First Dorsal Interosseus Muscle Research Articles

Are age-related changes in cortical motor representations linked with facilitation/inhibition in the primary motor cortex?

Are age-related changes in cortical motor representations linked with facilitation/inhibition in the primary motor cortex?

Differential Corticomotor Excitability Responses to Hypertonic Saline-Induced Muscle Pain in Forearm and Hand Muscles.

Experimental muscle pain inhibits corticomotor excitability (CE) of upper limb muscles. It is unknown if this inhibition affects overlapping muscle representations within the primary motor cortex to the same degree. This study explored CE changes of the first dorsal interosseus (FDI) and extensor carpi radialis (ECR) muscles in response to muscle pain. Participants (n = 13) attended two sessions (≥48 hours in-between). Hypertonic saline was injected in the ECR (session one) or the FDI (session two) muscle. CE, assessed by transcranial magnetic stimulation (TMS) motor-evoked potentials (MEPs), was recorded at baseline, during pain, and twenty minutes postinjection together with pain intensity ratings. Pain intensity ratings did not differ between the two pain sites (p = 0.19). In response to FDI muscle pain, the MEPs of the FDI muscle were reduced at 2 and 4 min postinjection (p ≤ 0.03), but not after ECR muscle pain. No significant MEP change was detected for the ECR muscle (p = 0.62). No associations between MEPs and pain intensity were found (p > 0.2). The present results indicate that the output from overlapping cortical representations of two muscles differentially adapts to acute muscle pain.

Acute muscle pain alters corticomotor output of the affected muscle stronger than a synergistic, ipsilateral muscle

Abstract Aims Muscle pain affects corticomotor areas representing the affected muscle, by changing the size of representation and reduces the corticospinal output as assessed by transcranial magnetic stimulation (TMS). Less work has been done to understand how pain in one muscle group may affect synergistic ipsilateral muscles distal to the pain. This study aimed to explore the effects of acute extensor carpi radialis (ECR) muscle pain on TMS motor-evoked potentials (MEPs) of the ECR and the first dorsal interosseus (FDI) muscle, which are known to strongly overlap within the corticomotor area. Methods Eight healthy volunteers (1 woman) were injected with hypertonic saline (5.8%, 0.5 mL) into the ECR muscle. Pain intensity was assessed by the visual analogue scale (VAS) every minute for 10 min. TMS was applied at 120% of ECR resting motor threshold, and MEPs were acquired from the ECR and the FDI muscles. At baseline, 10 TMS pulses were delivered. Temporal mapping of ECR and FDI MEPs over 10 min duration was performed by delivering 100 single-pulses of TMS, at 6 s interstimulus-interval. The MEPs for each muscle were averaged at baseline, peak-pain (1 –2 min epoch), and 10 min post-injection Results Pain intensity reduced significantly at 10 min postinjection as compared to peak-pain (P = 0.011). Further, one-way repeated measures analysis of variance revealed that ECR MEPs were altered at peak-pain compared to baseline (P > 0.05), but not 10 min post-injection (P > 0.05). Baseline and 10 min post-injection of ECR MEPs did not differ significantly (P = 0.67). The MEPs of the FDI muscle did not show a similar alteration over time (P = 0.1). Conclusions Despite the overlap between ECR and FDI representations, acute muscle pain of the ECR only significantly altered cortical excitability of the ECR muscle representation.

Effect Of Reduced Energy Availability By Either Diet Or Exercise On Muscle Force.

