DIFFERENCES IN THE EFFECT OF SENSORY AFFERENT INHIBITION ON OUTPUT MECHANISMS OF AGONIST AND ANTAGONIST MUSCLES

K. Sugawara¹, Y. Takenaka², H. Matsumoto², T. Suzuki¹

¹Kanagawa University of Human Services, Physical Therapy, Yokosuka, Japan, ²Kanagawa University of Human Services, Ph.D. Course, Yokosuka, Japan

Background: Short-latency afferent inhibition (SAI) is a phenomenon in which the motor cortex excitability decreases when the electrical stimulation given to peripheral nerves precedes transcranial magnetic stimulation (TMS) at intervals of -20 to 28ms. Here, we examined whether contraction of either agonist or antagonist muscles plays a role in the effect of SAI on motor cortex excitability (M1). Furthermore, we analyzed their functional relationship.

Purpose: To understand the motor control mechanisms of SAI in relation to agonist and antagonist muscles.

Methods: A total of 27 healthy participants were included in this study. They were divided in two groups for experiment 1 (n = 13) and experiment 2 (n = 14). All participants were right-handed with no history of neurological or psychiatric disease and were screened for any contraindications of TMS. Electrical stimulation for SAI was applied to the right wrist joint for 20ms; the intensity was 1.1 times the muscle contraction threshold. Subsequently, a TMS pulse of 1.2 times the motor evoked potential (MEP) threshold was applied to the motor cortex of the right first dorsal interosseous (FDI). SAI was measured in two different background conditions: during constant muscle output (Experiment 1) and before contraction onset at the electromyographic reaction time (Experiment 2). In each condition (Experiment 1 and 2), we observed the differences between the effects of SAI on the agonist and antagonist muscles innervated by the FDI.

Results: Experiment 1: MEP results showed that the effect of SAI decreased significantly with higher agonist muscle output during abduction in the M1 region innervating the FDI (P<0.05). However, SAI increased significantly in the M1 region during adduction as the antagonist muscle output increased (P<0.05).
Experiment 2: SAI decreased significantly 30ms prior to the onset of abduction (P<0.05). Contrastingly, SAI showed no change before the onset of adduction, regardless of the difference in the reaction times.

Conclusions: Even when the stimulus input is the same, the input effect can be modified by differences in the direction of the muscle's functional activity or the muscle performance. Namely, we identified the existence of a mechanism that modifies the input effect on the muscle functional action according to the output force.

Implications: It is important to analyze the effect of SAI decrease or increase during motor output performance in patients with motor disabilities to better understand the pathological conditions caused by central nervous system disorders.

Funding acknowledgements: This work was supported by the Japan Society for the Promotion of Science KAKENHI Program [Grant Number 20K11287].

Keywords:
Motor evoked potential
Short-latency afferent inhibition
Motor control

Topics:
Neurology

Did this work require ethics approval? Yes

Institution: Kanagawa University of Human Services

Committee: The Office of Research Ethics at Kanagawa University of Human Services

Ethics number: No.7-20-57