Transcranial magnetic stimulation (TMS) is used to apply brief magnetic pulses to the brain and the pulses are administered by passing high currents through an electromagnetic coil placed adjacent to a patient's scalp. These pulses induce an electric field in the underlying brain tissue and when the induced field is above a certain threshold, it is directed in an appropriate orientation relative to the brain's neuronal pathways. Through this localized axonal depolarisations are produced, therefore activating the neurons in the relevant brain structure. Basically Transcranial magnetic stimulation (TMS) is a non invasive technique.
Standard TMS coils are limited to activation of only cortical brain regions, up to a depth of about 1.5 cm. When treating the depression with a standard TMS system, the limbic system related to mood regulation which is generally deeper than 1.5 cm, is indirectly affected through the secondary processes involving cortical structures. These are directly activated by TMS and in turn affect the deeper limbic system structures.