Short-latency afferent inhibition during selective finger movement

Persistent adaptations in sensorimotor interneuron circuits in the motor cortex with a history of sport-related concussion

Recent studies highlight a persistent increase in subsequent injury risk following a sport-related concussion (SRC) despite clinical recovery. However, markers of persistent alterations in sensorimotor integration have yet to be identified. One possibility is that compensatory adaptation following SRC may only be unmasked during transient periods of high task complexity in specific sensorimotor circuits. The current study used short-latency afferent inhibition (SAI) to investigate the long-term sequelae of sport-related concussion (SRC) in different short-latency sensorimotor circuits converging in the motor cortex. Specific sensorimotor circuits sensitive to posterior-anterior current with a positive phase lasting 120µs (PA120) and anterior-posterior current with a positive phase lasting 30µs (AP30) were assessed using controllable pulse parameter transcranial magnetic stimulation (cTMS) while young adults with and without a history of SRC were at rest or responded to valid and invalid sensorimotor cues. SAI was quantified as the ratio of the motor-evoked potential (MEP) elicited by peripherally conditioned cTMS stimuli to the unconditioned MEP for each cTMS configuration. Individuals with a SRC history demonstrated persistent adaptation in AP30 SAI, but only in response to invalid cues. Persistent adaptation in AP30 SAI was not apparent at rest or during simple sensorimotor transformations in response to valid cues. PA120 SAI demonstrated similar responses at rest and in response to both valid and invalid cues, regardless of SRC history. AP30-sensitive sensorimotor circuits may mark the long-term SRC sequelae and the increased susceptibility to momentary breakdowns in sensorimotor integration during periods of high cognitive-motor demands.

The effects of verbal and spatial working memory on short- and long-latency sensorimotor circuits in the motor cortex

Multiple sensorimotor loops converge in the motor cortex to create an adaptable system capable of context-specific sensorimotor control. Afferent inhibition provides a non-invasive tool to investigate the substrates by which procedural and cognitive control processes interact to shape motor corticospinal projections. Varying the transcranial magnetic stimulation properties during afferent inhibition can probe specific sensorimotor circuits that contribute to short- and long-latency periods of inhibition in response to the peripheral stimulation. The current study used short- (SAI) and long-latency (LAI) afferent inhibition to probe the influence of verbal and spatial working memory load on the specific sensorimotor circuits recruited by posterior-anterior (PA) and anterior-posterior (AP) TMS-induced current. Participants completed two sessions where SAI and LAI were assessed during the short-term maintenance of two- or six-item sets of letters (verbal) or stimulus locations (spatial). The only difference between the sessions was the direction of the induced current. PA SAI decreased as the verbal working memory load increased. In contrast, AP SAI was not modulated by verbal working memory load. Visuospatial working memory load did not affect PA or AP SAI. Neither PA LAI nor AP LAI were sensitive to verbal or spatial working memory load. The dissociation of short-latency PA and AP sensorimotor circuits and short- and long-latency PA sensorimotor circuits with increasing verbal working memory load support multiple convergent sensorimotor loops that provide distinct functional information to facilitate context-specific supraspinal control.

Disinhibition of short-latency but not long-latency afferent inhibition of the lower limb during upper-limb muscle contraction

Research has demonstrated that motor and sensory functions of the lower limbs can be modulated by upper-limb muscle contractions. However, whether sensorimotor integration of the lower limb can be modulated by upper-limb muscle contractions is still unknown. [AQ: NR Original articles do not require structured abstracts. Hence, abstract subsections have been deleted. Please check.]Human sensorimotor integration has been studied using short- or long-latency afferent inhibition (SAI or LAI, respectively), which refers to inhibition of motor-evoked potentials (MEPs) elicited via transcranial magnetic stimulation by preceding peripheral sensory stimulation. In the present study, we aimed to investigate whether upper-limb muscle contractions could modulate the sensorimotor integration of the lower limbs by examining SAI and LAI. Soleus muscle MEPs following electrical tibial nerve stimulation (TSTN) during rest or voluntary wrist flexion were recorded at inter-stimulus intervals (ISIs) of 30 (i.e. SAI), 100, and 200 ms (i.e. LAI). The soleus Hoffman reflex following TSTN was also measured to identify whether MEP modulation occurred at the cortical or the spinal level. Results showed that lower-limb SAI, but not LAI, was disinhibited during voluntary wrist flexion. Furthermore, the soleus Hoffman reflex following TSTN during voluntary wrist flexion was unchanged when compared with that during the resting state at any ISI. Our findings suggest that upper-limb muscle contractions modulate sensorimotor integration of the lower limbs and that disinhibition of lower-limb SAI during upper-limb muscle contractions is cortically based.

