TY - UNPB

T1 - Multi-pulse transcranial magnetic stimulation of human motor cortex produces short-latency corticomotor facilitation via two distinct mechanisms

AU - Kesselheim, Janine

AU - Takemi, Mitsuaki

AU - Christiansen, Lasse

AU - Karabanov, Anke Ninija

AU - Siebner, Hartwig Roman

N1 - bioRxiv preprint posted February 22, 2022.

PY - 2022/2/22

Y1 - 2022/2/22

N2 - Background: Single-pulse transcranial magnetic stimulation of the precentral hand representation (M1HAND) can elicit indirect waves in the corticospinal tract at a periodicity of ~660 Hz, called indirect or I-waves. These synchronized descending volleys are produced by transsynaptic excitation of fastconducting monosynaptic corticospinal axons in M1-HAND. Paired-pulse TMS can induce short-interval intracortical facilitation (SICF) of motor evoked potentials (MEPs) at inter-pulse intervals that match I-wave periodicity.Objective: To examine whether short-latency corticospinal facilitation engages additional mechanisms independently of I-wave periodicity.Methods: In 19 volunteers, one to four biphasic TMS pulses were applied to left M1-HAND with interpulse interval was adjusted to the first peak or first trough of the individual SICF curve. TMS was applied at different intensities to probe the intensity-response relationship.Results: Pairs, triplets, or quadruplets at individual peak-latency facilitated MEP amplitudes across a wide range of TMS intensities compared to single pulses. Multi-pulse TMSHAND at individual trough-latency also produced a consistent facilitation of MEP amplitude. Short-latency facilitation at trough-latency was less pronounced than short-latency facilitation at peak-latency, but the relative difference in facilitation decreased with increasing stimulus intensity. Increasing the number of pulses from two to four pulses had only a modest effect on MEP facilitation.Conclusion: Two mechanisms underly short-latency corticomotor facilitation caused by biphasic multi-pulse TMS. An intracortical mechanism is related to I-wave periodicity and engages fast-conducting direct projections to spinal motoneurons. A second corticospinal mechanism does not rely on I-wave rhythmicity and may be mediated by slower conducting indirect pyramidal tract projections from M1-HAND to spinal interneurons. The latter mechanism deserves more attention in TMS studies of the corticomotor system.

AB - Background: Single-pulse transcranial magnetic stimulation of the precentral hand representation (M1HAND) can elicit indirect waves in the corticospinal tract at a periodicity of ~660 Hz, called indirect or I-waves. These synchronized descending volleys are produced by transsynaptic excitation of fastconducting monosynaptic corticospinal axons in M1-HAND. Paired-pulse TMS can induce short-interval intracortical facilitation (SICF) of motor evoked potentials (MEPs) at inter-pulse intervals that match I-wave periodicity.Objective: To examine whether short-latency corticospinal facilitation engages additional mechanisms independently of I-wave periodicity.Methods: In 19 volunteers, one to four biphasic TMS pulses were applied to left M1-HAND with interpulse interval was adjusted to the first peak or first trough of the individual SICF curve. TMS was applied at different intensities to probe the intensity-response relationship.Results: Pairs, triplets, or quadruplets at individual peak-latency facilitated MEP amplitudes across a wide range of TMS intensities compared to single pulses. Multi-pulse TMSHAND at individual trough-latency also produced a consistent facilitation of MEP amplitude. Short-latency facilitation at trough-latency was less pronounced than short-latency facilitation at peak-latency, but the relative difference in facilitation decreased with increasing stimulus intensity. Increasing the number of pulses from two to four pulses had only a modest effect on MEP facilitation.Conclusion: Two mechanisms underly short-latency corticomotor facilitation caused by biphasic multi-pulse TMS. An intracortical mechanism is related to I-wave periodicity and engages fast-conducting direct projections to spinal motoneurons. A second corticospinal mechanism does not rely on I-wave rhythmicity and may be mediated by slower conducting indirect pyramidal tract projections from M1-HAND to spinal interneurons. The latter mechanism deserves more attention in TMS studies of the corticomotor system.

U2 - 10.1101/2022.02.19.481138

DO - 10.1101/2022.02.19.481138

M3 - Preprint

BT - Multi-pulse transcranial magnetic stimulation of human motor cortex produces short-latency corticomotor facilitation via two distinct mechanisms

PB - bioRxiv

ER -