Rate-coding of spinal motoneurons with high-frequency magnetic stimulation of human motor cortex

Rate-coding in spinal motoneurons was studied using high-frequency magnetic stimulation of the human motor cortex. The subject made a weak contraction to cause rhythmic (i.e., tonic) discharge of a single motor unit in flexor (or extensor) carpi radialis or tibialis anterior, while the motor cortical representation of that muscle was stimulated with brief trains of pulses from a Pyramid stimulator (4 Magstim units connected by 3 BiStim modules). An "m@n" stimulus train consisted of m number of pulses (1-4), with an interpulse interval (IPI) of n ms (1-6). Peristimulus time histograms were constructed for each stimulus condition of a given motor unit, and related to the average rectified surface electromyography (EMG) from that muscle. Surface EMG responses showed markedly more facilitation than single-pulse stimulation, with increasing numbers of pulses in the train; responses also tended to increase in magnitude for the longer IPI values (4 and 6 ms) tested. Motor-unit response probability increased in a manner comparable to that of surface EMG. In particular, motoneurons frequently responded twice to a given stimulus train. In addition to recruitment of new motor units, the increased surface EMG responses were, in part, a direct consequence of short-term rate-coding within the tonically discharging motoneuron. Our results suggest that human corticomotoneurons are capable of reliably following high-frequency magnetic stimulation rates, and that this activity pattern is carried over to the spinal motoneuron, enabling it to discharge at extremely high rates for brief periods of time, a pattern known to be optimal for force generation at the onset of a muscle contraction.