COMPARISON OF ACTIVATION OF CORTICOSPINAL NEURONS AND SPINAL MOTOR-NEURONS BY MAGNETIC AND ELECTRICAL TRANSCRANIAL STIMULATION IN THE LUMBOSACRAL CORD OF THE ANESTHETIZED MONKEY

To illuminate the action of non-invasive stimuli on the human cerebral cortex, responses of corticospinal axons and of plantar alpha-motor n eurons following transcranial magnetic (TMS) and electrical stimulatio n (TES) were recorded in the lumbosacral cord in the anaesthetized mac aque monkey. A round coil was used for TMS, and the anode was located at the vertex for TES. The responses of 175 identified corticospinal a xons (conduction velocities of 24-95 m/s) were recorded from the later al corticospinal tract at the T-12-L-3 Spinal level. A single magnetic or electrical stimulus could evoke an early spike corresponding to th e direct (D) wave in surface recorded volleys and was termed a D respo nse. In the same axon, up to four further spikes, termed indirect (I) responses, could also be evoked At a given intensity of stimulation, D responses had clear thresholds and fixed latencies, whereas I respons es were labile in both respects. For TMS and TES, the thresholds of bo th D and I responses were inversely correlated with axonal conduction velocity. For TMS, fast conducting axons (>75 m/s) had lower threshold s for D responses, while more slowly conducting axons (<55 m/s) had lo wer thresholds for I responses. Very few of the axons with a conductio n velocity of <40 m/s (three out of 23) gave a D response to TMS. For TES, the majority of axons had lower thresholds for D responses or a s imilar threshold for both D and I responses. At threshold, the latenci es of D responses evoked by TMS and TES were consistent with activatio n within the cortex, while TES also excited some corticospinal axons d eep to the cortex. At 2.5 times threshold for the D response, TMS stil l excited axons mostly within the cortex, but with TES the site of act ivation shifted by as much as 65 mm below the cortex (mode 20 mm). Int racellular responses were recorded in 23 plantar alpha motor neurons s upplying intrinsic muscles of the foot All showed monosynaptic excitat ory post-synaptic potentials (EPSPs) to both TMS and TES with no signi ficant differences in the rise times of the evoked EPSPs. At threshold for a surface corticospinal volley, the average EPSP to TES began 0.5 ms earlier than that to TMS, and 2.0 ms earlier at 2.5 times this thr eshold The different sites of activation of corticospinal neurons by T MS and TES, as well as the different distribution of D and I responses that they evoke, may both contribute to the differences in the onset latencies of the EMG responses evoked by these methods in human subjec ts.