Reflex Latency Research Articles

Two-Stage Nerve Graft in Severe Scar: A Time-Course Study in a Rat Model.

Peripheral nerve repair outcomes are suboptimal in the presence of severe soft tissue injury and excessive scarring paralleling the process in tendon reconstruction of the hand. Inspired by the advantages of the two-stage technique in tendon grafting and with encouraging preliminary results, we aimed to investigate the two-stage nerve grafting technique as an alternative method of secondary nerve repair. Thirty female rats (~200 g) were randomly distributed into two groups (n=15). A 15 mm gap was created in the sciatic nerve of the animals and an excessive extraneural scar was induced using the "mincing" method. In this method, a thin strip of muscle was removed, minced in a petri dish and returned to the peripheral nerve. In the two-stage nerve graft group, a silicone tube was interposed in the first stage. After 4 weeks, in the second stage, the silicone tube was removed and a median nerve autograft was interposed through the newly formed vascularized sheath. In the conventional graft group, two nerve ends were protected with silicone caps in the first stage. After 4 weeks the caps were removed and the median graft was interposed. Behavioral assessments were performed at week 15 after surgery with the withdrawal reflex latency (WRL) and extensor postural thrust (EPT) and at the 3, 6 and 15-week time points with the TOA (toe out angle). Masson Trichrome staining method was used for histological assessments at week 15. According to the EPT and WRL, the two-stage nerve graft showed significant improvement (P=0.020 and P=0.017 respectively). The TOA showed no significant difference between the two groups. The total vascular index was significantly higher in the two-stage nerve graft group (P<0.001). Two-stage nerve graft using a silicone tube enhances vascularity of the graft and improves functional recovery.

Sympathetic Axonal Recruitment During Severe Chemoreflex Stress in Humans

We tested the hypothesis that severe chemoreflex stress stimulates the recruitment of sub‐populations of larger, faster conducting sympathetic neurones that are otherwise silent at baseline. Muscle sympathetic nerve activity (MSNA; microneurography) was measured in 11 healthy individuals (6 females; 25 ± 3 yrs) at baseline and during a maximal end‐inspiratory apnea, which was preceded by a rebreathing period designed to maximize the chemoreflex stress during apnea (End‐apnea: PO2 = 58 ± 4 Torr; PCO2 = 55 ± 5 Torr). Compared to baseline, action potential (AP) frequency (174 ± 151 to 1083 ± 487 spikes/min; P &lt; 0.001) and mean AP content per burst (11 ± 4 to 24 ± 12 spikes/burst; P &lt; 0.01) were elevated during apnea. When detected APs were binned according to their peak‐to‐peak amplitude, the number of total ‘clusters' of APs detected (13 ± 4 to 20 ± 8 total clusters; P &lt; 0.001), as well as the number of distinct ‘clusters' per burst (5 ± 2 to 8 ± 2 clusters/burst; P &lt; 0.001), were increased during apnea. At baseline (R2 = 0.91; P &lt; 0.001) and during apnea (R2 = 0.98; P &lt; 0.001), a pattern emerged whereby AP cluster latency decreased as cluster amplitude increased. Furthermore, the cluster latency‐size profile was shifted downwards for every corresponding cluster during apnea versus baseline (range −14.9 to −99.4 ms; mean = −53.1 ms), such that APs with similar amplitudes were detected approximately 53.1 ms faster during apnea. In conclusion, there appears a unique ability of the sympathetic nervous system to recruit sub‐populations of previously dormant larger, faster conducting sympathetic neurones, as well as acutely modify reflex latency of all APs during severe chemoreflex stress. Supported by NSERC and CIHR.

Estimation of operators' fatigue using optical methods for determination of pupil activity.

