Withdrawal reflexes in adductor muscles elicited by electrical and magnetic stimulation of the obturator nerve

The withdrawal reflex in the short head of the biceps femoris muscle after electrical stimulation of the sural nerve at the ankle has been investigated in numerous studies. These studies have described two distinct responses: early (R-II) and late (R-III). However, withdrawal reflex activity of the adductor muscles in the legs has not been studied systematically. Adductor muscle reflex activity is important because it can produce serious clinical problems, such as adductor spasticity and spasms, during bladder surgery. The present study examined withdrawal reflex features of adductor muscles obtained by electrical and magnetic stimulation of the obturator nerve (ON) in 34 normal healthy subjects. Early adductor muscle withdrawal reflex responses were elicited by ipsilateral ON electrical stimulation with a mean latency of 45.7+/-2.0 ms (responses in 94% of subjects). Reflex responses were also obtained using magnetic stimulation at a similar incidence rate. Contralateral ON electrical stimulation resulted in a similar reflex, but with a lower incidence. ON and femoral nerve electrical and magnetic coil stimulation produced similar low-incidence responses in the vastus medialis. These findings indicate that short latency adductor withdrawal reflexes are easily obtained on both sides following electrical or magnetic stimulation of the ON, and they can be elicited by both nociceptive and nonnociceptive stimuli. These reflexes prepare the body for a proper response to incoming signals and likely serve to protect the pelvic floor and pelvic organs.     

Transcranial magnetic stimulation of the trigeminal nerve: Intraoperative study on stimulation characteristics in man

We studied responses from the masseter and nasalis muscles following magnetic stimulation (magStim) and compared these responses with those obtained by direct electrical stimulation of the trigeminal (NV) and facial (NVII) nerve near the root exit zone during microvascular decompression operations of NVII. We found that (1) magStim threshold to excite the nerve is high for NV and low for NVII; (2) excitation of all motor fibers is impossible for NV, and easy for NVII; (3) optimal coil placement is critical for NV, but not critical for NVII; and (4) between and within subjects, the excitation site is variable on NV, but stable on NVII. We estimated that the anatomical location of magStim to be either within or outside the cerebrospinal fluid for NV, and to be in the labyrinthine segment of the facial canal for NVII. Physical models explain and clinical lesion models support these differences found between NV and NVII. © 1995 John Wiley & Sons, Inc.     

Magnetic stimulation excites skeletal muscle via motor nerve axons in the cat

Surface magnetic stimulation has been applied directly over skeletal muscle (triceps surae) in decerebrated cats. Recordings were made of the twitch contraction and electromyographic responses in triceps surae, and of the centripetal nerve volley in the sciatic nerve or spinal roots. Stimulus/response curves were established up to the maximum output of the magnetic stimulator. Neuromuscular blockade abolished the twitch contraction and muscle action potential leaving the nerve volley unaffected. Raising the stimulator output to its maximum increased the size of the nerve volley but failed to produce any muscle response. We conclude that magnetic stimulation applied directly to skeletal muscle excites via stimulation of motor nerve axons. The maximum output of the stimulator was incapable of exciting muscle fibers by direct depolarization. Use of magnetic stimulation in the clinical appraisal of muscle function should be interpreted in the knowledge that the stimulator elicits contraction only indirectly through nerve stimulation. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1108–1114, 1997     

A lack of effect from transcranial magnetic stimulation (TMS) on the vagus nerve stimulator (VNS).