Athletes often undergo periods of low energy availability (EA) and it is important to know whether muscle function is compromised as a consequence. PURPOSE: To determine the differential effects on muscle force production when low EA is attained by reduced dietary energy intake (DEI) or increased exercise energy expenditure (EEE). METHODS: Using a randomised, counterbalanced design, 22 females (mean ± SD age: 26 ± 5 y; 11 oral contraceptive [OC] users, 11 eumenorrheic [EU]) were assessed for body composition using DXA. They then completed three 3-day experimental conditions consisting of: controlled energy balance (EB); dietary-induced energy restriction (ER-D); and exercise-induced energy restriction (ER-E). EB was achieved by a DEI of 45 kcal·kg·LBM-1·day-1 with 0 kcal·kg·LBM-1·day-1 EEE. ER-D was achieved by a DEI of 15 kcal·kg·LBM-1·day-1 and an EEE of 0 kcal·kg·LBM-1·day-1, resulting in an EA of 15 kcal·kg·LBM-1·day-1. The same level of EA was achieved in ER-E with DEI of 45 kcal·kg·LBM-1·day-1 and treadmill running at 70% aerobic capacity until 30 kcal·kg·LBM-1·day-1 was expended. Body mass and maximal voluntary isometric force (MVIF) of the quadriceps and first dorsal interosseus (FDI) muscles were measured in the morning prior to and following each 3 day condition. Muscle force data were analysed using three way mixed-model ANOVA and body mass %change from pre to post in each condition was analysed using a one-way repeated measures ANOVA, with Bonferroni adjusted post hoc t-tests. RESULTS: There was no group (OC vs. EU) effect for any measure (P>0.05). ER-D (-1.8 ± 0.6%) and ER-E (-0.9 ± 0.6%) resulted in a greater reduction in body mass than EB (-0.6 ± 0.5%), with ER-D resulting in a greater reduction in body mass than ER-E (P≤0.05). MVIF of the FDI muscle was maintained from pre to post in the EB, ER-D and ER-E conditions (P>0.05). Quadriceps MVIF was maintained from pre to post in the EB and ER-D conditions, however there was a 13.1% decline in quadriceps MVIF from pre (372.6 ± 84.7N) to post (323.9 ± 99.2N) ER-E (P=0.017). CONCLUSION: Low EA achieved through dietary restriction had no effect on MVIF of the quadriceps or FDI muscle, however low EA caused by increased EEE resulted in a decline in MVIF of the fatigued quadriceps, which was not observed in the fresh FDI muscle. These effects were similar for both the OC and EU groups.

Cerebellar brain inhibition in the target and surround muscles during voluntary tonic activation

Motor surround inhibition is the neural mechanism that selectively favours the contraction of target muscles and inhibits nearby muscles to prevent unwanted movements. This inhibition was previously reported at the onset of a movement, but not during a tonic contraction. Cerebellar brain inhibition (CBI) is reduced in active muscles during tonic activation; however, it has not been studied in the surround muscles. CBI was evaluated in the first dorsal interosseus (FDI) muscle as the target muscle, and the abductor digiti minimi, flexor carpi radialis and extensor carpi radialis muscles as surround muscles, during rest and tonic activation of the FDI muscle in 21 subjects. Cerebellar stimulation was performed under magnetic resonance imaging-guided neuronavigation targeting lobule VIII of the cerebellar hemisphere. Stimulus intensities for cerebellar stimulation were based on the resting motor cortex threshold (RMT) and adjusted for the depth difference between the cerebellar and motor cortices. We used 90-120% of the adjusted RMT as the conditioning stimulus intensity during rest. The intensity that generated the best CBI at rest in the FDI muscle was selected for use during tonic activation. During selective tonic activation of the FDI muscle, CBI was significantly reduced only for the FDI muscle, and not for the surround muscles. Unconditioned motor evoked potential sizes were increased in all muscles during FDI muscle tonic activation as compared with rest, despite background electromyography activity increasing only for the FDI muscle. Our study suggests that the cerebellum may play an important role in selective tonic finger movement by reducing its inhibition in the motor cortex only for the relevant agonist muscle.

