Abstract
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.
Highlights
Short-latency afferent inhibition (SAI) occurs when a single transcranial magnetic stimulation (TMS) pulse over the primary motor cortex (M1) is preceded by peripheral electrical nerve stimulation at a short inter-stimulus interval (i.e.,20–28 ms) such that the corticospinal output to the targeted hand muscle is reduced [1,2]
SAI may contribute to another circuit known as surround inhibition that is important to performing individual finger movement
Surround inhibition in the motor system may be a mechanism that allows for precise selective movements by enhancing neural activity for muscles performing a task, while inhibiting neural activity for those muscles uninvolved in the task
Summary
Short-latency afferent inhibition (SAI) occurs when a single transcranial magnetic stimulation (TMS) pulse over the primary motor cortex (M1) is preceded by peripheral electrical nerve stimulation at a short inter-stimulus interval (i.e., ,20–28 ms) such that the corticospinal output to the targeted hand muscle is reduced [1,2]. Surround inhibition is a powerful neurophysiological mechanism that focuses neural activity by inhibiting areas surrounding the intended neural response. This mechanism has been observed in the visual [3], somatosensory [4], and motor systems [5]. Surround inhibition in the motor system may be a mechanism that allows for precise selective movements by enhancing neural activity for muscles performing a task, while inhibiting neural activity for those muscles uninvolved in the task. It may be that the SAI circuit is involved in focussing neural activity differently depending on the specific digit that is or is uninvolved in the task How does SAI act before EMG onset and before movement initiation? It may be that the SAI circuit is involved in focussing neural activity differently depending on the specific digit that is or is uninvolved in the task
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