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Received July 9, 2004
Revised July 14, 2004
Accepted after revision July 20, 2004
1 Royal Brompton Hospital
2 Royal Brompton and Harefield NHS Trust
3 Hôpital Raymond Poincaré
4 King's College Hospital
* To whom correspondence should be addressed. E-mail: m.polkey{at}rbh.nthames.nhs.uk.
Functional imaging studies in normal humans have shown that the supplementary motor area (SMA) and the primary motor cortex (PMC) are co-activated 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 characterised by a higher diaphragm motor threshold, a greater proportional increase in MEP amplitude with voluntary facilitation and a shorter silent period. Stimulus response curves did not differ significantly different 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 diaphragm. 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.
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