Transcranial Magnetic Stimulation (TMS) is a non-invasive procedure that uses magnetic fields to influence brain activity. A device placed on the scalp delivers magnetic pulses, which then modulate the function of nerve cells within the brain. TMS has gained recognition as a tool for studying and modulating brain function. It operates on the principle of electromagnetic induction.
Magnetic Pulse to Electrical Current
The fundamental step in how TMS operates involves converting an external magnetic pulse into an electrical current within the brain. A TMS device generates a rapidly changing electrical current within a coil placed against the scalp. This current creates a strong, time-varying magnetic field that passes unimpeded through the scalp and skull.
According to Faraday’s Law, a changing magnetic field induces an electrical current in nearby conductive material. In TMS, brain tissue acts as this conductor. The magnetic field induces a localized electric field and subsequent electrical current within the targeted brain region. This induced electrical current directly stimulates neural tissue. The strength of this induced electric field is highest near the cortex surface, decreasing with depth, typically reaching about 2 to 4 centimeters into the brain, directly beneath the coil.
Neuronal Activation and Inhibition
The induced electrical current directly influences neuron activity. Localized electrical currents depolarize neuron membranes. If this depolarization is sufficiently strong, it can trigger action potentials, the electrical impulses neurons use to communicate. TMS primarily stimulates myelinated axons within the targeted cortical area.
The effect of TMS on neuronal populations varies depending on stimulation parameters like intensity and frequency. High-frequency TMS (above 5 Hz) increases cortical excitability, making neurons more active. Conversely, low-frequency TMS (1 Hz or less) decreases neuronal excitability, leading to an inhibitory effect. This ability to either activate or suppress neural firing patterns allows TMS to modulate communication.
Shaping Brain Circuitry
Repeated TMS (rTMS) can lead to enduring changes in brain function through neuroplasticity. Neuroplasticity is the brain’s ability to reorganize itself by modifying neural connections. Sustained changes in neuronal activity, induced by rTMS, can strengthen or weaken connections between neurons, analogous to long-term potentiation (LTP) or long-term depression (LTD).
LTP involves a lasting increase in communication efficiency between neurons, often associated with learning and memory. Conversely, LTD represents a sustained reduction in synaptic strength, crucial for brain function and adaptation. These plastic changes, influenced by rTMS, reorganize neural networks, impacting communication pathways within the brain. This network reorganization contributes to the lasting effects observed following TMS interventions.
Precision Targeting within the Brain
TMS effectiveness relies on precisely targeting specific brain areas. The TMS coil’s design determines the shape and focality of the magnetic and induced electric fields. Different coil types create varying magnetic field patterns, influencing stimulation depth and spread. Figure-of-eight coils are common for their focal stimulation.
Neuroimaging techniques like MRI or fMRI guide precise coil placement over the scalp. These methods identify exact anatomical and functional brain regions, ensuring induced currents focus on intended neural circuits. This precision is important for modulating specific brain functions. Accurate coil positioning allows tailored modulation of neural activity in localized areas, central to how TMS affects brain function.