activity toward the flexor motoneurones, thus favouring the development of, for example, flexor spasms and flexor hypertonia. Since inhibitory effects from cutaneous stimulation are associated with activation of higher threshold afferents in normal man, the present results may reflect a decrease in the threshold for flexor withdrawal reflexes commonly associated with spasticity of spinal origin.     

Contralateral influences on triceps surae motoneuron excitability.

In an effort to more fully investigate spinal reflex pathways in humans, we measured the isometric force-time curve of the tibial nerve H-reflex in 12 college age subjects. We also conditioned the reflex with a contralateral H-reflex stimulus or a contralateral tendon-tap, to ascertain the effects of crossed spinal segmental inputs on alpha motoneuron excitability. The conditioning stimulus preceded the test reflex by 10, 25, 40, 55, 70, 85, 100, 115, 130 or 145 msec. The results demonstrate that a conditioning tibial nerve H-reflex produced marked facilitation onto the contralateral triceps surae motoneurons, predominantly at longer-latency intervals. Conversely, a conditioning Achilles tendon-tap produced long-latency inhibition to the triceps surae. These results demonstrate that differential motoneuron excitability changes can be produced by electrical and mechanical conditioning stimuli. Moreover, these excitability changes may be long lasting and only appear after a relatively long latency. Several neurophysiological mechanisms are proposed to contribute to these changes.     

Facilitation of acetylcholine secretion at a mouse neuromuscular junction

Facilitation of transmitter secretion from motor nerve terminals following one or more conditioning stimuli was examined in mouse sternomastoid muscles. Following a single conditioning stimulus at 20 degrees C, facilitation decayed exponentially during the first 70-80 msec with a time constant of 60 msec. After 70--80 msec, a small slower component of facilitation was apparent. Initial facilitation (obtained by extrapolation back to 'zero' time) had a value of approximately 0.5. Following more than one conditioning stimulus (2-6), initial facilitation was greater but the pattern of decay was similar, the slower component becoming more obvious as the number of conditioning stimuli was increased. The slow decay phase also appeared exponential. The pattern of decay of facilitation could be well fitted by the sum of two exponentials, F1 (0)exp(--t/tau 1) + F2(0)exp(--t/tau 2). After a single stimulus at 20 degrees C, tau 1 and tau 2 had mean values of 35 and 163 msec. The main effect of increasing the number of conditioning stimuli was to increase (F1(0) and F2(0) with little change in tau 1 or tau 2. Changing temperature from 30 degrees C to 10 degrees C increased F1(0) and tau 2 but had relatively little effect on F2(0) and tau 1.     

Effects of H-reflex conditioning upon the contralateral alpha motoneuron pool.

H-reflex recovery curves have been elicited by subliminal conditioning stimuli applied to the ipsilateral and contralateral posterior tibial nerves in 10 healthy female subjects. In both types of recovery curve there was clear evidence of a period of facilitation in the ipsilateral soleus motoneuron pool 75-250 msec after the conditioning stimulus. These results indicate the bilateral nature of the facilitation and show it to be most probably produced by stimulus-evoked inputs as opposed to twitch-evoked inputs. If the facilitation is produced by descending long-loop reflex influences, then complementary evidence is provided for previous electromyographic data showing the bilaterality of long-loop reflexes evoked by percutaneous electrical stimulation. It is impossible, however, at the present time, to rule out the possible involvement of cutaneous afferent discharges or other stimulus evoked inputs in the late facilitation.     

MEASURING VARIOUS SIZES OF H-REFLEX WHILE MONITORING THE STIMULUS CONDITION

The purpose of this study was to assess the usefulness of a new technique that measured various sizes of the soleus H-reflex, while monitoring the stimulus condition. Eight healthy volunteers participated in this experiment. In the new technique, an above-motor-threshold conditioning stimulus was given to the tibial nerve 10-12 ms after a below-motor-threshold test stimulus. The conditioning stimulus evoked a direct M-wave, which was followed by a test-stimulus-evoked H-reflex. This reflex was followed by a conditioning stimulus-evoked H-reflex. The amount of the voluntary-contraction-induced facilitation of the H-reflex was similar for both the new technique and conventional technique, in which an above-motor-threshold test stimulus was given without a conditioning stimulus. Using the new technique, we found that the amount of facilitation increased linearly with the size of the test H-reflex. This technique allows us to evoke various sizes of H-reflex while monitoring a stimulus condition, and is useful for measuring H-reflexes during voluntary movement.     

