Cortical excitability is not depressed in movement-modulated stretch response of human thumb flexor

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.