Introduction - Robotized Transcranial Magnetic Stimulation - page 2

Fig. 1.1 For TMS, a rapidly
changing magnetic field B is
produced by a coil and passes
through the skull. This way, B
induces an electric field E in
the cortex that depolarizes
neurons. In the target region,
the principle component of B
can be assumed to be in z-
direction
geometry and electrical conductivity of the tissue. Note that the induced current
stimulates the tissues similar to electrical stimulation (Transcranial Electrical
Stimulation (TES)) [ 66 ]. For TMS, the magnetic field just functions as a carrier to
induce an electric current inside the cortex. Thus, TMS does not produce high
currents in the skin and therefore does not result in pain. Figure 1.1 illustrates the
basic principle of TMS for non-invasive brain stimulation.
For TMS, the induced electric field is perpendicular to the magnetic field and in
opposite direction to the electrical current in the coil. In principle, assuming
homogeneous conductivity, the induced electric field is parallel to the plane of the
coil [ 24 ]. Hence, the TMS coil is tangentially placed on the head for (optimal)
stimulation. However, the human brain is inhomogeneous and local conductivity
differences occur. Therefore, only complex models and extensive simulations are
capable to predict the real current distribution inside the tissue [ 55 , 87 ].
Simulations have shown that for identical magnetic fields the magnitude of the
induced current in the brain critically depends on the orientation of the coil relative
to underlying gyri and sulci. In fact, the induced electric field in the tissue is
maximal when perpendicular to the underlying gyrus [ 82 ]. Furthermore, it has
been hypothesized that pyramidal neurons are stimulated most effectively when in
alignment with the current direction [ 21 ]. Thus, both, orientation and position of
the stimulation, provide information on the location of cells and structures that are
influenced by TMS.
Neuronal activation will ensue if the current density at the position of a
pyramidal neuron exceeds a threshold value to depolarize (or hyperpolarize) the
axon membrane [ 72 ]. This will cause an Action Potential. Even though pyramidal
axons are likely to be stimulated near bends [ 44 ], also other geometrical factors,
e.g., terminals and branches, may change the neuronal excitability [ 67 ]. In prin-
ciple, those axons are most likely to be activated that change their orientation in
relation to the induced electric field direction [ 24 ].