Modulation of corticospinal output in agonist and antagonist proximal arm muscles during motor preparation

Connecting the dots: harnessing dual-site transcranial magnetic stimulation to quantify the causal influence of medial frontal areas on the motor cortex

Dual-site transcranial magnetic stimulation has been widely employed to investigate the influence of cortical structures on the primary motor cortex. Here, we leveraged this technique to probe the causal influence of two key areas of the medial frontal cortex, namely the supplementary motor area and the medial orbitofrontal cortex, on primary motor cortex. We show that supplementary motor area stimulation facilitates primary motor cortex activity across short (6 and 8 ms) and long (12 ms) inter-stimulation intervals, putatively recruiting cortico-cortical and cortico-subcortico-cortical circuits, respectively. Crucially, magnetic resonance imaging revealed that this facilitatory effect depended on a key morphometric feature of supplementary motor area: individuals with larger supplementary motor area volumes exhibited more facilitation from supplementary motor area to primary motor cortex for both short and long inter-stimulation intervals. Notably, we also provide evidence that the facilitatory effect of supplementary motor area stimulation at short intervals is unlikely to arise from spinal interactions of volleys descending simultaneously from supplementary motor area and primary motor cortex. On the other hand, medial orbitofrontal cortex stimulation moderately suppressed primary motor cortex activity at both short and long intervals, irrespective of medial orbitofrontal cortex volume. These results suggest that dual-site transcranial magnetic stimulation is a fruitful approach to investigate the differential influence of supplementary motor area and medial orbitofrontal cortex on primary motor cortex activity, paving the way for the multimodal assessment of these fronto-motor circuits in health and disease.

Split-elbow sign in the PRO-ACT and Southern Italy ALS cohorts: a potential marker of disease severity and lower motor neuron involvement?

INTRODUCTION

Split phenomena in ALS refers to the preferential dysfunction of some groups of muscles over others. The split-elbow sign (SE) is characterized by the predominant weakness of the biceps compared to the triceps, but available results are conflicting.

OBJECTIVES

To evaluate the prevalence of the SE in two independent cohorts: the randomized controlled trial-based PRO-ACT cohort (n = 500) and a monocentric cohort of patients with ALS from Southern Italy (n = 144); to investigate the demographic and clinical variables associated with the SE sign.

METHODS

Wilcoxon signed-rank test was used to compare biceps with triceps power in the same limb measured by hand-held dynamometry in the PRO-ACT cohort and Medical Research Council (MRC) in our cohort. Each limb was considered independently and not paired within the same individual. The arm where the triceps was stronger than the biceps was defined SE + , whereas the arm where the biceps was stronger than the triceps was considered SE-. A backward stepwise multivariate logistic regression was used to analyze the relationship between clinical and demographic variables and SE. PENN Upper Motor Neuron and Devine scales were used to evaluate the different upper (UMN) and lower (LMN) motor neuron impairments between the SE + and SE- arms.

RESULTS

In both cohorts, the biceps were on average stronger than the triceps, and the SE sign was present in 41% of the PRO-ACT cohort and just 30% of the Southern Italy cohort. The multivariate logistic regression revealed that older age (OR: 1.45; p = 0.01), male gender (OR: 1.55; p = 0.002), spinal onset (OR: 1.59; p = 0.007), and higher disease severity (OR: 1.70; p = 0.001) were significant predictors of the SE sign in the PRO-ACT cohort. Conversely, in Southern Italy patients, only a lower ALSFRS-R score was a significant determinant of the SE (OR: 8.47; p = 0.008). Finally, SE + arms exhibited a significantly higher median Devine sub-score compared to SE- [1 vs 0, p = < 0.05], while arms SE- showed a significantly higher median PUMNS sub-score [2 vs 0; p = < 0.05)].

CONCLUSION

In our study, most patients with ALS do not show SE. Patients with SE are more likely older, males, with spinal onset, a higher degree of disease severity, and predominant and wider LMN impairment.

