Transcranial Magnetic Stimulation (TMS) is a non-invasive brain stimulation technique that has its roots in fundamental principles of physics, particularly Faraday’s Law of Electromagnetic Induction. This principle is central to how TMS therapy works, and it allows us as physicians to influence brain activity in targeted areas, without the need for invasive procedures.
Faraday’s Law of Electromagnetic Induction
Faraday’s Law of Induction, formulated in 1831, explains that a changing magnetic field induces an electric current in a conductor. This principle underpins the mechanism of TMS, where a rapidly changing magnetic field produced by the TMS coil induces small electrical currents in the brain. These electrical currents can activate neurons and stimulate neural circuits. This process is especially effective because neural tissue is electromagnetically sensitive—neurons rely on ion exchange to generate electrical signals, making them responsive to external electromagnetic forces.
The key point here is that the magnetic field changes must happen over time to induce current. This is why, during a TMS session, rapid pulses of magnetic fields are used to induce the necessary electrical activity in the brain. This aligns with Faraday’s principle that the induced electromotive force (EMF) is proportional to the rate of change of the magnetic flux through the circuit.
This foundational understanding allows us to use TMS as a tool to influence brain activity non-invasively, whether for treating depression, stimulating cognitive functions, or assessing motor pathways.
How TMS Stimulates the Brain
In TMS therapy, the changing magnetic field is generated by a coil placed near the scalp, and the “conductor” in this case is the neural tissue in the brain. Neural tissue is electromagnetically sensitive because neurons rely on the flow of ions (charged particles) to transmit electrical signals. By applying a rapidly changing magnetic field, TMS induces small electrical currents in the brain, influencing the activity of neurons in targeted regions.
The TMS coils used at Brain Aid Clinics are usually speaker-like (rather than mounted into a skull-cap), and generate brief, focused magnetic pulses. These pulses pass through the skull and induce electrical currents in the brain’s neurons. These are the nerve cells of the brain responsible for receiving input and send out information in the form of the electrical signals. Since neural tissue is highly responsive to electrical stimulation, the induced currents can either excite or inhibit neuronal activity depending on the frequency and intensity of the pulses.
When a magnetic field is applied to the brain, it causes the ions inside neurons to move, which creates an electrical potential. This change in electrical potential can trigger action potentials (nerve impulses) in neurons, which either become more likely to fire (excitatory effects) or less likely (inhibitory effects). This can be manipulated depending on the location and settings of the TMS coil, and the indication for treatment.
Neural Tissue Sensitivity to Electromagnetic Fields
Neurons in the brain communicate via electrical impulses, or action potentials, which are generated by the movement of ions across cell membranes. Because of this reliance on electrical signalling, neural tissue is particularly sensitive to changes in electromagnetic fields. TMS takes advantage of this sensitivity by using electromagnetic pulses to stimulate neurons in specific areas of the brain. In contrast, medications like antidepressants (eg SSRIs or Selective Serotonin Reuptake Inhibitors), operate by changing the chemical environment around neurons, which in turn affects electrical impulses or action potentials indirectly.
Different brain regions are associated with distinct functions—some areas control mood, while others are involved in memory, attention, or motor control. By adjusting the position of the TMS coil and the parameters of the magnetic pulse, TMS therapy can selectively target areas of the brain involved in specific mental health or cognitive functions. This ability to precisely influence brain activity makes TMS a versatile tool for treating a range of conditions, from depression to OCD to cognitive therapy (memory disorders, such as Alzheimers, or ADHD, both of which remains off-label or experimental in Australia).
TMS and Neuroplasticity
The brain’s ability to adapt to new experiences, environments, and injuries is known as neuroplasticity. TMS stimulates the growth and reorganisation of neural connections, an important step in neuroplasticity. In the context of depression treatment and anxiety therapy, where certain neural circuits may become dysfunctional. By stimulating these areas with TMS, it is possible to encourage the brain to “rewire” itself, leading to long-term improvements in symptoms.
