Transcranial Magnetic Stimulation (TMS) is a non-invasive procedure that uses magnetic fields to influence the activity of nerve cells in the brain. The technique works by applying a focused magnetic field to the scalp, which passes through the skull to stimulate the underlying brain tissue. This process modulates the electrical signaling of neurons, offering a way to adjust brain activity without surgery or medication.
Generating the Magnetic Pulse
The process begins with the TMS device, which uses an electromagnetic coil, often shaped like a figure-eight, placed against the patient’s scalp. A pulse generator delivers a high-intensity electrical current through this coil, creating a powerful, momentary magnetic field that lasts only a few hundred microseconds. This magnetic field is governed by electromagnetic induction and is comparable in strength to that of an MRI scanner. The magnetic field is able to pass through the skin, muscle, and skull without resistance or significant energy loss.
The figure-eight coil design focuses this magnetic energy. This shape causes the magnetic fields from the two loops to converge, creating a highly localized and concentrated field directly beneath the coil’s center. This precise focusing allows clinicians to target specific, small areas of the cerebral cortex.
Inducing Neuronal Activity
Once the focused magnetic field penetrates the skull, it reaches the brain tissue and generates a localized electrical current in the neurons. This transition from magnetic energy to electrical energy within the brain follows electromagnetic induction. This induced electrical current directly affects the nerve cells.
This induced electrical current causes the resting cell membranes of nearby neurons to depolarize, or fire, if the stimulus is strong enough. The stimulation primarily activates the axons of neurons, which are the signaling extensions. This rapid activation triggers an action potential, the electrical signal used for communication in the brain.
When TMS is applied repeatedly, known as repetitive TMS (rTMS), it causes long-lasting changes in brain function through neuroplasticity. Depending on the frequency of the magnetic pulses, rTMS can either increase or decrease the excitability of the targeted brain region. High-frequency pulses (e.g., 5 Hz or 10 Hz) increase neuronal activity, while low-frequency pulses (e.g., 1 Hz) decrease it.
These modulations leverage the brain’s natural mechanisms for strengthening or weakening synaptic connections, known as long-term potentiation (LTP) and long-term depression (LTD). By adjusting activity within specific neural circuits, rTMS aims to correct abnormal patterns of communication associated with various neurological and psychiatric conditions.
Clinical Application and Treatment Logistics
TMS has received regulatory approval for treating several conditions, including major depressive disorder, obsessive-compulsive disorder (OCD), and migraine headaches, especially when standard treatments have not been effective. The procedure offers a non-invasive alternative to medication or more invasive therapies.
For major depressive disorder, the magnetic coil is typically placed over the left dorsolateral prefrontal cortex (DLPFC). This area is chosen because it is involved in mood regulation and executive function, and depression often shows reduced activity there. Stimulating this region with high-frequency rTMS aims to increase its activity and alleviate depressive symptoms.
A standard course of treatment involves sessions performed daily, five days a week, lasting four to six weeks, totaling 20 to 30 sessions. Each session generally takes between 20 and 40 minutes, depending on the protocol used. Newer, faster protocols, such as intermittent theta-burst stimulation (iTBS), can significantly shorten the treatment time.
Patients remain fully awake and alert during the procedure and sit comfortably in a chair. The magnetic pulses are described as a tapping sensation on the scalp. While mild, temporary side effects like headache or scalp discomfort can occur, the treatment is well-tolerated and functions as a practical outpatient procedure.