Transcranial Magnetic Stimulation (TMS) is a non-invasive procedure that modulates brain activity, particularly for conditions like major depression and obsessive-compulsive disorder (OCD). This technique uses magnetic fields to gently influence nerve cells in targeted brain regions. The mechanism involves a precise physical process that leads to changes in neural function and long-term biological effects in the brain.
The Physics of Magnetic Induction
A TMS system relies on the principle of electromagnetic induction. It uses a specialized coil, acting as a powerful electromagnet, connected to a power source. When activated, a high-current, short-lasting electrical pulse is rapidly discharged into the coil. This intense flow of electrical energy generates a powerful, time-varying magnetic field around the coil.
The magnetic field can reach up to 2 or 3 Tesla, comparable to fields used in Magnetic Resonance Imaging (MRI). This field passes through non-conductive tissues, such as the scalp and the skull bone, without significant attenuation. The magnetic field itself does not directly stimulate the brain. The coil, often a figure-eight shape, is engineered to focus this magnetic field precisely on a small, localized area of the brain just beneath the scalp.
Converting Magnetic Fields to Electrical Impulses
The rapidly changing magnetic field that penetrates the skull makes Faraday’s Law of Induction biologically relevant. According to this law, a changing magnetic field induces a secondary electrical field in any nearby conductor. Brain tissue, containing electrically conductive neurons and fluid, acts as this conductor.
As the magnetic field collapses almost instantly, it generates a small, transient electrical current within the targeted brain region. This induced electrical current directly interacts with the neurons in the cerebral cortex. If the current is strong enough, it changes the electrical charge across the neuronal membrane, causing the neuron to depolarize. This depolarization triggers an action potential, the electrical signal that neurons use to communicate. By precisely controlling the magnetic pulse, this process can be used to either activate a population of neurons or, conversely, inhibit their activity.
Modulating Brain Networks and Neuroplasticity
The instantaneous firing of neurons from a single pulse does not account for the lasting therapeutic effects of the treatment. The true therapeutic benefit is achieved when the magnetic pulses are delivered repeatedly over a session, a method known as repetitive TMS (rTMS). The frequency of these repeated pulses is a defining factor in the resulting biological effect.
High-frequency rTMS (typically above 5 Hz) increases the excitability of the cortical tissue in the stimulated area. This effect is biologically similar to a process called long-term potentiation (LTP), which strengthens the connections between neurons. Conversely, low-frequency rTMS (usually at or below 1 Hz) decreases cortical excitability. This inhibitory effect resembles long-term depression (LTD), a mechanism that weakens synaptic connections.
By strategically applying high or low-frequency rTMS to specific dysfunctional brain regions, the treatment aims to rebalance or reorganize neural networks. For example, in depression, the left dorsolateral prefrontal cortex is often targeted with high-frequency stimulation to increase its activity. This sustained alteration in excitability is a form of neuroplasticity, the brain’s ability to reorganize itself by forming new synaptic connections. The cumulative effect of multiple rTMS sessions is the sustained modulation of these circuits, leading to lasting functional changes that improve symptoms in various neurological and psychiatric conditions.