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

We devised a method to investigate the cortical organization of corticomotoneurons (CMs) to upper limb muscles. A spike-triggering technique was used, in which a tonically discharging single motor unit (SMU) triggered transcranial magnetic stimulation (TMS) of motor cortex, and the probability of producing 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 showed a single optimum point of scalp stimulation and significant decreases in PER probability with shifts of 2 cm from this point, for all subjects. These findings suggest that the colony of CMs projecting to an individual SMN is contained within a small volume of motor cortex. Changes in PER probability with shifts in stimulation site may reflect the organization of other intracortical neurons mediating TMS activation of these CMs.