Transcranial Magnetic Stimulation (TMS): Clinical Application and Efficacy

Mechanism of Action and Physiological Effects

TMS delivers brief, focal electromagnetic pulses through the skull to stimulate target brain regions, inducing neuronal firing beneath the coil positioned over the scalp 1
The technique modulates cortical excitability through long-term potentiation (LTP) and long-term depression (LTD)-like changes in synaptic coupling of neurons 2
Effects depend critically on NMDA and AMPA receptor signaling within glutamatergic synapses, with dopaminergic transmission also playing a significant role 3, 4
TMS modulates expression of neurotrophic factors like BDNF, a key regulator of synaptic plasticity, and may influence plasticity-related gene expression and neurogenesis 5, 4
High-frequency stimulation (10 Hz) induces LTP-like effects, while low-frequency (1 Hz) produces LTD-like effects 2

Target location: Primarily left dorsolateral prefrontal cortex (DLPFC) for depression 3, 6
Frequency: High-frequency protocols (5-25 Hz) excite cortical neurons; low-frequency (1 Hz) produces inhibitory effects 3, 6
Stimulation at 5 Hz is particularly effective for disrupting reaction time in cognitive tasks 8, 9
Doubling the number of pulses enhances clinical effects in depressed patients 7
At least 4-6 weeks of daily treatment is required for significant clinical improvement compared to sham 7

Common side effects include clicking sounds, scalp sensations, and mild muscle contractions during stimulation 10
Significantly fewer systemic side effects compared to antidepressants 11

Systematic studies comparing different repetition intervals, frequencies, and intensities are lacking 7, 3
The relationship between stimulation parameters and outcomes remains incompletely understood 7
Homeostatic plasticity principles mean that excessive stimulation can paradoxically reduce efficacy 7, 2
Many online TMS studies are underpowered, with median sample sizes of only 5 when adjusted for multiple conditions 8, 9