Transcranial magnetic stimulation (TMS) is a method of neurostimulation and neuromodulation based on electromagnetic induction of an electric field in a given area of the brain.

Since the 90s of the XX century, the interest in TMS has steadily increased, since this method is non-invasive and potentially has broad diagnostic and therapeutic capabilities: the number of publications devoted to TMS increased from 67 in 1990 to 8699 in 2019 according to the PubMed database.

TMS is based on the relationship between electric and magnetic fields and the phenomenon of electromagnetic induction, discovered back in 1831 by the English physicist M. Faraday. An electromagnetic coil is installed over a certain area of the brain, in which, after a short discharge of a powerful magnetic stimulator (capacitor), a current of several thousand amperes is generated. This current generates a magnetic field perpendicular to the direction of the current in the coil, the intensity of which reaches 1.5-2 Tesla (T), and the duration is 100 ms. The dynamically changing magnetic field freely penetrates the skull to a depth of 1.5–3 cm and induces an electric field in the brain tissues (parallel but opposite to the current in the electromagnetic coil). Under the influence of an induction electric field, the membranes of cortical neurons are depolarized with the appearance of action potentials and the spread of excitation in the stimulated areas of the cerebral cortex.

The action potential arising in a neuron under the influence of a TMS impulse spreads along the axon and is capable of activating a variety of surrounding neurons of various modalities through synapses. Therefore, under the influence of TMS, the effects of both short-term excitation and inhibition can occur in the brain, and any stimulus can cause both of these effects, depending on its intensity and duration. Back in the 90s. it was shown that the effect of TMS with a frequency of ≥ 5 Hz acts excitingly, and at frequencies of 0.2-1 Hz in an inhibitory manner.

From the standpoint of functional systems, TMS has the ability to influence the processes of function regulation, influencing some neurotransmitters. Thus, stimulation of the frontal lobe with a frequency of 20 Hz led to a significant increase in the level of dopamine in the hippocampus, and stimulation in the projection of the posterior cranial fossa increases the concentration of dopamine both in the blood and in the cerebrospinal fluid, stimulation of the left dorsolateral prefrontal cortex (20 Hz, 20 min per day) changed the level of glutamate in the cortex not only on the stimulated side but also on the opposite side. An increase in the release of taurine, aspartate, and serine was noted. An increase in the concentration of cholecystokinin was found, which affects the exchange of neurotransmitters. According to studies after exposure to TMS, there is an increase in serotonin in the hippocampus, and the number of serotonin receptors increases. These findings may explain the mechanism of the therapeutic effects of TMS. And this is the treatment of Parkinson's disease, depression, burnout syndrome, chronic fatigue, addictions.

TMS activates B-adrenergic receptors and has been shown to increase glucose metabolism.

Affects the GABAergic systems of the brain, which is associated with a therapeutic effect in epilepsy.

Another mechanism is the activation of neurotrophic nerve growth factor, normalization of cerebral blood flow rate, an increase in glucose metabolism, which is used in the treatment of functional recovery after strokes, in psychiatry.

The state of the endocrine system deserves special attention as a potential factor that can explain some of the physiological effects of TMS. Several studies have found that exposure to TMS can lead to a short-term increase in the level of thyroid-stimulating hormone (TSH) in the blood plasma in both healthy individuals and in patients with depression. At the same time, subthreshold stimulation led to a transient decrease in the level of TSH and cortisol in the blood plasma of volunteers, indirectly indicating the relaxing effect of subthreshold TMS in healthy individuals.

Also, TMS has a modulating effect on the immune system, autonomic regulation of functions, cerebral hemodynamics (reactivity of cerebral vessels).

Impact with the help of TMS is able to "reboot" and restore damaged synapses and get rid of addictions. A group of scientists selected 29 volunteers with cocaine addiction. 16 people were treated with stimulation, 13 - with the traditional method, including drugs for anxiety. As a result of the experiment, 11 people in the first group were cured of addiction, and 3 in the second one.

Rhythmic TMS can have not only a momentary but also a delayed effect, the basis of which is probably the processes of neuroplasticity (modification of synapses under the influence of regularly repeated influences, production of neurotrophic factors).

TMS can cause changes in all cognitive functions in humans - speech, emotions, attention, memory, motivations, mood, visual perception.

The above information, based on research, gives a reason to understand that the effects of TMS can be considered as a result of the activation of neuroplasticity at different levels of the central nervous system (neuron, pathways of the brain, synapse, neurotransmitters, neurotransmitters, antioxidant systems, blood flow, and brain metabolism). This makes it possible to actively use TMS in the treatment of diseases of the central nervous system.


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