Transcranial magnetic stimulation (TMS) represents one of the most significant advances in clinical neuroscience and brain research since its introduction by Barker and colleagues in 1985. This non-invasive technique uses rapidly changing magnetic fields to induce electrical currents in neural tissue, providing a powerful tool for both investigating brain function and treating neurological and psychiatric disorders. TMS can excite or inhibit neural activity, modulate brain networks, and induce lasting neuroplastic changes that underlie its therapeutic effects. [@barker1985]
Transcranial magnetic stimulation (TMS) represents one of the most significant advances in clinical neuroscience and brain research since its introduction by Barker and colleagues in 1985. This non-invasive technique uses rapidly changing magnetic fields to induce electrical currents in neural tissue, providing a powerful tool for both investigating brain function and treating neurological and psychiatric disorders. TMS can excite or inhibit neural activity, modulate brain networks, and induce lasting neuroplastic changes that underlie its therapeutic effects. [@barker1985]
The neuronal effects of TMS are complex, involving direct activation of specific neural elements as well as network-level effects transmitted through synaptic connections. Understanding which neuronal populations are affected by TMS, and through what mechanisms, is essential for optimizing stimulation protocols and developing new applications. This knowledge also informs the interpretation of TMS studies and helps explain both therapeutic benefits and potential adverse effects. [@hallett2000]
TMS operates through electromagnetic induction, the same principle underlying transformers and electric generators. A pulse of electric current flowing through a coil placed on the scalp generates a rapidly changing magnetic field (reaching peak intensities of 1-2 Tesla within microseconds). This magnetic field penetrates the scalp and skull with minimal attenuation and induces secondary electric fields in the underlying brain tissue according to Faraday's law. These induced electric fields can depolarize neuronal membranes and trigger action potentials when they exceed threshold. [@rothwell1997]
The geometry of the stimulation coil determines the shape and intensity of the induced electric field. Circular coils produce a relatively diffuse field affecting a larger area, while figure-eight coils provide more focal stimulation with the highest field intensity at the junction of the two wing segments. Double-cone coils are designed to stimulate deeper structures at the cost of reduced focality. Advanced coil designs continue to improve the ability to target specific brain regions while minimizing effects on adjacent areas. [@valero-cabre2021]
The induced electric field in brain tissue preferentially activates certain neural elements based on their orientation, geometry, and position relative to the field. Large pyramidal neurons with their long vertical axes oriented perpendicular to the brain surface are particularly susceptible to TMS because the induced electric field is oriented parallel to the neuronal axis. This explains why cortical pyramidal cells in layer V, which give rise to the corticospinal tract, are among the most directly affected elements. [@pashut2011]
The threshold for activation depends on neuronal size, geometry, and membrane properties. Larger neurons require less current to reach threshold, and those oriented parallel to the induced electric field are more efficiently activated. The orientation of the coil relative to the brain surface can be adjusted to preferentially activate different neuronal populations, a technique known as current steering. [@hallett2000]
Beyond direct activation, TMS produces extensive network effects through synaptic connections. When directly activated neurons fire, they release neurotransmitters onto their postsynaptic targets, triggering activity in downstream circuits. These transsynaptic effects can spread far beyond the directly stimulated area, explaining how TMS can influence distributed brain networks. [@luber2019]
The magnitude and direction of transsynaptic effects depend on the state of the target neural circuits at the time of stimulation. For example, stimulating the motor cortex during a voluntary movement produces different effects than stimulation at rest. The brain's prior activity, attention state, and neurochemical milieu all modulate the propagation of TMS effects through neural networks. [@valero-cabre2021]
Layer V pyramidal neurons represent the primary direct target of TMS in the motor cortex. These large neurons have their cell bodies in the deep layers of the cortex and extend long apical dendrites to the superficial layers. Their axons project to subcortical structures including the thalamus, basal ganglia, and brainstem, as well as to spinal cord motor neurons through the corticospinal tract. TMS can directly activate the axons of these neurons, producing descending volleys that activate spinal motor neurons and generate muscle responses known as motor evoked potentials (MEPs). [@hallett2000]
The responsiveness of pyramidal neurons to TMS can be modulated by various factors including age, disease state, and pharmacological agents. In Parkinson's disease, for example, dopaminergic medications alter motor cortex excitability, which can be detected as changes in TMS measures. Similarly, GABAergic drugs increase the threshold for TMS-induced activation, reflecting the importance of inhibitory circuits in controlling pyramidal neuron output. [@rothwell1997]