The purpose of this study was to develop and initially validate an objective, yet quick, method for assessment of human fatigue. This aim can be achieved by the use of an optical, non-contact method of analysis of pupil activity. The study involved 2 phenomena typical for the behavior of the human pupil: pupillary reflex and pupillary movements. The 1st phenomenon is related to the pupillary light reflex (PLR), which presents the results of physiological adaptation mechanisms of the human eye. The pupillary unrest index (PUI) is an additional parameter referring to the tendency for instability of the pupil. Indicators of these 2 mechanisms were assessed under the same experimental laboratory conditions. Assessment was conducted on a group of volunteers (N = 10) during 4 controlled series of measurements performed at night. Pupillary reflex parameters associated with PLR and pupillary unrest index (PUI) were recorded using F²D Fit-For-Duty, a commercially available system made by AMTech Pupilknowlogy GmbH. Baseline pupil diameter, oscillations, reflex latency, maximum reaction time, pupil constriction time, pupil dilation time, and constriction amplitude were recorded. As a result of the study, we were able to demonstrate correlation and confirm the usefulness of PLR and PST methods in the assessment of sleep deprivation. Parameters of PLR that may indicate human fatigue were identified. The effect of light impulse sequences on the dynamics of pupillary reflex and the relationship between PUI (pupillary unrest index) measurement duration and sleepiness assessment validity, were assessed. The results of the pilot studies were sufficient to develop minimum requirements for a PLR sensor that would be capable of estimating the level of fatigue with accuracy of a PUI method, but at a 5 times faster rate.

A high-resolution binocular video-oculography system: assessment of pupillary light reflex and detection of an early incomplete blink and an upward eye movement

BackgroundThe pupillary light reflex characterizes the direct and consensual response of the eye to the perceived brightness of a stimulus. It has been used as indicator of both neurological and optic nerve pathologies. As with other eye reflexes, this reflex constitutes an almost instantaneous movement and is linked to activation of the same midbrain area. The latency of the pupillary light reflex is around 200 ms, although the literature also indicates that the fastest eye reflexes last 20 ms. Therefore, a system with sufficiently high spatial and temporal resolutions is required for accurate assessment. In this study, we analyzed the pupillary light reflex to determine whether any small discrepancy exists between the direct and consensual responses, and to ascertain whether any other eye reflex occurs before the pupillary light reflex.MethodsWe constructed a binocular video-oculography system two high-speed cameras that simultaneously focused on both eyes. This was then employed to assess the direct and consensual responses of each eye using our own algorithm based on Circular Hough Transform to detect and track the pupil. Time parameters describing the pupillary light reflex were obtained from the radius time-variation. Eight healthy subjects (4 women, 4 men, aged 24–45) participated in this experiment.ResultsOur system, which has a resolution of 15 microns and 4 ms, obtained time parameters describing the pupillary light reflex that were similar to those reported in previous studies, with no significant differences between direct and consensual reflexes. Moreover, it revealed an incomplete reflex blink and an upward eye movement at around 100 ms that may correspond to Bell’s phenomenon.ConclusionsDirect and consensual pupillary responses do not any significant temporal differences. The system and method described here could prove useful for further assessment of pupillary and blink reflexes. The resolution obtained revealed the existence reported here of an early incomplete blink and an upward eye movement.Electronic supplementary materialThe online version of this article (doi:10.1186/s12938-015-0016-6) contains supplementary material, which is available to authorized users.

Characterization of mitochondrial bioenergetics in neonatal anoxic model of rats.

Neonatal anoxia at the time of birth can lead to mitochondrial dysfunction and further neurodevelopmental abnormalities. The present study investigated the mitochondrial bioenergetics and associated sensorimotor changes in the anoxic neonatal rats. Rat pups after 30h to birth (2days) were subjected to anoxia of two episodes (10min in each) at a time interval of 24h by passing 100%N2 into an enclosed chamber. Brain mitochondrial respiration was measured using clark type oxygen electrode. A significant decrease in brain respiratory control ratio (RCR; State III/IV respiration) at all-time points, complex I (24h) and complex II (30min, 6 and 24h) enzyme activities indicated loss of mitochondrial integrity and function A significant increase in levels of nitric oxide was observed after second anoxic episode at all-time points. A significant change in sensorimotor activity in terms of increased reflex latency was observed 24h after second episode in this model, which is an indication of loss of subcortical maturation. All the above changes were observed after second but not after the first anoxic exposure. Therefore, this anoxic model shows significant changes in mitochondrial bioenergetics, nitric oxide levels and sensorimotor effects after second episode of anoxia. This model may be helpful to evaluate mitochondrial targeted pharmacological intervention for the treatment of anoxia.

