Objective: To investigate cortical excitability and surround cortical inhibition during movement compared with postural control task in healthy subjects.

Background: Motor control has traditionally been divided into the pyramidal and the extrapyramidal systems. The pyramidal system is conceptualized as the corticospinal pathways, thought to be responsible for fine motor and goal-directed movements, whereas the extrapyramidal system is responsible for the speed, selection, amplitude of movement and posture1. More recently separate systems within M1 for postural and movement control were discovered in healthy primate2.

Methods: Short interval cortical inhibition (SICI), resting motor threshold (RMT) and cortical silent period (CSP) are measures derived from motor evoked potentials measured over the resting abductor pollicis brevis muscle (APB) of the pronated hand. We used threshold tracking technique and transcranial magnetic stimulation(TMS) over M1, recording over the contralateral resting abductor pollicis brevis muscle (APB) of the pronated hand: while at rest, performance of a postural control task (constant extension of ipsilateral extensor indicis (EI)) and movement control task(activation of EI at 1Hz) in healthy subjects. A separate channel monitored APB muscle activity ensured its resting state during data acquisition. Studies were undertaken on 17 healthy controls were recruited (13 women; 4 men; age 35±1.5). All results are expressed as mean±SEM.

Results: SICI, between ISI 1 to 7ms, was significantly higher during the postural task (1.6±3.4%) when compared to the movement control task (-3.5±5.6%, p=0.03). In addition, cortical silent period during movement (151.0±13.7ms) was lower than the postural task (172.8±9.7ms, p=0.04).  Of further relevance, resting motor threshold was significantly lower during movement control (39.3±7.2%) than the postural task (43.7±7.8%, p=0.01). MEP amplitude was higher at rest (1.9±0.3mV,p=0.03) than during postural task  (1.3±0.8mV) or movement task (1.8±1.3mV,p=0.07). There was no difference between postural and movement control task MEP amplitude. 

Conclusions: Voluntary limb movement is associated with motor cortical disinhibition, contrasting with limb posture. Taken together, the present finding supports the notion that distinct motor control systems mediate movement and postural tasks.

References: [1]       Kurtzer, I.; Herter, T. M.; Scott, S. H., Random change in cortical load representation suggests distinct control of posture and movement. Nat Neurosci 2005, 8 (4), 498-504.

[2]       Hallett, M.; Khoshbin, S., A physiological mechanism of bradykinesia. Brain 1980, 103 (2), 301-14.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/cortical-excitability-during-a-movement-compared-with-postural-control-task-in-healthy-subjects/