Transcranial Magnetic Stimulation (TMS) is a non-invasive procedure that applies magnetic fields to stimulate specific parts of the brain. It is a therapeutic technique for various psychiatric and neurological conditions, most notably major depressive disorder. During a session, a specialized coil is placed against the scalp to deliver targeted magnetic pulses. This process modulates nerve cell activity in precise regions of the brain associated with mood regulation.
Inducing Electrical Currents in the Brain
The TMS process begins with a device that generates focused magnetic pulses. An electromagnetic coil, often in a figure-of-eight shape for focal stimulation, is held against the head over a predetermined area. When activated, a rapidly changing electrical current flows through the coil, creating a powerful magnetic field perpendicular to the coil itself that can reach 2 to 3 teslas. This magnetic field passes unimpeded through the scalp and skull.
Based on Faraday’s law of electromagnetic induction, the changing magnetic field creates an electrical current within the conductive brain tissue below. The magnetic pulses induce small, precise electrical currents localized to a depth of about 2 to 3 centimeters under the coil. The orientation of the coil is also a factor, as different current directions can target distinct populations of neurons.
Activating Individual Neurons
The electrical current induced by TMS directly influences neurons in the targeted cortical region. Neurons have a natural electrical balance called the resting membrane potential. The TMS-induced current disrupts this balance by causing a rapid influx of positive ions, a process known as depolarization.
If this depolarization passes a specific voltage threshold, it triggers an action potential—an all-or-nothing electrical signal that travels down the neuron’s axon. A single TMS pulse can cause populations of neurons to fire, while even weaker currents can alter a neuron’s excitability and influence its response to other brain signals.
Driving Long-Term Neuroplasticity
The therapeutic effects of TMS are achieved through repeated sessions, known as repetitive TMS (rTMS), which produce durable changes in brain function via neuroplasticity. Neuroplasticity is the brain’s ability to reorganize its structure and connections in response to experience. The frequency of the magnetic pulses determines the nature of these long-term changes.
High-frequency rTMS (10 Hz or higher) can induce long-term potentiation (LTP). This process strengthens synaptic connections between neurons, making their communication more efficient. Conversely, low-frequency rTMS (1 Hz or less) can lead to long-term depression (LTD), which weakens synaptic connections to decrease activity in overactive brain regions. These lasting changes in synaptic strength allow the effects of TMS to persist after treatment ends.
Altering Brain Networks and Chemical Signals
Changes in synaptic strength from rTMS in a targeted area can have ripple effects across an entire neural network. For example, in depression treatment, TMS often targets the dorsolateral prefrontal cortex (DLPFC). Altering the activity of this hub in the brain’s mood-regulating circuits can influence the entire network, helping to restore healthier patterns of activity. This stimulation propagates through neural pathways, affecting distant but connected brain structures.
This altered network activity also affects the brain’s chemical signaling systems. Modulating neuronal activity with TMS can influence the release of neurotransmitters like serotonin, dopamine, and glutamate in connected regions. This change in the brain’s chemical environment contributes to the therapeutic outcomes, linking the electrical effects of TMS to the chemical basis of mood regulation.