Cortical Silent Period Reveals Differences Between Adductor Spasmodic Dysphonia and Muscle Tension Dysphonia

Background. The pathophysiology of adductor spasmodic dysphonia (AdSD), like other focal dystonias, is largely unknown. Objective. The purposes of this study were to determine (a) cortical excitability differences between AdSD, muscle tension dysphonia (MTD), and healthy controls; (b) distribution of potential differences in cranial or skeletal muscle; and (c) if cortical excitability measures assist in the differential diagnosis of AdSD and MTD. Methods. Ten participants with adductor spasmodic dysphonia, 8 with muscle tension dysphonia, and 10 healthy controls received single and paired pulse transcranial magnetic stimulation (TMS) to the primary motor cortex contralateral to tested muscles, first dorsal interosseus (FDI), and masseter. We tested the hypothesis that cortical excitability measures in AdSD would be significantly different from those in MTD and healthy controls. In addition, we hypothesized that there would be a correlation between cortical excitability measures and clinical voice severity in AdSD. Results. Cortical silent period duration in masseter and FDI was significantly shorter in AdSD than MTD and healthy controls. Other measures failed to demonstrate differences. Conclusion. There are differences in cortical excitability between AdSD, MTD, and healthy controls. These differences in the cortical measure of both the FDI and masseter muscles in AdSD suggest widespread dysfunction of the GABAB mechanism may be a pathophysiologic feature of AdSD, similar to other forms of focal dystonia. Further exploration of the use of TMS to assist in the differential diagnosis of AdSD and MTD is warranted.

Reliability of force per unit cross-sectional area measurements of the first dorsal interosseus muscle

The purpose of this study was to determine the reliability of maximum voluntary isometric force (MVIF), cross-sectional area (CSA) and force per unit CSA measures, of the first dorsal interosseus (FDI) muscle, using a custom-built dynamometer and ultrasonography. Twenty-seven participants completed MVIF and CSA measurements on two separate occasions under the same conditions. Reliability was determined using paired samples t-tests, systematic bias ratio and ratio limits of agreement (RLoA), intra-class correlation (ICC) and coefficient of variation (CV). MVIF of the FDI muscle (mean ± s; 31.8 ± 7.6 N and 31.6 ± 7.3 N) was not different between trials (P = 0.63); RLoA between trials were 1.00 ×/÷ 1.09, ICC = 0.990 and CV = 3.22%. CSA of the FDI muscle (22.6 ± 6.9 and 22.9 ± 6.9 mm2) was also not different between trials (P = 0.31); RLoA between trials were 0.98 ×/÷ 1.19, ICC = 0.979 and CV = 6.61%. Force per unit CSA was not different between trials (1.49 ± 0.43 and 1.46 ± 0.44 N·mm2; P = 0.18), RLoA were 1.02 ×/÷ 1.17, ICC = 0.985 and CV = 5.76%. The techniques used to determine MVIF and CSA of the FDI muscle were reliable and can be combined to calculate force per unit CSA.

Motor 'surround inhibition' is not correlated with activity in surround muscles.

Surround inhibition (SI) is a neural process that has been extensively investigated in the sensory system and has been recently probed in the motor system. Muscle-specific modulation of corticospinal excitability at the onset of an isolated finger movement has been assumed to reflect the presence of SI in the motor system. This study attempted to characterise this phenomenon in a large cohort of normal volunteers and investigate its relationship with muscle activity in the hand. Corticospinal excitability of the pathways projecting to three hand muscles [first dorsal interosseus (FDI), abductor pollicis brevis (APB) and abductor digiti minimi (ADM)] and electromyographic (EMG) activity of the same muscles were assessed in 31 healthy volunteers during an isolated index finger movement. In the agonist FDI muscle both corticospinal excitability and EMG activity were found to be increased at the onset of the movement (P < 0.001 and P < 0.001, respectively). On the contrary, in the surround ADM, there was dissociation between the corticospinal excitability (decreased: P < 0.001) and EMG activity (increased: P < 0.001). Cross-correlation analysis of the EMG activity showed that neuronal signals driving the agonist and surround muscles are not synchronised when SI is present. The results suggest a distinctive origin of the neuronal signals driving the agonist and surround muscles. In addition, they indicate that cortical output might be simultaneously modulated by voluntary and non-voluntary activity, generated in cortical and subcortical structures, respectively.

