Vestibulospinal influences on lower limb motoneurons

Galvanic vestibular stimulation (GVS) is a research tool used to activate the vestibular system in human subjects. When a low-intensity stimulus (1-4 mA) is delivered percutaneously to the vestibular nerve, a transient electromyographic response is observed a short time later in lower limb muscles. Typically, galvanically evoked responses are present when the test muscle is actively engaged in controlling standing balance. However, there is evidence to suggest that GVS may be able to modulate the activity of lower limb muscles when subjects are not in a free-standing situation. The purpose of this review is to examine 2 studies from our laboratory that examined the effects of GVS on the lower limb motoneuron pool. For instance, a monopolar monaural galvanic stimulus modified the amplitude of the ipsilateral soleus H-reflex. Furthermore, bipolar binaural GVS significantly altered the onset of activation and the initial firing frequency of gastrocnemius motor units. The following paper examines the effects of GVS on muscles that are not being used to maintain balance. We propose that GVS is modulating motor output by influencing the activity of presynaptic inhibitory mechanisms that act on the motoneuron pool.     

Enhancing human balance control with galvanic vestibular stimulation

 With galvanic vestibular stimulation (GVS), electrical current is delivered transcutaneously to the vestibular afferents through electrodes placed over the mastoid bones. This serves to modulate the continuous firing levels of the vestibular afferents, and causes a standing subject to lean in different directions depending on the polarity of the current. Our objective in this study was to test the hypothesis that the sway response elicited by GVS can be used to reduce the postural sway resulting from a mechanical perturbation. Nine subjects were tested for their postural responses to both galvanic stimuli and support-surface translations. Transfer-function models were fit to these responses and used to calculate a galvanic stimulus that would act to counteract sway induced by a support-surface translation. The subjects' responses to support-surface translations, without and with the stabilizing galvanic stimulus, were then measured. With the stabilizing galvanic stimulus, all subjects showed significant reductions in both sway amplitude and sway latency. Thus, with GVS, subjects maintained a more erect stance and followed the support-surface displacement more closely. These findings suggest that GVS could possibly form the basis for a vestibular prosthesis by providing a means through which an individual's posture can be systematically controlled. Received: 11 May 2000 / Accepted in revised form: 20 November 2000     

Probing the human vestibular system with galvanic stimulation.

Galvanic vestibular stimulation (GVS) is a simple, safe, and specific way to elicit vestibular reflexes. Yet, despite a long history, it has only recently found popularity as a research tool and is rarely used clinically. The obstacle to advancing and exploiting GVS is that we cannot interpret the evoked responses with certainty because we do not understand how the stimulus acts as an input to the system. This paper examines the electrophysiology and anatomy of the vestibular organs and the effects of GVS on human balance control and develops a model that explains the observed balance responses. These responses are large and highly organized over all body segments and adapt to postural and balance requirements. To achieve this, neurons in the vestibular nuclei receive convergent signals from all vestibular receptors and somatosensory and cortical inputs. GVS sway responses are affected by other sources of information about balance but can appear as the sum of otolithic and semicircular canal responses. Electrophysiological studies showing similar activation of primary afferents from the otolith organs and canals and their convergence in the vestibular nuclei support this. On the basis of the morphology of the cristae and the alignment of the semicircular canals in the skull, rotational vectors calculated for every mode of GVS agree with the observed sway. However, vector summation of signals from all utricular afferents does not explain the observed sway. Thus we propose the hypothesis that the otolithic component of the balance response originates from only the pars medialis of the utricular macula.     