The effect of elbow flexor fatigue on spine kinematics and muscle activation in response to sudden loading at the hands

Sudden loads, originating at either the hands or the feet, can cause injury to spine structures. As muscles are primarily responsible for stabilization following a perturbation, the effect of spine muscle fatigue in this context has been well investigated. However, the effect of fatigue of arm muscles, which can help control perturbations originating at the hands, on the spine is unknown. The purpose of this study was to determine if the magnitude of spine flexion or the pre-activation, reflex amplitude, and reflex latency of spine muscles were altered by elbow flexor fatigue during a sudden loading (6.8kg) perturbation at the hands. Elbow flexor fatigue was induced by an isometric 30% maximal elbow flexion moment until failure. Results demonstrate that spine kinematics were not altered in the presence of elbow flexor fatigue. Small magnitude differences in trunk muscle pre- and peak activation indicate that the presence of elbow flexor fatigue does not necessitate substantially greater spine muscle action under the tested conditions. Despite fatigued elbow flexors, the arm muscles were sufficiently able to control the perturbation. Interestingly, 5/14 participants demonstrated altered reflex latencies in all observed muscles that lasted up to 10min after the fatiguing task.

Rapid voluntary clench induces bite force reflex in human

Objective: The purpose of this study was to test the occlusal analysis system T-Scan II for the detection of a bite force reflex and to determine the net jaw reflex modulation during a rapid voluntary clench. Materials and Methods: Eleven subjects were asked to bite as fast as possible on the pressure-sensitive sheet using occlusal scanning system T-Scan II . The bite force reflex parameters were determined from the occlusal force curve. Results :The inhibitory bite force reflex occurred in nine subjects with a latency of 34.0 ± 7.8 ms, duration of 17.0 ± 2.8 ms and strength of 1.6% ± 0.3%. A strong correlation was detected between the latency of the reflex and the time to reach 80% of the maximal relative bite force (r = 0.833; p < 0.01). A moderate correlation was found between the relative bite force at the moment of the first turning point of the reflex and the change of this force during the reflex (r = 0.677; p < 0.05). Conclusion: The reflex modulation confirms the protective role of the jaw reflex: during faster bites the latency of the bite force reflex was shorter and with a higher bite force the strength of the reflex was greater. Keywords: Bite force reflex, Human, Occlusal scanning, Rapid voluntary clench, Relative bite force

Long Latency Reflex (LLR) of Abductor Digiti Minimi (ADM) in the Dominant and non Dominant Hand of Healthy Adult Males

The LONG LATENCY REFLEX is an electrical response following the H reflex on stimulation of a mixed nerve. It assesses the sensory and motor pathways travelling via cortex. It helps to determine the conduction velocity within the spinal cord. LLR is clinically useful in investigating movement disorders like Parkinson's disease, Huntington's chorea. As very few Indian studies are available on LLR, the present study was undertaken to determine latency and amplitude of LLR of abductor digiti minimi of the dominant and non-dominant hand. Twenty healthy male subjects in the age group of 25–35 years were studied by stimulating ulnar nerve while the subject was maintaining 10–20% of maximum voluntary contraction of the abductor digiti minimi. A stimulus intensity of 5-10mA (submaximal) of 1 ms duration, delivered from a constant current stimulator through bipolar stimulating electrodes was used to stimulate the muscle. The latency of the first deflection from the baseline and the peak to peak amplitude of the evoked LLR responses were measured digitally using a digitalized nerve conduction/EMG/EP machine (Aleron, Recorders Medicare systems, Chandigarh, India).

Effects of various head postures on flexor carpi radialis H reflex in healthy subjects

Background and Aim: The H reflex is a useful measure of the nerve conduction through the entire length of the afferent and efferent pathways, especially at the proximal segment of the peripheral nerve, which is inaccessible by routine surface stimulating and recording techniques. Therefore, the objective of the present study is to determine the effect of head postural modification on flexor carpi radialis H reflex in healthy subjects. Methods: This study was done on 40 healthy individuals with a mean age of 22.37 ± 1.23. The H reflex was then recorded by the use of the electromyography machine with the subject in a sitting position. The subjects were asked to maintain 30-s hold into the end range of flexion, extension, rotation (to the right and then to the left), lateral bending (to the right and then to the left), retraction, and protraction of neck. The H reflex amplitude, latency and H/M ratio were measured. Results: Repeated multivariate analysis of variance was used to evaluate the significance of the difference among the H reflex, amplitude, and latency, in various head positions. Head postural modification does not significantly influences amplitude, latency and H/M ratio in normal healthy subjects. Conclusion: The H reflex latencies, amplitudes and H/M ratios did not show any significant difference between various head positions when compared with neutral head position.