OBJECTIVE: The effects of transcranial magnetic stimulation (TMS) on vagus nerve stimulation (VNS) are unknown. Understanding these effects is important before exposing individuals with an implanted VNS to TMS, as could occur in epilepsy or depression TMS research. To explore this issue, the TMS-induced current in VNS leads and whether TMS has an effect on the VNS pulse generator was assessed. METHODS: Ex vivo measurement of current in VNS leads during single-pulse TMS and pulse generator function before, during, and after single-pulse TMS was assessed. RESULTS: At the highest intensity and with the TMS coil held approximately 5 mm from the VNS wires, a 200 nA, 1.0 ms current was induced by TMS. This translates to an induced charge density of 3.3 nC/cm2/phase. The function of the pulse generator was unaffected by single-pulse TMS, even when its case was directly stimulated by the coil. CONCLUSIONS: TMS-induced current in VNS electrodes was not only well outside of the range known to be injurious to peripheral nerve, but also below the activation threshold of nerve fibers. SIGNIFICANCE: Using single-pulse TMS in individuals with VNS should not result in nerve stimulation or damage. Furthermore, single-pulse TMS does not affect the VNS pulse generator's function.     

Magnetic stimulation in hemifacial spasm and post–facial palsy synkinesis

The facial nerve was stimulated trascranially with a magnetic stimulator in 14 normal controls, 14 hemifacial spasm patients, and 16 post–facialpalsy synkinesis patients. Magnetic stimulation in normal controls revealed muscle responses which had latencies with a mean value of 4.99 ± 0.49 ms and amplitudes of 2.41 ± 1.08 mV. In the same group, transosseal conduction time was calculated to be 1.20 ± 0.13 ms. In the hemifacial spasm group, the amplitudes of the responses on the affected sides were lower as compared to the unaffected sides (mean values 1.78 vs. 2.41 mV, P = 0.01). Also, the threshold to magnetic stimulation was elevated on the affected sides. These findings are suggestive of the presence of a hypoexcitability to magnetic stimulation in the root entry zone. In the post–facial-palsy synkinesis patients, magnetic stimulation of the affected sides resulted in responses with long latencies and low amplitudes (mean latency 6.34 ms, mean amplitude 0.90 mV). In the recordings made with magnetic stimulation, the difference of the latencies between the two sides was larger as compared to those obtained by electrical stimulation. The transosseal conduction time was also remarkably prolonged on the affected side. These findings may suggest that magnetic stimulation can be an effective method of showing intracranially located lesions of the facial nerve. © 1993 John Wiley & Sons, Inc.     

Lumbosacral nerve root stimulation comparing electrical with surface magnetic coil techniques.

Stimulation of lumbosacral nerve roots using a monopolar needle electrode was compared with magnetic stimulation using a 7-cm diameter surface coil. Compound muscle action potentials were recorded from the tibialis anterior (TA) and flexor hallucis brevis (FHB) muscles. Although the mean latency of CMAPs did not differ using the two techniques, amplitudes were considerably larger using a needle. Mean amplitudes were 66% (TA) and 64% (FHB) of the direct M response obtained by distal, supramaximal stimulation compared with mean values using maximal magnetic coil stimulation of 36% (TA) and 25% (FHB). Minimum F-wave latencies from FHB were used to estimate the site of nerve root stimulation using both techniques. Although there was a large amount of variability in the data from individual subjects, the results suggested that, on the average, both forms of stimulation act proximal to the intervertebral foramen. We conclude that a needle electrode is a more suitable technique for stimulating lumbosacral nerve roots.     

Relevance of stimulus duration for activation of motor and sensory fibers: implications for the study of H-reflexes and magnetic stimulation.

Electric stimuli with durations of 0.5-1.0 msec are optimal for studies of H-reflexes. It is more difficult to obtain H-reflexes with shorter duration stimuli or with magnetic stimulation. In order to understand this behavior, we studied the excitation thresholds for motor and sensory fibers in the ulnar, median and tibial nerves using both electric and magnetic stimulation. For short duration electrical stimuli (0.1 msec) the threshold for motor fibers is lower than for sensory fibers. For longer duration electric stimuli (1.0 msec) the threshold for sensory fibers is lower. For magnetic stimulation the threshold for motor fibers is much lower than for sensory fibers. Thus, stimulus duration is a critical parameter for sensory fiber excitation, and current magnetic stimulators are not optimal.     

