Age-related Differences in Short- and Long-interval Intracortical Inhibition in a Human Hand Muscle

Changes in the cortical GABAergic inhibitory system with ageing and ageing-related neurodegenerative diseases

The human cortical inhibitory system is known to play a vital role for normal brain development, function, and plasticity. GABA is the most prominent inhibitory neurotransmitter in the CNS and is a key regulator not only for motor control and motor learning, but also for cognitive processes. With ageing and many neurodegenerative pathologies, a decline in GABAergic function in several cortical regions together with a reduced ability to task-specifically modulate and increase inhibition in the primary motor cortex has been observed. This decline in intracortical inhibition is associated with impaired motor control but also with diminished motor-cognitive (i.e. dual-tasking) and cognitive performance (e.g. executive functions). Furthermore, more general well-being such as sleep quality, stress resistance or non-specific pain perception are also associated with reduced GABA functioning. The current review highlights the interplay between changes in GABAergic function and changes in motor control, motor-cognitive and cognitive performance associated with healthy ageing, as well as in seniors with neurodegenerative diseases such as mild cognitive impairment. Furthermore, recent evidence highlighting the ability to up- or downregulate cortical inhibition by means of physical exercise programs is presented and discussed.

Considerable effects of lateralization and aging in intracortical excitation and inhibition

Introduction

Findings based on the use of transcranial magnetic stimulation and electromyography (TMS-EMG) to determine the effects of motor lateralization and aging on intracortical excitation and inhibition in the primary motor cortex (M1) are inconsistent in the literature. TMS and electroencephalography (TMS-EEG) measures the excitability of excitatory and inhibitory circuits in the brain cortex without contamination from the spine and muscles. This study aimed to investigate the effects of motor lateralization (dominant and non-dominant hemispheres) and aging (young and older) and their interaction effects on intracortical excitation and inhibition within the M1 in healthy adults, measured using TMS-EMG and TMS-EEG.

Methods

This study included 21 young (mean age = 28.1 ± 3.2 years) and 21 older healthy adults (mean age = 62.8 ± 4.2 years). A battery of TMS-EMG measurements and single-pulse TMS-EEG were recorded for the bilateral M1.

Results

Two-way repeated-measures analysis of variance was used to investigate lateralization and aging and the lateralization-by-aging interaction effect on neurophysiological outcomes. The non-dominant M1 presented a longer cortical silent period and larger amplitudes of P60, N100, and P180. Corticospinal excitability in older participants was significantly reduced, as supported by a larger resting motor threshold and lower motor-evoked potential amplitudes. N100 amplitudes were significantly reduced in older participants, and the N100 and P180 latencies were significantly later than those in young participants. There was no significant lateralization-by-aging interaction effect in any outcome.

Conclusion

Lateralization and aging have independent and significant effects on intracortical excitation and inhibition in healthy adults. The functional decline of excitatory and inhibitory circuits in the M1 is associated with aging.

Differences in motor inhibition in young and older musicians and non-musicians at rest

Introduction

Older adults experience a decline in motor inhibition. These declines have been implicated in instrumental activities of daily living. However, studies have revealed that older musicians have behavioral and neurophysiological enhancements in various motor domains compared to non-musicians. This suggests that music training may delay the decline in motor inhibition with aging. Nevertheless, motor inhibition has not been studied in young or older musicians and non-musicians. Thus, the present study aimed to investigate the neurophysiological differences in motor inhibition in aging musicians and non-musicians.

Methods

A total of 19 healthy young adult musicians, 16 healthy young non-musicians, 13 healthy older adult musicians, and 16 healthy older adult non-musicians were recruited for the study. Transcranial magnetic stimulation single-pulse (SP) and short interval cortical inhibition (SICI) were performed at rest and then converted into inhibition percentage.

Results

We did not observe significant differences between young and older musicians and non-musicians in resting SP MEP. Older adults had lower resting SICI MEP than young adults. Older adults (36%) had a greater percentage of inhibition than young adults (16%). However, when controlling for background EMG activity, musicians had a lower inhibition percentage than non-musicians.

Discussion

The results revealed that, despite the greater use of spinal mechanisms, decreased SICI, and increased inhibition percentage in older adults, motor inhibitory circuitry remains intact and functional in both young and older musicians and non-musicians. Future studies will reveal whether there are differences in motor inhibition during movement in musicians across a person's lifespan.

Long-interval intracortical inhibition in primary motor cortex related to working memory in middle-aged adults

Introduction

Excitability of the primary motor cortex measured with TMS has been associated with cognitive dysfunctions in patient populations. However, only a few studies have explored this relationship in healthy adults, and even fewer have considered the role of biological sex.

Methods

Ninety-seven healthy middle-aged adults (53 male) completed a TMS protocol and a neuropsychological assessment. Resting Motor Threshold (RMT) and Long-Interval Intracortical Inhibition (LICI) were assessed in the left motor cortex and related to attention, episodic memory, working memory, reasoning, and global cognition composite scores to evaluate the relationship between cortical excitability and cognitive functioning.

Results

In the whole sample, there was a significant association between LICI and cognition; specifically, higher motor inhibition was related to better working memory performance. When the sample was broken down by biological sex, LICI was only associated with working memory, reasoning, and global cognition in men. No associations were found between RMT and cognitive functions.

Conclusion

Greater intracortical inhibition, measured by LICI, could be a possible marker of working memory in healthy middle-aged adults, and biological sex plays a critical role in this association.