Short-and long-latency afferent inhibition of the human leg motor cortex by H-reflex subthreshold electrical stimulation at the popliteal fossa

In humans, peripheral sensory stimulation inhibits subsequent motor evoked potentials (MEPs) induced by transcranial magnetic stimulation; this process is referred to as short- or long-latency afferent inhibition (SAI or LAI, respectively), depending on the inter-stimulus interval (ISI) length. Although upper limb SAI and LAI have been well studied, lower limb SAI and LAI remain under-investigated. Here, we examined the time course of the soleus (SOL) muscle MEP following electrical tibial nerve (TN) stimulation at the popliteal fossa at ISIs of 20-220 ms. When the conditioning stimulus intensity was three-fold the perceptual threshold, MEP amplitudes were inhibited at an ISI of 220 ms, but not at shorter ISIs. TN stimulation just below the Hoffman (H)-reflex threshold intensity inhibited MEP amplitudes at ISIs of 30, 35, 100, 180 and 200 ms. However, the relationship between MEP inhibition and the P30 latency of somatosensory evoked potentials (SEPs) did not show corresponding ISIs at the SEP P30 latency that maximizes MEP inhibition. To clarify whether the site of afferent-induced MEP inhibition occurs at the cortical or spinal level, we examined the time course of SOL H-reflex following TN stimulation. H-reflex amplitudes were not significantly inhibited at ISIs where MEP inhibition occurred but at an ISI of 120 ms. Our findings indicate that stronger peripheral sensory stimulation is required for lower limb than for upper limb SAI and LAI and that lower limb SAI and LAI are of cortical origin. Moreover, the direct pathway from the periphery to the primary motor cortex may contribute to lower limb SAI.

Specific sensorimotor interneuron circuits are sensitive to cerebellar-attention interactions

Background: Short latency afferent inhibition (SAI) provides a method to investigate mechanisms of sensorimotor integration. Cholinergic involvement in the SAI phenomena suggests that SAI may provide a marker of cognitive influence over implicit sensorimotor processes. Consistent with this hypothesis, we previously demonstrated that visual attention load suppresses SAI circuits preferentially recruited by anterior-to-posterior (AP)-, but not posterior-to-anterior (PA)-current induced by transcranial magnetic stimulation. However, cerebellar modulation can also modulate these same AP-sensitive SAI circuits. Yet, the consequences of concurrent cognitive and implicit cerebellar influences over these AP circuits are unknown.

Objective: We used cerebellar intermittent theta-burst stimulation (iTBS) to determine whether the cerebellar modulation of sensory to motor projections interacts with the attentional modulation of sensory to motor circuits probed by SAI.

Methods: We assessed AP-SAI and PA-SAI during a concurrent visual detection task of varying attention load before and after cerebellar iTBS.

Results: Before cerebellar iTBS, a higher visual attention load suppressed AP-SAI, but not PA-SAI, compared to a lower visual attention load. Post-cerebellar iTBS, the pattern of AP-SAI in response to visual attention load, was reversed; a higher visual attention load enhanced AP-SAI compared to a lower visual attention load. Cerebellar iTBS did not affect PA-SAI regardless of visual attention load.

Conclusion: These findings suggest that attention and cerebellar networks converge on overlapping AP-sensitive circuitry to influence motor output by controlling the strength of the afferent projections to the motor cortex. This interaction has important implications for understanding the mechanisms of motor performance and learning.

The Interactions Between Primary Somatosensory and Motor Cortex during Human Grasping Behaviors

Afferent inputs to the primary somatosensory cortex (S1) are differentially processed during precision and power grip in humans. However, it remains unclear how S1 interacts with the primary motor cortex (M1) during these two grasping behaviors. To address this question, we measured short-latency afferent inhibition (SAI), reflecting S1-M1 interactions via thalamo-cortical pathways, using paired-pulse transcranial magnetic stimulation (TMS) during precision and power grip. The TMS coil over the hand representation of M1 was oriented in the posterior-anterior (PA) and anterior-posterior (AP) direction to activate distinct sets of corticospinal neurons. We found that SAI increased during precision compared with power grip when AP, but not PA, currents were applied. Notably, SAI tested in the AP direction were similar during two-digit than five-digit precision grip. The M1 receives movement information from S1 through direct cortico-cortical pathways, so intra-hemispheric S1-M1 interactions using dual-site TMS were also evaluated. Stimulation of S1 attenuated M1 excitability (S1-M1 inhibition) during precision and power grip, while the S1-M1 inhibition ratio remained similar across tasks. Taken together,our findings suggest that distinct neural mechanisms for S1-M1 interactions mediate precision and power grip, presumably by modulating neural activity along thalamo-cortical pathways.