Motor empathy is a consequence of misattribution of sensory information in observers

Human behavior depends crucially on the ability to interact with others and empathy has a critical role in enabling this to occur effectively. This can be an unconscious process and based on natural instinct and inner imitation (Montag et al., 2008) responding to observed and executed actions (Newman-Norlund et al., 2007). Motor empathy relating to painful stimuli is argued to occur via the mirror system in motor areas (Rizzolatti and Luppino, 2001). Here we investigated the effects of the location of emotional information on the responses of this system. Motor evoked potential (MEP) amplitudes from the right first dorsal interosseus (FDI) muscle in the hand elicited by single pulses of transcranial magnetic stimulation (TMS) delivered over the left motor cortex were measured while participants observed a video of a needle entering a hand over the FDI muscle, representing a painful experience for others. To maintain subjects’ internal representation across different viewing distances, we used the same size of hand stimuli both in peripersonal and extrapersonal space. We found a reduced MEP response, indicative of inhibition of the corticospinal system, only for stimuli presented in peripersonal space and not in extrapersonal space. This empathy response only occurring for near space stimuli suggests that it may be a consequence of misidentification of sensory information as being directly related to the observer. A follow up experiment confirmed that the effect was not a consequence of the size of the stimuli presented, in agreement with the importance of the near space/far space boundary for misattribution of body related information. This is consistent with the idea that empathy is, at least partially, a consequence of misattribution of perceptual information relating to another to the observer and that pain perception is modulated by the nature of perception of the pain.

P 8. Bringing TMS-based corticomotor mapping into shape: Shape-based TMS mapping reveals motor finger somatotopy in M1-HAND

Movements of the fingers involve overlapping neural representations in the hand region of the primary motor cortex (M1 HAND ). Despite of these overlapping representations, somatotopic gradients of cortical motor representations exist in M1 HAND , but cortical motor somatotopy is difficult to capture with conventional transcranial magnetic stimulation (TMS) mapping techniques. To develop a novel TMS mapping approach, which takes into account the sulcal shape to map motor finger somatotopy in human M1 HAND . In nine healthy young subjects, neuronavigated single-pulse TMS was given to the left M1 HAND via a small figure-of-eight coil to map muscle-specific corticomotor representations. We compared three different mapping approaches which differed in terms of target grids and coil orientations: (1) Line-45°condition: Six targets located on a straight medio-to-lateral line which corresponded to the overall orientation of the central sulcus using a fixed coil orientation that induced a posterior-to-anterior current having a direction of 45° relative to the mid-sagittal line, (2) Shape-45°condition: Seven targets which followed the bending of the central sulcus with a fixed coil orientation as in the first condition, and (3) Shape-90°condition: Seven targets which were individually positioned following the bending of the central sulcus as in condition 2 but with the coil orientation adjusted to produce a current direction perpendicular to the central sulcus. We compared cortico-motor representations of the abductor digiti minimi (ADM) and the first dorsal interosseus (FDI) muscle at rest, during isometric contraction of either the FDI or ADM muscles or during co-contraction of both muscles. Based on the amplitudes of the motor evoked potential (MEP) at each target site, we generated muscle excitability profiles, the position of the weighted mean MEP amplitude. In contrast to the other mapping conditions, the shape-90°condition demonstrated clear somatotopy of the FDI and ADM muscle representations in M1 HAND and yielded state-dependant shifts in the weighted mean positions of the curves when switching from rest to active conditions. Within-hand motor somatotopy in M1 HAND can be readily studied with neuronavigated TMS that follows the sulcal shape and creates a tissue current perpendicular to the central sulcus at all mapping sites.

