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Spinal μ‐opioid receptor‐sensitive lower limb muscle afferents determine corticospinal responsiveness and promote central fatigue in upper limb muscle
Author(s) -
Sidhu Simranjit K.,
Weavil Joshua C.,
Venturelli Massimo,
Garten Ryan S.,
Rossman Matthew J.,
Richardson Russell S.,
Gmelch Benjamin S.,
Morgan David E.,
Amann Markus
Publication year - 2014
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2014.275438
Subject(s) - transcranial magnetic stimulation , medicine , muscle fatigue , silent period , upper limb , physical medicine and rehabilitation , elbow , stimulation , muscle contraction , electromyography , motor unit recruitment , anesthesia , anatomy
Key pointsWe aimed to elucidate the role of group III/IV locomotor muscle afferents in the development of central fatigue and the responsiveness of the corticospinal tract in relation to an unexercised arm muscle. Intrathecal fentanyl, a μ‐opioid receptor agonist, was employed to attenuate afferent feedback from the leg muscles during intense cycling exercise characterized by either no or severe peripheral locomotor muscle fatigue. In the absence of locomotor muscle fatigue, group III/IV‐mediated leg afferent feedback facilitates the responsiveness of the motor pathway to upper limb flexor muscles. By contrast, in the presence of leg fatigue, group III/IV locomotor muscle afferents facilitate supraspinal fatigue in a remote muscle not involved in the exercise and disfacilitate the responsiveness of associated corticospinal projections.Abstract We investigated the influence of group III/IV lower limb muscle afferents on the development of supraspinal fatigue and the responsiveness of corticospinal projections to an arm muscle. Eight males performed constant‐load leg cycling exercise (80% peak power output) for 30 s (non‐fatiguing) and to exhaustion (∼9 min; fatiguing) both under control conditions and with lumbar intrathecal fentanyl impairing feedback from μ‐opioid receptor‐sensitive lower limb muscle afferents. Voluntary activation (VA) of elbow flexors was assessed via transcranial magnetic stimulation (TMS) during maximum voluntary contraction (MVC) and corticospinal responsiveness was monitored via TMS‐evoked potentials (MEPs) during a 25% MVC. Accompanied by a significant 5 ± 1% reduction in VA from pre‐ to post‐exercise, elbow flexor MVC progressively decreased during the fatiguing trial ( P < 0.05). By contrast, with attenuated feedback from locomotor muscle afferents, MVC and VA remained unchanged during fatiguing exercise ( P > 0.3). MEPs decreased by 36 ± 6% ( P < 0.05) from the start of exercise to exhaustion under control conditions, but this reduction was prevented with fentanyl blockade. Furthermore, fentanyl blockade prevented the significant increase in elbow flexor MEP observed from rest to non‐fatiguing exercise under control conditions and resulted in a 14% lower corticospinal responsiveness during this short bout ( P < 0.05). Taken together, in the absence of locomotor muscle fatigue, group III/IV‐mediated leg muscle afferents facilitate responsiveness of the motor pathway to upper limb flexor muscles. By contrast, in the presence of cycling‐induced leg fatigue, group III/IV locomotor muscle afferents facilitate supraspinal fatigue in remote muscle not involved in the exercise and disfacilitate, or inhibit, the responsiveness of corticospinal projections to upper limb muscles.