Association of short- and long-latency afferent inhibition with human behavior

Transcranial magnetic stimulation (TMS) paired with nerve stimulation evokes short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI), which are non-invasive assessments of the excitability of the sensorimotor system. SAI and LAI are abnormally reduced in various special populations in comparison to healthy controls. However, the relationship between afferent inhibition and human behavior remains unclear. The purpose of this review is to survey the current literature and synthesize observations and patterns that affect the interpretation of SAI and LAI in the context of human behavior. We discuss human behaviour across the motor and cognitive domains, and in special and control populations. Further, we discuss future considerations for research in this field and the potential for clinical applications. By understanding how human behavior is mediated by changes in SAI and LAI, this can allow us to better understand the neurophysiological underpinnings of human motor control.

Effect of the brain-derived neurotrophic factor gene Val66Met polymorphism on sensory-motor integration during a complex motor learning exercise

The brain-derived neurotrophic factor (BDNF) gene Val66Met polymorphism may cause impairment in short-term motor learning by reducing activity-dependent BDNF expression, which causes alterations in synaptic plasticity by changing glutamatergic and GABAergic synaptic transmissions. Sensory-motor integration (SMI) plays an important role in motor learning. In this study, we investigated the role of this polymorphism on SMI during a complex motor learning practice. Forty-three healthy participants performed standardized 5-day basketball shooting exercises under supervision. Electrophysiologic SMI studies were performed before the first day exercise (T0) and after the first and fifth day exercises (T1 and T2, respectively). SMI was studied using electrical median nerve stimulation at the wrist, followed by transcranial magnetic stimulation (TMS) of the contralateral motor cortex with various inter-stimulus intervals (ISIs). Recordings were made from the thenar and forearm flexor muscles. Participants were divided into two groups according to their BDNF genotype. Group 1 consisted of 26 subjects with the Val66Val genotype and group 2 included 17 subjects with the BDNF Met allele. Group 2 had a lower increase in basketball scores at day 5. Moreover, they had higher afferent facilitation for the responses recorded from both thenar and forearm flexor muscles at T1, but these changes could not be maintained until T2. This non-persistent early hyper-responsivity of the sensory-motor cortex in subjects with the BDNF Met allele might be explained by a transient upsurge of cortical excitability to compensate the insufficient cortical plasticity during motor learning, which could be considered as a sign of lower performance in motor skill learning.

Randomized controlled trial combining constraint-induced movement therapy and action-observation training in unilateral cerebral palsy: clinical effects and influencing factors of treatment response

Introduction:

Constraint-induced movement therapy (CIMT) improves upper limb (UL) motor execution in unilateral cerebral palsy (uCP). As these children also show motor planning deficits, action-observation training (AOT) might be of additional value. Here, we investigated the combined effect of AOT to CIMT and identified factors influencing treatment response.

Methods:

A total of 44 children with uCP (mean 9 years 6 months, SD 1 year 10 months) participated in a 9-day camp wearing a splint for 6 h/day and were allocated to the CIMT + AOT (n = 22) and the CIMT + placebo group (n = 22). The CIMT + AOT group received 15 h of AOT (i.e. video-observation) and executed the observed tasks, whilst the CIMT + AOT group watched videos free of biological motion and executed the same tasks. The primary outcome measure was bimanual performance. Secondary outcomes included measures of body function and activity level assessed before (T1), after the intervention (T2), and at 6 months follow-up (T3). Influencing factors included behavioural and neurological characteristics.

Results:

Although no between-groups differences were found (p > 0.05; η² = 0–16), the addition of AOT led to higher gains in children with initially poorer bimanual performance (p = 0.02; η² = 0.14). Both groups improved in all outcome measures after the intervention and retained the gains at follow up (p < 0.01; η² = 0.02–0.71). Poor sensory function resulted in larger improvements in the total group (p = 0.03; η² = 0.25) and high amounts of mirror movements tended to result in a better response to the additional AOT training (p = 0.06; η² = 0.18). Improvements were similar irrespective of the type of brain lesion or corticospinal tract wiring pattern.

Conclusions:

Adding AOT to CIMT, resulted in a better outcome for children with poor motor function and high amounts of mirror movements. CIMT with or without AOT seems to be more beneficial for children with poor sensory function.

Trial registration:

Registered at ClinicalTrials.gov on 22nd August 2017 (ClinicalTrials.gov identifier: NCT03256357).