Measuring various sizes of H-reflex while monitoring the stimulus condition

The purpose of this study was to assess the usefulness of a new technique that measured various sizes of the soleus H-reflex, while monitoring the stimulus condition. Eight healthy volunteers participated in this experiment. In the new technique, an above-motor-threshold conditioning stimulus was given to the tibial nerve 10-12 ms after a below-motor-threshold test stimulus. The conditioning stimulus evoked a direct M-wave, which was followed by a test-stimulus-evoked H-reflex. This reflex was followed by a conditioning stimulus-evoked H-reflex. The amount of the voluntary-contraction-induced facilitation of the H-reflex was similar for both the new technique and conventional technique, in which an above-motor-threshold test stimulus was given without a conditioning stimulus. Using the new technique, we found that the amount of facilitation increased linearly with the size of the test H-reflex. This technique allows us to evoke various sizes of H-reflex while monitoring a stimulus condition, and is useful for measuring H-reflexes during voluntary movement.     

Effect of stimulation of an upper limb on motor evoked potentials in lower limb muscles to transcranial magnetic stimulation in normal subjects and patients with thalamic infarction.

The effects of conditioning stimulation of an upper limb on motor evoked potentials (MEPs) of relaxed muscles in both lower limbs were studied in 7 normal subjects and two patients with left thalamic infarction. A possible mechanism for the Jendrassik maneuver (JM) is that induced proprioceptive input ascends supraspinally to facilitate the descending volleys. In order to mimic the JM with a more controlled influence, we used an electrical conditioning (C) stimulation (4 times sensory threshold) delivered to the left index finger preceding the transcranial (T) magnetic stimulation at C-T intervals of 0-200 ms. The MEP facilitation of bilateral tibialis anterior (TA) and gastrocnemius medialis (GC) was within C-T 70-110 ms. The peak facilitation was at C-T 80 ms for ipsilateral TA (309%) and GC (405%) and at C-T 90 ms for contralateral TA (207%) and GC (283%). In the two thalamic infarction patients with right-sided sensory loss, the facilitation did not occur when the conditioning stimulation was delivered to the affected index finger. Therefore, it is likely that the peripheral volley must be transmitted supraspinally to facilitate MEPs of the lower limbs. This method for studying sensory facilitation is more quantitative and reproducible than the JM and technically better than other previously described methods for somatosensory conditioning.     

Effects of transcranial electrical and magnetic stimulation on reciprocal inhibition in the human arm

We studied the effects of transcranial electrical stimulation (TES) and transcranial magnetic stimulation (TMS), delivered at intensities below the threshold for evoking an electromyographic response, on the disynaptic and presynaptic phases of reciprocal inhibition in 8 healthy subjects. After TES, the H-reflex evoked in the flexor carpi radialis (FCR) muscle was strongly facilitated when the cortical stimulus was given 4.0–4.5 ms after the test stimulus (median nerve stimulus). TES reduced the disynaptic phase of reciprocal inhibition most strongly when the cortical stimulus followed the test stimulus by 3.0–3.5 ms. TES also reduced presynaptic inhibition, but with a time course that was identical to that of the facilitation of the uninhibited H-reflex. After subthreshold TMS, the facilitation of the H-reflex showed at least 2 peaks, one occurring when the cortical stimulus was given 2 ms after the test stimulus and the other when the cortical stimulus followed the test stimulus by 0.5 to −1.5 ms. The effects of TMS on the 2 phases of reciprocal inhibition were similar, and in both cases the disinhibitory effects had essentially the same time course as the facilitatory effect of TMS on the uninhibited H-reflex. The different effects of TES on the 2 phases of reciprocal inhibition provide evidence of the presynaptic natuer of the second phase. The absence of a difference in the effect of TMS on the 2 phases could be due to the more temporally dispersed descending volley after TMS.     

Intracortical inhibition and facilitation in paired-pulse transcranial magnetic stimulation: effect of conditioning stimulus intensity on sizes and latencies of motor evoked potentials.

The influence of the intensity of the conditioning stimulus on intracortical inhibition (ICI) and intracortical facilitation (ICF) was assessed in a study using paired-pulse transcranial magnetic stimulation. Interstimulus intervals (ISIs) between conditioning and test stimuli were 3 msec and 13 msec. Latencies and areas of motor evoked potentials in response to the test stimulus were measured in the right extensor carpi radialis muscle. Motor evoked potential areas with ISIs of 3 msec and 13 msec showed a different dependence on the intensity of the conditioning stimulus. In contrast, the changes of motor evoked potential latencies were fairly similar with both ISIs. The findings point to a parallel action of ICI and ICF. Furthermore, the latencies seem to be a more sensitive indicator for ICF action than the size parameters of motor evoked potentials.     