Twitch and M‐wave potentiation induced by intermittent maximal voluntary quadriceps contractions: Differences between direct quadriceps and femoral nerve stimulation

Introduction: The aim of this study was to investigate differences in twitch and M‐wave potentiation in the quadriceps femoris when electrical stimulation is applied over the quadriceps muscle belly versus the femoral nerve trunk. Methods: M‐waves and mechanical twitches were evoked using direct quadriceps muscle and femoral nerve stimulation between 48 successive isometric maximal voluntary contractions (MVC) from 10 young, healthy subjects. Potentiation was investigated by analyzing the changes in M‐wave amplitude recorded from the vastus medialis (VM) and vastus lateralis (VL) muscles and in quadriceps peak twitch force. Results: Potentiation of twitch, VM M‐wave, and VL M‐wave were greater for femoral nerve than for direct quadriceps stimulation (P < 0.05). Despite a 50% decrease in MVC force, the amplitude of the M‐waves increased significantly during exercise. Conclusions: In addition to enhanced electrogenic Na⁺‐K⁺ pumping, other factors (such as synchronization in activation of muscle fibers and muscle architectural properties) may significantly influence the magnitude of M‐wave enlargement. Muscle Nerve 48 : 920–929, 2013     

A comparison of magnetic and electrical stimulation of peripheral nerves

We compared magnetic stimulation using different coil designs (2 rounded coils and a butterfly-prototype coil) with electrical stimulation of the median and ulnar nerves in 5 normal subjects. Using magnetic stimulation we were able to record technically satisfactory maximal sensory and motor responses only with the butterfly coil. Submaximal electrical stimuli preferentially activated sensory rather than motor axons, but submaximal magnetic stimuli did not. The onset latency, amplitude, area and duration of responses elicited electrically or magnetically with the butterfly coil during routine sensory and motor nerve conduction studies were similar, and motor and sensory conduction velocities were comparable when studied over long segments of nerve. However, the motor conduction velocities with magnetic and electrical stimulation differed by as much as 18 m/sec in the across-elbow segment of ulnar nerve. Thus, recent developments in magnetic stimulator design have improved the focality of the stimulus, but the present butterfly coil design cannot replace electrical stimulation for the detection of focal changes in nerve conduction velocity at common entrapment sites, such as in the across-elbow segment of the ulnar nerve.     

Somatosensory cortex responses to median nerve stimulation: fMRI effects of current amplitude and selective attention.

OBJECTIVES: The aim of this study was to localize and to investigate response properties of the primary (SI) and the secondary (SII) somatosensory cortex upon median nerve electrical stimulation. METHODS: Functional magnetic resonance imaging (fMRI) was used to quantify brain activation under different paradigms using electrical median nerve stimulation in healthy right-handed volunteers. In total 11 subjects were studied using two different stimulus current values in the right hand: at motor threshold (I(max)) and at I(min) (1/2 I(max)). In 7 of these 11 subjects a parametric study was then conducted using 4 stimulus intensities (6/6, 5/6, 4/6 and 3/6 I(max)). Finally, in 10 subjects an attention paradigm in which they had to perform a counting task during stimulation with I(min) was done. RESULTS: SI activation increased with current amplitude. SI did not show significant activation during stimulation at I(min). SII activation did not depend on current amplitude. Also the posterior parietal cortex appeared to be activated at I(min). The I(min) response in SII significantly increased by selective attention compared to I(min) without attention. At I(max) significant SI activity was observed only in the contralateral hemisphere, the ipsilateral cerebellum, while other areas possibly showed bilateral activation. CONCLUSIONS: Distributed activation in the human somatosensory cortical system due to median nerve stimulation was observed using fMRI. SI, in contrast to SII, appears to be exclusively activated on the contralateral side of the stimulated hand at I(max), in agreement with the concept of SI's important role in processing of proprioceptive input. Only SII remains significantly activated in case of lower current values, which are likely to exclusively stimulate the sensible fibres mediating cutaneous receptor input. Selective attention only enhances SII activity, indicating a higher-order role for SII in the processing of somatosensory input.     