Are Neurophysiological Biomarkers Able to Discriminate Multiple Sclerosis Clinical Subtypes?

Secondary progressive multiple sclerosis (SPMS) subtype is retrospectively diagnosed, and biomarkers of the SPMS are not available. We aimed to identify possible neurophysiological markers exploring grey matter structures that could be used in clinical practice to better identify SPMS. Fifty-five people with MS and 31 healthy controls underwent a transcranial magnetic stimulation protocol to test intracortical interneuron excitability in the primary motor cortex and somatosensory temporal discrimination threshold (STDT) to test sensory function encoded in cortical and deep grey matter nuclei. A logistic regression model was used to identify a combined neurophysiological index associated with the SP subtype. We observed that short intracortical inhibition (SICI) and STDT were the only variables that differentiated the RR from the SP subtype. The logistic regression model provided a formula to compute the probability of a subject being assigned to an SP subtype based on age and combined SICI and STDT values. While only STDT correlated with disability level at baseline evaluation, both SICI and STDT were associated with disability at follow-up. SICI and STDT abnormalities reflect age-dependent grey matter neurodegenerative processes that likely play a role in SPMS pathophysiology and may represent easily accessible neurophysiological biomarkers for the SPMS subtype.

Submaximal isometric fatiguing exercise of the elbow flexors has no age-related effect on GABAB mediated inhibition

Age-related changes in the neuromuscular system can result in differences in fatigability between young and older adults. Previous research has shown that single joint isometric fatiguing exercise of small muscle results in an age-related compensatory decrease in GABA_B mediated inhibition. However, this has yet to be established in a larger muscle group. In 15 young (22 ± 4 years) and 15 older (65 ± 5 years) adults, long interval cortical inhibition (LICI; 100 ms ISI) and corticospinal silent period (SP) were measured in the biceps brachii during a 5% EMG contraction using transcranial magnetic stimulation (TMS) before, during and after a submaximal contraction (30% MVC force) held intermittently to task failure. Both age groups developed similar magnitude of fatigue (~24% decline in MVC; P = 0.001) and ~28% decline in LICI (P = 0.001) post fatiguing exercise. No change in SP duration was observed during and immediately following fatigue (P = 0.909) but ~ 6% decrease was seen at recovery in both age groups (P<0.001)." Contrary to previous work in a small muscle, these findings suggest no age-related differences in GABA_B mediated inhibition following single joint isometric fatiguing exercise of the elbow flexors, indicating that GABA_B modulation with ageing may be muscle group dependent. Furthermore, variations in SP duration and LICI modulation during and post fatigue in both groups suggest that these measures are likely mediated by divergent mechanisms.

Neural mechanisms mediating cross education: With additional considerations for the ageing brain

CALVERT, G.H.M., and CARSON, R.G. Neural mechanisms mediating cross education: With additional considerations for the ageing brain. NEUROSCI BIOBEHAV REV 21(1) XXX-XXX, 2021. - Cross education (CE) is the process whereby a regimen of unilateral limb training engenders bilateral improvements in motor function. The contralateral gains thus derived may impart therapeutic benefits for patients with unilateral deficits arising from orthopaedic injury or stroke. Despite this prospective therapeutic utility, there is little consensus concerning its mechanistic basis. The precise means through which the neuroanatomical structures and cellular processes that mediate CE may be influenced by age-related neurodegeneration are also almost entirely unknown. Notwithstanding the increased incidence of unilateral impairment in later life, age-related variations in the expression of CE have been examined only infrequently. In this narrative review, we consider several mechanisms which may mediate the expression of CE with specific reference to the ageing CNS. We focus on the adaptive potential of cellular processes that are subserved by a specific set of neuroanatomical pathways including: the corticospinal tract, corticoreticulospinal projections, transcallosal fibres, and thalamocortical radiations. This analysis may inform the development of interventions that exploit the therapeutic utility of CE training in older persons.

Shoulder position and handedness differentially affect excitability and intracortical inhibition of hand muscles

Individuals with stroke show distinct differences in hand function impairment when the shoulder is in adduction, within the workspace compared to when the shoulder is abducted, away from the body. To better understand how shoulder position affects hand control, we tested the corticomotor excitability and intracortical control of intrinsic and extrinsic hand muscles important for grasp in twelve healthy individuals. Motor evoked potentials (MEP) using single and paired-pulse transcranial magnetic stimulation were elicited in extensor digitorum communis (EDC), flexor digitorum superficialis (FDS), first dorsal interosseous (FDI), and abductor pollicis brevis (APB). The shoulder was fully supported in horizontal adduction (ADD) or abduction (ABD). Separate mixed-effect models were fit to the MEP parameters using shoulder position (or upper-extremity [UE] side) as fixed and participants as random effects. In the non-dominant UE, EDC showed significantly greater MEPs in shoulder ABD than ADD. In contrast, the dominant side EDC showed significantly greater MEPs in ADD compared to ABD; %facilitation of EDC on dominant side showed significant stimulus intensity x position interaction, EDC excitability was significantly greater in ADD at 150% of the resting threshold. Intrinsic hand muscles of the dominant UE received significantly more intracortical inhibition (SICI) when the shoulder was in ADD compared to ABD; there was no position-dependent modulation of SICI on the non-dominant side. Our findings suggest that these resting-state changes in hand muscle excitabilities reflect the natural statistics of UE movements, which in turn may arise from as well as shape the nature of shoulder-hand coupling underlying UE behaviors.

