How does TMS therapy work in the brain?
The biophysics and neurobiology of magnetic neuromodulation: from electromagnetic pulses to synaptic neuroplasticity. TMS is offered at our Orland Park office.
Key Clinical Takeaways
- TMS uses Faraday's principle of electromagnetic induction to generate localized cortical micro-currents.
- For depression, it usually targets the left dorsolateral prefrontal cortex (DLPFC), an area that brain-imaging research has linked with reduced activity in depression.
- Repeated sessions are thought to promote neuroplasticity, changing how mood-related brain circuits communicate; this is still being studied.
- Researchers think the effects may spread to deeper, connected brain regions involved in emotion, such as the anterior cingulate cortex.
The Physics of Magnetic Induction
To understand Transcranial Magnetic Stimulation, it helps to review basic physics. In 1831, Michael Faraday discovered electromagnetic induction: a rapidly changing magnetic field induces an electrical current in a nearby conductor. A brief electrical current passing through a wire coil creates such a changing magnetic field, and when that field reaches conductive tissue, such as the brain, it induces a small, localized electrical current.
In TMS, brief electrical pulses flow through a magnetic coil held against the head (many devices use a figure-8 shaped coil). The resulting magnetic pulses pass through the hair, scalp, and skull without the need for electrodes, surgery, or anesthesia. In the cortex, the outer layer of the brain beneath the coil, the induced current can cause nerve cells to fire.
Why the Left Dorsolateral Prefrontal Cortex?
Brain-imaging studies using PET and functional MRI have linked major depressive episodes with reduced activity in the left DLPFC, although findings vary from person to person. This region is involved in executive functions such as working memory, planning, and regulating emotional responses.
One leading theory is that reduced prefrontal activity weakens the brain's regulation of deeper emotional (limbic) regions, such as the amygdala, which may contribute to symptoms like rumination, anhedonia (loss of pleasure or interest), difficulty concentrating, and fatigue. Many standard TMS protocols for depression stimulate the left DLPFC with high-frequency (often 10 Hz) pulses, which is thought to help restore more balanced activity between the prefrontal cortex and these deeper regions.
Neuroplasticity: Why Repeated Sessions Matter
A single TMS session produces short-lived changes in brain activity. Repeated sessions over several weeks are thought to produce longer-lasting changes through processes similar to Long-Term Potentiation (LTP), a strengthening of the connections between nerve cells.
Research also suggests a role for Brain-Derived Neurotrophic Factor (BDNF), a protein that supports the growth and maintenance of connections between nerve cells. These mechanisms are still being studied. How long improvement lasts after a course of TMS varies from person to person, and some people need additional treatment later.