Magnetic stimulation of muscle evokes cerebral potentials

Somatosensory evoked potentials (SEPs) were recorded from the scalp in man to magnetic stimulation of various skeletal muscles. The potentials consisted of several components, the earliest of which decreased in latency as the stimulated site moved rostrally, ranging from 46 msec for stimulation of the gastrocnemius, to 14 msec for stimulation of the deltoid. Experiments were performed to distinguish the mechanisms by which magnetic stimulation of the muscle was effective in evoking these cerebral potentials. For the gastrocnemius, the intensity of the magnetic stimulus needed for evoking cerebral potentials was less than that required for activating mixed or sensory nerves in proximity to the muscle belly (eg, posterior tibial nerve in the popliteal fossa, sural nerve at the ankle). Vibration of the muscle or passive lengthening of the muscle, procedures which activate muscle spindles, were accompanied by a significant attenuation of the potentials evoked by magnetic stimulation of the muscle. Anesthesia of the skin underlying the stimulating coil had no effect on the latency or amplitude of the early components of the magnetically evoked potentials, whereas electrically evoked potentials from skin electrodes were abolished. Thus, the cerebral potentials accompanying magnetic stimulation of the muscle appear to be due to activation of muscle afferents. We suggest that magnetic stimulation of muscle can provide a relatively simple method for quantifying the function of muscle afferents originating from a wide variety of skeletal musculature.     

Evoked potentials elicited by stimulation of the lateral and anterior femoral cutaneous nerves in meralgia paresthetica

Seventy-five consecutive patients with clinical symptoms and signs of meralgia paresthetica underwent bilateral somatosensory evoked potential (SEP) studies involving stimulation of skin areas innervated by the lateral and anterior femoral cutaneous nerves of the thighs. The most common abnormality was an absolute lateral femoral cutaneous SEP latency > 40 ms in 35 patients (47%), followed by an absent response in 14 patients (19%), an absolute latency < 40 ms but amplitude reduction > 50% compared with the contralateral response in 8 patients (11%), and an absolute latency < 40 ms but > 5 ms interside latency difference in 5 patients (7%). Anterior femoral cutaneous SEPs were of value in distinguishing meralgia paresthetica from a proximal lumbar radiculopathy in an additional 4 patients and confirming bilateral meralgia paresthetica in 10 patients.     

Clinical and neurophysiological abnormalities before and after reconstruction of the anterior cruciate ligament of the knee

OBJECTIVES: We aimed to study knee proprioception and somatosensory evoked potentials (SEPs) to stimulation of the common peroneal nerve (CPN) in 7 patients with lesion of the anterior cruciate ligament (ACL) before and after ACL reconstruction. MATERIALS AND METHODS: We recorded the spinal N14 and scalp P27 potentials in 5 patients, while in the remaining 2 patients we calculated scalp SEP maps by 20 electrodes. The knee proprioception was tested by comparing the sensitivity to movement of both the knees. RESULTS: Before surgery, all patients showed decreased knee position sense and lack of the cortical P27 potential on the side of the ACL lesion. Arthroscopic reconstruction of the ligament improved neither the knee proprioception nor the somatosensory central conduction. CONCLUSION: We suggest that the loss of the knee mechanoreceptors can be followed by modifications of the central nervous system, which are not compensated by other nervous structures.     

Magnetic stimulation of muscle evokes cerebral potentials by direct activation of nerve afferents: A study during muscle paralysis

We tested the hypothesis that magnetic stimulation of muscle evokes cerebral potentials by causing a muscle contraction that then activates muscle receptors. We measured cerebral evoked potentials accompanying magnetic stimulation of muscle in 3 patients during surgery both before and after muscle paralysis with succinylcholine, a depolarizing agent. The magnetic stimulation was at low intensity (30%) and at a 2/s rate. The administration of succinylcholine sufficient to produce muscle paralysis did not alter cerebral potentials evoked by either low-intensity magnetic stimulation of muscle (gastrocnemius/soleus) or electrical stimulation of peripheral nerve (tibial nerve). In 1 normal subject, the S1 nerve root action potentials conducting at rapid velocity (> 60 m/s) were detected at the S1 foramen with a needle electrode using electrical stimulation of the tibial nerve. However, no S1 nerve root potentials could be identified to magnetic stimulation of muscle that evoked a cerebral potential. We conclude that magnetic stimulation of muscle activates terminal afferents in the muscle to provide the afferent drive for the cerebral potentials independent of muscle contraction. The failure to detect the afferent volley in S1 nerve root to magnetic stimulation suggests that only a few afferents are activated or that the activation of afferents is temporally dispersed. © 1996 John Wiley & Sons, Inc.     