Local inhibitory interneurons are critically involved in shaping the effects of TMS. GABAergic interneurons in the cortex receive input from thalamocortical afferents and other cortical neurons, providing feedback and feedforward inhibition that regulates pyramidal neuron activity. TMS activates inhibitory circuits, producing complex effects that include both direct excitation and indirect inhibition. [@luber2019]
The balance between excitation and inhibition determines the net effect of TMS on the stimulated cortex. Single-pulse TMS typically produces a brief excitatory effect followed by a longer period of reduced excitability, the latter likely reflecting activation of inhibitory circuits. This suppression of excitability can be measured using paired-pulse TMS protocols that assess intracortical inhibition. [@chen2003]
Thalamic neurons that project to the cortex are important intermediaries in TMS effects. Thalamocortical neurons have their cell bodies in the thalamus and their axon terminals in the cortex, forming the major input pathway to the cerebral cortex. When TMS activates cortical neurons, the resulting retrograde activity can influence thalamic neurons, creating bidirectional communication between cortex and thalamus. [@pashut2011]
The thalamus is also a direct target of TMS in some protocols. High-frequency stimulation of the motor thalamus can produce therapeutic effects in movement disorders, and thalamic TMS has been explored as a treatment for epilepsy. The thalamic effects of TMS may mediate some of the network-level changes observed in functional imaging studies. [@rossini2015]
While TMS primarily targets cortical neurons, sufficiently intense stimulation can also affect subcortical structures. The basal ganglia, cerebellum, and brainstem nuclei can be influenced either directly or through transsynaptic effects. These subcortical effects are particularly relevant for understanding the therapeutic actions of TMS in movement disorders such as Parkinson's disease and in psychiatric conditions involving basal ganglia circuits. [@strafella2003]
Single-pulse TMS over the motor cortex produces a characteristic response in target muscles: the motor evoked potential (MEP). The MEP amplitude reflects the excitability of the corticospinal pathway, from cortical motoneurons through spinal motor neurons to muscle fibers. MEP threshold, latency, and amplitude provide clinically useful information about corticospinal excitability and the integrity of motor pathways. [@rossini2015]
Paired-pulse TMS protocols reveal additional information about intracortical circuits. Short-interval intracortical inhibition (SICI) assesses GABA-A receptor-mediated inhibition, while intracortical facilitation (ICF) reflects excitatory mechanisms. These measures are abnormal in various neurological disorders, providing diagnostic information and insight into disease mechanisms. [@chen2003]
Repetitive TMS (rTMS) can produce lasting changes in cortical excitability, a phenomenon with important therapeutic implications. High-frequency rTMS (≥5 Hz) generally increases cortical excitability, while low-frequency rTMS (≤1 Hz) decreases it. These effects outlast the stimulation period by minutes to hours and are thought to reflect activity-dependent plasticity mechanisms similar to long-term potentiation (LTP) and depression (LTD). [@luber2019]
The induction of plasticity by rTMS depends on the pattern of stimulation. Theta burst stimulation (TBS), which mimics theta-frequency neural activity associated with learning, can produce powerful and lasting effects with much shorter stimulation sessions than conventional rTMS protocols. Intermittent TBS enhances excitability, while continuous TBS suppresses it, providing a versatile approach to modulating neural circuits. [@hum e2019]
TMS influences not only electrical activity but also neurochemical systems. Studies using positron emission tomography (PET) have shown that rTMS alters dopamine release in the striatum, providing a mechanism for its effects in Parkinson's disease and depression. Similar effects have been observed for serotonin and other neurotransmitters, suggesting that TMS influences the fundamental neurochemical balance of target circuits. [@strafella2003]
The neurochemical effects of TMS develop over time and may underlie the delayed therapeutic benefits observed in some patients. The initial direct neural effects of TMS trigger downstream molecular and cellular changes that gradually normalize dysfunctional neural circuits. Understanding these mechanisms is essential for optimizing stimulation protocols and predicting treatment response. [@valero-cabre2021]
The most established therapeutic application of TMS is in treatment-resistant depression. High-frequency rTMS applied to the left dorsolateral prefrontal cortex (DLPFC) reduces depressive symptoms in approximately 30-50% of patients who have failed to respond to antidepressant medications. The mechanism likely involves modulation of the prefrontal cortex and its connections to subcortical structures and the anterior cingulate cortex, regions implicated in depression pathophysiology. [@george2013]
The antidepressant effects of TMS are thought to involve normalization of dysregulated brain networks. Functional imaging studies have shown that successful TMS treatment reduces elevated activity in the DLPFC and associated networks while enhancing activity in the ventromedial prefrontal cortex and subgenual anterior cingulate. These changes reflect the restoration of healthy patterns of brain activity that characterize remission. [@mayberg1997]
