TMS delivers MRI strength (1.5 Tesla) magnetic energy to the region of the brain that is targeted. The magnetic energy acts directly only at a small area of the cortical surface.
The target is selected based on the condition being treated. In the case of MDD, this will be the left Dorsolateral Prefrontal Cortex (DLPFC).
In depression, the left DLPFC is under-active as seen on both EEG and fMRI studies. Deeper structures are also underactive.
The fluctuating magnetic field induces action potentials along the axons of the underlying neurons. This leads not only to immediate release of neurotransmitters but also to adjustments in neurotransmitter receptor site densities.
Since the DLPFC has many neuronal connections to the ipsilateral and contralateral limbic and hippocampal structures, simultaneous alterations in brain activity occur in these deeper structures with each treatment. TMS leads to alterations in these functions over time as long term potentiation occurs.