Tuning of the ocular vestibular evoked myogenic potential to bone-conducted sound stimulation

Ocular vestibular evoked myogenic potentials (oVEMPs) are a recently described clinical measure of the vestibulo-ocular reflex. Studies demonstrating differences in frequency tuning between air-conducted and bone-conducted (BC) oVEMPs suggest a separate vestibular (otolith) origin for each stimulus modality. In this study, 10 healthy subjects were stimulated with BC stimuli using a hand-held minishaker. Frequencies were tested in the range of 50-1,000 Hz using both a constant-force and constant-acceleration method. Subjects were stimulated at the mastoid process and the forehead. For constant-force stimulation at both sites, maximum acceleration occurred around 100 Hz, in differing axes. Both forms of stimulation had low-frequency peaks of oVEMP amplitudes (constant force: mastoid, 80-150 Hz; forehead, 50-125 Hz; constant acceleration: mastoid, 100-200 Hz; forehead, 80-150 Hz), for both sites of application, despite differences in the magnitude and direction of evoked head acceleration. For mastoid stimulation, ocular responses changed from out of phase to in phase for 400 Hz and above. Our results demonstrate that BC stimuli show tuning around 100 Hz, independent of stimulus site, that is not due to skull properties. The findings are consistent with an effect on a receptor with a resonance around 100 Hz, most likely the utricle.     

Stimulus parameters and temporal evolution of the olfactory evoked potential in rats

Evoked potentials were recorded from olfactory bulb, piriformcortex and scalp in urethane anesthetized rats in response tobrief odorant stimuli (amyl acetate, phenylethyl alcohol, eugenol)presented through a nasal cannula by means of a constant flowolfactometer. The effects of stimulus duration, nasal cannulaposition, flow rate, concentration and interstimulus intervalwere examined. The highest amplitude potentials were evokedby 10% amyl acetate at 20 ms duration, 1000 ml/min flow rateand a 60-s interstimulus interval with the stimulus deliveredat the nares. Odorant evoked potentials from deep within theolfactory bulb consisted of a triphasic wave with major componentsat 60 ms (P60), 90 ms (N90) and 140 ms (P140) with the lattertwo reversing polarity close to the surface of the bulb. Potentialsrecorded from layer I of piriform cortex were of similar amplitude,but opposite in polarity to the deep olfactory bulb potentials.Recordings from the skin over the nose elicited waveforms ofsimilar morphology to the deep olfactory bulb potentials, butone-quarter the amplitude and of opposite polarity The evokedpotentials changed with repetitive stimulation The N90 componentwas not present initially and only appeared after several stimuli.The appearance of the N90 component depended on the integrityof the olfactory peduncle. Thus, olfactory evoked potentialsto odorant stimuli reflect dynamic aspects of the encoding ofolfactory information dependent on connections between olfactorybulb and piriform cortex     

Neural Mechanisms Influencing Interlimb Coordination during Locomotion in Humans: Presynaptic Modulation of Forearm H-Reflexes during Leg Cycling

Presynaptic inhibition of transmission between Ia afferent terminals and alpha motoneurons (Ia PSI) is a major control mechanism associated with soleus H-reflex modulation during human locomotion. Rhythmic arm cycling suppresses soleus H-reflex amplitude by increasing segmental Ia PSI. There is a reciprocal organization in the human nervous system such that arm cycling modulates H-reflexes in leg muscles and leg cycling modulates H-reflexes in forearm muscles. However, comparatively little is known about mechanisms subserving the effects from leg to arm. Using a conditioning-test (C-T) stimulation paradigm, the purpose of this study was to test the hypothesis that changes in Ia PSI underlie the modulation of H-reflexes in forearm flexor muscles during leg cycling. Subjects performed leg cycling and static activation while H-reflexes were evoked in forearm flexor muscles. H-reflexes were conditioned with either electrical stimuli to the radial nerve (to increase Ia PSI; C-T interval  = 20 ms) or to the superficial radial (SR) nerve (to reduce Ia PSI; C-T interval  = 37–47 ms). While stationary, H-reflex amplitudes were significantly suppressed by radial nerve conditioning and facilitated by SR nerve conditioning. Leg cycling suppressed H-reflex amplitudes and the amount of this suppression was increased with radial nerve conditioning. SR conditioning stimulation removed the suppression of H-reflex amplitude resulting from leg cycling. Interestingly, these effects and interactions on H-reflex amplitudes were observed with subthreshold conditioning stimulus intensities (radial n., ∼0.6×MT; SR n., ∼ perceptual threshold) that did not have clear post synaptic effects. That is, did not evoke reflexes in the surface EMG of forearm flexor muscles. We conclude that the interaction between leg cycling and somatosensory conditioning of forearm H-reflex amplitudes is mediated by modulation of Ia PSI pathways. Overall our results support a conservation of neural control mechanisms between the arms and legs during locomotor behaviors in humans.     

