Localization of diaphragm motor cortical representation and determination of corticodiaphragmatic latencies by using magnetic stimulation in normal adult human subjects

The aims of this study were to identify the motor cortical representation of the diaphragm and to assess the corticodiaphragmatic pathway from both hemispheres. Specially designed bipolar surface electrodes were used to record the ipsilateral and contralateral compound motor evoked potentials (CMEPs) of the diaphragm after transcranial magnetic stimulation (TMS) of the motor cortex. In addition, the response to cervical magnetic stimulation of the phrenic nerve roots, effected using a figure-of-eight magnetic coil, was also recorded. The study involved 30 normal adult male volunteers. The average point of optimal excitability (POE) was determined to be 3.7 cm lateral to the mid-sagittal plane and 0.89 cm anterior to the preauricular plane. The largest response was obtained at a stimulus coil orientation of 0–90°. The TMS of either hemisphere produced CMEPs in the contralateral and ipsilateral diaphragm muscles. TMS of either hemisphere elicited CMEPs that had significantly greater amplitudes and shorter latencies from the contralateral muscles compared with the ipsilateral response (P<0.0001). The central motor conduction time of the crossed tract (8.8 ms) was significantly shorter than that of the uncrossed tract (12.2 ms). No significant interhemispheric differences were recorded. The recorded CMEPs recorded in response to TMS were facilitated during volitional inspiration. Phrenic nerve latency was 5.7 ms and 5.6 ms for the right and left phrenic nerves, respectively, with no significant difference between these values. Both bilateral crossed and uncrossed corticospinal connections to the diaphragm were usually present, with the crossed tract predominating. The technique used in this study may be useful for investigations into the function and integrity of central and peripheral pathway of the diaphragm muscles in various neurological disorders. Electronic Publication     

膈神经传导以及膈运动诱发电位对呼吸功能障碍评价的初步探讨

目的：初步探讨膈神经传导（ＰＮＣ）及膈运动诱发电位（ＭＥＰ）对评价各种呼吸功能障碍的价值。方法：对３４例病人（各种神经肌肉疾病１９例、呼吸系统疾病１５例）在胸锁乳突肌后缘中点用电刺激膈神经,于第７—８肋间和剑突处记录膈肌复合动作电位;用磁圈置于对侧头皮进行刺激,在深吸气状态下记录膈ＭＥＰ。结果：肌病患者的ＰＮＣ均正常;格林巴利综合征、重症肌无力危象以及遗传性运动感觉神经病者的ＰＮＣ均异常,动态观察结果均表现为ＰＮＣ与呼吸功能障碍显著相关;７例睡眠呼吸暂停综合征中４例ＰＮＣ异常;３例慢性阻塞性肺病有２例主要表现为膈肌电位波幅低;憋气、平卧时呼吸困难者患者５例中仅１例表现为单侧膈肌电位波幅低;５例作ＭＥＰ的睡眠呼吸暂停综合征者３例异常、３例慢性阻塞性肺病者的隔ＭＥＰ均正常。结论：ＰＮＣ不仅可客观地评价神经肌肉性呼吸功能障碍、预示疾病过程,而且还可为呼吸系统病变所致呼吸功能障碍提供膈肌功能障碍的其它信息;结合ＰＮＣ及隔ＭＥＰ两者的结果可能有助于判断睡眠呼吸暂停综合征的病变类型。     

膈神经传导时间在麻醉复苏中的应用

目的:探讨膈神经传导时间(PNCT)在麻醉复苏过程中对膈肌功能的监测作用。方法: 对8例手术病人观察全麻使用肌松药前后颤搐性跨膈压(Pdi_(t))和PNCT的变化。结果: 8例手术病人在全麻前:Pdi(t)为(23.7±2.4) cmH₂O,左、右侧PNCT分别为(5.7±1.3) ms和(5.6±0.9) ms;全麻使用肌松药后:Pdi_(t)下降到(11.5±3.4) cmH2O(下降率51.5%,P＜0.01),而左、右侧PNCT则分别延长为(6.1±1.3)ms和(6.4±0.6)ms,并随Pdi_(t)的恢复而逐步缩短。结论:肌松药诱发膈肌无力和引起双侧PNCT延长,且PNCT随着Pdi_(t)的恢复而缩短; PNCT的测定有助于间接监测全麻使用肌松药期间膈肌肌力的动态变化。     