Verbal working memory modulates afferent circuits in motor cortex

Verbal instruction and strategies informed by declarative memory are key to performance and acquisition of skilled actions. We previously demonstrated that anatomically distinct sensory-motor inputs converging on the corticospinal neurons of motor cortex are differentially sensitive to visual attention load. However, how loading of working memory shapes afferent input to motor cortex is unknown. This study used short-latency afferent inhibition (SAI) to probe the effect of verbal working memory upon anatomically distinct afferent circuits converging on corticospinal neurons in the motor cortex. SAI was elicited by preceding a suprathreshold transcranial magnetic stimulus (TMS) with electrical stimulation of the median nerve at the wrist while participants mentally rehearsed a two- or six-digit numeric memory set. To isolate different afferent intracortical circuits in motor cortex SAI was elicited, using TMS involving posterior-anterior (PA) or anterior-posterior (AP) monophasic current. Both PA and AP SAI were significantly reduced during maintenance of the six-digit compared to two-digit memory set. The generalized effect of working memory across anatomically distinct circuits converging upon corticospinal neurons in motor cortex is in contrast to the specific sensitivity of AP SAI to increased attention load. The common response across the PA and AP SAI circuits to increased working memory load may reflect an indiscriminate perisomatic mechanism involved in the voluntary facilitation of desired and/or suppression of unwanted actions during action selection or response conflict.

Effects of lorazepam and baclofen on short- and long-latency afferent inhibition

KEY POINTS

Short-latency afferent inhibition (SAI) is modulated by GABA_A receptor activity, whereas the pharmacological origin of long-latency afferent inhibition remains unknown. This is the first study to report that long-latency afferent inhibition (LAI) is reduced by the GABA_A positive allosteric modulator lorazepam, and that both SAI and LAI are not modulated by the GABA_B agonist baclofen. These findings advance our understanding of the neural mechanisms underlying afferent inhibition.

ABSTRACT

The afferent volley evoked by peripheral nerve stimulation has an inhibitory influence on transcranial magnetic stimulation induced motor evoked potentials. This phenomenon, known as afferent inhibition, occurs in two phases: short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI). SAI exerts its inhibitory influence via cholinergic and GABAergic activity. The neurotransmitter receptors that mediate LAI remain unclear. The present study aimed to determine whether LAI is contributed by GABA_A and/or GABA_B receptor activity. In a double-blinded, placebo-controlled study, 2.5 mg of lorazepam (GABA_A agonist), 20 mg of baclofen (GABA_B agonist) and placebo were administered to 14 males (mean age 22.7 ± 1.9 years) in three separate sessions. SAI and LAI, evoked by stimulation of the median nerve and recorded from the first dorsal interosseous muscle, were quantified before and at the peak plasma concentration following drug ingestion. Results indicate that lorazepam reduced LAI by ∼40% and, in support of previous work, reduced SAI by ∼19%. However, neither SAI, nor LAI were altered by baclofen. In a follow-up double-blinded, placebo-controlled study, 10 returning participants received placebo or 40 mg of baclofen (double the dosage used in Experiment 1). The results obtained indicate that SAI and LAI were unchanged by baclofen. This is the first study to show that LAI is modulated by GABA_A receptor activity, similar to SAI, and that afferent inhibition does not appear to be a GABA_B mediated process.

Exploring Behavioral Correlates of Afferent Inhibition

(1) Background: Afferent inhibition is the attenuation of the muscle response evoked from transcranial magnetic stimulation (TMS) by a prior conditioning electrical stimulus to a peripheral nerve. It is unclear whether the magnitude of afferent inhibition relates to sensation and movement; (2) Methods: 24 healthy, young adults were tested. Short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI) were obtained following median and digital nerve stimulation. Temporal tactile acuity was assessed with a temporal order judgement (TOJ) task, spatial tactile acuity was assessed using a grating orientation task (GOT), and fine manual dexterity was assessed with the Pegboard task; (3) Results: Correlation analyses revealed no association between the magnitude of SAI or LAI with performance on the TOJ, GOT, or Pegboard tasks; (4) Conclusion: The magnitude of SAI and LAI does not relate to performance on the sensory and motor tasks tested. Future studies are needed to better understand whether the afferent inhibition phenomenon relates to human behavior.