The Role of Frequency in TMS
The effects of transcranial magnetic stimulation depend largely on the frequency of the magnetic pulses. Low-frequency TMS (around 1 Hz) tends to have an inhibitory effect on neuronal activity, which can be useful for conditions like anxiety, where overactivity in certain brain regions may contribute to symptoms. High-frequency TMS (typically 10-20 Hz), on the other hand, is excitatory and is often used in depression treatment to stimulate under-active regions of the brain, such as the prefrontal cortex.
These varying effects make TMS a flexible tool that can be adjusted to meet the specific needs of each patient, depending on the condition being treated and the brain region being targeted.
TMS vs. Electrical Stimulation
While both TMS and electrical stimulation (such as electroconvulsive therapy, or ECT) work by influencing brain’s electrical activity, TMS has a few key advantages. Most importantly, TMS does not require the direct application of electrical currents to the brain. Instead, it uses magnetic fields to induce currents indirectly, which makes the treatment non-invasive and reduces the risk of side effects like memory loss, which can occur with ECT.
Because TMS can target specific areas of the brain without affecting other regions, it is a more precise treatment option compared to methods that rely on direct electrical stimulation.
Peer-reviewed papers
Here are some key peer-reviewed papers that delve into the mechanisms of Transcranial Magnetic Stimulation (TMS), how it works on the brain, and its use in neurostimulation for evaluating motor and spinal function:
Barker AT, Jalinous R, Freeston IL. Non-invasive magnetic stimulation of human motor cortex. Lancet. 1985 May 11;1(8437):1106-7. doi: 10.1016/s0140-6736(85)92413-4. PMID: 2860322.
Overview: This foundational paper details the first successful application of TMS in stimulating the motor cortex. The study introduced the idea of using magnetic fields to non-invasively stimulate brain regions and map motor pathways.
Takeaway: It demonstrated that TMS could be used to elicit motor responses in peripheral muscles, helping to map the cortical areas responsible for movement. This paper laid the groundwork for using TMS to assess motor function and its clinical applications in diagnosing motor system disorders.
More: Meyer BU, Benecke R, Dressler D, Haug B, Conrad B. Fraktionierte Bestimmung zentraler motorischer Leitungszeiten mittels Reizung von Kortex, spinalen Bahnen und Spinalnervenwurzeln: Möglichkeiten und Grenzen [Fractionated determination of central motor conduction times using stimulation of the cortex, spinal tract and spinal nerve roots: possibilities and limits]. EEG EMG Z Elektroenzephalogr Elektromyogr Verwandte Geb. 1988 Dec;19(4):234-40. German. PMID: 2850151.
Rossini PM, Rossi S. Clinical applications of motor evoked potentials. Electroencephalogr Clin Neurophysiol. 1998 Mar;106(3):180-94. doi: 10.1016/s0013-4694(97)00097-7. PMID: 9743275.
Overview: This paper explores the use of TMS in assessing motor function by eliciting motor evoked potentials (MEPs) from muscle responses. It focuses on the clinical applications of MEPs in evaluating motor pathways in diseases like multiple sclerosis and spinal cord injuries. It looks at transcranial stimulation of non-motor brain areas for the evaluation of lateralized hemispheric properties connected with higher cortical functions, and for the treatment of psychiatric disorders.
Takeaway: TMS-induced MEPs are described as a way to assess the integrity of the motor system and spinal cord, providing a quantitative measure of neural conduction and motor output. This is crucial in both diagnostic and rehabilitation settings for conditions affecting motor function.
More: Rossini PM, Caramia MD. Central conduction studies and magnetic stimulation. Curr Opin Neurol Neurosurg. 1992 Oct;5(5):697-703. PMID: 1327307.
Hallett M. Transcranial magnetic stimulation and the human brain. Nature. 2000 Jul 13;406(6792):147-50. doi: 10.1038/35018000. PMID: 10910346.