Mapping location of excitation during magnetic stimulation: Effects of coil position

We determined the location of excitation for different positions of a round and butterfly coil duringin vitro magnetic stimulation of cut peripheral nerves. We analyzed the conditions under which excitation occurs, either at the termination or at the peak of the field gradients (first spatial derivative of the electric field). These results were then compared to predictions about the location of excitation sites from a theoretical model of magnetic stimulation of finite neuronal structures. Excitation along a straight nerve occurred at terminations when 1) a coil was positioned close to the end of a nerve (at least one diameter length from the end), 2) a nerve ended in a finite terminating impedance much greater than the axial resistance of the nerve, 3) the induced electric field was of sufficient magnitude, pointing in a direction away from the axis of a nerve. Excitation occurred at the negative peak of the field gradients along a nerve when 1) a coil was positioned far away from the ends of a nerve, 2) there were no geometric or volume conductor inhomogeneities around a nerve, and 3) it was of sufficient magnitude. Threshold strengths for excitation at terminations were significantly lower than that for field gradient excitation and comparable to that due to geometric and volume conductor inhomogeneities.     

Magnetic stimulation of peripheral nerve; site of stimulation estimated from H reflex.

In order to find out the difference in the site of stimulation between electrical and magnetic stimulation of a peripheral nerve, H reflexes were recorded in 20 healthy persons. Electrical stimulation was performed with the stimulator placed along the tibial nerve (A) and at right angles to the tibial nerve (B). Magnetic stimulation (Magstim Model 200) was performed with a large (14 cm in diameter) magnetic coil (C) and a small (7 cm in diameter) magnetic coil (D). The magnetic coil was held parallel to the skin and its center was placed on the point where the cathode had been placed in the electrical stimulation. The mean value of the sum of M wave latency and H reflex latency in C was 1.4 msec, 1.7 msec and 0.8 msec shorter than those in A, B and D, respectively. The mean value of the sum of M wave latency and H reflex latency in D was 0.6 msec shorter than that in A and 0.9 msec shorter than that in B. When the center of the magnetic coil is placed over the peripheral nerve, two loci of the nerve, 7-8 cm apart from each other with the large magnetic coil and 3-4 cm apart from each other with the small magnetic coil, are considered to be stimulated simultaneously.     

Transcranial magnetic stimulation in the visual system. II. Characterization of induced phosphenes and scotomas

Transcranial magnetic stimulation (TMS) induces phosphenes and disrupts visual perception when applied over the occipital pole. Both the underlying mechanisms and the brain structures involved are still unclear. In the first part of this study we show that the masking effect of TMS differs to masking by light in terms of the psychometric function. Here we investigate the emergence of phosphenes in relation to perimetric measurements. The coil positions were measured with a stereotactic positioning device, and stimulation sites were characterized in four subjects on the basis of individual retinotopic maps measured by with functional magnetic resonance imaging. Phosphene thresholds were found to lie a factor of 0.59 below the stimulation intensities required to induce visual masking. They covered the segments in the visual field where visual suppression occurred with higher stimulation intensity. Both phosphenes and transient scotomas were found in the lower visual field in the quadrant contralateral to the stimulated hemisphere. They could be evoked from a large area over the occipital pole. Phosphene contours and texture remained quite stable with different coil positions over one hemisphere and did not change with the retinotopy of the different visual areas on which the coil was focused. They cannot be related exclusively to a certain functionally defined visual area. It is most likely that both the optic radiation close to its termination in the dorsal parts of V1 and back-projecting fibers from V2 and V3 back to V1 generate phosphenes and scotomas.     