Attention modulates specific motor cortical circuits recruited by transcranial magnetic stimulation

Skilled performance and acquisition is dependent upon afferent input to motor cortex. The present study used short-latency afferent inhibition (SAI) to probe how manipulation of sensory afference by attention affects different circuits projecting to pyramidal tract neurons in motor cortex. SAI was assessed in the first dorsal interosseous muscle while participants performed a low or high attention-demanding visual detection task. SAI was evoked by preceding a suprathreshold transcranial magnetic stimulus with electrical stimulation of the median nerve at the wrist. To isolate different afferent intracortical circuits in motor cortex SAI was evoked using either posterior-anterior (PA) or anterior-posterior (PA) monophasic current. In an independent sample, somatosensory processing during the same attention-demanding visual detection tasks was assessed using somatosensory-evoked potentials (SEP) elicited by median nerve stimulation. SAI elicited by AP TMS was reduced under high compared to low visual attention demands. SAI elicited by PA TMS was not affected by visual attention demands. SEPs revealed that the high visual attention load reduced the fronto-central P20-N30 but not the contralateral parietal N20-P25 SEP component. P20-N30 reduction confirmed that the visual attention task altered sensory afference. The current results offer further support that PA and AP TMS recruit different neuronal circuits. AP circuits may be one substrate by which cognitive strategies shape sensorimotor processing during skilled movement by altering sensory processing in premotor areas.

Short-latency afferent inhibition determined by the sensory afferent volley

Short-latency afferent inhibition (SAI) is characterized by the suppression of the transcranial magnetic stimulation motor evoked potential (MEP) by the cortical arrival of a somatosensory afferent volley. It remains unknown whether the magnitude of SAI reflects changes in the sensory afferent volley, similar to that observed for somatosensory evoked potentials (SEPs). The present study investigated stimulus-response relationships between sensory nerve action potentials (SNAPs), SAI, and SEPs and their interrelatedness. Experiment 1 (n = 23, age 23 ± 1.5 yr) investigated the stimulus-response profile for SEPs and SAI in the flexor carpi radialis muscle after stimulation of the mixed median nerve at the wrist using ∼25%, 50%, 75%, and 100% of the maximum SNAP and at 1.2× and 2.4× motor threshold (the latter equated to 100% of the maximum SNAP). Experiment 2 (n = 20, age 23.1 ± 2 yr) probed SEPs and SAI stimulus-response relationships after stimulation of the cutaneous digital nerve at ∼25%, 50%, 75%, and 100% of the maximum SNAP recorded at the elbow. Results indicate that, for both nerve types, SAI magnitude is dependent on the volume of the sensory afferent volley and ceases to increase once all afferent fibers within the nerve are recruited. Furthermore, for both nerve types, the magnitudes of SAI and SEPs are related such that an increase in excitation within somatosensory cortex is associated with an increase in the magnitude of afferent-induced MEP inhibition.

Attentional Control of Gait and Falls: Is Cholinergic Dysfunction a Common Substrate in the Elderly and Parkinson's Disease?

The aim of this study was to address whether deficits in the central cholinergic activity may contribute to the increased difficulty to allocate attention during gait in the elderly with heightened risk of falls. We recruited 50 participants with a history of two or more falls (33 patients with Parkinson's Disease and 17 older adults) and 14 non-fallers age-matched adults. Cholinergic activity was estimated by means of short latency afferent inhibition (SAI), a transcranial magnetic stimulation (TMS) technique that assesses an inhibitory circuit in the sensorimotor cortex and is regarded as a global marker of cholinergic function in the brain. Increased difficulty to allocate attention during gait was evaluated by measuring gait performance under single and dual-task conditions. Global cognition was also assessed. Results showed that SAI was reduced in patients with PD than in the older adults (fallers and non-fallers) and in older adults fallers with respect to non-fallers. Reduction in SAI indicates less inhibition i.e., less cholinergic activity. Gait speed was reduced in the dual task gait compared to normal gait only in our faller population and changes in gait speed under dual task significantly correlated with the mean value of SAI. This association remained significant after adjusting for cognitive status. These findings suggest that central cholinergic activity may be a predictor of change in gait characteristics under dual tasking in older adults and PD fallers independently of cognitive status.

Dynamic modulation of corticospinal excitability and short-latency afferent inhibition during onset and maintenance phase of selective finger movement

OBJECTIVE

During highly selective finger movement, corticospinal excitability is reduced in surrounding muscles at the onset of movement but this phenomenon has not been demonstrated during maintenance of movement. Sensorimotor integration may play an important role in selective movement. We sought to investigate how corticospinal excitability and short-latency afferent inhibition changes in active and surrounding muscles during onset and maintenance of selective finger movement.

METHODS

Using transcranial magnetic stimulation (TMS) and paired peripheral stimulation, input-output recruitment curve and short-latency afferent inhibition (SAI) were measured in the first dorsal interosseus and abductor digiti minimi muscles during selective index finger flexion.

RESULTS

Motor surround inhibition was present only at the onset phase, but not at the maintenance phase of movement. SAI was reduced at onset but not at the maintenance phase of movement in both active and surrounding muscles.