P 200. Coupling of the sensorimotor cortical activation explored by blood-oxygenation-level-dependent signal navigated and traditional magnetic evoked potential recordings

Introduction The blood-oxygenation-level-dependent (BOLD) signal in functional magnetic resonance imaging (fMRI) reflects associated hemodynamics for a specific motor task. Motor evoked potentials (MEPs) represent the excitability of the motor cortex. Recently the MEP recording can be visually guided by the BOLD signal in fMRI. However, the two methods may represent diverse physiological mechanisms ( Krivanekova et al., 2012 , Antal et al., 2011 ). Objectives To study the coupling of the sensorimotor cortical activation observed in fMRI BOLD signals and measured by traditional MEP recordings. Materials and methods Ten healthy, right-handed subjects (7 men, mean age: 34.6 ± 7.6 years) participated in this study. Functional imaging was acquired by using a 3.0T GE MR scanner with a single-shot EPI sequence. The subjects were requested to perform simple, repetitive abduction movement at right index finger while they saw a go-signal appeared on the projection screen and to take a rest during no-go signal periods. Transcranial magnetic stimulation (TMS) was delivered through a focal figure-of-eight stimulating coil. The coil was held tangential to the scalp with the handle pointing backwards and ∼45° away from the midline. Twenty single TMS trials were measured with a randomly varying intertrial interval ranging from 7.5 to 12.5 s at four coil positions. The electrophysiological optimal (EO) position was determined as the site where TMS at a slightly suprathreshold intensity produced consistently the largest MEPs in right first dorsal interosseus (FDI). The site was marked on the scalp to assure a constant placement of the coil throughout this session. The other three positions were determined by a frameless MR-navigation system (Visor™). The maximal BOLD signals at left primary motor cortex (BM1), left primary sensory cortex (BS1) and the lateral margin of the BOLD activity (BL) were targeted by the guidance of the navigation system. Two TMS intensities which were adjusted to produce MEPs of about 0.5 mV (low intensity) and 1 mV (high intensity) in peak-to-peak amplitude in the right FDI muscle were tested. In total 8 recording sessions including 4 positions and 2 intensities were completed by a randomized order for each subject. The mean MEP amplitude of each session was analyzed by a two-way repeated measures analysis of variance (rmANOVA) with the within-subject factors of POSITION and INTENSITY. Results All subjects demonstrated a clear BOLD signal in the left sensorimotor cortex with response to the right finger movement. RmANOVA of the MEP amplitudes revealed significant main effects of POSITION and INTENSITY, and a significant interaction between POSITION and INTENSITY. The post hoc analysis revealed a significant MEP difference between the four positions except EO-BM1 and BS1-BL at high intensity stimulation (all p Conclusion Findings suggest that the coupling of the BOLD signals and the MEPs can be TMS-intensity dependent. A distinct localization of the BOLD signal in M1 or S1 by the TMS-navigation system may significantly vary the MEP findings under a high TMS-intensity condition.

A Higher Number of TMS-Elicited MEP from a Combined Hotspot Improves Intra- and Inter-Session Reliability of the Upper Limb Muscles in Healthy Individuals

We aimed to determine, using transcranial magnetic stimulation (TMS), the number of elicited motor evoked potentials (MEPs) that induces the highest intra- and inter-sessions reliability for the extensor carpi radialis (ECR) and first dorsal interosseus (FDI) muscles. Twelve healthy subjects participated in this study on two separate days. Single pulse magnetic stimuli were triggered with Magstim 2002 to obtain MEPs from the muscles of interest, with the subjects in a relaxed position. Reliability of MEP responses was investigated in three blocks of 5, 10 and 15 trials. The intra- and inter-session reliability of the MEPs' amplitudes and latencies were assessed using intraclass correlation coefficients (ICCs). Repeated measures ANOVA and paired t-tests revealed no significant time effect in the MEP amplitude and latency measurements (P>0.05). The ICCs indicated high intra-session reliability in the MEPs' amplitudes for the ECR and FDI muscles (0.77 to 0.99, 0.90 to 0.99, respectively) and latency (0.80 to 1.00, 0.75 to 0.97, respectively). The MEPs' amplitudes also had high inter-session reliability (0.84 to 0.97, 0.88 to 0.93, respectively), as did their latency (0.80 to 0.90, 0.75 to 0.97, respectively). Highest intra- and inter-session reliability was achieved for blocks of 10 and 15 trials. Our data suggest that intra- and inter-session comparisons should be performed using at least 10 MEPs in “combined hotspot” stimulation technique to ensure highest reliability.