The split-elbow index: A biomarker of the split elbow sign in ALS

OBJECTIVE

The split elbow sign is a clinical feature of amyotrophic lateral sclerosis (ALS), characterised by preferential weakness of biceps brachii muscle compared to triceps. A novel neurophysiological index, termed the split elbow index (SEI), was developed to quantify the split-elbow sign, and assess its utility in ALS.

METHODS

Clinical and neurophysiological assessment was prospectively undertaken on 34 ALS patients and 32 ALS mimics. Compound muscle action potential (CMAP) amplitude was recorded from biceps brachii and triceps muscles from which the SEI was calculated using the following formula: SEI = CMAPamplitudeBICEPSBRACHII CMAPamplitudeTRICEPSBRACHII .

RESULTS

The split elbow sign was significantly more common in ALS patients when compared to ALS mimic patients (P < 0.05). The SEI was significantly reduced in ALS patients when compared to ALS mimics (P < 0.01). This reduction was evident in spinal and bulbar onset ALS. A SEI cut-off value of ≤0.62 exhibited a sensitivity of 71% and specificity of 61%.

CONCLUSIONS

The split elbow sign is significantly more common in ALS patients, and was supported by a reduction in the SEI.

SIGNIFICANCE

The SEI may be utilised as a surrogate biomarker of the split elbow sign in future ALS studies.

Exploring cortico-cortical interactions during action preparation by means of dual-coil transcranial magnetic stimulation: A systematic review

Action preparation is characterized by a set of complex and distributed processes that occur in multiple brain areas. Interestingly, dual-coil transcranial magnetic stimulation (TMS) is a relevant technique to probe effective connectivity between cortical areas, with a high temporal resolution. In the current systematic review, we aimed at providing a detailed picture of the cortico-cortical interactions underlying action preparation focusing on dual-coil TMS studies. We considered four theoretical processes (impulse control, action selection, movement initiation and action reprogramming) and one task modulator (movement complexity). The main findings highlight 1) the interplay between primary motor cortex (M1) and premotor, prefrontal and parietal cortices during action preparation, 2) the varying (facilitatory or inhibitory) cortico-cortical influence depending on the theoretical processes and the TMS timing, and 3) the key role of the supplementary motor area-M1 interactions that shape the preparation of simple and complex movements. These findings are of particular interest for clinical perspectives, with a need to better characterize functional connectivity deficiency in clinical population with altered action preparation.

Corticospinal excitability related to reciprocal muscles during the motor preparation period: effect of movement repetition

OBJECTIVES

Voluntary movements require a preparatory phase before action initiation to elaborate optimal motor commands. This study investigated the time course of change in corticospinal excitability related to each agonist or antagonist muscle during the motor preparation period and the influence of movement repetition on time course change in corticospinal excitability.

METHODS

The participants were instructed to perform wrist flexion as quickly as possible in response to the appearance of a red circle. One hundred fifty milliseconds after the presentation of the red circle cue, we delivered transcranial magnetic stimulation at the primary motor cortex related to the agonist flexor and the antagonist extensor carpi radialis muscles.

RESULTS

The motor evoked potential amplitudes for both the flexor and extensor carpi radialis during the motor preparation period which were regressed on the logarithm of time, and there were no significant differences in the slope of the logarithmic curves in the early half trials of movement repetition. However, the slope of the logarithmic curve of the flexor carpi radialis was steeper than that of the extensor carpi radialis in the late half trials of movement repetition.

CONCLUSION

These results implied that corticospinal excitability for agonist muscles during the motor preparation period is consistently modulated by movement repetition, whereas that for antagonist muscles is weakened by movement repetition.