Inhibition of dorsal column by nuclei stimulation of trigeminal afferents in decerebrate—decerebellate cats

In decerebrate—decerebellate cats, stimulation of trigeminal afferents inhibited neurons in dorsal column (DC) nuclei driven by activation of DC input and produced primary afferent depolarization in DC primary afferent terminals. This inhibition was most likely mediated by a trigeminal—brainstem—DC nuclear loop.     

Neurophysiological evaluation of sensorimotor functions of the leg: comparison of evoked cortical potentials following electrical and mechanical stimulation,long-latency muscle responses,and transcranial magnetic stimulation

Summary Twenty-two patients with localized lesions of the central nervous system (unilateral cerebral ischaemia, cervical myelopathy, spinal tumour, familial spastic paraplegia) underwent neurophysiological evaluation of sensorimotor deficits of the leg. Functional methods using muscle stretch as stimulus, i.e. long-latency muscle responses and cortical potentials evoked by dorsiflection of the foot, were compared with transcranial magnetic stimulation and somatosensory evoked cortical potentials following electrical stimulation of the posterior tibial nerve. The functional neurophysiological methods yielded no diagnostic superiority with respect to the procedures using artificial (i.e. magnetic and electrical) stimulation. However, in most cases of missing compound motor action potentials following transcranial magnetic stimulation or missing electrically evoked cortical potentials, the long-latency muscle responses still allowed quantitative assessment of sensorimotor function.     

Rapid modulation of cortical proprioceptive activity induced by transient cutaneous deafferentation: neurophysiological evidence of short-term plasticity across different somatosensory modalities in humans

Single cell recording in non-human primates shows plastic changes of cortical somatic representations across different types of somatic inputs originating from the same peripheral territory. In humans, muscle afferents from first dorsal interosseus are supplied by the ulnar nerve while the cutaneous territory overlying this muscle is supplied by the radial nerve. This peculiar anatomical nervous distribution allowed us to devise an experimental model which provided a unique opportunity to assess, in humans with a non-invasive technique, the functional relationships between cutaneous and muscle afferent inputs originating from the same peripheral territory. We recorded spinal, brainstem and cortical somatosensory potentials evoked by stimulation of muscle afferents of the right first dorsal interosseus before, during and after anaesthetic block of the sensitive branch of the ipsilateral radial nerve. Amplitude of parietal N20 and P27 and frontal N30 somatosensory evoked potential components showed an increase of amplitudes with more profound anaesthesia. Amplitudes returned to pre-anaesthetic values several minutes after anaesthesia. By contrast, spinal N13 and brainstem P14 potentials did not change throughout the experiment. Results show, for the first time in humans, that a transient cutaneous deafferentation may induce rapid modulation of cortical activity evoked by stimulation of muscle afferents originating in the anaesthetic territory.     

Structural muscular adaptations in upper limb after stroke: a systematic review

Background: Stroke is a leading cause of disability in the adult population, impairing upper limb (UL) movements affecting activities of daily living. Muscle weakness has been associated to disabilities in this population, but much attention is given to central nervous system alterations and less to skeletal muscles.

Objective: The objective of this review is to carry out a systematic literature review to identify structural muscle alterations in the UL of poststroke individuals.

Method: The search was performed in December, 2017. MEDLINE, PubMed, SCOPUS, CINAHL, and Science Direct were used as electronic databases. There was no restriction regarding language and publication dates. Studies conducted on poststroke subjects and results on UL skeletal muscle alterations identified by imaging tests were included.

Results: Seven studies were included. The sample size and the variables varied among the studies. All the studies compared the paretic UL with the nonparetic UL and one of the studies also compared healthy subjects. Ultrasonography was the most used measurement tool to assess muscle adaptation.