Mechanisms underlying reversal of motor unit activation order in electrically evoked contractions after spinal cord injury

Extracellular stimulation normally activates larger-diameter axons, innervating motor units producing higher force, at lower stimulation intensities than required to activate small-diameter axons innervating motor units producing low force. However, activation of weaker thenar motor units at lower stimulation intensities than required to activate strong motor units has been reported during extracellular stimulation of the median nerve in persons with chronic cervical spinal cord injury. We used a computational model that reproduced this experiment to identify the potential mechanisms for the observed reversal of the inverse recruitment order, including preferential death of large motoneurons, demyelination and remyelination, and denervation and reinnervation of muscle fibers. Five sets of simulations assessed these mechanisms with seven simulated subjects. Preferential reinnervation, with small-diameter axons reinnervating more abandoned muscle fibers than larger-diameter axons, accounted for the apparent reversal of the inverse recruitment order observed previously. Preferential death of larger axons enhanced the reversal, but alone could not account for the observed reversal. Further, demyelination and remyelination, even in an extreme case and when combined with preferential death of large motoneurons, could not reproduce the reversal of inverse recruitment order. Thus, the apparent reversal of the inverse recruitment order was not a reversal of activation order across different diameter nerve fibers, but rather was a consequence of the redistributed force-generating capacity of the motor units resulting from denervation and reinnervation.     

Effects of 10-kHz Subthreshold Stimulation on Human Peripheral Nerve Activation

ObjectiveThe mechanisms of action of high-frequency stimulation (HFS) are unknown. We investigated the possible mechanism of subthreshold superexcitability of HFS on the excitability of the peripheral nerve.Materials and MethodsThe ulnar nerve was stimulated at the wrist in six healthy participants with a single (control) stimulus, and the responses were compared with the responses to a continuous train of 5 seconds at frequencies of 500 Hz, 2.5 kHz, 5 kHz, and 10 kHz. Threshold intensity for compound muscle action potential (CMAP) was defined as intensity producing a 100-μV amplitude in ten sequential trials and “subthreshold” as 10% below the CMAP threshold. HFS threshold was defined as stimulation intensity eliciting visible tetanic contraction.ResultsComparing the threshold of single pulse stimulation for eliciting CMAP vs threshold for HFS response and pooling data at different frequencies (500 Hz–10 kHz) revealed a significant difference (p = 0.00015). This difference was most obvious at 10 kHz, with a mean value for threshold reduction of 42.2%.ConclusionsHFS with a stimulation intensity below the threshold for a single pulse induces axonal superexcitability if applied in a train. It can activate the peripheral nerve and produce a tetanic muscle response. Subthreshold superexcitability may allow new insights into the mechanism of HFS.     

Central and peripheral motor conduction to cremasteric muscle

The few electrophysiologic studies of the cremasteric muscle (CM) have mainly been restricted to the cremaster reflex with no reference to central and peripheral nerve conduction to the muscle, probably for technical reasons.Twenty-six normal adult male volunteers were studied by transcranial magnetic cortical stimulation (TMS) and stimulation of thoracolumbar roots. The genitofemoral nerve (GFN) was stimulated electrically at the anterior superior iliac spine and a needle electrode was inserted into the CM for conduction studies. The motor latency to the CM from the cortical TMS ranged from 20 to 33 ms among the subjects (25.8 +/- 2.9 ms, mean +/- SD). Magnetic stimulation of the lumbar roots produced a motor response of the CM within 9.6 +/- 1.9 ms (range, 6-15). The central motor conduction time to the CM was 16.5 +/- 2.8 ms (range, 10-21). Stimulation of the GFN produced a compound muscle action potential with a mean value of 6.4 +/- 1.8 (range, 4-10) ms in 23 of the 26 cases. Thus, central motor nerve fibers to the CM motor neurons exist, and there may be a representation area for the CM in the cerebral cortex. The GFN motor conduction time to the CM may have clinical utility, such as in the evaluation of the groin pain due to surgical procedures in the lower abdomen.     