Contribution of TMS and TMS-EEG to the Understanding of Mechanisms Underlying Physiological Brain Aging

In the human brain, aging is characterized by progressive neuronal loss, leading to disruption of synapses and to a degree of failure in neurotransmission. However, there is increasing evidence to support the notion that the aged brain has a remarkable ability to reorganize itself, with the aim of preserving its physiological activity. It is important to develop objective markers able to characterize the biological processes underlying brain aging in the intact human, and to distinguish them from brain degeneration associated with many neurological diseases. Transcranial magnetic stimulation (TMS), coupled with electromyography or electroencephalography (EEG), is particularly suited to this aim, due to the functional nature of the information provided, and thanks to the ease with which it can be integrated with behavioral manipulation. In this review, we aimed to provide up to date information about the role of TMS and TMS-EEG in the investigation of brain aging. In particular, we focused on data about cortical excitability, connectivity and plasticity, obtained by using readouts such as motor evoked potentials and transcranial evoked potentials. Overall, findings in the literature support an important potential contribution of TMS to the understanding of the mechanisms underlying normal brain aging. Further studies are needed to expand the current body of information and to assess the applicability of TMS findings in the clinical setting.

Cortical disinhibition in Parkinson's disease

In Parkinson's disease, striatal dopamine depletion produces profound alterations in the neural activity of the cortico-basal ganglia motor loop, leading to dysfunctional motor output and parkinsonism. A key regulator of motor output is the balance between excitation and inhibition in the primary motor cortex, which can be assessed in humans with transcranial magnetic stimulation techniques. Despite decades of research, the functional state of cortical inhibition in Parkinson's disease remains uncertain. Towards resolving this issue, we applied paired-pulse transcranial magnetic stimulation protocols in 166 patients with Parkinson's disease (57 levodopa-naïve, 50 non-dyskinetic, 59 dyskinetic) and 40 healthy controls (age-matched with the levodopa-naïve group). All patients were studied OFF medication. All analyses were performed with fully automatic procedures to avoid confirmation bias, and we systematically considered and excluded several potential confounding factors such as age, gender, resting motor threshold, EMG background activity and amplitude of the motor evoked potential elicited by the single-pulse test stimuli. Our results show that short-interval intracortical inhibition is decreased in Parkinson's disease compared to controls. This reduction of intracortical inhibition was obtained with relatively low-intensity conditioning stimuli (80% of the resting motor threshold) and was not associated with any significant increase in short-interval intracortical facilitation or intracortical facilitation with the same low-intensity conditioning stimuli, supporting the involvement of cortical inhibitory circuits. Short-interval intracortical inhibition was similarly reduced in levodopa-naïve, non-dyskinetic and dyskinetic patients. Importantly, intracortical inhibition was reduced compared to control subjects also on the less affected side (n = 145), even in de novo drug-naïve patients in whom the less affected side was minimally symptomatic (lateralized Unified Parkinson's Disease Rating Scale part III = 0 or 1, n = 23). These results suggest that cortical disinhibition is a very early, possibly prodromal feature of Parkinson's disease.

Pinging the brain with transcranial magnetic stimulation reveals cortical reactivity in time and space

BACKGROUND

Single-pulse transcranial magnetic stimulation (TMS) elicits an evoked electroencephalography (EEG) potential (TMS-evoked potential, TEP), which is interpreted as direct evidence of cortical reactivity to TMS. Thus, combining TMS with EEG can be used to investigate the mechanism underlying brain network engagement in TMS treatment paradigms. However, controversy remains regarding whether TEP is a genuine marker of TMS-induced cortical reactivity or if it is confounded by responses to peripheral somatosensory and auditory inputs. Resolving this controversy is of great significance for the field and will validate TMS as a tool to probe networks of interest in cognitive and clinical neuroscience.

OBJECTIVE

Here, we delineated the cortical origin of TEP by spatially and temporally localizing successive TEP components, and modulating them with transcranial direct current stimulation (tDCS) to investigate cortical reactivity elicited by single-pulse TMS and its causal relationship with cortical excitability.

METHODS

We recruited 18 healthy participants in a double-blind, cross-over, sham-controlled design. We collected motor-evoked potentials (MEPs) and TEPs elicited by suprathreshold single-pulse TMS targeting the left primary motor cortex (M1). To causally test cortical and corticospinal excitability, we applied tDCS to the left M1.

RESULTS

We found that the earliest TEP component (P25) was localized to the left M1. The following TEP components (N45 and P60) were largely localized to the primary somatosensory cortex, which may reflect afferent input by hand-muscle twitches. The later TEP components (N100, P180, and N280) were largely localized to the auditory cortex. As hypothesized, tDCS selectively modulated cortical and corticospinal excitability by modulating the pre-stimulus mu-rhythm oscillatory power.

CONCLUSION

Together, our findings provide causal evidence that the early TEP components reflect cortical reactivity to TMS.