Short latency vestibular evoked potentials (VsEPs) to linear acceleration impulses in rats

In this study, short latency (t<12.7 ms) vestibular evoked potentials (VsEPs) in response to linear acceleration impulses were recorded in 37 rats. A new technique (based on a solenoid) was used for generating linear force impulses that were delivered to the animal's head. The impulse had a maximal peak acceleration of 12 g. During the impulse, the displacement was 50 μm (at 4 g) and the rise time was 1.0 ms. A stimulation rate of 2/s was usually used. The VsEPs (averaged responses to 128 stimulations, digital filter: 300–1500 Hz) were recorded with electrodes on pinna and vertex, and were composed of 4–6 clear waves with mean amplitudes (for a 4 g stimulus) of 1–5 μV. The VsEPs were resistant to white noise masking, and were significantly suppressed (P<0.05) following bilateral application of a saturated KCl solution to the inner ear, showing that contributions of the auditory and somatosensory systems are negligible. The latency of the response decreased as a power law function of stimulus magnitude, and the amplitude of the first wave increased as a sigmoid function of stimulus magnitude. VsEP responses were still present at the lowest intensities attainable (0.06–0.4 g) and reached saturation at 9 g. The amplitude of the later components was reduced when stimulus rate was elevated to 20/s. These results suggest that VsEPs in response to linear accelerations are similar in their nature to VsEPs in response to angular acceleration impulses that were previously recorded. These VsEPs to linear accelerations are most likely initiated in the otolith organs.     

Ankle extensor proprioceptors contribute to the enhancement of the soleus EMG during the stance phase of human walking

A rapid plantar flexion perturbation applied to the ankle during the stance phase of the step cycle during human walking unloads the ankle extensors and produces a marked decline in the soleus EMG. This demonstrates that sensory activity contributes importantly to the enhancement of the ankle extensor muscle activation during human walking. On average, the EMG begins to decline approximately 52 ms after the perturbation. In contrast, a rapid dorsi flex ion perturbation produces a group Ia mediated short-latency stretch reflex burst with an onset latency of approximately 36 ms. The transmission of sensory traffic from the foot and ankle was suppressed in 10 subjects by an anaesthetic nerve block produced with local injections of lidocaine hydrochloride. The anaesthetic block had no effect on the stance phase soleus EMG, the latencies of the EMG responses, or the magnitude of the EMG decline following the plantar flexion perturbation. Therefore, it is more likely that proprioceptive afferents, rather than cutaneous afferents, contribute to the background soleus EMG during the late stance phase of the step cycle. The large difference in onset latencies between the short-latency reflex and unload responses suggests that the largest of the active group Ia afferents might not contribute strongly to the background soleus EMG, although it remains to be determined which of the proprioceptive pathways provide the more important contributions.     

Short and middle latency vestibular evoked responses to acceleration in man

We have succeeded in recording short and middle latency vestibular evoked responses in human subjects. The head was held rigidly in a special, patented head holder, constructed individually for each subject, which gripped the teeth of the upper jaw. The stimulus consisted of 2/sec steps of angular acceleration impulses produced by a special motor with intensities of about 10,000°/sec² and with a rise time of 1–2 msec. The electrical activity was recorded as the potential difference between special forehead and mastoid electrodes having a large, secure contact area with the skin. The activity was digitally filtered and averaged in 2 separate channels by means of a Microshev 2000 evoked response system. The short latency responses, with peaks at about 3.5 msec (forehead positive), 6.0 msec (forehead negative) and 8.4 msec (forehead positive; bandpass: 200–2000 Hz; average of 1024 trials), had amplitudes of about 0.5 μV. The middle latency responses had peaks at about 8.8 msec (forehead positive), 18.8 msec (forehead negative) and 26.8 msec (forehead positive; 30–300 Hz; N = 128 trials), with larger amplitudes (about 15 μV). These responses were consistently recorded in the same subject at different times and were similar in different normal subjects. Strenuous control experiments were conducted in order to ensure that these responses are not artefacts due to the movement of conducting media (head, electrodes and leads) in the electromagnetic field of the motor and are elicited by activation of normal labyrinths. Among other controls, they were not present in a cadaver, in patients with bilateral absence of nystagmus to caloric stimuli and in conducting volumes the size of the human head. They were also not masked by white noise.     