Clinical, neurophysiological and neuroimaging study of tremor in multiple sclerosis

To identify clinical types of tremor in multiple sclerosis (MS) and clarify their pathophysiological mechanisms. We examined 124 patients with MS, including 58 patients with tremor, using clinical (digital spiralography), neurophysiological (tremor electromyography, visual and sensory evoked potentials, transcranial magnetic stimulation with tremor resetting, long latency reflexes, electroencephalography) and neuroimaging (MRI, morphometry) methods. Five main variants of tremor were identified: distal postural and postural-intention (variant 1), distal intention (variant 2), proximal and distal intention and postural-intention (variant 3), Holmes (variant 4), axial (variant 5). Postural tremor (variants 1, 3) and rest tremor (variant 4) are caused by the central oscillators. Intention tremor (variants 2, 3), postural-intention tremor (variant 4), axial (variant 5) are caused by the pathology of cerebellar feedback loops. Clarification of mechanisms for the development of tremor in MS allowed to develop a scheme of differential treatment.

Whole-body vibration-induced muscular reflex: Is it a stretch-induced reflex?

[Purpose] Whole-body vibration (WBV) can induce reflex responses in muscles. A number of studies have reported that the physiological mechanisms underlying this type of reflex activity can be explained by reference to a stretch-induced reflex. Thus, the primary objective of this study was to test whether the WBV-induced muscular reflex (WBV-IMR) can be explained as a stretch-induced reflex. [Subjects and Methods] The present study assessed 20 healthy males using surface electrodes placed on their right soleus muscle. The latency of the tendon reflex (T-reflex) as a stretch-induced reflex was compared with the reflex latency of the WBV-IMR. In addition, simulations were performed at 25, 30, 35, 40, 45, and 50 Hz to determine the stretch frequency of the muscle during WBV. [Results] WBV-IMR latency (40.5 ± 0.8 ms; 95% confidence interval [CI]: 39.0–41.9 ms) was significantly longer than T-reflex latency (34.6 ± 0.5 ms; 95% CI: 33.6–35.5 ms) and the mean difference was 6.2 ms (95% CI of the difference: 4.7–7.7 ms). The simulations performed in the present study demonstrated that the frequency of the stretch signal would be twice the frequency of the vibration. [Conclusion] These findings do not support the notion that WBV-IMR can be explained by reference to a stretch-induced reflex.

Evaluation of biodegradable electric conductive tube-guides and mesenchymal stem cells.

To study the therapeutic effect of three tube-guides with electrical conductivity associated to mesenchymal stem cells (MSCs) on neuro-muscular regeneration after neurotmesis. Rats with 10-mm gap nerve injury were tested using polyvinyl alcohol (PVA), PVA-carbon nanotubes (CNTs) and MSCs, and PVA-polypyrrole (PPy). The regenerated nerves and tibialis anterior muscles were processed for stereological studies after 20 wk. The functional recovery was assessed serially for gait biomechanical analysis, by extensor postural thrust, sciatic functional index and static sciatic functional index (SSI), and by withdrawal reflex latency (WRL). In vitro studies included cytocompatibility, flow cytometry, reverse transcriptase polymerase chain reaction and karyotype analysis of the MSCs. Histopathology of lung, liver, kidneys, and regional lymph nodes ensured the biomaterials biocompatibility. SSI remained negative throughout and independently from treatment. Differences between treted groups in the severity of changes in WRL existed, showing a faster regeneration for PVA-CNTs-MSCs (P < 0.05). At toe-off, less acute ankle joint angles were seen for PVA-CNTs-MSCs group (P = 0.051) suggesting improved ankle muscles function during the push off phase of the gait cycle. In PVA-PPy and PVA-CNTs groups, there was a 25% and 42% increase of average fiber area and a 13% and 21% increase of the "minimal Feret's diameter" respectively. Stereological analysis disclosed a significantly (P < 0.05) increased myelin thickness (M), ratio myelin thickness/axon diameter (M/d) and ratio axon diameter/fiber diameter (d/D; g-ratio) in PVA-CNT-MSCs group (P < 0.05). Results revealed that treatment with MSCs and PVA-CNTs tube-guides induced better nerve fiber regeneration. Functional and kinematics analysis revealed positive synergistic effects brought by MSCs and PVA-CNTs. The PVA-CNTs and PVA-PPy are promising scaffolds with electric conductive properties, bio- and cytocompatible that might prevent the secondary neurogenic muscular atrophy by improving the reestablishment of the neuro-muscular junction.