Motor cortex dysfunction in problem gamblers

Impairments in response inhibition have been implicated in gambling psychopathology. This behavioral impairment may suggest that the neural mechanisms involved in response inhibition, such as GABA_A -mediated neurotransmission in the primary motor cortex (M1), are also impaired. The present study obtained paired-pulse transcranial magnetic stimulation markers of GABA_A and glutamate receptor activity from the left M1 of three groups-problem gamblers (n = 17, 12 males), at-risk gamblers (n = 29, 19 males), and controls (n = 23, six males)-with each group matched for alcohol use, substance use, and attention-deficit hyperactivity disorder (ADHD) symptomology. Response inhibition was measured using the stop signal task. Results showed that problem gamblers had weaker M1 GABA_A receptor activity relative to controls and elevated M1 glutamate receptor activity relative to at-risk gamblers and controls. Although there were no differences in response inhibition between the groups, poorer response inhibition was correlated with weaker M1 GABA_A receptor activity. These findings are the first to show that problem gambling is associated with alterations in M1 GABA_A and glutamate-mediated neurotransmission.

Short-interval intracortical inhibition as a function of inter-stimulus interval: Three methods compared

BACKGROUND

Short-interval intracortical inhibition (SICI), as measured by threshold-tracking as a function of inter-stimulus interval (ISI), has been proposed as a useful biomarker for amyotrophic lateral sclerosis (ALS), but its relationship to conventional amplitude measurements has not been established.

METHODS

Serial tracking of SICI at increasing ISIs from 1 to 7 ms (T-SICIs) was compared in 50 healthy control subjects with the same ISIs tracked in parallel (T-SICIp), and with conventional amplitude measurements (A-SICI). For T-SICIp and A-SICI, pairs of conditioning and test stimuli with different ISIs were pseudo-randomised and interspersed with test-alone stimuli given at regular intervals. Thresholds were estimated by regression of log peak-to-peak amplitude on stimulus.

RESULTS

T-SICIp and A-SICI were closely related: a ten-fold reduction in amplitude corresponding to an approximately 18% increase in threshold. Threshold increases were greater for T-SICIs than for T-SICIp at 3.5-5 ms (P < 0.001). This divergence depended on the initial settings and whether ISIs were progressively increased or decreased, and was attributed to the limitations of the serial tracking protocol. SICI variability between subjects was greatest for T-SICIs estimates and least for A-SICI, and only A-SICI estimates revealed a significant decline in inhibition with age.

CONCLUSIONS

The serial tracking protocol did not accurately show the dependence of inhibition on ISI. Randomising ISIs gives corresponding SICI measures, whether tracking thresholds or measuring amplitude measurements. SICI variability suggested that A-SICI measurements may be the most sensitive to loss of inhibition.

Intracortical Circuits in the Contralesional Primary Motor Cortex in Patients With Chronic Stroke After Botulinum Toxin Type A Injection: Case Studies

Spasticity and motor recovery are both related to neural plasticity after stroke. A balance of activity in the primary motor cortex (M1) in both hemispheres is essential for functional recovery. In this study, we assessed the intracortical inhibitory and facilitatory circuits in the contralesional M1 area in four patients with severe upper limb spasticity after chronic stroke and treated with botulinum toxin-A (BoNT-A) injection and 12 weeks of upper limb rehabilitation. There was little to no change in the level of spasticity post-injection, and only one participant experienced a small improvement in arm function. All reported improvements in quality of life. However, the levels of intracortical inhibition and facilitation in the contralesional hemisphere were different at baseline for all four participants, and there was no clear pattern in the response to the intervention. Further investigation is needed to understand how BoNT-A injections affect inhibitory and facilitatory circuits in the contralesional hemisphere, the severity of spasticity, and functional improvement.

Single joint fatiguing exercise decreases long but not short-interval intracortical inhibition in older adults

Ageing is accompanied by neuromuscular changes which may alter fatigue in older adults. These changes may include changes in corticospinal excitatory and inhibitory processes. Previous research has suggested that single joint fatiguing exercise decreases short-(SICI) and long-(LICI) interval intracortical inhibition in young adults. However, this is yet to be established in older adults. In 19 young (23 ± 4 years) and 18 older (69 ± 5 years) adults, SICI (2 ms interstimulus interval; ISI) and LICI (100 ms ISI) were measured in a resting first dorsal interosseous (FDI) muscle using transcranial magnetic stimulation (TMS) before and after a 15 min sustained submaximal contraction at 25% of their maximum EMG. Subsequent ten 2-min contractions held at 25% EMG were also performed to sustain fatigue for a total of 30 min, while SICI and LICI were taken immediately after each contraction. There was no change in SICI post-fatiguing exercise compared to baseline in both young and older adults (P = 0.4). Although there was no change in LICI post-fatiguing exercise in younger adults (P = 1.0), LICI was attenuated in older adults immediately post-fatiguing exercise and remained attenuated post-fatigue (PF)1 and PF2 (P < 0.05). Contrary to previous studies, the lack of change in SICI and LICI in young adults following a sustained submaximal EMG contraction suggests that GABA modulation may be dependent on the type of fatiguing task performed. The reduction in LICI in older adults post-fatiguing exercise suggests an age-related decrease in GABA_B-mediated activity with sustained submaximal fatiguing exercise.