Electrical Vestibular Stimuli to Enhance Vestibulo-Motor Output and Improve Subject Comfort

Electrical vestibular stimulation is often used to assess vestibulo-motor and postural responses in both clinical and research settings. Stochastic vestibular stimulation (SVS) is a recently established technique with many advantages over its square-wave counterpart; however, the evoked muscle responses remain relatively small. Although the vestibular-evoked responses can be enhanced by increasing the stimulus amplitude, subjects often perceive these higher intensity electrical stimuli as noxious or painful. Here, we developed multisine vestibular stimulation (MVS) signals that include precise frequency contributions to increase signal-to-noise ratios (SNR) of stimulus-evoked muscle and motor responses. Subjects were exposed to three different MVS stimuli to establish that: 1) MVS signals evoke equivalent vestibulo-motor responses compared to SVS while improving subject comfort and reducing experimentation time, 2) stimulus-evoked vestibulo-motor responses are reliably estimated as a linear system and 3) specific components of the cumulant density time domain vestibulo-motor responses can be targeted by controlling the frequency content of the input stimulus. Our results revealed that in comparison to SVS, MVS signals increased the SNR 3–6 times, reduced the minimum experimentation time by 85% and improved subjective measures of comfort by 20–80%. Vestibulo-motor responses measured using both EMG and force were not substantially affected by nonlinear distortions. In addition, by limiting the contribution of high frequencies within the MVS input stimulus, the magnitude of the medium latency time domain motor output response was increased by 58%. These results demonstrate that MVS stimuli can be designed to target and enhance vestibulo-motor output responses while simultaneously improving subject comfort, which should prove beneficial for both research and clinical applications.     

Gender differences in knee stability in response to whole-body vibration

The purpose of this study was to determine whether there are kinematic and electromyographic (EMG) differences between men and women in how the knee is controlled during a single-legged drop landing in response to whole-body vibration (WBV). Forty-five healthy volunteers, 30 men (age 22 ± 3 years; weight 76.8 ± 8.8 kg; height 179.0 ± 6.8 cm) and 15 women (age 22 ± 3 years; weight 61.0 ± 7.7 kg; height 161.9 ± 7.2 cm) were recruited for this study. Knee angles, vertical ground reaction forces, and the time to stabilize the knee were assessed after single-legged drop landings from a 30-cm platform. Surface EMG data in rectus femoris (RF) and hamstrings (H) and knee and ankle accelerometry signals were also acquired. The participants performed 3 pretest landings, followed by a 3-minute recovery and then completed 1 minute of WBV (30 Hz to 4 mm). Before vibration, the female subjects had a significantly higher peak vertical force value, knee flexion angles, and greater H preactivity (EMG(RMS) 50 milliseconds before activation) than did the male subjects. In addition, although not significant, the medial-lateral (ML) acceleration in both knee and ankle was also higher in women. After WBV, no significant differences were found for any of the other variables. However, there was a decrease in the RF to H activation ratio during the precontact phase and an increase in the ratio during the postcontact phase just in women, which leads to a decrement in ML acceleration. The gender differences reported in knee stability in response to WBV underline the necessity to perform specific neuromuscular training programs based on WBV together with instruction of the proper technique, which can assist the clinician in the knee injury prevention.     