The effect of digital nerve stimulation on recruitment order of motor units in the first deep lumbrical muscle of the cat

The normal recruitment order of EMG spikes of the first deep lumbrical muscle of the cat's hindpaw, usually seen during cortical stimulation, pad pinch and weak plantar nerve stimulation, was temporarily reversed after stimulation of the medial digital nerve of the foot at 50 Hz for 2 min, and normal order was recovered in 2 to 10 min. The longer the period of stimulation of the medial digital nerve was, the longer the time for recovery. min. After prolonged medial digital nerve stimulation, EMG responses to pad pinch and plantar nerve stimulation were facilitated. Combination of some stimuli (e.g. cortical stimulation and plantar nerve stimulation, pad pinch and plantar nerve stimulation, plantar nerve and medial digital nerve stimulation) also produced reversal of recruitment order during the period of stimulation.A functionally single motor nerve fiber to the first deep lumbrical muscle was isolated from the motor nerve axons in L7 and S1 of the spinal cord of cats, and physiological properties of pairs of motor units whose recruitment order was temporarily altered by plantar nerve stimulation after prolonged stimulation of the medial digital nerve of a foot were examined. Motor units with large action potentials were more facilitated than motor units with small action potentials after prolonged stimulation of the medial digital nerve. The former motor units showed fast contraction time, large twitch tension, low resistance to fatigue and presence of sag-behavior. The latter motor units showed slow contraction time, small twitch tension and high resistance to fatigue.High threshold motor units with middle-sized surface EMG records were recruited after motor units with large potentials had been recruited; motor units with middle-size action potentials were beta motor axons branching to both intrafusal and extrafusal muscle fibers. The beta motor axons also showed fast conduction velocity and the presence of sag-behavior.     

Submaximal stimuli activate different nerve fiber populations at different sites

Theoretically, the largest and fastest nerve fibers are preferentially stimulated with submaximal stimuli. However, it is also well known that intraneural fascicular topography changes substantially along a proximal to distal axis. Because of this change in fascicular topography, we hypothesized that percutaneous submaximal stimuli applied to a nerve at different locations would stimulate different subpopulations of large fibers. We performed a series of collision studies by stimulating the ulnar nerve submaximally at proximal and distal sites at varying levels of stimulation intensity from motor threshold to supramaximal stimulation. The results suggest that variation in intraneural topography at different sites allows different large diameter nerve fiber subpopulations to be activated at submaximal stimuli, and emphasizes the importance of supramaximal stimulation to determine a valid conduction velocity. © 1994 John Wiley & Sons, Inc.     

The silent period produced by electrical stimulation of mixed peripheral nerves.

The electromyographic silent period (SP) produced by electrical stimulation of the median and ulnar nerve, before and after lidocaine block of the ulnar nerve at the elbow, was recorded in the voluntarily contracting abductor pollicis brevis (APB) muscle of 4 normal subjects. Prior to block, stimulation of the corresponding segment in either nerve (e.g., wrist and elbow) elicited SPs with similar end points. With more proximal stimulation, the SPs consistently ended earlier. Following the block, ulnar nerve stimulation below the elbow failed to elicit a SP in any subject, despite a mechanical twitch caused by the antagonistic contraction of adductor pollicis muscle. Ulnar nerve stimulation above the block elicited a SP in all subjects similar to the preblock SP. In all 4 subjects, submaximal median nerve stimulation at the wrist produced an H-reflex, followed by a SP in the absence of the direct M-response. This SP, due to selective activation of sensory fibers, lacked a collision component, but, was otherwise similar to the SP elicited by supramaximal wrist stimulation. These findings indicate that the ascending volley following electrical stimulation of a mixed peripheral nerve produces the SP without apparent contribution from the descending motor volley.     