Conclusions: This review demonstrated little evidence with poor to fair quality on the structural muscle adaptations in the poststroke subjects, showing muscle atrophy, a higher stiffness, and amount of fibrous and fat tissue without alterations in lean tissue of distal muscles of the paretic UL compared to the nonparetic limb. However, the nonparetic side also presented alterations, which makes it an inappropriate comparison. Thus, well-designed studies addressing this issue are required.     

术中皮质体感诱发电位与电刺激术定位脑功能区

目的探讨脑功能区手术中利用脑皮质体感诱发电位(SEP)及直接皮质电刺激定位脑功能区的方法及意义。方法对10例脑功能区病变病人在唤醒麻醉下进行手术，利用皮质SEP及皮质直接电刺激定位感觉区、运动区及语占区，住保护脑功能区的前提下，手术切除病变。结果7例病人利用SEP及皮质电刺激确定出运动感觉区，其中4例利用SEP位相倒置确定出中央沟，3例病变位于左侧额颞叶的病人通过皮质直接电刺激确定出语言区?术后功能均较术前明显好转。结论术中SEP及直接皮质电刺激可准确、实时确定脑功能区，最大程度地保护功能，切除病变。     

Recovery functions of spinal cord and subcortical somatosensory evoked potentials to posterior tibial nerve stimulation: intrasurgical recordings

The recovery function of evoked potentials to posterior tibial nerve stimulation was studied. Intrasurgical recordings were made from interspinous ligaments at lumbar levels and from high thoracic-low cervical level. In addition, surface recordings were obtained from neck-scalp derivations. The recovery function of the potentials recorded from lumbar and from high thoracic-low cervical spinal cord were very similar, showing an early period of supernormality (5-20 ms) followed by a period of subnormality which reached its lowest point at 40-60 ms. Assuming that the potentials recorded at the lumbar level reflect activity in the cauda equina, we conclude that the results support the hypothesis that the potentials recorded from the thoraco-cervical level reflect activity in the dorsal columns. The recovery curve of the amplitude between the far field potentials P27 (which most probably reflects activity of the afferent volley at the level of foramen magnum) and N30 (which, by latency criteria, would reflect lemniscal or thalamic activity) showed a similar shape but with a shorter duration of the periods of super- and subnormality. It is likely that this modification was due to the synapse at the gracilis nucleus. The first cortical component (P32) recorded in the neck-scalp derivation was totally abolished within the recovery period studied (50 ms interval).     

Orbicularis oculi responses to stimulation of nerve afferents from upper and lower limbs in normal humans

A brief mechanical or electrical stimulus to peripheral nerve afferents from the upper and lower limbs elicited a small and inconsistent EMG response of the orbicularis oculi muscles. This response was facilitated when the stimuli were delivered at fixed leading time intervals, of 45–300 ms, with respect to a supraorbital nerve electrical stimulus. Also, the peripheral nerve stimulus modified the conventional blink reflex responses, inducing facilitation of R1 and inhibition of R2. These results suggest a complex processing of sensory inputs from the face and the limbs at the brainstem, where they are probably integrated in a network of interneurons influencing the excitability of facial motoneurons.     

The effect of stimulation type,head modeling,and combined EEG and MEG on the source reconstruction of the somatosensory P20/N20 component