Motor neuroplasticity: A MEG-fMRI study of motor imagery and execution in healthy ageing

Age-related decline in motor function is associated with over-activation of the sensorimotor circuitry. Using a multimodal MEG-fMRI paradigm, we investigated whether this neural over-recruitment in old age would be related to changes in movement-related beta desynchronization (MRBD), a correlate of the inhibitory neurotransmitter γ-aminobutyric acid (GABA), and whether it would characterize compensatory recruitment or a reduction in neural specialization (dedifferentiation). We used MEG to assess age-related changes in beta band oscillations in primary motor cortices, fMRI to localize age-related changes in brain activity, and the Finger Configuration Task to measure task performance during overt and covert motor processing: motor execution (ME) and motor imagery (MI). The results are threefold: first, showing age-related neuroplasticity during ME of older adults, compared to young adults, as evidenced by increased MRBD in motor cortices and over-recruitment of sensorimotor areas; second, showing similar age-related neuroplastic changes during MI; and finally, showing signs of dedifferentiation during ME in older adults whose performance negatively correlated with connectivity to bilateral primary motor cortex. Together, these findings demonstrate that elevated MRBD levels, reflecting greater GABAergic inhibitory activity, and over-activation of the sensorimotor network during ME may not be compensatory, but rather might reflect an age-related decline of the quality of the underlying neural signal.

Alterations of hand sensorimotor function and cortical motor representations over the adult lifespan

Using a cross sectional design, we aimed to identify the effect of aging on sensorimotor function and cortical motor representations of two intrinsic hand muscles, as well as the course and timing of those changes. Furthermore, the link between cortical motor representations, sensorimotor function, and intracortical inhibition and facilitation was investigated. Seventy-seven participants over the full adult lifespan were enrolled. For the first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscle, cortical motor representations, GABAA-mediated short-interval intracortical inhibition (SICI), and glutamate-mediated intracortical facilitation (ICF) were assessed using transcranial magnetic stimulation over the dominant primary motor cortex. Additionally, participants' dexterity and force were measured. Linear, polynomial, and piecewise linear regression analyses were conducted to identify the course and timing of age-related differences. Our results demonstrated variation in sensorimotor function over the lifespan, with a marked decline starting around the mid-thirties. Furthermore, an age-related reduction in cortical motor representation volume and maximal MEP of the FDI, but not for ADM, was observed, occurring mainly until the mid-forties. Area of the cortical motor representation did not change with advancing age. Furthermore, cortical motor representations, sensorimotor function, and measures of intracortical inhibition and facilitation were not interrelated.

Age-Related Changes in the Plasticity of Neural Networks Assessed by Transcranial Magnetic Stimulation With Electromyography: A Systematic Review and Meta-Analysis

Objective: The excitability of cerebral cortical cells, neural pathway, and neural networks, as well as their plasticity, are key to our exploration of age-related changes in brain structure and function. The combination of transcranial magnetic stimulation (TMS) with electromyography (EMG) can be applied to the primary motor cortex; it activates the underlying neural group and passes through the corticospinal pathway, which can be quantified using EMG. This meta-analysis aimed to analyze changes in cortical excitability and plasticity in healthy elderly individuals vs. young individuals through TMS-EMG.

Methods: The Cochrane Library, Medline, and EMBASE databases were searched to identify eligible trials published from database inception to June 3, 2019. The Cochrane Risk of Bias Tool and improved Jadad scale were used to assess the methodological quality. A meta-analysis of the comparative effects was conducted using the Review Manager 5.3 software and Stata 14.0 software.

Results: The pooled results revealed that the resting motor threshold values in the elderly group were markedly higher than those reported in the young group (mean difference [MD]: −2.35; 95% confidence interval [CI]: −3.69 to −1.02]; p < (0.00001). The motor evoked potential amplitude significantly reduced in the elderly group vs. the young group (MD: 0.18; 95% CI: 0.09–0.27; p < 0.0001). Moreover, there was significantly longer motor evoked potential latency in the elderly group (MD: −1.07; 95% CI: −1.77 to −0.37]; p =(0.003). There was no significant difference observed in the active motor threshold between the elderly and young groups (MD: −1.52; 95% CI: −3.47 to −0.42]; p =(0.13). Meanwhile, only two studies reported the absence of adverse events.

Conclusion: We found that the excitability of the cerebral cortex declined in elderly individuals vs. young individuals. The findings of the present analysis should be considered with caution owing to the methodological limitations in the included trials. Additional high-quality studies are warranted to validate our findings.

Investigations on maladaptive plasticity in the sensorimotor cortex of unilateral upper limb CRPS I patients

BACKGROUND

Patients with a complex regional pain syndrome (CRPS) in the upper limb show a sensory and motor impairment of the hand. Decreased intra-cortical-inhibition (ICI) of the motor representation of the affected hand muscle and decreased somatosensory hand representation size were related to maladaptive plasticity.

OBJECTIVE

To achieve new insights about CRPS we examined whether these alterations were present in a single cohort.

METHODS

We used a multi-modal approach comprising behavioral testing, transcranial magnetic stimulation, and high resolution fMRI combined with a new analysis technique for improved neuronal specificity.

RESULTS

We found a decreased pinch-grip performance, two-point discrimination on the fingertips, ICI in the motor cortex, and representation size of the hand in Brodmann Area 3b (BA3b) in the somatosensory cortex. Our analysis further showed that correlations with ICI on the non-affected side were absent on the affected side.

CONCLUSIONS

This study is the first to gather behavioral, neurophysiologic and imaging measurements for one patient cohort and it therefore enables a comprehensive view of collapsed associations of function and representation focused on the hemisphere contralateral to the affected hand.

Neuromuscular adaptations to healthy aging

Even in the absence of disease or disability, aging is associated with marked physiological adaptations within the neuromuscular system. An ability to perform activities of daily living and maintain independence with advanced age is reliant on the health of the neuromuscular system. Hence, it is critical to elucidate the age-related adaptations that occur within the central nervous system and the associated muscles to design interventions to maintain or improve neuromuscular function in the elderly. This brief review focuses on the neural alterations observed at both spinal and supraspinal levels in healthy humans in their seventh decade and beyond. The topics addressed are motor unit loss and remodelling, neural drive, and responses to transcranial magnetic stimulation of the motor cortex.