Posterior Interosseus Palsy with an Incidental Froment-Rauber Nerve Presenting as a Pseudoclaw Hand

IntroductionAnatomic variations of peripheral nerves are well recog-nized and reported in literature [4]. It is important tounderstand these variations because they can explainunusual clinical deficits or findings. The Froment–Raubernerve is a distal continuation of the motor branch of theposterior interosseous nerve, which innervates the firstdorsal interosseus muscle. This report discusses a case ofthis rare anomalous connection that initially confused thediagnosis of a posterior interosseus palsy.Case ReportA 17-year-old left-hand dominant baseball pitcher pre-sented with an 8-month history of progressive pain in hisleft proximal forearm and elbow. His symptoms proceededto subjective weakness of the small finger that progressedradially to include the ring and middle fingers withposturing in flexion at the metacarpal−phalangeal andinterphalangeal joints. He was given the presumptivediagnosis of an ulnar palsy. Prior to the onset of handweakness, there were previous episodes of elbow pain thatresolved without sequelae. He was referred to a handsurgeon. Initial physical examination was notable forclawing of the middle, ring, and small digits (Fig. 1) withfull passive mobility. Additionally, he had subtle weaknessof the wrist and extrinsic digit extensors as well as of theextensor pollicis longis and the abductor pollicis longus. Hedenied any sensory changes but slightly decreased protec-tive sensibility was noted in the ulnar nerve distribution bySemmes–Weinstein monofilament testing (3.61), includingthe dorsal ulnar cutaneous nerve. A mildly positive Tinel’ssign over the ulnar nerve at the elbow was detected.Nonoperative treatments of splinting and nonsteroidal anti-inflammatory agents had been attempted and had failed toimprove his symptoms.Electrodiagnostic studies (nerve conduction and electro-myography (EMG)) were consistent with a posteriorinterosseous nerve (PIN) neuropathy. These findings in-cluded denervation in the extensor carpi ulnaris and firstdorsal interosseus with complex repetitive discharges in thelatter. The number of motor units in all muscles innervatedby the posterior interosseus nerve as well as in the firstdorsal interosseus were decreased. Nerve conductionstudies of the median and ulnar nerves were normal,including the amplitude of the thenar and hypothenarcompound action potentials. The amplitude of the extensorindicis proprius compound muscle action potential wassignificantly decreased. Notable is that although no neuro-physiologic evidence of ulnar nerve pathology was found atthe elbow or wrist, evidence of denervation was noted inhis first dorsal interosseus muscle on EMG. A completeneurological evaluation including magnetic resonance im-aging of the brain, cervical spine, elbow, and forearm were

Greater amount of visual feedback decreases force variability by reducing force oscillations from 0–1 and 3–7 Hz

The purpose was to determine the relation between visual feedback gain and variability in force and whether visual gain-induced changes in force variability were associated with frequency-specific force oscillations and changes in the neural activation of the agonist muscle. Fourteen young adults (19-29 years) were instructed to accurately match the target force at 2 and 10% of their maximal voluntary contraction with abduction of the index finger. Force was maintained at specific visual feedback gain levels that varied across trials. Each trial lasted 20 s and the amount of visual feedback was varied by changing the visual gain from 0.5 to 1,474 pixels/N (13 levels; equals approximately 0.001-4.57 degrees ). Force variability was quantified as the standard deviation of the detrended force data. The neural activation of the first dorsal interosseus (FDI) was measured with surface electromyography. The mean force did not vary significantly with the amount of visual feedback. In contrast, force variability decreased from low gains compared to moderate gains (0.5-4 pixels/N: 0.09 +/- 0.04 vs. 64-1,424 pixels/N: 0.06 +/- 0.02 N). The decrease in variability was predicted by a decrease in the power of force oscillations from 0-1 Hz (approximately 50%) and 3-7 Hz (approximately 20%). The activity of the FDI muscle did not vary across the visual feedback gains. These findings demonstrate that in young adults force variability can be decreased with increased visual feedback gain (>64 pixels/N vs. 0.5-4 pixels/N) due to a decrease in the power of oscillations in the force from 0-1 and 3-7 Hz.