TMS has been explored extensively in Parkinson's disease, with evidence supporting benefits for both motor and non-motor symptoms. High-frequency stimulation of the motor cortex can improve bradykinesia and rigidity, possibly through activation of the supplementary motor area and normalization of basal ganglia output. Effects on gait and postural stability are more variable. [@rossini2015]
The mechanisms of TMS in PD likely involve multiple pathways. Direct activation of the motor cortex can compensate for reduced dopaminergic input to the premotor and supplementary motor areas. Additionally, TMS may influence the thalamus and basal ganglia through transsynaptic effects, normalizing pathological patterns of activity in these structures. [@strafella2003]
TMS is increasingly used in stroke rehabilitation to enhance recovery of motor function. Low-frequency rTMS applied to the contralesional motor cortex can reduce interhemispheric inhibition from the healthy side onto the affected side, facilitating recovery. Alternatively, high-frequency rTMS applied to the affected motor cortex can enhance excitability and promote reorganization of motor networks. [@lefaucheur2014]
The effectiveness of TMS in stroke depends on factors including time since stroke, severity of impairment, and specific stimulation parameters. Optimal effects are generally observed in subacute stroke patients with moderate deficits, while chronic stroke patients with severe impairments show more variable responses. Combination with physical therapy enhances the benefits of TMS. [@luber2019]
TMS can modulate pain perception through effects on motor cortex and associated pain-processing networks. High-frequency rTMS of the primary motor cortex (M1) produces analgesic effects in chronic pain conditions including fibromyalgia, neuropathic pain, and complex regional pain syndrome. The analgesia involves both sensory-discriminative and affective-motivational systems, with activation of the anterior cingulate and insula contributing to the subjective experience of pain relief. [@lefaucheur2014]
The analgesic effects of TMS develop gradually and persist beyond the stimulation period, suggesting involvement of neuroplastic mechanisms. Repeated stimulation sessions produce more robust and lasting benefits than single sessions, consistent with the induction of long-term changes in pain-processing circuits. The optimal stimulation target varies across pain conditions, with the M1 hand area effective for widespread pain and the M1 face area effective for orofacial pain. [@valero-cabre2021]
TMS is being investigated for numerous additional applications. In epilepsy, low-frequency rTMS can reduce seizure frequency by suppressing hyperexcitable cortical tissue. In tinnitus, TMS targeting the auditory cortex can reduce the phantom sounds that characterize this condition. In obsessive-compulsive disorder, high-frequency stimulation of the DLPFC or supplementary motor area may reduce symptoms by modulating frontal-striatal circuits. [@lefaucheur2014]
TMS is generally well-tolerated, with the most common side effects being mild and transient. Headache occurs in approximately 10-30% of patients and typically responds to standard analgesics. Scalp discomfort at the site of stimulation is also common and may limit treatment tolerability in some individuals. Transient hearing changes have been reported but are prevented by using earplugs during stimulation. [@rossi2009]
Absolute contraindications to TMS include the presence of metallic implants in or near the head (except titanium), implanted electronic devices such as pacemakers or vagus nerve stimulators, and a history of seizures. Relative contraindications include pregnancy, medications that lower seizure threshold, and significant neurological or psychiatric comorbidities. Careful screening before treatment is essential to ensure patient safety. [@rossi2009]
The primary serious risk of TMS is the induction of seizures, which has been reported primarily with high-frequency or high-intensity rTMS protocols. Seizure risk is influenced by stimulation parameters, individual susceptibility factors, and medication status. Adherence to established safety guidelines, which specify maximum stimulation intensities and train lengths for different protocols, effectively minimizes this risk. The overall seizure rate with TMS is estimated at less than 1 per 30,000 treatments. [@rossi2009]
Research continues to optimize TMS protocols for different applications. Variable pulse protocols, which interleave different pulse types within a session, may enhance efficacy while reducing the total number of pulses needed. Paired associative stimulation (PAS), which combines TMS with peripheral nerve stimulation, can more selectively induce plasticity and may enhance therapeutic effects. Theta burst protocols offer the advantage of much shorter treatment times while producing comparable effects to conventional rTMS. [@hum e2019]
Advances in neuronavigation and functional imaging allow more precise targeting of TMS. Using individual structural MRI scans to guide coil placement improves focality and consistency of stimulation. Functional imaging can identify the optimal target based on each patient's pattern of brain activity, enabling personalized treatment approaches. Combined TMS-fMRI allows real-time assessment of TMS effects on brain networks. [@valero-cabre2021]
New applications for TMS continue to emerge. In Alzheimer's disease, TMS combined with cognitive training may enhance memory function by modulating hippocampal-cortical networks. In minimally conscious states, TMS may promote recovery of consciousness by activating frontal attention networks. The development of wearable, portable TMS devices may enable at-home treatment for chronic conditions, expanding access to this therapy. [@luber2019]