Transcranial magnetic stimulation (TMS) is a method of evoking compound motor potentials through non-invasive stimulation of the central nervous system. Developed for clinical use about 20 years ago, TMS has become an important tool for studying the conductivity and excitability of the corticospinal system, abnormal cortical circuitry in neurologic and other diseases and the reorganization of motor systems after peripheral and central nervous system lesions. TMS is based on the principle that electromagnetic induction can stimulate neural tissue. TMS is accomplished through the generation of high intensity current pulses built up from a series capacitors and discharged through wire coils.
The coils induce magnetic flux of short duration and high intensity. The magnetic fields, in turn, induce electrical current in the underlying cerebral cortex. TMS delivery devices are embedded in a non-conductive plastic or rubber material and typically fashioned into a figure eight, butterfly, flat or concave circular shapes and applied near the part of the nervous system under study. The various shapes and sizes are designed to distribute or focus the induced magnetic fields; round coils generate more diffuse magnetic fields where figure eight coils produce a narrower window of neural activation.TMS is relatively painless compared to the clinically similar electrical stimulation. Unlike transcranial electrical stimulation which directly excites cortical long tracts, the induced electrical fields with TMS preferentially stimulate neural elements oriented parallel to the surface of the brain, i.e. primarily interneurons. TMS pulses are associated with benign acoustic clicks and mild scalp/facial muscle activation. A momentary sense of disorientation at maximal stimulation may occur; however, there are virtually no clinically apparent cognitive or uncomfortable effects at the TMS intensities used.
The motor evoked potential (MEP) latency and amplitude are thought to measure most directly the integrity of pathways and related membrane excitability characteristics of the upper motor neuron. The central motor conduction time is obtained by a single maximal TMS over the primary motor cortex, and is derived by subtracting peripheral motor latencies at the levels of the cervical and lumbosacral proximal roots from the total TMS conduction time. Central motor conduction time prolongation may be due to the loss of large myelinated motor axons subsequent to degeneration and often correlates with upper motor neuron dysfunction.