Intracranial measurement of current densities induced by transcranial magnetic stimulation in the human brain

Transcranial magnetic stimulation (TMS) is a non-invasive technique that uses the principle of electromagnetic induction to generate currents in the brain via pulsed magnetic fields. The magnitude of such induced currents is unknown. In this study we measured the TMS induced current densities in a patient with implanted depth electrodes for epilepsy monitoring. A maximum current density of 12 microA/cm2 was recorded at a depth of 1 cm from scalp surface with the optimum stimulation orientation used in the experiment and an intensity of 7% of the maximal stimulator output. During TMS we recorded relative current variations under different stimulating coil orientations and at different points in the subject's brain. The results were in accordance with current theoretical models. The induced currents decayed with distance form the coil and varied with alterations in coil orientations. These results provide novel insight into the physical and neurophysiological processes of TMS.     

Cortical localization of external urethral sphincter activation by transcranial electrical stimulation

PURPOSE: Repetitive transcranial electrical stimulation (rTES) was used to activate descending output to the external urethral sphincter muscle. METHODS: Motor evoked potentials (MEPs) were recorded from external urethral sphincter (EUS), and anterior tibial (TA) muscles following high voltage rTES in 9 consecutive patients undergoing spine surgery. Anesthesia was achieved by continuous propofol/narcotic infusion without paralytic agents. Anodal cortical stimulation was delivered at C4/C3, C2/C1, and Cz/Fz locations in each patient. Latency and amplitude of the MEP was measured and compared for each bipolar stimulation montage. RESULTS: The mean latency was 20.24 +/- 1.3 msec. for Cz/Fz; 20.19 +/- 1.1 msec. for C4/C3 and 20.19 +/- 1.1 msec. for C2/C1. Statistical analysis showed no significant difference in latency between the three sites (F(2,15) = 0.004; p > 0.05). The mean amplitude was 37.14 +/- 24.3 microV for Cz/Fz; 113.33 +/- 100.6 microV for C4/C3; and 85 +/- 73.9 microV for C2/C1. A significant difference between the amplitudes at three sites was observed (F(2,8) = 5.2; p < 0.05). The amplitude at C4/C3 was significantly greater than amplitude at CzlFz (t (8) = 3.08; p < 0.05), but data did not give enough evidence to believe that difference between amplitudes for site C4/C3 & C2/C1 was significant (p > 0.05). CONCLUSIONS: This study shows that the intraoperative MEP monitoring of the EUS is a feasible method. Furthermore, activation of descending axonal outflow to the EUS muscle is best achieved by cortical stimulation directed from C4 to C3 or C2 to C1 points.     

Asymmetry of magnetic motor evoked potentials recorded in calf muscles of the dominant and non-dominant lower extremity

The aim of the study was to determine the applicability of magnetic stimulation and magnetic motor evoked potentials (MEPs) in motor asymmetry studies by obtaining quantitative and qualitative measures of efferent activity during low intensity magnetic stimulation of the dominant and non-dominant lower extremities. Magnetic stimulation of the tibial nerve in the popliteal fossa was performed in 10 healthy male right-handed and right-footed young adults. Responses were recorded from the lateral head of the gastrocnemius muscles of the right and left lower extremities. Response characteristics (duration, onset latency, amplitude) were analyzed in relation to the functional dominance of the limbs and in relation to the direction of the current in the magnetic coil by use of the Wilcoxon pair sequence test. The CCW direction of coil current was related to reduced amplitudes of recorded MEPs. Greater amplitudes of evoked potentials were recorded in the non-dominant extremity, both in the CW and CCW coil current directions, with the statistical significance of this effect (p = 0.005). No differences in duration of response were found in the CW current direction, while in CCW the time of the left-side response was prolonged (p = 0.01). In the non-dominant extremity longer onset latencies were recorded in both current directions, but only for the CW direction the side asymmetries showed a statistical significance of p = 0.005. In the dominant extremity the stimulation correlated with stronger paresthesias, especially using the CCW direction of coil current. The results indicate that low intensity magnetic stimulation may be useful in quantitative and qualitative research into the motor asymmetry.     