Modeling and experimental parameters influence the Electro‐ (EEG) and Magnetoencephalography (MEG) source analysis of the somatosensory P20/N20 component. In a sensitivity group study, we compare P20/N20 source analysis due to different stimulation type (Electric‐Wrist [EW], Braille‐Tactile [BT], or Pneumato‐Tactile [PT]), measurement modality (combined EEG/MEG – EMEG, EEG, or MEG) and head model (standard or individually skull‐conductivity calibrated including brain anisotropic conductivity). Considerable differences between pairs of stimulation types occurred (EW‐BT: 8.7 ± 3.3 mm/27.1° ± 16.4°, BT‐PT: 9 ± 5 mm/29.9° ± 17.3°, and EW‐PT: 9.8 ± 7.4 mm/15.9° ± 16.5° and 75% strength reduction of BT or PT when compared to EW) regardless of the head model used. EMEG has nearly no localization differences to MEG, but large ones to EEG (16.1 ± 4.9 mm), while source orientation differences are non‐negligible to both EEG (14° ± 3.7°) and MEG (12.5° ± 10.9°). Our calibration results show a considerable inter‐subject variability (3.1–14 mS/m) for skull conductivity. The comparison due to different head model show localization differences smaller for EMEG (EW: 3.4 ± 2.4 mm, BT: 3.7 ± 3.4 mm, and PT: 5.9 ± 6.8 mm) than for EEG (EW: 8.6 ± 8.3 mm, BT: 11.8 ± 6.2 mm, and PT: 10.5 ± 5.3 mm), while source orientation differences for EMEG (EW: 15.4° ± 6.3°, BT: 25.7° ± 15.2° and PT: 14° ± 11.5°) and EEG (EW: 14.6° ± 9.5°, BT: 16.3° ± 11.1° and PT: 12.9° ± 8.9°) are in the same range. Our results show that stimulation type, modality and head modeling all have a non‐negligible influence on the source reconstruction of the P20/N20 component. The complementary information of both modalities in EMEG can be exploited on the basis of detailed and individualized head models.     

Comparison of cortical potentials evoked by mechanical and electrical stimulation of the rectum

Patients with irritable bowel syndrome have heightened perception of gut sensation. The mechanisms responsible for this remain unknown, due to current poor knowledge of the central processing of gut sensation. Cortical evoked potentials (CEPs) have been recorded following both electrical rectal stimulation (ERS) and mechanical rectal stimulation (MRS). Because of the lack of a direct comparison of these two methods, their robustness for future clinical use remains unknown. The aim of our study was to compare the characteristics of CEPs following ERS and MRS. CEPs were recorded from the vertex in 14 healthy volunteers following ERS with bipolar ring electrodes, and MRS by repeated rectal distension. CEPs were recorded in all subjects following electrical stimulation, but only in 11 subjects following mechanical stimulation. In comparison with electrical stimulation, mechanical stimulation produced CEPs with a smaller amplitude and longer latency. However, the morphology of CEPs following electrical and mechanical rectal stimulation was similar, with no difference in the interpeak latencies. In conclusion, we have demonstrated that electrical rectal stimulation is a more reliable stimulus for recording CEPs. The similarity of the morphology and interpeak latencies of the CEPs suggests that both stimuli are activating a similar network of cortical neurones.     

Decreased cortical excitability during motor imagery after disuse of an upper limb in humans

OBJECTIVE: The present study investigated the effect of joint immobilization on corticomotoneuronal excitability to only intracortical input from a hierarchical level above the primary motor cortex. METHODS: Motor evoked potentials (MEPs) and H-reflexes in the flexor carpi radialis muscle were elicited from 8 orthopedic patients with splints and 8 healthy volunteers. Each patient was examined on the day of splint removal (disuse stage) and 2 months after that day (recovery stage). Both potentials were recorded under 3 conditions: at rest, while imagining motor movement (during motor imagery), and during 10% of maximum voluntary contraction (10% MVC). RESULTS: In the patient group, the amplitude of surface electromyography during voluntary maximum wrist flexion was lower at the disuse stage than at the recovery stage, although the supra-maximum M-wave amplitude did not change between stages. Compared to both the patient group at the recovery stage and the control group, patients at the disuse stage recorded significantly lower MEPs, but only during motor imagery. In contrast, the H-reflex amplitudes were not significantly changed under any of the 3 conditions for both patients and control. CONCLUSIONS: The present results indicated a strict parallelism between motor execution (the reduction of electromyography during mvc after immobilization) and motor imagery (the reduction of MEP-amps after immobilization). This parallelism suggests that a functional reorganization or decreased excitability in the cerebral cortex area involved in executing movement likely decreases the motor capability to produce voluntary muscular output after immobilization.     

Long-term electrical stimulation of muscles in children with Duchenne and Becker muscular dystrophy.

