Paired-Pulse TMS as a Window into Cortical Microcircuits: Mechanistic Insights and Translational Challenges

Transcranial magnetic stimulation (TMS) generates cortical electrical currents that, when applied to the primary motor cortex, depolarise neurons, producing motor evoked potentials. Rather than selectively interrogating isolated networks, paired-pulse TMS (PP-TMS) paradigms provide sensitivity to different overlapping inhibitory and facilitatory processes. Conventionally, short-interval Intracortical Inhibition has been assumed to reflect fast GABAA receptor-mediated transmission, long-interval intracortical inhibition to rely on slower GABAB receptor mechanisms, and intracortical facilitation on glutamatergic AMPA/NMDA receptor activity. Despite decades of use, clinical translation of these protocols is limited. Interpretation is constrained by reliance on in vivo and pharmacological models, which only partially capture human cortical microcircuitry. Crucially, the precise cellular and synaptic substrates underlying PP-TMS measures remain incompletely understood, limiting mechanistic insight. This review integrates evidence from human and experimental models to dissect neurotransmitter-specific and cellular mechanisms of PP-TMS. We examine these measures from complementary perspectives— postsynaptic receptor pharmacology, presynaptic vesicular and calcium dynamics, non-synaptic axonal excitability, spinal and subcortical integration, neuromodulatory control, and computational modelling—and integrate them within a translational, multi-level framework in which each macroscopic measure is treated as the net result of overlapping excitatory and inhibitory processes rather than the signature of a single cellular substrate. This “overlap” framework is used throughout to judge which cellular mechanisms can, and cannot, plausibly account for human short-interval intracortical inhibition, long-interval intracortical inhibition, intracortical facilitation, and short-interval intracortical facilitation, and to foreground current gaps, translational limitations, and priorities for clinical application.