The effect of selective attention on pattern-specific visual evoked potentials

In a complex choice reaction time experiment, patterned stimuli without luminance change were presented, and pattern-specific visual evoked potentials to lower half-field stimulation were recorded. Two experimental conditions were used. The first was the between-field selection, where square patterns were presented in either the lower or the upper half of the visual field. In a given stimulus run one of the half-fields was task-relevant, and the subjects' task was to press a microswitch to stimuli of higher duration value (GO stimuli), while they had to ignore shorter ones, i. e. stimuli of lower apparent spatial contrast (NOGO stimuli). They had to ignore the stimuli appearing in the irrelevant half-field (IRR stimuli). In order to ensure proper fixation, the subjects had to press another microswitch at the onset of a dim light at the fixation point (CRT stimuli). Our second experimental condition was the within-field selection, where the GO, NOGO, and IRR stimuli appeared in the lower half of the visual field. GO and NOGO were square patterns while IRR stimuli were constructed of circles, or vice versa. (The CRT stimuli were the same as in the previous condition.) Three pattern-specific visual evoked potential components were identified, i. e. CI (70 ms latency), CII (100 ms latency), and CIII (170 ms latency). There were marked selective attention effects on both the CI-CII and CII-CIII peak-to-peak amplitudes. In both experimental conditions, responses with the highest amplitude were evoked by the GO type of stimuli, while the IRR stimuli evoked the smallest responses. According to these results, attention effects on the pattern-specific visual evoked potentials in the first 200 ms cannot be attributed to a simple stimulus set kind of selection.     

Central Adaptation to Repeated Galvanic Vestibular Stimulation: Implications for Pre-Flight Astronaut Training

Healthy subjects (N = 10) were exposed to 10-min cumulative pseudorandom bilateral bipolar Galvanic vestibular stimulation (GVS) on a weekly basis for 12 weeks (120 min total exposure). During each trial subjects performed computerized dynamic posturography and eye movements were measured using digital video-oculography. Follow up tests were conducted 6 weeks and 6 months after the 12-week adaptation period. Postural performance was significantly impaired during GVS at first exposure, but recovered to baseline over a period of 7–8 weeks (70–80 min GVS exposure). This postural recovery was maintained 6 months after adaptation. In contrast, the roll vestibulo-ocular reflex response to GVS was not attenuated by repeated exposure. This suggests that GVS adaptation did not occur at the vestibular end-organs or involve changes in low-level (brainstem-mediated) vestibulo-ocular or vestibulo-spinal reflexes. Faced with unreliable vestibular input, the cerebellum reweighted sensory input to emphasize veridical extra-vestibular information, such as somatosensation, vision and visceral stretch receptors, to regain postural function. After a period of recovery subjects exhibited dual adaption and the ability to rapidly switch between the perturbed (GVS) and natural vestibular state for up to 6 months.     

Galvanic vestibular stimulation improves the results of vestibular rehabilitation

Here, we present findings from a three-step investigation of the effect of galvanic vestibular stimulation (GVS) in normal subjects and in subjects undergoing vestibular rehabilitation (VR). In an initial study, we examined the body sway of 10 normal subjects after one minute of 2 mA GVS. The effect of the stimulation lasted for at least 20 minutes in all subjects and up to two hours in 70% of the subjects. We then compared a group of patients who received conventional VR (40 patients) with a group that received a combination of VR and GVS. Results suggest a significant improvement in the second group. Finally, we attempted to establish the optimal number of GVS sessions and to rule out a placebo effect. Fifteen patients received "systematic" GVS: five sessions, once a week. Five patients received "nonsystematic" galvanic stimulation in a sham protocol, which included two stimulations of the clavicle. These data were analyzed with Fisher's exact test and indicated that the best results were obtained after three sessions of GVS and no placebo effect was observed.     

Transition from slow to ballistic movement: development of triphasic electromyogram patterns.