Overview: This seminal paper provides a comprehensive review of how TMS works on the brain and its potential applications in neuroscience. It details how TMS can modulate brain activity, especially in motor areas, by inducing electric currents in the cortical tissue. It explores how TMS is used to map motor cortex function and can assess motor pathways by producing motor-evoked potentials (MEPs).
Takeaway: TMS is highlighted as a non-invasive tool for studying brain connectivity and motor function, with applications in both clinical and research settings.
More: Hallett M. Transcranial magnetic stimulation: a primer. Neuron. 2007 Jul 19;55(2):187-99. doi: 10.1016/j.neuron.2007.06.026. PMID: 17640522.
Kobayashi M, Pascual-Leone A. Transcranial magnetic stimulation in neurology. Lancet Neurol. 2003 Mar;2(3):145-56. doi: 10.1016/s1474-4422(03)00321-1. PMID: 12849236.
Overview: This review paper examines the use of TMS in both research and clinical neurology. It details how TMS can be used to assess the function of the motor system, its ability to induce plastic changes in the brain, and its therapeutic potential in conditions like stroke, Parkinson’s disease, and multiple sclerosis.
Takeaway: The paper highlights the clinical applications of TMS in assessing motor pathways and its potential to improve recovery in neurodegenerative and motor system diseases. It discusses the use of TMS in tracking motor recovery after spinal cord injuries or strokes.
Di Lazzaro V, Rothwell JC. Corticospinal activity evoked and modulated by non-invasive stimulation of the intact human motor cortex. J Physiol. 2014 Oct 1;592(19):4115-28. doi: 10.1113/jphysiol.2014.274316. Epub 2014 Aug 28. PMID: 25172954; PMCID: PMC4215763.
Overview: This paper focuses on how TMS can assess corticospinal excitability and synaptic plasticity in the human brain. It discusses the neurophysiological mechanisms behind TMS-induced changes in brain activity and how this relates to motor control.
Takeaway: TMS is a valid tool for measuring the excitability of the corticospinal tract, which is critical for motor control: “it is potentially possible to test and condition specific neural circuits in motor cortex that could be affected differentially by disease, or be used in different forms of natural behaviour”. It explores the role of TMS in modulating synaptic plasticity, which is essential for both learning and recovery after neural injury.
Ridding, M. C., & Rothwell, J. C. (2007). “Is There a Future for Therapeutic Use of Transcranial Magnetic Stimulation?” Nature Reviews Neuroscience, 8(7), 559-567.
Overview: This paper reviews the therapeutic potential of TMS, including its role in inducing plasticity in motor and cognitive circuits. It explores the ability of TMS to facilitate motor recovery and assess functional integrity in patients with neurological disorders.
Takeaway: The authors discuss how TMS can be used to enhance motor function through long-term potentiation-like effects in the brain, making it a promising tool for rehabilitation in motor dysfunction and spinal cord injury cases.
Ziemann, U., & Siebner, H. R. (2008). “Modifying Motor Learning Through GABAergic Inhibition in the Human Motor Cortex.”Journal of Neuroscience, 28(37), 9213-9220.
Overview: This paper looks at the effects of TMS on motor learning and motor cortex plasticity. It specifically examines how TMS can modulate inhibitory and excitatory mechanisms in the brain, particularly through GABAergic (inhibitory) pathways.
Takeaway: The paper discusses how TMS can enhance or inhibit motor learning and control, depending on the stimulation protocol used. This has important implications for rehabilitation in spinal injuries and neurodegenerative diseases affecting motor function.
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Other references:
Faraday:
https://www.livescience.com/53509-faradays-law-induction.html
These papers provide a strong foundation for understanding how **TMS therapy** works on the brain, particularly in the context of neurostimulation and assessing motor and spinal function. Each study highlights different aspects of how TMS influences cortical excitability, motor pathways, and neural plasticity, offering insights into its clinical applications and future potential in neurology and rehabilitation.