No relation between afferent facilitation induced by digital nerve stimulation and the latency of cutaneomuscular reflexes and somatosensory evoked magnetic fields.

Primary motor cortex (M1) excitability can be assessed using transcranial magnetic stimulation (TMS) and can be modulated by a conditioning electrical stimulus delivered to a peripheral nerve prior to TMS. This is known as afferent facilitation (AF). The aim of this study was to determine whether AF can be induced by digital nerve stimulation and to evaluate the relation between the interstimulus interval (ISI) required for AF and the latency of the E2 component of the cutaneomuscular reflex (CMR) and the prominent somatosensory evoked field (SEF) deflection that occurs approximately 70 ms after digital nerve stimulation (P60m). Stimulation of the digital nerve of the right index finger was followed, at various time intervals, by single-pulse TMS applied to the contralateral hemisphere. The ISI between digital nerve stimulation and TMS was 20, 30, 40, 50, 60, 70, 80, 100, 140, 180, 200, or 220 ms. Single-pulse TMS was performed alone as a control. SEFs were recorded following digital nerve stimulation of the index finger, and the equivalent current dipole of prominent deflections that occurred around 70 ms after the stimulation was calculated. CMRs were recorded following digital nerve stimulation during muscle contraction. Motor evoked potentials (MEPs) were facilitated at an ISI between 50 and 100 ms in 11 of 13 subjects, and the facilitated MEP amplitude was larger than the unconditioned MEP amplitude (p < 0.01). There was no significant correlation between the ISI at which AF was maximal and the latency of the P60m component of the SEF (r = −0.50, p = 0.12) or the E2 component of the CMR (r = −0.54, p = 0.88). These results indicate that the precise ISI required for AF cannot be predicted using SEF or CMR.

Air Conducted Cervical and Ocular Vestibular Evoked Myogenic Potentials in Peudotumor Cerebri Patients

Objective : Pseudotumor cerebri is a disorder of unknown etiology that results in raised intracranial pressure (ICP) and characterized by attendant signs and symptoms of increased ICP in an alert and oriented patient but without localizing neurologic findings. Vestibular evoked myogenic potentials (VEMPs) are short latency reflex responses produced by stimulation of the vestibular apparatus. The purpose of this study was to determine whether group of pseudotumor cerebri patients have abnormal changes in VEMPs (cervical and ocular) and to analyze the results and compare them with matched group of normal subjects. Design : Ocular and cervical VEMPs were performed to the entire study groups. The responses were obtained from thirteen pseudotumor cerebri patients and compared to fifteen age matched normal subjects. All participants were subjected to audiological assessments that include pure tone audiometry, tympanometry and acoustic reflexes. Results : Ocular and cervical VEMPs amplitudes were reduced in response in pseudotumor cerebri patients than normal but still of non-statistical significant difference p>0.05. Discussion and Conclusion : The results of this study suggest that patients with pseudotumor cerebri do not have statistically significant changes in comparison with the normal control as regard ocular and cervical VEMPs testing. Hence, asserting a query whether there is affection of VEMPs in pseudotumor cerebri patients.