Technical approaches to hemisphere-selective transcranial magnetic brain stimulation

Clinical application of transcranial magnetic brain stimulation is mainly used to determine central motor conduction times. With the stimulation coil (Magstim 200, Novametrix) centered conventionally over the midline of the skull convexity and using high stimulus intensities, which are often required in pathological states, the motor cortices of both hemispheres are usually activated simultaneously. Under this condition it is not possible to determine from which hemisphere the descending excitatory volleys to a particular motoneurone pool originate and how the input to the lower motoneurons is organized (uni-/bilateral, ipsi-/contralateral). This limitation can be overcome by two different techniques for selective stimulation of the motor cortex of one hemisphere without coactivation of the other even when using maximal stimulus intensities: 1. A large 12 cm (outer diameter) stimulation coil could be used for selective stimulation when a) the magnetic field radiated over the non-stimulated hemisphere is modified by using a prototype coil shield covering the half of the coil over the nonstimulated hemisphere in combination with b) placing the coil away from the midline towards the preferentially excited hemisphere. The coil shield consists of a sheet of a nickel iron alloy which alters the time course of the induced currents by reducing the initial rate of current intensity change (dI/dt). 2. The use of a smaller 6.5 cm (outer diameter) coil also provided a useful tool for selective stimulation of one hemisphere but was restricted to subjects with low excitation thresholds. In subjects with high excitation thresholds the described use of the large stimulation coil is advisable.     

The effect on corticospinal volleys of reversing the direction of current induced in the motor cortex by transcranial magnetic stimulation

Descending corticospinal volleys were recorded from a bipolar electrode inserted into the cervical epidural space of four conscious human subjects after monophasic transcranial magnetic stimulation over the motor cortex with a figure-of-eight coil. We examined the effect of reversing the direction of the induced current in the brain from the usual posterior-anterior (PA) direction to an anterior-posterior (AP) direction. The volleys were compared with D waves evoked by anodal electrical stimulation (two subjects) or medio-lateral magnetic stimulation (two subjects). As reported previously, PA stimulation preferentially recruited I1 waves, with later I waves appearing at higher stimulus intensities. AP stimulation tended to recruit later I waves (I3 waves) in one of the subjects, but, in the other three, I1 or D waves were seen. Unexpectedly, the descending volleys evoked by AP stimulation often had slightly different peak latencies and/or longer duration than those seen after PA stimulation. In addition the relationship between the size of the descending volleys and the subsequent EMG response was often different for AP and PA stimulation. These findings suggest that AP stimulation does not simply activate a subset of the sites activated by PA stimulation. Some sites or neurones that are relatively inaccessible to PA stimulation may be the low-threshold targets of AP stimulation.     

The excitability of human cortical inhibitory circuits responsible for the muscle silent period after transcranial brain stimulation

The silent period after transcranial magnetic brain stimulation mainly reflects the activity of inhibitory circuits in the human motor cortex. To assess the excitability of the cortical inhibitory mechanisms responsible for the silent period after transcranial stimulation, we studied, in 15 healthy human subjects, the recovery cycle of the silent period evoked by transcranial and mixed nerve stimulation delivered with a paired stimulation technique. The recovery cycle is defined as the time course of the changes in the size or duration of a conditioned test response when pairs of stimuli (conditioning and test) are used at different conditioning-test intervals. The recovery cycle of the duration of the silent period in the first dorsal interosseous (FDI) muscle during maximum voluntary contraction after transcranial magnetic stimulation was studied by delivering paired magnetic shocks (a conditioning shock and a test shock) at 120% motor-threshold intensity. Conditioning-test intervals ranged from 20-550 ms. The recovery cycle of the silent period in the FDI muscle during maximum voluntary contraction after nerve stimulation was evaluated by paired, supramaximum bipolar electrical stimulation of the ulnar nerve at the wrist (conditioning-test intervals ranging from 20 to 550 ms). Electromyographic activity was recorded by a pair of surface-disk electrodes over the FDI muscle. The recovery cycle of the silent period after transcranial magnetic stimulation delivered through the large round coil showed two phases of facilitation (lengthening of the silent period), one at 20-40 ms and the other at 180-350 ms conditioning-test intervals, with an interposed phase of inhibition (shortening of the silent period) at 80-160 ms. The conditioning magnetic shock left the size of the test motor-evoked potentials statistically unchanged during maximum voluntary contraction. Paired transcranial stimulation with a figure-of-eight coil increased the duration of the test silent period only at short conditioning-test intervals. Conditioning nerve stimulation left the silent period produced by test nerve stimulation unchanged. In conclusion, after a single transcranial magnetic shock, inhibitory circuits in the human motor cortex undergo distinctive short-term changes in their excitability, probably involving different mechanisms.     