CONCLUSIONS

Our study showed dynamic changes in corticospinal excitability and sensorimotor modulation for active and surrounding muscles in different movement states. SAI does not appear to contribute to motor surround inhibition at the movement onset phase. Also, there seems to be different inhibitory circuit(s) other than SAI for the movement maintenance phase in order to delineate the motor output selectively when corticospinal excitability is increased in both active and surrounding muscles.

SIGNIFICANCE

This study enhances our knowledge of dynamic changes in corticospinal excitability and sensorimotor interaction in different movement states to understand normal and disordered movements.

Reduced Short- and Long-Latency Afferent Inhibition Following Acute Muscle Pain: A Potential Role in the Recovery of Motor Output

OBJECTIVE

Corticomotor output is reduced in response to acute muscle pain, yet the mechanisms that underpin this effect remain unclear. Here the authors investigate the effect of acute muscle pain on short-latency afferent inhibition, long-latency afferent inhibition, and long-interval intra-cortical inhibition to determine whether these mechanisms could plausibly contribute to reduced motor output in pain.

DESIGN

Observational same subject pre-post test design.

SETTING

Neurophysiology research laboratory.

SUBJECTS

Healthy, right-handed human volunteers (n = 22, 9 male; mean age ± standard deviation, 22.6 ± 7.8 years).

METHODS

Transcranial magnetic stimulation was used to assess corticomotor output, short-latency afferent inhibition, long-latency afferent inhibition, and long-interval intra-cortical inhibition before, during, immediately after, and 15 minutes after hypertonic saline infusion into right first dorsal interosseous muscle. Pain intensity and quality were recorded using an 11-point numerical rating scale and the McGill Pain Questionnaire.

RESULTS

Compared with baseline, corticomotor output was reduced at all time points (p = 0.001). Short-latency afferent inhibition was reduced immediately after (p = 0.039), and long-latency afferent inhibition 15 minutes after (p = 0.035), the resolution of pain. Long-interval intra-cortical inhibition was unchanged at any time point (p = 0.36).

CONCLUSIONS

These findings suggest short- and long-latency afferent inhibition, mechanisms thought to reflect the integration of sensory information with motor output at the cortex, are reduced following acute muscle pain. Although the functional relevance is unclear, the authors hypothesize a reduction in these mechanisms may contribute to the restoration of normal motor output after an episode of acute muscle pain.

Short-latency afferent inhibition in chronic spinal cord injury

Background

Short-latency afferent inhibition (SAI) results when somatosensory afferent input inhibits the corticospinal output from primary motor cortex (M1). The present study examined SAI in the flexor carpi radialis (FCR) muscle in individuals with spinal cord injury (SCI) and uninjured controls.

Methods

Short-latency afferent inhibition (SAI) was evoked by stimulating the median nerve at the elbow at intervals of 15, 20 and 25 ms in advance of a transcranial magnetic stimulation (TMS) pulse over M1. SAI was tested with the FCR at rest and also during ~20% of maximum voluntary contraction. Corticospinal output was assessed through measuring both motor thresholds and motor evoked potential (MEP) recruitment curves. The afferent volley was assessed via the N20–P25 amplitude of the somatosensory evoked potential (SEP) and the amplitude of sensory nerve action potentials (SNAP) recorded over the median nerve at the elbow.

Results

SAI is reduced in SCI in both the contracted and non-contracted FCR muscle. MEP recruitment curves and thresholds were decreased in SCI only in the active state and not the resting state. N20–P25 amplitude was similar between groups in both the resting and active states although SNAP was significantly reduced in SCI at rest.

Conclusions

We conclude that reduced SAI in SCI is likely attributed to neuroplasticity altering the intrinsic M1 circuitry mediating SAI and/or reduced afferent input traversing a direct thalamocortical route to M1. These data provide a new avenue of research aimed at identifying therapeutic approaches to alter SAI to improve upper limb function in individuals with SCI.

Afferent inhibition and cortical silent periods in shoulder primary motor cortex and effect of a suprascapular nerve block in people experiencing chronic shoulder pain

OBJECTIVE

To characterise short afferent inhibition (SAI) and the cortical silent period (CSP) in the primary motor cortex representations of the infraspinatus muscle in healthy adults and people experiencing chronic shoulder pain, to determine the impact of a suprascapular nerve block (SSNB).

METHODS

Neurophysiological measures were obtained in 18 controls and 8 patients with chronic shoulder pain, pre and post SSNB and 1 week later. Pain intensity was assessed by a visual analogue scale.

