Electrical current flowing through a coil induces a magnetic field. This is the basis for MRI, a technology that has been groundbreaking in furthering our understanding of the human brain with respect to structure and function. Correspondingly, changes in magnetic fields can also induce electrical currents. When applied in proximity to live brain tissue, alternating magnetic fields can induce electrical currents that will lead to depolarization in neurons, causing them to fire. Thus, we can use electromagnetism not only to image the brain, but to probe it.
Transcranial magnetic stimulation (TMS) uses coils of various shapes and sizes held near the scalp to stimulate the brain beneath the skull; a figure-8 coil is commonly used (Figure). Electric current flows through the coil, quickly reversing the direction of flow and inducing alternating magnetic fields. Magnetic fields easily cross the scalp and skull and induce electrical current in the underlying cortex, with downstream effects in functionally connected brain regions. The depth of penetration of the magnetic field is approximately 2 centimeters, reaching the junction between gray and white matter, at which point it is dissipated to about half of its initial strength. Interneurons and GABA fibers are more amenable to TMS as they are parallel to this induced electrical field, whereas pyramidal neurons are less so, because they are more perpendicular in orientation with respect to the scalp.
The cerebral cortex is excitable and responds to electrical and magnetic stimulation. The excitability of the cortex is influenced by many factors, including medications, caffeine, sleep deprivation, and even expectancy of response. When the motor cortex is stimulated with TMS pulses, motor responses can be evoked. Think back to the homunculus you learned about in medical school that lies along the motor cortex, which begins inferiorly with the mouth, then rises to the face, then hands and arms, then torso, with the feet dangling from the top of the homunculus into the interhemispheric medial longitudinal fissure that separates the 2 hemispheres. When you stimulate the area that corresponds to the thumb, the thumb will move. In doing so, you can estimate a person’s cortical excitability at a certain point by calculating the resting motor threshold —the minimum pulse strength necessary to elicit a twitch in at least 6 of 10 stimulation pulses.
TMS is an important tool in brain mapping, as direct activation of circuits can immediately elicit or disrupt observable responses; it can be particularly informative when used with brain imaging and electrophysiology. With respect to therapeutics, the delivery of a series of repetitive pulses (rTMS) has more lasting modulatory effects on neuroplasticity and functional connectivity of the brain and has been developed to treat neuropsychiatric disorders. When pulses are delivered at a high frequency, more than once per second, the result is excitation (increased firing) of the target neural tissue. When pulses are delivered at a low frequency, fewer than 1 per 10 seconds, the result is inhibition (decreased firing) of the target neural tissue (see Figure).
Dr. Corcoran is Associate Professor, Department of Psychiatry, Icahn School of Medicine at Mount Sinai, New York. Dr. Stanford is Medical Director, Alkermes, Inc, Waltham, MA. Dr. Friel is Director, Clinical Laboratory for Early Brain Injury Recovery, Burke Medical Research Institute, White Plains, NY, and Assistant Professor, Brain Mind Research Institute, Weill Cornell Medical Center, New York.
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