Functional imaging studies in normal humans have shown that the supplementary motor area (SMA) and the primary motor cortex (PMC) are coactivated during various breathing tasks. It is not known whether a direct pathway from the SMA to the diaphragm exists, and if so what properties it has. Using transcranial magnetic stimulation (TMS) a site at the vertex, representing the diaphragm primary motor cortex, has been identified. TMS mapping revealed a second area 3 cm anterior to the vertex overlying the SMA, which had a rapidly conducting pathway to the diaphragm (mean latency 16.7 ± 2.4 ms). In comparison to the vertex, the anterior position was characterized by a higher diaphragm motor threshold, a greater proportional increase in motor-evoked potential (MEP) amplitude with voluntary facilitation and a shorter silent period. Stimulus–response curves did not differ significantly between the vertex and anterior positions. Using paired TMS, we also compared intracortical inhibition/facilitation (ICI/ICF) curves. In comparison to the vertex, the MEP elicited from the anterior position was not inhibited at short interstimulus intervals (1–5 ms) and was more facilitated at long interstimulus intervals (9–20 ms). The patterns of response were identical for the costal and crural diaphragms. We conclude that the two coil positions represent discrete areas that are likely to be the PMC and SMA, with the latter wielding a more excitatory effect on the diaphragm.