Multiple descending corticospinal volleys demonstrated by changes of the wrist flexor H-reflex to magnetic motor cortex stimulation in intact human subjects

To study noninvasively multiple descending volleys on alpha motoneurons, we used a 30–50% maximum H-reflex in the wrist flexor muscle conditioned by a subthreshold magnetic stimulus of the contralateral motor cortex as a parameter of motoneuronal excitability in seven neurologically healthy subjects. The time interval of the median nerve (test) stimulus and the magnetic (conditioning) stimulus was varied. In all subjects the conditioned H-reflex significantly (P < 0.05) enhanced at conditioning-test intervals between -1 ms and +10 ms. Superimposed on the overall facilitation there were at least two short duration periods of enhanced facilitation in all but one subject. These periods were separated by 2–4 ms and suggest increased excitability at the alpha motoneuronal pool brought on by the descending volleys. We conclude that with subthreshold magnetic cortical stimuli it is possible to produce multiple descending corticospinal volleys, which probably represent descending I waves. This should be taken into account when analyzing motor evoked potentials (MEPs) in clinical studies. © 1993 John Wiley & Sons, Inc.     

Conditioning stimulation techniques for enhancement of transcranially elicited evoked motor responses

INTRODUCTION: In spite of the use of multipulse, transcranial electrical stimulation (TES) is still insufficient in a subgroup of patients to elicit motor-evoked potentials during intraoperative neurophysiological monitoring (IONM). Classic facilitation methods used in awake patients are precluded under general anaesthesia. Conditioning techniques can be used in this situation. OBJECTIVE: To present clinical experimental data and models of motor-neuron (MN) excitability for homonymous and heteronymous conditioning and discuss their applications in IONM. MATERIAL AND METHODS: Data were obtained in a prospective study on multipulse TES-conditioning of the monosynaptic H-reflex and double multipulse TES. DISCUSSION: The principle of facilitation by conditioning stimulation is to apply a test stimulus when motor neurons (MNs) have been made maximally excitable by a conditioning stimulus. Both conditioning and test stimuli recruit separate populations of MNs. The overlapping fraction of MNs controls the efficacy of facilitation. Heteronymous conditioning stimulation, which is performed at a different site from the test stimulus, is illustrated by the TES-conditioned H-reflex (HR). Autonomous conditioning stimulation, which is performed at the same stimulation site, is illustrated by double-train TES (dt-TES). The facilitating curves obtained by conditioning stimulation are often 3-modal and show peaks of facilitation at short intertrain intervals (S-ITIs) of 10ms and between 15 and 20ms and at longer intertrain intervals (L-ITI) of over 100ms. The facilitation curves from HR and dt-TES are not always identical since different alphaMN pools are involved. Dt-TES is often successful in neurologically impaired patients whereas facilitation of the HR can be used when conditioned by TES at subthreshold levels allowing continuous IONM without movement in the surgical field. Alternatively, facilitation by conditioning from peripheral-nerve stimulation can be used for selective transmission of subthreshold TES motor responses to peripheral muscles, permitting motor-monitoring by a so-called selective motor-gating technique. CONCLUSIONS: Facilitation techniques offer many possibilities in IONM by enhancing low-amplitude TES-MEP responses. They can also selectively enhance responses in a few muscle groups for the reduction of movement.     

Late facilitations of motor evoked potentials by contralateral mixed nerve stimulation

The effect on motor evoked potentials (MEPs) of a peripheral afferent volley (a single square pulse delivered to the contralateral median nerve at the wrist) was studied in abductor digiti minimi muscle of 9 normal patients following magnetic stimulation of the motor cortex. Long intervals were used (200–700 cosec) between the electrical conditioning stimulus and magnetic test stimulation. An MEP amplitude increase, with no change in latency, was observed when the C-T interval was about 500 cosec, and the motor threshold was decreased by the conditioning stimulation. The observed long latency and duration of the facilitation effect would seem consistent with activation of suprasegmental areas.     

Sensory group Ia proximal conduction velocity

The fastest median and ulnar velocities derived by recording motor and mixed nerve action potentials, F waves, H-reflexes, and somatosensory evoked potentials (SEPs) were compared. H-reflex recording was facilitated by employing selective group Ia excitation during voluntary muscular contraction. Mixed nerve, SEP, and H velocities, considered to predominantly reflect group Ia conduction, measured 63.2 +/- 3.2 m/sec, 63.4 +/- 4.5 m/sec, and 67.2 +/- 4.3 m/sec, respectively, between the wrist and elbow. Conventional motor conduction velocity was significantly slower (58.3 +/- 5.1 msec), but F velocity, which although nonuniform is also a measure of motor conduction, was 68.4 m/sec. Mean F latency was considered more reliable and representative than minimum F latency. F and H velocities accelerated proximally by 4.5 m/sec. They complement each other when evaluating motor and sensory group Ia conduction. The H-reflex and SEP use identical stimulus characteristics and when simultaneously recorded allow direct comparison of the fastest conducting peripheral and central sensory pathways.