Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive neuromodulation technique used for the treat-ment of a great variety of neurological disorders. The technique involves applying a magnetic field in certain areas of the cerebral cortex in order to modify neuronal excitability outside the skull.^[1]

Technical principle of TMS

Basic circuit of transcranial magnetic stimulation

The capacitor is first charged to a high voltage and then is discharged into the inductor (the stimulation coil) when the switch is closed. Additional components are needed to shape monophasic TMS pulses and to stop biphasic TMS pulses after a single cycle.

The resulting pulse waveforms are determined by the resonant frequency of the capacitor plus coil. These waveforms fall into two categories: (a) monophasic and (b) biphasic. “Monophasic” magnetic pulses have the general shape illustrated in FIGURE 2A, with a large initial peak and a much smaller, longer-lasting tail that presumably has no biologic effect. Biphasic pulses form a cosine wave, as shown in FIGURE 2B, with the later peaks smaller than the first because of energy losses in the circuitry.

Working principle of transcranial magnetic stimulation

Electromagnetic induction and electromagnetic conversion are the basic principles of transcranial magnetic stimulation. The pulse current passing through the TMS stimulation coil generates a strong transient magnetic field. The changing magnetic field induces an inductive electric field in nearby conductors, including human tissues and brain neurons, and then generates an inductive current. The biological effects of magnetic field stimulation are all generated by the stimulation of this inductive electric field and inductive current on the nerves.

The fundamental principles of electromagnetic induction are that an electric current produces a magnetic field and a changing magnetic field induces a flow of electric current in nearby conductors— including human tissue. A static magnetic field, however large, produces no current in the brain. Thus, the key parameter for a magnetic stimulator is not the absolute magnetic field strength in teslas but its first time derivative, dB/dt, combined with the duration of the stimulating pulse.

Mechanism of transcranial magnetic stimulation

TMS is an external cause of non-invasive nerve stimulation, which must play its role through the inner part of cortical neurons. It is generally believed that the mechanism of TMS is to affect the information processing process of the nervous system, including the excitation, inhibition and plasticity of synapses. For more than 30 years from animal experiments and clinical applications, TMS has reached a consensus that high frequency stimulation can cause long-term potentiation (LTP) like increase in nerve excitability in cortex, and low frequency stimulation can cause long-term inhibition (LTD) like decrease in excitability.

Neuromodulation mechanism

rTMS holds promise in the treatment of a variety of neurological disorders, perhaps through the induction of LTP and LTD-like plasticity. Advances in technology, allowing reliable, safe stimulation of deep structures and careful individual tailoring of stimulation sites and protocols using neuro-navigated TMS, will only expand this potential.^[4]

Multiple mechanisms interact

Through the combined use with PET, MRI and electroencephalogram (EEG), it is found that TMS not only has the stimulation effect on the local cortex, but also has a remote effect through the neural network connection of the stimulation area, causing changes in neurotransmitters, hormones, brain-derived neurotrophic factors, blood flow and metabolism, changes in the basic activity frequency and resonance frequency of brain waves, and modulating brain function through a variety of mechanisms.

Reference

[1] Pernia, Alberto M., Zorzo, C&ela., Prieto, Miguel J., Martinez, Juan A.,  & Higarza, Sara G.. (2020). Equipment for Repetitive Transcranial Magnetic Stimulation. IEEE transactions on biomedical circuits and systems, 14(3), 525-534.
[2]Figure1&2 by Charles M. Epstein, Roberto Michelucci, Mark Hallett Transcranial Magnetic Stimulation.Retrieved from https://neupsykey.com/transcranial-magnetic-stimulation/
[3]Funke K. 2018. Transcranial magnetic stimulation of rodents: repetitive transcranial magnetic stimulation—a noninva-sive way to induce neural plasticity in vivo and in vitro. Handbook of Behavioral Neuroscience 28:365–87.
[4]Bliss, Timothy V P.,  & Cooke, Sam F.. (2011). Long-term potentiation and long-term depression: a clinical perspective. Clinics (Sao Paulo, Brazil), 66 Suppl 1, 3-17.