Corticospinal excitability is modulated by temporal feedback gaps

The integration of sensorimotor information is important for accurate goal-directed movement and affects corticospinal excitability (CE). This study investigated CE during the motor preparation period in a goal-directed movement task with temporal feedback gaps. Each trial began with a pair of first-informative and second-response beeps presented successively as cues. Trials with temporal feedback gaps showed that virtual hand movements lagged 400 ms behind actual performed movements. The participants were instructed to prepare for movement in accordance with the first beep, start the movement upon hearing the second beep, and perform movements that were both fast and accurate to the virtual target. We delivered a single-pulse of transcranial magnetic stimulation to the first dorsal interosseous muscle 250 ms before the presentation of the response beep. Motor-evoked potential amplitudes with temporal feedback gaps were significantly higher than those without temporal feedback gaps. Moreover, motor-evoked potential amplitudes with temporal feedback gaps gradually decreased over the course of the trials, whereas those without temporal feedback gaps did not change. In summary, CE during the motor preparation period was increased by temporal feedback gaps, and this excitation decreased in accordance with adaptation to temporal feedback gaps.

Effect of movement-related pain on behaviour and corticospinal excitability changes associated with arm movement preparation

KEY POINTS

Experimental pain or its anticipation influence motor preparation processes as well as upcoming movement execution, but the underlying physiological mechanisms remain unknown. Our results showed that movement-related pain modulates corticospinal excitability during motor preparation. In accordance with the pain adaptation theory, corticospinal excitability was higher when the muscle has an antagonist (vs. an agonist) role for the upcoming movement associated with pain. Anticipation of movement-related pain also affects motor initiation and execution, with slower movement initiation (longer reaction times) and faster movement execution compared to movements that do not evoke pain. These results confirm the implementation of protective strategies during motor preparation known to be relevant for acute pain, but which may potentially have detrimental long-term consequences and lead to the development of chronic pain.

ABSTRACT

When a movement repeatedly generates pain, we anticipate movement-related pain and establish self-protective strategies during motor preparation, but the underlying mechanisms remains poorly understood. The current study investigated the effect of movement-related pain anticipation on the modulation of behaviour and corticospinal excitability during the preparation of arm movements. Participants completed an instructed-delay reaction-time (RT) task consisting of elbow flexions and extensions instructed by visual cues. Nociceptive laser stimulations (unconditioned stimuli) were applied to the lateral epicondyle during movement execution in a specific direction (CS+) but not in the other (CS-), depending on experimental group. During motor preparation, transcranial magnetic stimulation was used to measure corticospinal excitability in the biceps brachii (BB). RT and peak end-point velocity were also measured. Neurophysiological results revealed an opposite modulation of corticospinal excitability in BB depending on whether it plays an agonist (i.e. flexion) or antagonist (i.e. extension) role for the CS+ movements (P < 0.001). Moreover, behavioural results showed that for the CS+ movements RT did not change relative to baseline, whereas the CS- movements were initiated more quickly (P = 0.023) and the CS+ flexion movements were faster relative to the CS- flexion movements (P < 0.001). This is consistent with the pain adaptation theory which proposes that in order to protect the body from further pain, agonist muscle activity is reduced and antagonist muscle activity is increased. If these strategies are initially relevant and lead to short-term pain alleviation, they may potentially have detrimental long-term consequences and lead to the development of chronic pain.

Muscle coactivation: definitions, mechanisms, and functions

The phenomenon of agonist-antagonist muscle coactivation is discussed with respect to its consequences for movement mechanics (such as increasing joint apparent stiffness, facilitating faster movements, and effects on action stability), implication for movement optimization, and involvement of different neurophysiological structures. Effects of coactivation on movement stability are ambiguous and depend on the effector representing a kinematic chain with a fixed origin or free origin. Furthermore, coactivation is discussed within the framework of the equilibrium-point hypothesis and the idea of hierarchical control with spatial referent coordinates. Relations of muscle coactivation to changes in one of the basic commands, the c-command, are discussed and illustrated. A hypothesis is suggested that agonist-antagonist coactivation reflects a deliberate neural control strategy to preserve effector-level control and avoid making it degenerate and facing the necessity to control at the level of signals to individual muscles. This strategy, in particular, allows stabilizing motor actions by covaried adjustments in spaces of control variables. This hypothesis is able to account for higher levels of coactivation in young healthy persons performing challenging tasks and across various populations with movement impairments.