Dynamic influence of wrist flexion and extension on the intracortical inhibition of the first dorsal interosseus muscle during precision grip

This work questioned further the influence of wrist movements on the control of precision grip. Seated subjects wearing a full-arm orthosis with the wrist and hand free were instructed to maintain a thumb/index finger opposition corresponding to 15% of maximal voluntary contraction for the first dorsal interosseus (FDI). Paired-pulse transcranial magnetic stimulation eliciting conditioned MEPs of FDI was used to determine the modulation of short intracortical inhibition (SICI) during cyclic active and passive wrist flexion and extension and during a static condition (no wrist movement, hand in the neutral position). The FDI active motor threshold (AMT) and the conditioning stimulus (0.8 AMT) were assessed in each series of FDI SICI measurements and the test stimulus (TS) was adjusted to match the amplitudes of test FDI MEPs across conditions. An increase of FDI background EMG during active wrist flexion compared to extension in some subjects did not influence FDI SICI as tested at matched EMG levels in the static condition. FDI SICI was reduced during wrist flexion (whether active or passive) compared to wrist extension, the latter being of equivalent FDI SICI as in the static condition. We suggest that wrist flexion and precision grip could be linked in a functional proximo-distal synergy. Indeed, coupling the activity between M1 sites of wrist flexors and FDI muscle via cortico-cortical disinhibition of FDI site may help recruit the interjoint synergy. Also, the salience of afferent information from wrist muscles may contribute to the phase-dependent modulation of SICI in the preactivated FDI muscle.

Systematic assessment of training-induced changes in corticospinal output to hand using frameless stereotaxic transcranial magnetic stimulation

Measuring changes in the characteristics of corticospinal output has become a critical part of assessing the impact of motor experience on cortical organization in both the intact and injured human brain. In this protocol we describe a method for systematically assessing training-induced changes in corticospinal output that integrates volumetric anatomical MRI with transcranial magnetic stimulation (TMS). A TMS coil is sited to a target grid superimposed onto a 3D MRI of cortex using a stereotaxic neuronavigation system. Subjects are then required to exercise the first dorsal interosseus (FDI) muscle on two different tasks for a total of 30 min. The protocol allows for reliably and repeatedly detecting changes in corticospinal output to FDI muscle in response to brief periods of motor training.

Hemispheric asymmetry and somatotopy of afferent inhibition in healthy humans

A conditioning electrical stimulus to a digital nerve can inhibit the motor-evoked potentials (MEPs) in adjacent hand muscles elicited by transcranial magnetic stimulation (TMS) to the contralateral primary motor cortex (M1) when given 25-50 ms before the TMS pulse. This is referred to as short-latency afferent inhibition (SAI). We studied inter-hemispheric differences (Experiment 1) and within-limb somatotopy (Experiment 2) of SAI in healthy right-handers. In Experiment 1, conditioning electrical pulses were applied to the right or left index finger (D2) and MEPs were recorded from relaxed first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscles ipsilateral to the conditioning stimulus. We found that SAI was more pronounced in right hand muscles. In Experiment 2, electrical stimulation was applied to the right D2 and MEPs were recorded from ipsilateral FDI, extensor digitorum communis (EDC) and biceps brachii (BB) muscles. The amount of SAI did not differ between FDI, EDC and BB muscles. These data demonstrate inter-hemispheric differences in the processing of cutaneous input from the hand, with stronger SAI in the dominant left hemisphere. We also found that SAI occurred not only in hand muscles adjacent to electrical digital stimulation, but also in distant hand and forearm and also proximal arm muscles. This suggests that SAI induced by electrical D2 stimulation is not focal and somatotopically specific, but a more widespread inhibitory phenomenon.