The purpose of this investigation was to determine how the triphasic electromyogram (EMG) pattern of muscle activation developed from the agonist muscle only pattern as movement time (tmov) decreased. Six adult women produced a series of 30 degrees elbow extension movements in the horizontal plane at speeds ranging from ballistic (less than 400-ms tmov) to very slow (greater than 800-ms tmov). Surface EMG from triceps brachii (agonist) and biceps brachii (antagonist) muscles were recorded, together with elbow angle, on a microcomputer. The results showed that triphasic EMG patterns developed systematically as tmov decreased from 1000 ms to less than 200 ms. In trials with very long tmov, many elbow extension movements were produced by a single continuous activation of the agonist triceps brachii muscle. As tmov decreased however, agonist activation became predominantly burst-like and other components of the triphasic EMG pattern [activation of the antagonist (Ant) and second agonist activation (Ag2)] began to appear. At the fastest movement speeds, triphasic EMG patterns (Ag1-Ant-Ag2, Ag1 being first activation of agonist muscle) were always present. This data indicated that the triphasic pattern of muscle activation was not switched on when a particular tmov was achieved. Rather, each component systematically developed until all were present, as distinctive bursts of activity, in most trials with tmov less than 400 ms.     

Functionally independent components of early event-related potentials in a visual spatial attention task.

Spatial visual attention modulates the first negative-going deflection in the human averaged event-related potential (ERP) in response to visual target and non-target stimuli (the N1 complex). Here we demonstrate a decomposition of N1 into functionally independent subcomponents with functionally distinct relations to task and stimulus conditions. ERPs were collected from 20 subjects in response to visual target and non-target stimuli presented at five attended and non-attended screen locations. Independent component analysis, a new method for blind source separation, was trained simultaneously on 500 ms grand average responses from all 25 stimulus-attention conditions and decomposed the non-target N1 complexes into five spatially fixed, temporally independent and physiologically plausible components. Activity of an early, laterally symmetrical component pair (N1aR and N1aL) was evoked by the left and right visual field stimuli, respectively. Component N1aR peaked ca. 9 ms earlier than N1aL. Central stimuli evoked both components with the same peak latency difference, producing a bilateral scalp distribution. The amplitudes of these components were no reliably augmented by spatial attention. Stimuli in the right visual field evoked activity in a spatio-temporally overlapping bilateral component (N1b) that peaked at ca. 180 ms and was strongly enhanced by attention. Stimuli presented at unattended locations evoked a fourth component (P2a) peaking near 240 ms. A fifth component (P3f) was evoked only by targets presented in either visual field. The distinct response patterns of these components across the array of stimulus and attention conditions suggest that they reflect activity in functionally independent brain systems involved in processing attended and unattended visuospatial events.     

成对短声不同间隔对单、双耳听觉脑干反应的影响——衍生听觉脑干反应方法的应用

采用具有不同间隔(0～32ms)的65dB nHL(正常听力水平)的成对短声刺激,记录20名正常人的单侧耳和两耳交替刺激的听觉脑干反应(ABR)。用计算机从成对短声反应中减去单一短声反应以提取衍生ABR。结果表明,单、双耳的衍生ABR V波振幅在成对短声间隔为0.2～1.5ms时,受到明显影响。单耳的减小54％～65％(P<0.01),两耳的减少46％～53％(P<0.01),但单、双耳的衍生ABR I波振幅未显示显著差异(P>0.05)。该结果说明,高位脑干通路在成对短声间隔为0.2～1.5ms时,不但对同侧耳的第2个短声反应能力降低,而且对来自对侧耳的第2个短声也如此。从而推断,在两耳交替刺激的耳间短声间隔小于2ms范围时,在下丘部位可能存在两耳交互作用。结果还提示,临床检查ABR时,采用的短声刺激间隔至少不应小于30ms。     

Muscle responses and monosynaptic reflexes in falling monkey. Role of the vestibular system.

