Mapping motor cortex projections to single motor units in humans with transcranial magnetic stimulation

We devised a method to investigate the cortical organization of corticomoto neurons (CMs) to upper limb muscles. A spike-triggering technique was used, in which a tonically discharging single motor unit (SMU) triggered transcr anial magnetic stimulation (TMS) of motor cortex, and the probability of pr oducing short-latency discharges (primary excitatory responses [PERs]) was measured. PER probabilities were mapped in 34 SMUs, using a 16 cm(2) scalp grid with the central reference point having a probability of 0.5. Maps sho wed a single optimum point of scalp stimulation and significant decreases i n PER probability with shifts of 2 cm from this point, for all subjects. Th ese findings suggest that the colony of CMs projecting to an individual SMN is contained within a small volume of motor cortex. Changes in PER probabi lity with shifts in stimulation site may reflect the organization of other intracortical neurons mediating TMS activation of these CMs. (C) 2000 John Wiley & Sons, Inc.