Repetitive exercise training changes input–output property of the corticospinal pathway during lengthening and shortening contractions in human first dorsal interosseus muscle

We investigated the modulation of the corticospinal excitability during lengthening (LEN) and shortening (SHO) contractions in the first dorsal interosseus (FDI) muscle. The maximum slope of input–output property (the relationship between stimulus intensities vs. motor evoked potentials) during LEN contractions was significantly higher than that during SHO contractions. However, after repetitive exercise training of LEN and SHO contractions for 2 weeks, the maximum slope during LEN contractions was significantly reduced. This result suggests that the FDI muscle is inexperienced in a significant number of LEN contractions in daily life.

Effect of cold-induced vasodilatation in the index finger on temperature and contractile characteristics of the first dorsal interosseus muscle during cold-water immersion

We investigated whether cyclic elevations in index finger temperature (cold-induced vasodilatation, CIVD) during prolonged cold exposure correlated with hand temperature and neuromuscular function. Evoked twitch force of the first dorsal interosseus (FDI) muscle was measured every minute in eight males and four females [age 25.4 (5.7) years, mean (SD)] during cooling of the hand for 30 min in 9 degrees C water, and in thermoneutral 30 degrees C water. During cooling, index finger temperature increased from 9.4 (0.9) degrees C at the nadir to 13.3 (2.4) degrees C (P<0.01) at the apex of the CIVD. However, the minimum skin temperature above the FDI muscle was 14.2 (2.1) degrees C, with no CIVD detected in any of the subjects. Peak twitch force was 2.5 (0.7) N at the nadir of the finger CIVD and 2.0 (0.8) N at the apex (P=0.07), time-to-peak increased from 189 (18) ms to 227 (26) ms (P<0.01), and half relaxation time increased from 135 (14) ms to 183 (32) ms (P<0.01). We conclude that CIVD is a local phenomenon isolated to the fingers, and that it does not have beneficial effects on either temperature or neuromuscular function of the FDI muscle during cold exposure.

Hemispheric Asymmetry and Somatotopy of Short-Latency Cutaneomotor Inhibition in Healthy Humans

A conditioning electrical stimulus to a digital nerve can inhibit the motor response in adjacent hand muscles elicited by a supra-motor threshold transcranial magnetic stimulation (TMS) test stimulus to the contralateral primary motor cortex (M1) hand area when given 25 to 50 ms before the TMS pulse. Such sensorimotor inhibition is referred to as short-latency afferent inhibition (SAI). There is good evidence that SAI is mediated through sensory-motor interaction at a cortical level. In right-handed healthy volunteers, we studied inter-hemispheric differences and within-limb somatotopy of SAI in two experiments. In experiment 1, conditioning electrical pulses were applied to the right or left index finger and motor evoked potentials (MEPs) were recorded from relaxed first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscles ipsilateral to the conditioning stimulus. Conditioning electrical stimulation of the right index finger was more effective in producing SAI in ipsilateral intrinsic hand muscles than stimulation of the left index finger. In experiment 2, electrical stimulation was applied to the right index finger only and MEPs were recorded from ipsilateral FDI, ADM, extensor digitorum communis (EDC) and extensor digiti minimi (EDM) muscles (experiment 2a) or from right FDI, ADM and biceps brachii (BB) muscles (experiment 2b). Experiment 2a revealed that the amount of SAI did not differ between right FDI, ADM, EDC and EDM muscles. In contrast, in experiment 2b SAI was also present in right BB muscle but significantly smaller as compared to FDI and ADM muscles. These data demonstrate inter-hemispheric differences in cortical processing of cutaneous input from the hand with stronger SAI in the dominant left hemisphere. In addition, these results show that apart from intrinsic hand muscles adjacent to electrical digital stimulation SAI occurs to the same extent in distant hand and forearm muscles and is also present in proximal arm muscles, albeit less pronounced. This can be taken as evidence that SAI is not a focal and somatotopically specific phenomenon.