SUPPRESSION AND LONG-LATENCY EXCITATION OF SINGLE SPINAL MOTONEURONS BY TRANSCRANIAL MAGNETIC STIMULATION IN HEALTH, MULTIPLE-SCLEROSIS, AND STROKE

Whether or not suppression at the level of the spinal motoneuron plays a role in motor deficits such as central paresis is unknown. In this study suppression in the firing of tonically active low threshold sing le motoneurons following low intensity transcranial magnetic stimulati on is described in health and disease. Changes in firing probability i n the absence of an early excitatory response were studied in a total of 14 motor units from 4 healthy subjects, 5 patients with multiple sc lerosis, and 1 patient with stroke. Firing probability began to fall 1 8-59 ms after the stimulus and remained low for a period of 27-133 ms. There were no obvious differences between the three subject groups. T he change in firing probability was not associated with specific physi cal signs. Late rises in firing probability were seen in 7 of the 14 m otor units at latencies that were similar to the secondary peak which is known to occur with higher stimulus intensities. It is argued that the mechanism of partial suppression is not dependent on the full inte grity of the pyramidal tract and is likely to involve a transient with drawal of descending excitatory drive rather than an inhibitory postsy naptic potential at the spinal motoneuron. (C) 1994 John Wiley & Sons, Inc.