Trunk active response and spinal forces in sudden forward loading – analysis of the role of perturbation load and pre-perturbation conditions by a kinematics-driven model

Understanding the central nervous system (CNS) response strategy to trunk perturbations could help in prevention of back injuries and development of rehabilitation and treatment programs. This study aimed to investigate biomechanical response of the trunk musculoskeletal system under sudden forward loads, accounting for pre-perturbation conditions (preloading, initial posture and abdominal antagonistic coactivation) and perturbation magnitudes. Using a trunk kinematics-driven iterative finite element (FE) model, temporal profiles of measured kinematics and external load along with subjects’ weights were prescribed to predict thoracolumbar muscle forces/latencies and spinal loads for twelve healthy subjects when tested in six conditions during pre- and post-perturbation periods. Results demonstrated that preloading the trunk significantly (i.e., p<0.05) increased pre-perturbation back muscle forces but significantly decreased post-perturbation peak muscle active forces and muscle latencies. Initial trunk flexion significantly increased muscle active and passive forces before the perturbation and their peak values after the perturbation, which in turn caused much larger spinal loads. Abdominal muscles antagonistic pre-activation did not alter the internal variables investigated in this study. Increase in sudden applied load increased muscle reflex activities and spinal forces; a 50N increase in sudden load (i.e., when comparing 50N to 100N) increased the L5-S1 compression force by 1327N under 5N preload and by 1374N under 50N preload. Overall, forces on the spine and hence risk of failure substantially increased in sudden forward loading when the magnitude of sudden load increased and when the trunk was initially in a flexed posture. In contrast, a higher initial preload diminished reflex latencies and compression forces.

Contributions of motoneuron hyperexcitability to clinical spasticity in hemispheric stroke survivors

ObjectiveMuscle spasticity is one of the major impairments that limits recovery in hemispheric stroke survivors. One potential contributing mechanism is hyperexcitability of motoneurons. Previously, the response latency of the surface electromyogram (EMG) record evoked by joint rotation has been used to characterize motoneuron excitability. Given the limitations of this method, the objective of the current study was to reexamine the excitability of motoneurons in chronic stroke survivors by estimating reflex latency using single motor unit discharge. MethodsWe quantified the excitability of spastic motoneurons using the response latency of a single motor unit discharge elicited by a position controlled tap on the biceps brachii tendon. We applied tendon taps of different amplitudes on the biceps tendons of both arms of the stroke survivors. Unitary reflex responses were recorded using intramuscular EMG recordings. ResultsOur results showed that the latency of unitary discharge was systematically shorter in the spastic muscle compared with the contralateral muscle, and this effect was consistent across multiple tap amplitudes. ConclusionsThis method allowed us to quantify latencies more accurately, potentially enabling a more rigorous analysis of contributing mechanisms. SignificanceThe findings provide evidence supporting a contribution of hyperexcitable motoneurons to muscle spasticity.

Does electrode impedance affect the recording of ocular vestibular-evoked myogenic potentials?

In evoked potential testing, it is common practice to abrade the skin surface as a means of reducing and balancing electrode impedance. The effects of skin preparation and electrode impedance on ocular vestibular-evoked myogenic potential (oVEMP) prevalence and amplitudes are not known. We sought to determine whether comparable oVEMP waveforms could be recorded without excessive skin preparation. This was a prospective study with a repeated-measures (within-subjects) design. The study group comprised 20 healthy participants (12 females and 8 males) ages 21-57 yr. oVEMP reflexes were recorded in response to air-conducted and bone-conducted (AC and BC) stimuli in three conditions. In the first condition (no skin preparation), electrodes were simply placed over the skin surface. For the second condition (moderate skin preparation), oVEMP testing was repeated after the skin had been prepared with 70% isopropyl alcohol swabs. oVEMPs recorded in these two conditions were then compared with those recorded using a third conventional protocol whereby the skin was abraded with skin preparation gel until electrode impedances were low and balanced. For BC stimuli, reflex amplitudes and latencies were compared using a repeated-measures general linear model. For AC stimuli, rates of reflex detection were analyzed using a generalized estimating equation. Nonparametric Friedman tests were used to compare AC oVEMP amplitudes across the three conditions. There was no significant effect of electrode impedance on reflex amplitudes, latencies, or rates of detection (p > 0.05). The results indicated significant stimulus-related artifact (≥3 μV) in 25 of 40 recordings under the high-impedance condition in response to BC stimulation. The stimulus artifact was detectable in 12 of 40 recordings after moderate skin preparation and in 5 recordings after skin abrasion. Comparable rates of reflex detection and oVEMP amplitudes were recorded in the three conditions, implying that rigorous rubbing of the facial skin is largely unnecessary in clinical oVEMP testing. However, for oVEMPs recorded in response to a single-polarity stimulus, reducing and balancing electrode impedances with either isopropyl alcohol wipes or skin abrasion may help reduce unwanted stimulus artifact.

