Cj. Wallace et Ts. Miles, Cortical excitability is not depressed in movement-modulated stretch response of human thumb flexor, EXP BRAIN R, 139(4), 2001, pp. 448-453
There is strong evidence that the predominant pathway of the long-latency s
tretch reflex for flexor pollicis longus crosses the motor cortex. This ref
lex response is diminished during, active thumb movements. We tested the hy
pothesis that this could be due to a decrease in the excitability of the tr
anscortical component during movement. During isometric, concentric and ecc
entric thumb movements, transcranial magnetic stimulation (TMS) of the moto
r cortex was given at a time when the reflex signal was traversing the moto
r cortex. TMS was also given earlier in separate runs when the signal was t
raversing the spinal cord under each of the three contractile conditions. T
he electromyogram was analysed for non-linear summation between stretch res
ponses and the potential evoked by the cortical stimulus. The response to T
MS alone was uniform across the three types of contraction, and the lack of
cortical involvement in the short-latency reflex was confirmed. The TMS-ev
oked response summed in a non-linear manner with the long-latency reflex re
sponse, confirming that the excitability of the motor cortex was increased
as the reflex signal passed through it. The long-latency response was marke
dly depressed during isotonic compared with isometric contractions. However
, the non-linear summation was not greater during the isometric contraction
s. Thus, the depressed reflex responses during isotonic movements do not st
em from reduced motor cortical responsiveness or afferent input to the tran
scortical pathway, and may instead reflect modulation of cutaneous reflexes
during isotonic contractions.