What is the mechanical function of a transcranial magnetic stimulation (TMS) machine?

Mechanical Function of a TMS Machine

A TMS machine operates through electromagnetic induction: a brief, high-intensity electrical current passes through a magnetic coil positioned over the scalp, generating a rapidly changing magnetic field that penetrates the skull and induces an electrical current in underlying brain tissue, causing neuronal depolarization and firing. 1

Core Physical Mechanism

The fundamental physics underlying TMS involves Faraday's law of electromagnetic induction 2, 3:

• A capacitor discharges a brief pulse of electrical current (typically lasting microseconds) through a wire coil 4, 3
• This current generates a magnetic field perpendicular to the coil that reaches peak intensities of approximately 1-2 Tesla 2
• The rapidly changing magnetic field penetrates the scalp and skull without attenuation 5, 4
• This time-varying magnetic field induces a secondary electrical current in the conductive brain tissue beneath the coil 1, 2
• The induced electrical current is strong enough to depolarize neuronal membranes and trigger action potentials 1

Coil Design and Spatial Targeting

The geometry of the magnetic coil critically determines the spatial distribution and focality of stimulation 1:

• Figure-8 coils (two circular windings joined at their centers) produce a focal electric field concentrated beneath the central junction, allowing selective targeting of specific brain regions 1
• Circular coils generate a non-focal electric field distributed under the coil's perimeter, making them less capable of precise regional targeting 1
• The induced electric field strength decreases with distance from the coil, limiting effective stimulation depth to approximately 2-3 cm below the skull surface 2

Stimulation Patterns and Neurophysiological Effects

TMS can be delivered in different temporal patterns that produce distinct neurophysiological effects 1:

• Single-pulse TMS (spTMS) delivers isolated pulses used to measure cortical excitability and motor thresholds 1
• Paired-pulse TMS (PP-TMS) applies two pulses with variable inter-pulse intervals to assess intracortical inhibitory and facilitatory mechanisms 1, 5
• Repetitive TMS (rTMS) delivers continuous trains of pulses at specific frequencies to induce lasting changes in cortical excitability 1, 6
• High-frequency rTMS (≥5 Hz) facilitates cortical excitability through LTP-like mechanisms 7, 6
• Low-frequency rTMS (≤1 Hz) inhibits cortical excitability through LTD-like mechanisms 7, 6
• Theta-burst stimulation (TBS) uses patterned pulse delivery with shorter stimulation duration than conventional rTMS 1, 6

Amplitude Dosing and Individualization

Stimulation intensity is a critical parameter affecting both the depth and focality of neuronal activation 1:

• Motor threshold (MT) represents the lowest stimulation intensity that reliably induces a motor evoked potential (MEP) ≥50 μV in at least 50% of trials when stimulating the motor cortex 1
• Resting motor threshold (rMT) is measured with the target muscle at rest 1
• Active motor threshold (aMT) is measured during voluntary muscle contraction at 20% maximum strength, typically requiring MEP ≥200 μV 1
• Phosphene threshold (PT) is the lowest intensity inducing visual phosphenes in at least 5 of 10 pulses when stimulating visual cortex 1
• Subsequent therapeutic stimulation is typically dosed as a percentage of the individual's MT (e.g., 80%, 110%, 120% of rMT) 1
• Fixed intensity dosing uses a percentage of maximum stimulator output (MSO) without individualization 1

Biological Mechanism of Action

While the electromagnetic physics are well-understood, the downstream neurobiological mechanisms remain incompletely characterized 8, 3:

• TMS modulates cortical excitability through long-term potentiation (LTP) and long-term depression (LTD)-like changes in synaptic coupling 7
• Effects depend on NMDA and AMPA receptor signaling within glutamatergic synapses, with dopaminergic transmission also contributing 7
• TMS influences expression of brain-derived neurotrophic factor (BDNF), a key regulator of synaptic plasticity 7
• Repetitive stimulation may affect plasticity-related gene expression and neurogenesis 7, 5

Critical Technical Considerations

Several factors affect the precision and reproducibility of TMS effects 1, 6:

• Coil-to-cortex distance varies with individual skull thickness and scalp anatomy, affecting stimulation intensity 1
• Individual cortical excitability varies substantially between subjects 1
• Targeting accuracy depends on the localization method: scalp-based measurements (e.g., EEG coordinates) are less accurate than neuroimaging-guided approaches 1, 6
• fMRI-guided targeting is associated with increased degree of disruption compared to scalp-based targeting 6

Common Pitfalls

The relationship between stimulation parameters and behavioral/clinical outcomes remains incompletely understood despite decades of research 7:

• Systematic studies comparing different frequencies, intensities, and repetition intervals are lacking 7
• Homeostatic plasticity principles mean excessive stimulation can paradoxically reduce efficacy 7
• Motor and phosphene thresholds may not correlate, suggesting MT may be inappropriate for guiding amplitude selection in non-motor brain regions 1