Non-invasive Brain Stimulation: Probing Intracortical Circuits and Improving Cognition in the Aging Brain

The impact of cognitive aging on brain function and structure is complex, and the relationship between aging-related structural changes and cognitive function are not fully understood. Physiological and pathological changes to the aging brain are highly variable, making it difficult to estimate a cognitive trajectory with which to monitor the conversion to cognitive decline. Beyond the information on the structural and functional consequences of cognitive aging gained from brain imaging and neuropsychological studies, non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) can enable stimulation of the human brain in vivo, offering useful insights into the functional integrity of intracortical circuits using electrophysiology and neuromodulation. TMS measurements can be used to identify and monitor changes in cortical reactivity, the integrity of inhibitory and excitatory intracortical circuits, the mechanisms of long-term potentiation (LTP)/depression-like plasticity and central cholinergic function. Repetitive TMS and tDCS can be used to modulate neuronal excitability and enhance cortical function, and thus offer a potential means to slow or reverse cognitive decline. This review will summarize and critically appraise relevant literature regarding the use of TMS and tDCS to probe cortical areas affected by the aging brain, and as potential therapeutic tools to improve cognitive function in the aging population. Challenges arising from intra-individual differences, limited reproducibility, and methodological differences will be discussed.

GABA levels and measures of intracortical and interhemispheric excitability in healthy young and older adults: an MRS-TMS study

Edited magnetic resonance spectroscopy (MRS) and transcranial magnetic stimulation (TMS) have often been used to study the integrity of the GABAergic neurotransmission system in healthy aging. To investigate whether the measurement outcomes obtained with these 2 techniques are associated with each other in older human adults, gamma-aminobutyric acid (GABA) levels in the left sensorimotor cortex were assessed with edited MRS in 28 older (63-74 years) and 28 young adults (19-34 years). TMS at rest was then used to measure intracortical inhibition (short-interval intracortical inhibition/long-interval intracortical inhibition), intracortical facilitation, interhemispheric inhibition from left to right primary motor cortex (M1) and recruitment curves of left and right M1. Our observations showed that short-interval intracortical inhibition and long-interval intracortical inhibition in the left M1 were reduced in older adults, while GABA levels did not significantly differ between age groups. Furthermore, MRS-assessed GABA within left sensorimotor cortex was not correlated with TMS-assessed cortical excitability or inhibition. These observations suggest that healthy aging gives rise to altered inhibition at the postsynaptic receptor level, which does not seem to be associated with MRS-assessed GABA+ levels.

Cortical inhibition in symptomatic and remitted mania compared to healthy subjects: A cross-sectional study

OBJECTIVES

Transcranial magnetic stimulation (TMS)-derived cortical reactivity studies provide a unique opportunity to non-invasively study gamma amino butyric acid (GABA)-mediated inhibitory neurotransmission in bipolar disorder (BD). Earlier studies were conducted in smaller samples and on patients who were on medications that can potentially confound the results. We aimed to study short-interval (SICI) and long-interval intracortical inhibition (LICI) in medication-naïve/free symptomatic (manic) BD patients (n=39), first episode mania (FEM) patients who had recently (≤6 months) remitted with treatment (remitted FEM; n = 28) and healthy subjects (HSs; n = 45).

METHODS

Resting motor threshold (RMT), stimulation intensity to elicit a 1-mV motor evoked potential (MEP) (SI_1 mV ), SICI and LICI were measured in three groups using single- and paired-pulse TMS.

RESULTS

Motor thresholds were higher in the manic BD and HS groups compared to the remitted FEM group (P < .001). SICI was lower (P = .026) but LICI was higher (P = .044) in the manic BD and remitted FEM groups compared to the HS group.

CONCLUSIONS

Lower motor thresholds in remitted FEM perhaps reflect the effect of treatment, and could be studied as potential prognostic neuromarkers. Inverse findings for SICI (reduced) and LICI (increased) in BD indicate a possible differential involvement of the GABA_A and GABA_B subreceptor systems. These could be trait markers as they are impaired in both mania and euthymia.

Long-interval intracortical inhibition is asymmetric in young but not older adults

Aging is typically accompanied by a decline in manual dexterity and handedness; the dominant hand executes tasks of manual dexterity more quickly and accurately than the nondominant hand in younger adults, but this advantage typically declines with age. Age-related changes in intracortical inhibitory processes might play a role in the age-related decline in manual dexterity. Long-interval intracortical inhibition (LICI) is asymmetric in young adults, with more sensitive and more powerful LICI circuits in the dominant hemisphere than in the nondominant hemisphere. Here we investigated whether the hemispheric asymmetry in LICI in younger adults persists in healthy older adults. Paired-pulse transcranial magnetic stimulation was used to measure LICI in the dominant and nondominant hemispheres of younger and older adults; LICI stimulus-response curves were obtained by varying conditioning stimulus intensity at two different interstimulus intervals [100 ms (LICI₁₀₀) and 150 ms]. We have replicated the finding that LICI₁₀₀ circuits are more sensitive and more powerful in the dominant than the nondominant hemisphere of young adults and extend this finding to show that the hemispheric asymmetry in LICI₁₀₀ is lost with age. In the context of behavioral observations showing that dominant hand movements in younger adults are more fluent than nondominant hand movements in younger adults and dominant hand movements in older adults, we speculate a role of LICI₁₀₀ in the age-related decline in manual dexterity.NEW & NOTEWORTHY In younger adults, more sensitive and more powerful long-interval intracortical inhibitory circuits are evident in the hemisphere controlling the more dexterous hand; this is not the case in older adults, for whom long-interval intracortical inhibitory circuits are symmetric and more variable than in younger adults. We speculate that the highly sensitive and powerful long-interval intracortical inhibition circuits in the dominant hemisphere play a role in manual dexterity.