Nine children suffering from progressive muscular dystrophy (7 Duchenne and 2 Becker) were included in a program of low-frequency electrical stimulation (LFES) of the right tibialis anterior (TA) muscle. Muscle strength and muscle fatigue were estimated by measuring torques in the ankle during attempts of maximal voluntary contraction (MVC) in the direction of dorsal flexion of the foot and during electrically evoked contractions (EEC). No important increase in the strength of the stimulated muscles was noticed in 4 boys whose muscles were stimulated for 3 months. The muscles of 5 boys who were subjected to electrical stimulation for 9 months showed an improvement; 6 measurements made during the stimulation program revealed that changes of torques in the ankle of the right stimulated extremity were significantly different (P less than 0.001) from the changes of torques in the ankle of the left nonstimulated extremity.     

Parietal and prerolandic somatosensory evoked responses in carotid artery reversible ischemic attacks (RIA)

Somatosensory potentials evoked by median nerve stimulation were recorded bilaterally from the prerolandic and parietal scalp in 36 patients with reversible ischemic attacks in one carotid artery distribution. Responses recorded from the affected and unaffected hemisphere were compared. SEP abnormalities were observed over the affected hemisphere in 22 subjects (61.1%), irrespective of number and/or duration of episodes, and appeared selectively correlated to their clinical features. SEP studies proved to be more sensitive than conventional EEG recordings and are suitable for evaluation of the functional impairment of specific cerebral areas following transient cerebral ischemia. The occurrence of SEP abnormalities in RIA may represent an unfavourable sign, being more frequent in patients with evidence of internal carotid artery disease.     

Larger muscle mass of the upper limb correlates with lower amplitudes of deltoid MEPs following transcranial stimulation

To perform spinal surgery safely, it is important to understand the risk factors, including factors that negatively influence intraoperative neuromonitoring (IONM). Transcranial motor evoked potentials (TcMEPs) are important in IONM. Therefore, we aimed to investigate whether muscle mass affects the waveforms of TcMEPs to understand the risk factors influencing TcMEPs. We enrolled 48 patients with thoracolumbar spinal diseases who underwent surgery at our facility between April 2015 and March 2018. Before surgery, the body composition, including muscle mass and fat mass, of all patients was measured using bioelectrical impedance analysis (BIA). During surgery, cranial stimulation under general anesthesia was used to derive TcMEPs, enabling us to measure the amplitude, using the control wave of the TcMEPs of the deltoid muscles and the abductor digiti minimi (ADM) muscles. We found a negative correlation between the amplitude of deltoid-muscle TcMEPs and muscle mass of the upper limb. The amplitude of deltoid-muscle TcMEPs did not correlate with the skeletal muscle index (SMI), muscle mass of the lower limb, or body fat mass. The amplitude of ADM-muscle TcMEPs did not correlate with SMI, muscle mass of any limb, or body fat mass. In conclusion, a larger muscle mass of the upper limb correlated with a lower amplitude of deltoid-muscle TcMEPs. By contrast, there was no correlation between the muscle mass of the upper limb and the amplitude of ADM-muscle TcMEPs. These findings suggest that TcMEPs of the ADM are less influenced by muscle mass and are more stable than those of the deltoid.     

Effects of electrical and chemical stimulation of nucleus raphe magnus on responses to renal nerve stimulation

Electrical stimulation of the nucleus raphe magnus (NRM) inhibits some somatic and visceral input at the spinal level. This study was designed to examine the effects of electrical and chemical stimulation of NRM on neuronal responses to afferent renal nerve (ARN) stimulation. In chloralose-anesthetized rats, electrical stimulation of ARN elicited predominantly excitatory responses in spinal gray neurons. In 10 neurons studied, electrical stimulation of the NRM elicited an inhibition of spontaneous activity of 8 neurons and inhibited evoked responses to ARN stimulation in 6 neurons. Microinjection of glutamate (5-10 nmol in 0.5-1 microliter) into the NRM elicited an inhibition of spontaneous activity in 9 neurons, a facilitation in 6 neurons and no response in 8 neurons receiving ARN input. Responses evoked by ARN stimulation were inhibited in 12 neurons, facilitated in 4 neurons and not affected in 8 neurons. We conclude that renal input can be modulated at the spinal level by activation of the NRM and adjacent tissue. Furthermore, the inhibition of spinal gray neuronal responses elicited by stimulation of the NRM appears to be due, at least in part, to activation of fibers of passage since non-selective electrical stimulation is more efficacious than selective chemical stimulation of neuronal soma and dendrites.