Investigation into non-monosynaptic corticospinal excitation of macaque upper limb single motor units

There has been considerable recent debate as to relative importance, in the primate, of propriospinal transmission of corticospinal excitation to upper limb motoneurons. Previous studies in the anesthetized macaque monkey suggested that, compared with the cat, the transmission of such excitation via a system of C3-C4 propriospinal neurons may be relatively weak. However, it is possible that in the anesthetized preparation, propriospinal transmission of cortical inputs to motoneurons may be depressed. To address this issue, the current study investigated the responses of single motor units (SMUs) to corticospinal inputs in either awake (n = 1) or lightly sedated (n = 3) macaque monkeys. Recordings in the awake state were made during performance of a precision grip task. The responses of spontaneously discharging SMUs to electrical stimulation of the pyramidal tract (PT) via chronically implanted electrodes were examined for evidence of non-monosynaptic, presumed propriospinal, effects. Single PT stimuli (up to 250 microA; duration, 0.2 ms, 2 Hz) were delivered during steady discharge of the SMU (10-30 imp/s). SMUs were recorded from muscles acting on the thumb (adductor pollicis and abductor pollicis brevis, n = 18), wrist (extensor carpi radialis, n = 29) and elbow (biceps, n = 9). In all SMUs, the poststimulus time histograms to PT stimulation consisted of a single peak at a fixed latency and with a brief duration [0.74 +/- 0.25 (SD) ms, n = 56], consistent with the responses being mediated by monosynaptic action of cortico-motoneuronal (CM) impulses. Later peaks, indicating non-monosynaptic action, were not present even when the probability of the initial peak response was low and when there was no evidence for suppression of ongoing SMU activity following this peak (n = 20 SMUs). Even when repetitive (double-pulse) PT stimuli were used to facilitate transmission through oligosynaptic linkages, no later peaks were observed (16 SMUs). In some thumb muscle SMUs (n = 8), responses to PT stimulation were compared with those evoked by transcranial magnetic stimulation, using a figure-eight coil held over the motor cortex. Responses varied according the orientation of the coil: in the latero-medial position, single peak responses similar to those from the PT were obtained; their latencies confirmed direct excitation of CM cells, and there were no later peaks. In the posterio-anterior orientation, responses had longer latencies and consisted of two to three subpeaks. At least under the conditions that we have tested, the results provide no positive evidence for transmission of cortical excitation to upper limb motoneurons by non-monosynaptic pathways in the macaque monkey.     

Magnetic stimulation of the human peripheral nerves

In this report some preliminary data after magnetic stimulation of the peripheral nerves in 8 normal individuals are presented. Compound muscle action potentials were recorded by surface electrodes after magnetic and conventional electrical stimulation of the median, ulnar, peroneal, tibial, femoral and sciatic nerves, and the lumbosacral roots. The data clearly show the ability of the magnetic coil to stimulate the roots in sites relatively inaccessible to electrical stimulation and obtain consistent muscle response suggesting important clinical application of magnetic stimulation in lumbosacral root disorders. However, before the magnetic coil can be recommended for general use, a very careful study comparing the responses after conventional electrical stimulation with those obtained after magnetic stimulation of each nerve in the upper and lower limbs must be evaluated.     

