The cell membrane sustains a potential difference between intra- and extracellular space. In static cells the transmembrane potential is about 70 mV (intracellural more negative).
An externally applied electric field may deviate the cell's membrane potential, that is, depolarize the membrane and hence activate excitable tissue.
Electric fields suitable for neural stimulation can be generated non-invasively applying electromagnetic induction. In magnetic stimulation a time-varying magnetic field induces a current flow in the tissue.
The magnetic field strengths of up to several Tesla required in magnetic stimulation are achieved by driving the stimulating coil with brief current pulses of several kiloamperes.
The neurons in the motor cortex are more readily excited. Placing the coil with one stimulating edge about 5 cm lateral from the vertex often causes the thumb to twitch quickly after stimulus onset. There will be a clicking sound from the coil and the subject may feel a usually not uncomfortable sensation of scalp being drawn up, which probably results from attractive force between the coil current and the induced current in the scalp.
The basic electrical
circuit of the MS instrument is simple (image on right, top), consisting of a
capacitor, a thyristor switch and the stimulating coil. Together with the
resistance R in the components and cables, the capacitor and coil form an
oscillating RLC circuit that, when fired, sets up a brief exponentially decaying
sinusoidal, or biphasic, current pulse I(t) (bottom). The energy is
returned trough the diode D from the coil to the instrument, which
reduces coil heating and power consumption. Without D or with great
R, the current polarity reversal is absent or suppressed (monophasic
pulse).
Figure below: the induced electric field in a plane 10 mm below (a) small
circular coil, 21 turns, (b) large circular coil, 11 turns, and (c) 8-shaped
coil, each wing 11 turns. The coil outlines are shown as circles. The current
rises at a rate of 100 A/ms. Maximum values are given in the bottom left corner.
The coil current grows in clockwise direction in (a) and (b) and in the upper
wing in (c). In the lower wing of (c) the current direction is opposite.
Original content by Jarmo Ruohonen, updated and converted to HTML 4.0 by Jussi Nurminen
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