Reduced Prefrontal Short-Latency Afferent Inhibition in Older Adults and Its Relation to Executive Function: A TMS-EEG Study

Combining transcranial magnetic stimulation (TMS) with electroencephalography (EEG) allows for the assessment of various neurophysiological processes in the human cortex. One of these paradigms, short-latency afferent inhibition (SAI), is thought to be a sensitive measure of cholinergic activity. In a previous study, we demonstrated the temporal pattern of this paradigm from both the motor (M1) and dorsolateral prefrontal cortex (DLPFC) using simultaneous TMS-EEG recording. The SAI paradigm led to marked modulations at N100. In this study, we aimed to investigate the age-related effects on TMS-evoked potentials (TEPs) with the SAI from M1 and the DLPFC in younger (18-59 years old) and older (≥60 years old) participants. Older participants showed significantly lower N100 modulation in M1-SAI as well as DLPFC-SAI compared to the younger participants. Furthermore, the modulation of N100 by DLPFC-SAI in the older participants correlated with executive function as measured with the Trail making test. This paradigm has the potential to non-invasively identify cholinergic changes in cortical regions related to cognition in older participants.

GABA and primary motor cortex inhibition in young and older adults: a multimodal reliability study

The effects of healthy aging on γ-aminobutyric acid (GABA) within primary motor cortex (M1) remain poorly understood. Studies have reported contrasting results, potentially due to limitations with the common assessment technique. The aim of the present study was to investigate the effect of healthy aging on M1 GABA concentration and neurotransmission using a multimodal approach. Fifteen young and sixteen older adults participated in this study. Magnetic resonance spectroscopy (MRS) was used to measure M1 GABA concentration. Single-pulse and threshold-tracking paired-pulse transcranial magnetic stimulation (TMS) protocols were used to examine cortical silent period duration, short- and long-interval intracortical inhibition (SICI and LICI), and late cortical disinhibition (LCD). The reliability of TMS measures was examined with intraclass correlation coefficient analyses. SICI at 1 ms was reduced in older adults (15.13 ± 2.59%) compared with young (25.66 ± 1.44%; P = 0.002). However, there was no age-related effect for cortical silent period duration, SICI at 3 ms, LICI, or LCD (all P > 0.66). The intersession reliability of threshold-tracking measures was good to excellent for both young (range 0.75-0.96) and older adults (range 0.88-0.93). Our findings indicate that extrasynaptic inhibition may be reduced with advancing age, whereas GABA concentration and synaptic inhibition are maintained. Furthermore, MRS and threshold-tracking TMS provide valid and reliable assessment of M1 GABA concentration and neurotransmission, respectively, in young and older adults.NEW & NOTEWORTHY γ-Aminobutyric acid (GABA) in primary motor cortex was assessed in young and older adults using magnetic resonance spectroscopy and threshold-tracking paired-pulse transcranial magnetic stimulation. Older adults exhibited reduced extrasynaptic inhibition (short-interval intracortical inhibition at 1 ms) compared with young, whereas GABA concentration and synaptic inhibition were similar between age groups. We demonstrate that magnetic resonance spectroscopy and threshold-tracking provide valid and reliable assessments of primary motor cortex GABA concentration and neurotransmission, respectively.

Proactive modulation of long-interval intracortical inhibition during response inhibition

Daily activities often require sudden cancellation of preplanned movement, termed response inhibition. When only a subcomponent of a whole response must be suppressed (required here on Partial trials), the ensuing component is markedly delayed. The neural mechanisms underlying partial response inhibition remain unclear. We hypothesized that Partial trials would be associated with nonselective corticomotor suppression and that GABAB receptor-mediated inhibition within primary motor cortex might be responsible for the nonselective corticomotor suppression contributing to Partial trial response delays. Sixteen right-handed participants performed a bimanual anticipatory response inhibition task while single- and paired-pulse transcranial magnetic stimulation was delivered to elicit motor evoked potentials in the left first dorsal interosseous muscle. Lift times, amplitude of motor evoked potentials, and long-interval intracortical inhibition were examined across the different trial types (Go, Stop-Left, Stop-Right, Stop-Both). Go trials produced a tight distribution of lift times around the target, whereas those during Partial trials (Stop-Left and Stop-Right) were substantially delayed. The modulation of motor evoked potential amplitude during Stop-Right trials reflected anticipation, suppression, and subsequent reinitiation of movement. Importantly, suppression was present across all Stop trial types, indicative of a "default" nonselective inhibitory process. Compared with blocks containing only Go trials, inhibition increased when Stop trials were introduced but did not differ between trial types. The amount of inhibition was positively correlated with lift times during Stop-Right trials. Tonic levels of inhibition appear to be proactively modulated by task context and influence the speed at which unimanual responses occur after a nonselective "brake" is applied.