磁刺激八字线圈变形对空间磁场与感应电场分布的影响

在经颅磁刺激中,尽管传统八字线圈的聚焦性要优于圆形线圈的聚焦性,但是八字线圈的两侧边缘产生的磁场与感应电场的峰值仍然较高。设计一种基于折叠变形的八字线圈,通过数值计算分析变形八字线圈空间磁场与感应电场的分布,并与传统八字线圈进行对比。结果表明,当变形八字线圈的折叠角度在45°～60°范围内时,其中心位置产生的空间磁场和感应电场占传统八字线圈在中心位置的90%左右,而变形线圈两侧产生的空间磁场与感应电场占传统八字线圈两侧的30%左右。因此,在空间磁场和感应电场的聚焦度方面,折叠变形八字线圈均优于传统八字线圈。所提出的折叠变形八字线圈为设计新型经颅磁刺激线圈提供了新的思路。     

Effect of tonic voluntary activity on the excitability of human motor cortex.

1. The threshold for obtaining EMG responses after transcranial magnetic stimulation of the brain is reduced by voluntary contraction of the target muscle. The present experiments tested whether some of this effect is due to increased cortical, as opposed to spinal, excitability during the contraction. 2. Magnetic stimulation was delivered with a figure-of-eight coil oriented with the junction region along the interaural line and also (in 4 of 7 subjects) with a circular coil centred at the vertex. The intensity of the conditioning stimulus was subthreshold for evoking a motor response in the relaxed wrist flexor muscles of the forearm. The presence of a small descending corticospinal volley in both the relaxed and active conditions was detected by measuring the facilitation of test H reflexes elicited in the flexor muscles of the forearm. 3. In all subjects, magnetic stimulation with either coil facilitated the H reflex at conditioning-test intervals of -1 to -3 ms (median nerve stimulus before magnetic). This was followed by a long-lasting facilitation. In three of the seven subjects stimulation with the figure-of-eight coil elicited an additional, earlier peak of facilitation at a conditioning-test interval of -3 to -5 ms. 4. In all subjects, the threshold for obtaining facilitation of the H reflex using a conditioning-test interval of -1 to -3 ms was reduced, and the amount of facilitation was larger, if subjects performed a weak tonic voluntary contraction. In contrast, with a conditioning-test interval of -3 to -5 ms voluntary contraction had no effect on the threshold. 5. It is suggested that H reflex facilitation at the conditioning-test interval of -1 to -3 ms was produced by indirect activation of corticospinal neurones by the magnetic stimulus, whereas at -3 to -5 ms, the facilitation was produced by direct activation of corticospinal axons. It is concluded that tonic voluntary contraction of a target muscle decreases the threshold for indirect activation of corticospinal neurones but not for direct stimulation of their axons.     

Reproducibility of twitch and sniff transdiaphragmatic pressures

Twitch transdiaphragmatic pressure (Tw Pdi) measured with magnetic stimulation of the phrenic nerve is used to follow up patients and to assess the effect of clinical treatments on diaphragm function. However the reproducibility of Tw Pdi on different occasions has been little studied. We investigated 32 normal subjects, measuring Tw Pdi elicited by bilateral magnetic stimulation of the phrenic nerves on two to 14 occasions. Sniff transdiaphragmatic pressure (sniff Pdi) was also measured. The mean value of Tw Pdi and sniff Pdi were 28+/-5 and 134+/-24 cm H(2)O, respectively. The within subjects coefficient of variation was 11% for both Tw Pdi and sniff Pdi. We conclude that there is a variability of Tw Pdi and the variability of Tw Pdi is the same as that of sniff Pdi.     

