Motor cortical dis - inhibition with baroreceptor unloading 3 induced by orthostatic stress 4

19 Unloading of the baroreceptors due to orthostatic stress increases corticospinal excitability. The 20 purpose of this study was to examine the effects of baroreceptor unloading due to orthostatic 21 stress on intracortical excitatory and inhibitory pathways in the motor cortex. Using transcranial 22 magnetic stimulation, measures of intracortical excitability for a hand muscle were tested on two 23 days in healthy young adults. Lower body negative pressure (LBNP) of 40 mmHg was applied 24 during one of the days and not during the Control day. During application of LBNP, heart rate 25 and the low frequency component of heart rate variability increased, while mean arterial blood 26 pressure was maintained. In the resting state, LBNP decreased short-interval intracortical 27 inhibition (SICI) and had no effect on intracortical facilitation (ICF) or short-interval 28 intracortical facilitation (SICF) compared with the Control day. During isometric contraction, no 29 effects of LBNP were observed on tested measures of intracortical excitability including SICI, 30 SICF, and cortical silent period. It was concluded that baroreceptor unloading due to orthostatic 31 stress results in diminished intracortical inhibition, at least in the resting muscle. 32 33