Evidence for sustained cortical involvement in peripheral stretch reflex during the full long latency reflex period

Adaptation of reflexes to environment and task at hand is a key mechanism in optimal motor control, possibly regulated by the cortex. In order to locate the corticospinal integration, i.e. spinal or supraspinal, and to study the critical temporal window of reflex adaptation, we combined transcranial magnetic stimulation (TMS) and upper extremity muscle stretch reflexes at high temporal precision. In twelve participants (age 49±13 years, eight male), afferent signals were evoked by 40ms ramp and subsequent hold stretches of the m. flexor carpi radialis (FCR). Motor conduction delays (TMS time of arrival at the muscle) and TMS-motor threshold were individually assessed. Subsequently TMS pulses at 96% of active motor threshold were applied with a resolution of 5–10ms between 10ms before and 120ms after onset of series of FCR stretches. Controlled for the individually assessed motor conduction delay, subthreshold TMS was found to significantly augment EMG responses between 60 and 90ms after stretch onset. This sensitive temporal window suggests a cortical integration consistent with a long latency reflex period rather than a spinal integration consistent with a short latency reflex period. The potential cortical role in reflex adaptation extends over the full long latency reflex period, suggesting adaptive mechanisms beyond reflex onset.

Proactive and reactive neuromuscular control in subjects with chronic ankle instability: Evidence from a pilot study on landing

To understand why subjects with chronic ankle instability (CAI) have frequent sprains, one must study the preparation/reactions of these subjects to situations related to ankle inversion in real life. In the present pilot study, we examined whether subjects with CAI altered their neuromuscular control and reflex responses during and after ankle perturbations in landing. EMG signals were collected from the tibialis anterior (TA), peroneus longus (PL), medial gastrocnemius (MG), and gluteus medius (GLM) of both legs in 9 subjects with CAI and 9 subjects with intact ankles (control). A trapdoor was used to produce an ankle inversion of 25° with the left leg (control) or the affected leg (CAI) in 0%, 50% or 100% of the landing trials. As compared to controls, subjects with CAI had increased proactive activity in the contralateral side prior to touchdown during landing trials with 50% (PL) and 100% (PL and MG) chance of inversion (all, p<0.05). The increase proactive control on the contralateral side could be part of a strategy to smooth the impact of landing on the affected side in subjects with CAI. Following touchdown, the CAI group showed decreased ipsilateral short latency reflex (SLR) responses in all test conditions both in distal (PL and MG) and in proximal muscles (GLM) on the affected side (all, p<0.05). Finally, subjects with CAI adjusted their reflex gain differently as compared to controls when exposed to a possible inversion. Overall, individuals with CAI displayed different neuromuscular strategies from controls while landing.

Sensorimotor modulation differs with load type during constant finger force or position.

During submaximal isometric contraction, there are two different load types: production of a constant force against a rigid restraint (force task), and maintenance of position against a constant load (position task). Previous studies reported that the time to task failure during a fatigue task was twice as long in the force task compared with the position task. Sensory feedback processing may contribute to these differences. The purpose of the current study was to determine the influence of load types during static muscle contraction tasks on the gating effect, i.e., attenuation of somatosensory-evoked potentials (SEPs) and the cortical silent period (cSP). Ten healthy subjects contracted their right first dorsal interosseus muscle by abducting their index finger for 90 s, to produce a constant force against a rigid restraint that was 20% of the maximum voluntary contraction (force task), or to maintain a constant position with 10° abduction of the metacarpophalangeal joint against the same load (position task). Somatosensory evoked potentials (SEPs) were recorded from C3′ by stimulating either the right ulnar or median nerve at the wrist while maintaining contraction. The cortical silent period (cSP) was also elicited by transcranial magnetic stimulation. Reduction of the amplitude of the P45 component of SEPs was significantly larger during the position task than during the force task and under control rest conditions when the ulnar nerve, but not the median nerve, was stimulated. The position task had a significantly shorter cSP duration than the force task. These results suggest the need for more proprioceptive information during the position task than the force task. The shorter duration of the cSP during the position task may be attributable to larger amplitude of heteronymous short latency reflexes. Sensorimotor modulations may differ with load type during constant finger force or position tasks.