Differential effects of low-intensity motor cortical stimulation on the inspiratory activity in scalene muscles during voluntary and involuntary breathing

To assess the cortical contribution to breathing, low-intensity transcranial magnetic stimulation (TMS) was delivered over the motor cortex in 10 subjects during: (i) voluntary static inspiratory efforts, (ii) hypocapnic voluntary ventilation (end-tidal CO(2), 2.7±0.4% mean±SD), and (iii) hypercapnic involuntary ventilation (end-tidal CO(2), 6.0±0.7%). Electromyographic activity (EMG) was recorded from the scalene muscles (obligatory inspiratory muscles) and was significantly suppressed by TMS at short latency (17.2±1.7ms). The scalene EMG was reduced to 76±8% and 76±7% in voluntary breathing and the static inspiratory effort, respectively, but only to 91±10% during the involuntary ventilation, significantly less than during the two voluntary tasks (p<0.005). Thus, with differences in chemical drive to breathe, TMS shows differences in the cortical contribution to inspiratory activity in scalene muscles. Voluntary breathing showed larger suppression than involuntary breathing, when the suppression was marginal. The results strongly suggest that drive from fast-conducting corticospinal neurones contributes to inspiratory activity in scalenes during voluntary breathing but is not required during involuntary breathing.