RESULTS

SAI was apparent in controls (all P<0.03) and a CSP was observed which reduced in the presence of SAI (all P<0.0001). Compared to controls, shoulder pain patients demonstrated higher active motor threshold (P=0.046), less SAI (P=0.044), a longer CSP (P=0.048) and less modulation of the CSP by SAI (P=0.045). Higher motor thresholds were related to higher pain scores (P=0.009). The SSNB immediately restored SAI (P=0.013), with a positive relationship between increased SAI and reduced pain (P=0.031). The SSNB further reduced modulation of CSP by SAI at 1 week post injection (P=0.006).

CONCLUSIONS

SAI and the CSP were present and demonstrated robust interaction in controls, which was aberrant in patients. The SSNB transiently restored SAI but had no effect on the CSP; however CSP modulation by SAI was further attenuated 1 week post injection.

SIGNIFICANCE

The current findings improve understanding of the neurophysiology of the shoulder motor cortex and its modulation by chronic pain. The effect of SSNB in shoulder pain patients should be interpreted with caution until proven in a larger population. Interventions that target intracortical inhibition might increase efficacy in people with chronic shoulder pain.

Modulation of short-latency afferent inhibition depends on digit and task-relevance

Short-latency afferent inhibition (SAI) occurs when a single transcranial magnetic stimulation (TMS) pulse delivered over the primary motor cortex is preceded by peripheral electrical nerve stimulation at a short inter-stimulus interval (∼ 20-28 ms). SAI has been extensively examined at rest, but few studies have examined how this circuit functions in the context of performing a motor task and if this circuit may contribute to surround inhibition. The present study investigated SAI in a muscle involved versus uninvolved in a motor task and specifically during three pre-movement phases; two movement preparation phases between a "warning" and "go" cue and one movement initiation phase between a "go" cue and EMG onset. SAI was tested in the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles in twelve individuals. In a second experiment, the origin of SAI modulation was investigated by measuring H-reflex amplitudes from FDI and ADM during the motor task. The data indicate that changes in SAI occurred predominantly in the movement initiation phase during which SAI modulation depended on the specific digit involved. Specifically, the greatest reduction in SAI occurred when FDI was involved in the task. In contrast, these effects were not present in ADM. Changes in SAI were primarily mediated via supraspinal mechanisms during movement preparation, while both supraspinal and spinal mechanisms contributed to SAI reduction during movement initiation.

Short-latency afferent inhibition modulation during finger movement

When somatosensory input via electrical stimulation of a peripheral nerve precedes a transcranial magnetic stimulation (TMS) pulse over the primary motor cortex (M1) the corticospinal output is substantially reduced, a phenomenon known as short-latency afferent inhibition (SAI). The present study investigated SAI during rest and during pre-movement, phasic and tonic components of movement. Participants were required to perform an index finger flexion reaction time task in response to an auditory cue. In a series of experiments, SAI was evoked from the mixed, median nerve at the wrist or the cutaneous, digital nerve stimulation of the index finger. To assess the spinal versus cortical origin of movement-related modulation of SAI, F-wave amplitudes were measured during rest and the three movement components. Results indicated that SAI was reduced during all movement components compared to rest, an effect that occurred for both nerves stimulated. Pre-movement SAI reduction was primarily attributed to reduced cortical inhibition, while increased spinal excitability additionally contributed to reduced SAI during tonic and phasic components of movement. SAI was differentially modulated across movement components with mixed but not cutaneous nerve stimulation. These findings reveal that SAI is reduced during movement and this reduction begins as early as the preparation to move. Further, these data suggest that the degree of SAI reduction during movement may be specific to the volume and/or composition of afferent input carried by each nerve.

Postsynaptic nigrostriatal dopamine receptors and their role in movement regulation

The article presents the hypothesis that nigrostriatal dopamine may regulate movement by modulation of tone and contraction in skeletal muscles through a concentration-dependent influence on the postsynaptic D1 and D2 receptors on the follow manner: nigrostriatal axons innervate both receptor types within the striatal locus somatotopically responsible for motor control in agonist/antagonist muscle pair around a given joint. D1 receptors interact with lower and D2 receptors with higher dopamine concentrations. Synaptic dopamine concentration increases immediately before movement starts. We hypothesize that increasing dopamine concentrations stimulate first the D1 receptors and reduce muscle tone in the antagonist muscle and than stimulate D2 receptors and induce contraction in the agonist muscle. The preceded muscle tone reduction in the antagonist muscle eases the efficient contraction of the agonist. Our hypothesis is applicable for an explanation of physiological movement regulation, different forms of movement pathology and therapeutic drug effects. Further, this hypothesis provides a theoretical basis for experimental investigation of dopaminergic motor control and development of new strategies for treatment of movement disorders.

