Commentary on “Facilitation of the Lesioned Motor Cortex During Tonic Contraction of the Unaffected Limb Corresponds to Motor Status After Stroke”

2016 ◽  
Vol 40 (1) ◽  
pp. 22-23
Author(s):  
Kelly P. Westlake ◽  
Brian P. Johnson
Keyword(s):  
Motor Cortex ◽  
Tonic Contraction

Corticospinal Excitability Is Lower During Rhythmic Arm Movement Than During Tonic Contraction

2006 ◽  
Vol 95 (2) ◽  
pp. 914-921 ◽  
Author(s):  
Timothy J. Carroll ◽  
Evan R. L. Baldwin ◽  
David F. Collins ◽  
E. Paul Zehr
Keyword(s):  
Motor Cortex ◽  
Neural Control ◽  
Motor Evoked Potentials ◽  
Arm Movement ◽  
Voluntary Contraction ◽  
Tonic Contraction ◽  
H Reflex ◽  
Corticospinal Excitability ◽  
Arm Cycling ◽  
Subcortical Regions

Humans perform rhythmic, locomotor movements with the arms and legs every day. Studies using reflexes to probe the functional role of the CNS suggest that spinal circuits are an important part of the neural control system for rhythmic arm cycling and walking. Here, by studying motor-evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and H-reflexes induced by electrical stimulation of peripheral nerves, we show a reduction in corticospinal excitability during rhythmic arm movement compared with tonic, voluntary contraction. Responses were compared between arm cycling and tonic contraction at four positions, while participants generated similar levels of muscle activity. Both H-reflexes and MEPs were significantly smaller during arm cycling than during tonic contraction at the midpoint of arm flexion ( F = 13.51, P = 0.006; F = 11.83, P = 0.009). Subthreshold TMS significantly facilitated the FCR H-reflex during tonic contractions, but did not significantly modulate H-reflex amplitude during arm cycling. The data indicate a reduction in the responsiveness of cells constituting the fast, monosynaptic, corticospinal pathway during arm cycling and suggest that the motor cortex may contribute less to motor drive during rhythmic arm movement than during tonic, voluntary contraction. Our results are consistent with the idea that subcortical regions contribute to the control of rhythmic arm movements despite highly developed corticospinal projections to the human upper limb.

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