Measurement of electric fields induced in a human subject due to natural movements in static magnetic fields or exposure to alternating magnetic field gradients

A dual dipole electric field probe has been used to measure surface electric fields in vivo on a human subject over a frequency range of 0.1-800 Hz. The low-frequency electric fields were induced by natural body movements such as walking and turning in the fringe magnetic fields of a 3 T magnetic resonance whole-body scanner. The rate-of-change of magnetic field (dB/dt) was also recorded simultaneously by using three orthogonal search coils positioned near to the location of the electric field probe. Rates-of-change of magnetic field for natural body rotations were found to exceed 1 T s(-1) near the end of the magnet bore. Typical electric fields measured on the upper abdomen, head and across the tongue for 1 T s(-1) rate of change of magnetic field were 0.15+/-0.02, 0.077+/-0.003 and 0.015+/-0.002 V m(-1) respectively. Electric fields on the abdomen and chest were measured during an echo-planar sequence with the subject positioned within the scanner. With the scanner rate-of-change of gradient set to 10 T m(-1) s(-1) the measured rate-of-change of magnetic field was 2.2+/-0.1 T s(-1) and the peak electric field was 0.30+/-0.01 V m(-1) on the chest. The values of induced electric field can be related to dB/dt by a 'geometry factor' for a given subject and sensor position. Typical values of this factor for the abdomen or chest (for measured surface electric fields) lie in the range of 0.10-0.18 m. The measured values of electric field are consistent with currently available numerical modelling results for movement in static magnetic fields and exposure to switched magnetic field gradients.     

Transcranial facial nerve stimulation by magnetic stimulator in normal subjects

Magnetic stimulation provides a new method to stimulate facial nerve transcranially. Stimulation can be directed to the intracranial part of the facial nerve, whereas the conventional electric stimuli are delivered extracranially to a more peripheral part of the nerve. Fourty healthy volunteers were examined to determine the normal responses for transcranial facial nerve stimulation. The center of the inducing coil ring was located so that its center was 3 cm posterior and 6 cm lateral to the vertex. Responses were recorded on the nasolabial fold. Latencies were 4.5 +/- 0.4 ms on both sides, being 1.1 ms longer than those elicited by electric stimulation of the nerve at the stylomastoid foramen. Amplitudes with magnetic stimuli were equal to those obtained with electric stimuli. The transcranial magnetic stimulation seems to be an accurate and promising method to examine the facial nerve.     

Responses of ankle extensor and flexor motoneurons to transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) of the motor cortex excites limb muscles of the contralateral side of the body. Reports of poorly defined, or a complete lack of systematic excitatory responses of soleus motoneurons compared with those of tibialis anterior (TA) motoneurons has led to the proposal that while all ankle flexor motoneurons receive strong corticomotoneuronal connections, very few soleus motoneurons do. In addition, the connections to these few motoneurons are weak. The nature of corticomotoneuronal connections onto these two motoneuron pools was re-evaluated in the following experiments. The leg area of the left motor cortex was stimulated with a large double-cone coil using Magstim 200, while surface electromyographic (EMG) and single motor unit (SMU) responses were recorded from soleus and TA muscles of healthy adult subjects. Under resting conditions, the onset (25-30 ms) and duration of concomitantly recorded short latency motor evoked potentials (MEPs) in surface EMG from both muscles were similar. The input-output relationships of the simultaneously recorded soleus and TA EMG responses showed much greater increases in TA MEPs compared with soleus MEPs with identical increases in stimulus intensity. Under resting and nonisometric conditions, a later peak with onset latency of approximately 100 ms was observed in soleus. During isometric conditions or with vibration of the TA tendon, the second soleus peak was abolished indicating reflex origin of this peak. Recordings from 42 soleus and 39 TA motor units showed clear response peaks in the peristimulus time histograms (PSTHs) of every unit. Two statistical tests were done to determine the onset and duration of response peaks in the PSTHs. With chi(2) test, the duration was 6.9 +/- 4.2 ms (mean +/- SD) for soleus and 5.1 +/- 2.1 ms for TA. Using the criterion of discerning a peak by bin counts being three SDs above background, the duration was 10.0 +/- 4.4 ms for soleus and 7.8 +/- 2.6 ms for TA. Results of these experiments do not suggest a lack of systematic corticomotoneuronal connections on soleus motoneurons when compared with those on TA, though some differences in the strengths of corticomotoneuronal connections onto the two pools do exist.