A meta-analysis of the effects of aging on motor cortex neurophysiology assessed by transcranial magnetic stimulation

OBJECTIVE

Transcranial magnetic stimulation (TMS) is a non-invasive tool used for studying cortical excitability and plasticity in the human brain. This review aims to quantitatively synthesize the literature on age-related differences in cortical excitability and plasticity, examined by TMS.

METHODS

A literature search was conducted using MEDLINE, Embase, and PsycINFO from 1980 to December 2015. We extracted studies with healthy old (50-89years) versus young (16-49years) individuals that utilized the following TMS measures: resting motor threshold (RMT), short-interval cortical inhibition (SICI), short-latency afferent inhibition (SAI), cortical silent period (CSP), intracortical facilitation (ICF), and paired associative stimulation (PAS).

RESULTS

We found a significant increase in RMT (g=0.414, 95% confidence interval (CI) [0.284, 0.544], p<0.001), a significant decrease in SAI (g=0.778, 95% CI [0.478, 1.078], p<0.001), and a trending decrease in LTP-like plasticity (g=-0.528, 95% CI [-1.157, 0.100] p<0.1) with age.

CONCLUSIONS

Our findings suggest an age-dependent reduction in cortical excitability and sensorimotor integration within the human motor cortex.

SIGNIFICANCE

Alterations in the ability to regulate cortical excitability, sensorimotor integration and plasticity may underlie several age-related motor deficits.

The effects of a protein enriched diet with lean red meat combined with a multi-modal exercise program on muscle and cognitive health and function in older adults: study protocol for a randomised controlled trial

Background

Age-related muscle wasting has been strongly implicated with falls and fractures in the elderly, but it has also been associated with cognitive decline and dementia. Progressive resistance training (PRT) and adequate dietary protein are recognised as important contributors to the maintenance of muscle health and function in older adults. However, both factors also have the potential to improve brain function and prevent cognitive decline via several pathways, including the regulation of various growth and neurotrophic factors [insulin-like growth factor-1 (IGF-1)]; brain-derived growth factor (BDNF)] and/or the modulation of systemic inflammation. The primary aim of this study is to investigate whether a modest increase in dietary protein achieved through the consumption of lean red meat three days per week, when combined with PRT, can enhance muscle mass, size and strength and cognitive function in community-dwelling older people.

Methods/Design

The study design is a 48-week randomised controlled trial consisting of a 24-week intervention with a 24-week follow-up. Men and women (n=152) aged 65 years and over residing in the community will be randomly allocated to: 1) PRT and provided with 220 g (raw weight) of lean red meat to be cooked and divided into two 80 g servings on each of the three days that they complete their exercise session, or 2) control PRT in which participants will be provided with and advised to consume ≥1 serving (~1/2 cup) of rice and/or pasta or 1 medium potato on each of the three training days. The primary outcome measures will be muscle mass, size and strength and cognitive function. Secondary outcomes will include changes in: muscle function, neural health (corticospinal excitability and inhibition and voluntary activation), serum IGF-1 and BDNF, adipokines and inflammatory markers, fat mass and inter-/intra-muscular fat, blood pressure, lipids and health-related quality of life. All outcome measures will be assessed at baseline and 24 weeks, with the exception of cognitive function and the various neurobiological and inflammatory markers which will also be assessed at week 12.

Discussion

The findings from this study will provide important new information on whether a modest increase in dietary protein achieved through the ingestion of lean red meat can enhance the effects of PRT on muscle mass, size and strength as well as cognitive function in community-dwelling older adults. If successful, the findings will form the basis for more precise exercise and nutrition guidelines for the management and prevention of age-related changes in muscle and neural health and cognitive function in the elderly.

Trial registration

Australian New Zealand Clinical Trials Registry: ACTRN12613001153707. Date registered 16^th October, 2013.

A Single Session of Repetitive Transcranial Magnetic Stimulation Over the Dorsolateral Prefrontal Cortex in Patients With Unresponsive Wakefulness Syndrome

Background. The level of consciousness is regulated by the activation of the frontal parietal network since a disruption within this interregional cortical connectivity is associated with a consciousness breakdown. Objective. The primary aim of the present study was to examine the feasibility and the safety of a single session of repetitive transcranial magnetic stimulation (rTMS), delivered over the dorsolateral prefrontal cortex (DLPFC), in patients with unresponsive wakefulness syndrome (UWS). As a secondary aim, we investigated the efficacy of rTMS over DLPFC on cortico-cortical connectivity as tested with paired-pulse and dual-coil TMS techniques. Methods. We enrolled 10 healthy and 10 postanoxic UWS subjects. After clinical assessment, subjects underwent a single/paired pulse TMS paradigm evaluating the inhibitory and facilitatory intracortical circuits. In addition, several interregional interactions between primary motor, dorsal and ventral premotor, supplementary motor, and posterior parietal cortex areas were investigated by means of dual-coil TMS technique. These parameters were evaluated at baseline, immediately after, and 60 minutes after a protocol of 10-Hz rTMS delivered at the level of the DLPFC. Results. rTMS over DLPFC did not cause any adverse events. We showed that rTMS over DLPFC did not induce, at group level, any clinical improvement or intra-/intercortical connectivity changes. Interestingly, in 3 patients rTMS induced a significant, although transient, clinical improvement associated with a short-lasting reshaping of brain connectivity. Conclusions. We demonstrated that a single session of 10-Hz rTMS over the right DLPFC may transiently improve consciousness and partially restore the connectivity within several cortical areas in some patients with UWS.