Information is transmitted in the brain in the form of electrical signals. These excitation transmissions do not run through a nucleus, but through the sheaths that are present in the organism as myelin sheaths. These can be stimulated and inhibited by magnetic fields. For this purpose, there is a non-invasive procedure designed as a tool for basic research of the human brain and for diagnosis. It is called transcranial magnetic stimulation, which uses a time-varying magnetic field to affect the electrical activity in the brain, and thus should lead to positive changes in various ailments and disorders.
What is transcranial magnetic stimulation?
Transcranial magnetic stimulation is used to influence electrical activity in the brain through a time-varying magnetic field, leading to positive changes in various ailments and disorders. Diseases of the central nervous system often affect the medullary sheaths. These are a multi-layered structure of myelin that spirals around a nerve fiber, also known as an axon. There, stimuli are transmitted more slowly due to diseases. On the other hand, there are diseases in which there is a failure of all nerve cells. Transcranial magnetic stimulation makes it possible to distinguish between the two diseases and to measure the processes taking place there. As early as the 19th century, the French physician Jacques-Arsène d’Arsonval experimented with this method, using high-voltage coils to prove that impulses in the brain trigger electrical reactions. The physician carried out experiments on himself and on test subjects, who experienced circulatory disturbances to loss of consciousness as a result. The method was first presented in a modern version in 1985 by the physicist Anthony Barker. Here, magnetic stimulation was used to stimulate the motor cortex in order to study the course of motor pathways, which soon became accepted as a neurological diagnostic method, as this procedure is almost without discomfort for the patient. Direct electrical stimulation of the skull, on the other hand, which was also frequently used in practice, causes pain and side effects. The motor cortex, in turn, is the brain region responsible for the control of all muscles. Therefore, the stimulation acts as a short muscle twitch. If this results in measurable delays in the brain or spinal cord, it can be used to determine the extent to which conduction time is slowed or blocked altogether and whether there are associated functional disorders.
Function, effect, and goals
Transcranial magnetic stimulation is based on the physical principle of induction. A magnetic coil held directly over the patient’s skull generates a magnetic field that passes unimpeded through the skull into the brain, where it causes electrical current. The magnetic field is oriented at right angles to the electric field of action and to the plane of the coil, is not attenuated by the skull, and serves as an input for electrical stimulation of the cortex. If the current frequency exceeds the stimulus threshold of the pyramidal fibers running in the motor cortex, a transaxonal current flow occurs. This leads to excitation of the nerve cells located there and triggers action potentials in the brain. If regular and rapidly successive individual stimulations are applied, this is referred to as repetitive transcranial magnetic stimulation. The effects in the brain differ depending on frequency and application. The exact mechanism is complex. Inter- and intracortical inhibitions also occur in different brain regions. Inside the skull, more precisely in the axon, a depolarization starts, which spreads over the cell body of the neurons and leads to an excitation threshold. One problem with magnetic stimulation is spatial resolution, as it is unclear to what extent interconnected regions actually reach the target region through stimulation. Therefore, diagnosis can only be vague about the stimulated brain region. Transcranial magnetic stimulation is used in neurology and psychiatry, as well as in the field of neuroscience research. It is mainly used for studies of pathways in the spinal cord and cerebral cortex. The motor cortex is stimulated by single impulses.Transcranial magnetic stimulation is not only used to make neurological diagnoses, but also to treat neurological diseases in particular. These include, for example, epilepsy, apoplexy, Parkinson’s disease or tinnitus. Stimulation is also helpful for affective disorders, schizophrenia and depression. This has been particularly well demonstrated in severe forms of depression, where even the use of psychotropic drugs has not brought about any improvement. The antidepressant efficacy may be due to the fact that parallels exist between electroconvulsive therapy and transcranial magnetic stimulation, although differences exist, such that, for example, generalized electrical excitation contrasts with region-specific cortical stimulation. However, studies showed that in severely depressed patients there is reduced glucose metabolism and reduced neuronal activity in various brain areas that can be stimulated or activated and increased by magnetic stimulation, once in blood flow as well as in glucose metabolism. The effect starts at the neurotransmitter level, similar to the effect of taking antidepressants in the brain. Nevertheless, the method has not yet been able to establish itself in general psychiatric practice. Diseases like multiple sclerosis are diseases exactly in the region that can be measured, in the brain and spinal cord, so magnetic stimulations lead to changes and can be diagnosed. Migraine or epilepsy also show a change in stimulus thresholds. Transcranial magnetic stimulation also shows good results, although not yet sufficiently researched, in manias, post-traumatic stress disorder, here in a low-frequency application, in obsessive-compulsive disorder as a high-frequency application, and in catatonia cases.
Risks, side effects, and hazards
The tolerability of magnetic stimulation is, by and large, less distressing and painless for the patient. Some side effects have nevertheless been described; for example, patients complained of severe headaches, but these subsided. Another side effect of the treatment can be an epileptic seizure triggered by the stimulation and irritation of the nerve cells, which in turn makes its use especially in the field of epilepsy a greater risk.