Changes in short afferent inhibition during phasic movement in focal dystonia

Impaired surround inhibition could account for the abnormal motor control seen in patients with focal hand dystonia, but the neural mechanisms underlying surround inhibition in the motor system are not known. We sought to determine whether an abnormality of the influence of sensory input at short latency could contribute to the deficit of surround inhibition in patients with focal hand dystonia (FHD). To measure digital short afferent inhibition (dSAI), subjects received electrical stimulation at the digit followed after 23 ms by transcranial magnetic stimulation (TMS). Motor evoked potentials (MEPs) were recorded over abductor digiti minimi (ADM) during rest and during voluntary phasic flexion of the second digit. F-waves were also recorded. We studied 13 FHD patients and 17 healthy volunteers. FHD patients had increased homotopic dSAI in ADM during flexion of the second digit, suggesting that this process acts to diminish overflow during movement; this might be a compensatory mechanism. No group differences were observed in first dorsal interosseous. Further, no differences were seen in the F-waves between groups, suggesting that the changes in dSAI are mediated at the cortical level rather than at the spinal cord. Understanding the role of these inhibitory circuits in dystonia may lead to development of therapeutic agents aimed at restoring inhibition.

Contribution of transcranial magnetic stimulation to the understanding of cortical mechanisms involved in motor control

Transcranial magnetic stimulation (TMS) was initially used to evaluate the integrity of the corticospinal tract in humans non-invasively. Since these early studies, the development of paired-pulse and repetitive TMS protocols allowed investigators to explore inhibitory and excitatory interactions of various motor and non-motor cortical regions within and across cerebral hemispheres. These applications have provided insight into the intracortical physiological processes underlying the functional role of different brain regions in various cognitive processes, motor control in health and disease and neuroplastic changes during recovery of function after brain lesions. Used in combination with neuroimaging tools, TMS provides valuable information on functional connectivity between different brain regions, and on the relationship between physiological processes and the anatomical configuration of specific brain areas and connected pathways. More recently, there has been increasing interest in the extent to which these physiological processes are modulated depending on the behavioural setting. The purpose of this paper is (a) to present an up-to-date review of the available electrophysiological data and the impact on our understanding of human motor behaviour and (b) to discuss some of the gaps in our present knowledge as well as future directions of research in a format accessible to new students and/or investigators. Finally, areas of uncertainty and limitations in the interpretation of TMS studies are discussed in some detail.

Corticomotor facilitation associated with observation, imagery and imitation of hand actions: a comparative study in young and old adults

In the present report, we extent our previous findings (Clark et al. in Neuropsychologia 42:105-122, 2004) on corticomotor facilitation associated with covert (observation and imagery) and overt execution (action imitation) of hand actions to better delineate the selectivity of the effect in the context of an object-oriented action. A second aim was to examine whether the pattern of facilitation would be affected by age. Corticomotor facilitation was determined in two groups of participants (young n = 21, 24 +/- 2 years; old n = 19, 62 +/- 6 years) by monitoring changes in the amplitude and latency of motor evoked potentials (MEPs) elicited in hand muscles by transcranial magnetic stimulation. MEP responses were measured from both the first dorsal interosseous (FDI, task selective muscle) and the abductor digiti minimi (ADM) of the right hand while participants attended to four different video presentations. Each of four videos provided specific instructions for participants to either: (1) close their eyes and relax (REST), (2) observe the action attentively (OBS), (3) close their eyes and mentally simulate the action (IMAG), or (4) imitate the action (IMIT). The action depicted in the videos represented a male subject cutting a piece of material with scissors. In the young group, the pattern of results revealed selective facilitation in the FDI in conditions involving either covert (OBS and IMAG) or overt action execution (IMIT). In the ADM, only overt execution with action imitation was associated with significant MEP facilitation. In the old group, a similar pattern of results was observed, although the modulation was less selective than that seen in the young group. In fact, older individuals often exhibited concomitant facilitation in both the FDI and ADM during either covert (OBS and IMAG conditions) or overt action execution (IMIT condition). Taken together, these results further corroborate the notion that the corticomotor system is selectively active when actions are covertly executed through internal simulation triggered by observation or by motor imagery, as proposed by Jeannerod (Neuroimage 14:S103-S109, 2001). With aging, the ability to produce corticomotor facilitation in association with covert action execution appears to be largely preserved, although there seems to be a loss in selectivity. This lack of selectivity may, in turn, reflect age-related alterations in the function of the corticospinal system, which may impair the ability to individuate finger movements either in the covert or overt stage of action execution.