Voluntary contraction and responses to submaximal cervical nerve root stimulation

Voluntary contraction of hand muscles increases compound muscle action potential (CMAP) amplitudes evoked by submaximal electrical percutaneous cervical stimulation (EPCS). This has been reported to be due to an intraspinal, presynaptic mechanism. We studied the effects of voluntary contraction on hypothenar CMAP amplitudes in 5 volunteers following electrical peripheral nerve stimulation at the wrist, EPCS, magnetic stimulation at the neck and the effects of a conditioning subthreshold cortical magnetic stimulus on CMAPs evoked by EPCS at rest. CMAP amplitudes increased with voluntary contraction of the target muscle, regardless of type or location of stimulus (P < 0.001). No increase in CMAP amplitude occurred when a conditioning transcranial stimulus was employed with EPCS (P = 0.35). Our findings indicate a peripheral rather than central mechanism underlies this effect of voluntary contraction. It is probably related to the recruitment order of motor axons, comparing voluntary activation with electrical or magnetic stimulation. © 1994 John Wiley & Sons, Inc.     

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.     

Anatomical, Physiological, and Theoretical Basis for the Antiepileptic Effect of Vagus Nerve Stimulation

Summary: The vagus is a mixed nerve carrying somatic and visceral afferents and efferents. The majority of vagal nerve fibers are visceral afferents and have a wide distribution throughout the central nervous system (CNS) either monosynaptically or via the nucleus of the solitary tract. Besides activation of well-defined reflexes, vagal stimulation produces evoked potentials recorded from the cerebral cortex, the hippocampus, the thalamus, and the cerebellum. Activation of vagal afferents can depress monosynaptic reflexes, decrease the activity of spinothalamic neurons, and increase pain threshold. Depending on the stimulation parameters, vagal afferent stimulation in experimental animals can produce electroencephalo-graphic (EEG) synchronization or desynchronization and has been shown to affect sleep states. The desynchronization of the EEG appears to depend on activation of afferent fibers that have conduction velocities of ≤ 15 m/s. Vagal afferent stimulation can also influence the activity of interictal cortical spikes produced by topical strychnine application, and either attenuate or stop seizures produced by pentylenetetrazol, 3-mercaptoproprionic acid, maximal electroshock, and topical alumina gel. The mechanisms for the antiepileptic effects of vagal stimulation are not fully understood but probably relate to effects on the reticular activating system. The vagus provides an easily accessible, peripheral route to modulate CNS function.     

Anatomical,Physiological, and Theoretical Basis for the Antiepileptic Effect of Vagus Nerve Stimulation

Summary: The vagus is a mixed nerve carrying somatic and visceral afferents and efferents. The majority of vagal nerve fibers are visceral afferents and have a wide distribution throughout the central nervous system (CNS) either monosynaptically or via the nucleus of the solitary tract. Besides activation of well-defined reflexes, vagal stimulation produces evoked potentials recorded from the cerebral cortex, the hippocampus, the thalamus, and the cerebellum. Activation of vagal afferents can depress monosynaptic reflexes, decrease the activity of spinothalamic neurons, and increase pain threshold. Depending on the stimulation parameters, vagal afferent stimulation in experimental animals can produce electroencephalo-graphic (EEG) synchronization or desynchronization and has been shown to affect sleep states. The desynchronization of the EEG appears to depend on activation of afferent fibers that have conduction velocities of ≤ 15 m/s. Vagal afferent stimulation can also influence the activity of interictal cortical spikes produced by topical strychnine application, and either attenuate or stop seizures produced by pentylenetetrazol, 3-mercaptoproprionic acid, maximal electroshock, and topical alumina gel. The mechanisms for the antiepileptic effects of vagal stimulation are not fully understood but probably relate to effects on the reticular activating system. The vagus provides an easily accessible, peripheral route to modulate CNS function.