The free fall has been used in our laboratory as a way to test vestibular function in baboons in order to quantify vestibular compensation in the hemilabyrinthectomized animal. This study presents only those results that concern the contribution of the vestibular system to muscle responses due to sudden fall. EMG activity was recorded from the fully conscious animal using chronic electrodes implanted in various muscles. Spinal monosynaptic reflexes (Hoffmann's and tendon reflexes) were studied in the soleus muscle. Baboons were seated in a special chair suspended from an electromagnet and unexpectedly dropped 90 cm. Experiments were performed in normal, unilateral and bilateral vestibular neurectomized baboons. 1. In normal baboons, results showed a first short-latency response in all tested muscles, followed by a second peak of EMG activity in these muscles. Comparison with data from bilateral vestibular neurectomized baboons demonstrates that normal vestibular function is essential for the appearance of the first peak; the second peak rapidly disappears in our experimental situation where the animal's fall is mechanically braked and interrupted, so the animal does not have to make the postural adjustments necessary for landing, It is suggested that the first peak is concerned with the automatic and reflex control of landing, the second with the voluntary breaking of landing. 2. The modulation of monosynaptic spinal reflexes is closely related to the EMG response in soleus muscle. Facilitation of the H-reflex begins just prior to the onset of the EMG activity and continues as long as the baboon is falling. The T-reflex modulation presents a similar time course except in its early phase where it is depressed. Decrease in T and increase in H-reflexes suggest that the EMG response is most likely due to direct activation of alpha-motoneurons and not by means of the gamma-loop. 3. In unilateral vestibular neurectomized baboons, EMG and reflexological data show the classical asymmetry characterized by a strong decrease of the responses on the side of the lesion, and by a pronounced increase on the contralateral side. It is concluded that this represents the imbalance between the resting discharge of the vestibular neurons, and discloses the influence of labyrinthine afferences at the spinal level. We suggest consequently the use of EMG responses and modulation of spinal reflexes to fall in order to quantify vestibular compensation.     

Startle stimuli exert opposite effects on human cortical and spinal motor system excitability in leg muscles

Increased excitability of the spinal motor system has been observed after loud and unexpected acoustic stimuli (AS) preceding H-reflexes. The paradigm has been proposed as an electrophysiological marker of reticulospinal tract activity in humans. The brainstem reticular formation also maintains dense anatomical interconnections with the cortical motor system. When a startling AS is delivered, prior to transcranial magnetic stimulation (TMS), the AS produces a suppression of motor evoked potential (MEP) amplitude in hand and arm muscles of healthy subjects. Here we analyzed the conditioning effect of a startling AS on MEP amplitude evoked by TMS to the primary motor leg area. Ten healthy volunteers participated in two experiments that used a conditioning-test paradigm. In the first experiment, a startling AS preceded a suprathreshold transcranial test stimulus. The interstimulus interval (ISI) varied between 20 to 160 ms. When given alone, the test stimulus evoked a MEP amplitude of approximately 0.5 mV in the slightly preinervated soleus muscle (SOL). In the second experiment, the startling AS was used to condition the size of the H-reflex in SOL muscle. Mean MEP amplitude was calculated for each ISI. The conditioning AS suppressed MEP amplitude at ISIs of 30-80 ms. By contrast, H-reflex amplitude was augmented at ISIs of 100-200 ms. In conclusions, acoustic stimulation exerts opposite and ISI-specific effects on the amplitude of MEPs and H-reflex in the SOL muscle, indicating different mechanism of auditory-to-motor interactions at cortical and spinal level of motor system.     

Electromyography responses of pediatric and young adult volunteers in low-speed frontal impacts

No electromyography (EMG) responses data exist of children exposed to dynamic impacts similar to automotive crashes, thereby, limiting active musculature representation in computational occupant biomechanics models. This study measured the surface EMG responses of three neck, one torso and one lower extremity muscles during low-speed frontal impact sled tests (average maximum acceleration: 3.8 g; rise time: 58.2 ms) performed on seated, restrained pediatric (n = 11, 8–14 years) and young adult (n = 9, 18–30 years) male subjects. The timing and magnitude of the EMG responses were compared between the two age groups. Two normalization techniques were separately implemented and evaluated: maximum voluntary EMG (MVE) and neck cross-sectional area (CSA). The MVE-normalized EMG data indicated a positive correlation with age in the rectus femoris for EMG latency; there was no correlation with age for peak EMG amplitudes for the evaluated muscles. The cervical paraspinous exhibited shorter latencies compared with the other muscles (2–143 ms). Overall, the erector spinae and rectus femoris peak amplitudes were relatively small. Neck CSA-normalized peak EMG amplitudes negatively correlated with age for the cervical paraspinous and sternocleidomastoid. These data can be useful to incorporate active musculature in computational models, though it may not need to be age-specific in low-speed loading environments.