**4. Mechanism of actions and theories**

When current flows through the coil, magnetic field is generated by Ampere's law. When the coil is placed over the scalp, the magnetic field strength generated passes through the scalp and skull then affects a nearby neuron. The magnetic field generates current in that particular neuron by Faraday's law to cause depolarization of the neuron and propagation of action potential. If the neuron affected is located in motor cortex, the cascade process induced by TMS triggers a muscle movement which can be recorded by electromyography. This firing of impulse passes through the pathway to the end organ to produce a specific response. If this pathway is not intact, the impulse of action potential cannot reach the end organ, and no response can be produced. **Figure 5** shows how TMS triggers the cascade [13]. The magnetic field strength

#### *Transcranial Magnetic Stimulation, Connectome and Its Clinical Applications DOI: http://dx.doi.org/10.5772/intechopen.109963*

generated ranged from 1.5 to 3 Tesla [14, 15]. Typical figure-of-eight coil is able to activate cortical neurons 1.5–3 cm beneath the scalp [16].

The method of pulse delivery leads to different neuromodulative outcomes. Single-pulse delivery produces immediate effect without causing any long-lasting effect, i.e., no after effect. This mode of pulse delivery is best used for cortical mapping and checking the integrity of neuronal pathways. Pattern TMS, including repetitive TMS (rTMS) and theta burst stimulation (TBS) pattern, on the other hand produces persistent effect beyond the stimulation period, i.e., it possesses after effect. This property is useful in provision of treatment and rehabilitation. rTMS is the delivery of a number of trains of repetitive stimulation at a fixed frequency. rTMS may have intervals of breaks in between trains of stimulation. rTMS delivered at 1 Hertz (Hz) is considered as low frequency, while rTMS delivered at 5 Hz or above is considered as high frequency. TBS pattern describes the delivery of 3 pulses at 50 Hz in every 200 microseconds. TBS pattern can be given in an intermittent fashion or in a continuous fashion. In the intermittent theta burst stimulation pattern (iTBS), 600 pulses of stimulations are given in 190 seconds in a fashion that a 2-second train of TBS stimulation followed by 8 seconds of break in every 10 seconds. In the continuous theta burst stimulation paradigm (cTBS), 600 pulses of stimulations are given in TBS pattern uninterruptedly for 40 seconds [16]. Neuromodulation makes use of the after effect which can be classified as excitatory or inhibitory. High-frequency rTMS and iTBS are found to have excitatory effects, while, on the contrary, low-frequency rTMS and cTBS are associated with inhibitory effects [16]. **Figure 6** shows the summary of effects of rTMS and TBS [17].

The exact mechanism of how after effect of TMS helps in neuromodulation remains unclear. The most popular theory is interhemispheric balance theory, which is also known as interhemispheric rivalry theory [18, 19]. It is believed that each cerebral hemisphere inhibits its contralateral counterpart via the corpus callosum and the two sides are at a balanced status. When a pathology occurs in one side, this balance is disturbed. The inhibitory effect from the normal contralateral side will be uncounteracted in the lesioned side, leading to symptoms. Therefore, the aim of TMS is to restore the balance, either by inhibiting the normal contralateral side or stimulating the pathological side (**Figure 7**).

**Figure 6.** *Summary of effects of rTMS and TBS [17].*

**Figure 7.**

*Interhemispheric rivalry theory [18].*
