STIMULATION OF EXCITABLE TISSUES - Design and Development of Medical Electronic Instrumentation - page 346

nerve lesions, neuropathies) where the possibility exists that the patient would not feel
that the skin is being burned.
• People who have a spinal cord injury may be subject to episodes of autonomic
dysre exia. This is characterized by a rise in blood pressure elicited by a noxious stim-

ulus such as electrical stimulation (e.g., FNS) applied below the level of the lesion.
• Electrical stimulators should not be used while driving or operating dangerous
machinery.

fl

MAGNETIC STIMULATION

Excitable tissue can be stimulated by strong, time-varying magnetic fields. As shown in

Figure 7.23, a coil is placed over the tissue to be stimulated and a capacitor bank is dis-
charged into the coil. Because of Faraday's law of induction, the time-varying current in
the coil generates a time-varying magnetic

fi

field, which induces eddy currents in the tissue,
causing stimulation. Note that we are not talking about some nonmedical therapy involv-
ing weak magnets alleged to promote health. We are talking about producing eddy currents
that are strong enough to actually depolarize cell membranes and hence activate excitable
tissue. This is electrodeless electrical stimulation, where the magnetic

fi

field is only the
medium used to transfer electrical energy from the coil to the tissue. The magnetic

fi

fi field

strengths need to reach a peak of several tesla (comparable to the static magnetic fi eld of

MRI machines, some 40,000 times the Earth's magnetic field), which is usually achieved

by driving the stimulating coil with brief current pulses of several kiloamperes.
Plain electrical brain stimulation is possible noninvasively using scalp electrodes. How-
ever, transcranial electrical stimulation (TCES) is very painful because of activation of pain
fi

fi

fibers in the scalp and hence is of limited clinical value. On the other hand, magnetic stim-
ulation of the brain and peripheral nerves is painless. The reason for this di erence is that

in direct electrode stimulation, the stimulus current decays as a function of the impedance
of the tissue between the electrodes and the target tissue. On its way to the brain, the current
must pass through the highly resistive scalp and skull. Hence, to deliver su

ff

cient electric
current to neural tissue within the brain requires much higher currents to be delivered to the
scalp. The narrow (e.g., 50 to 100

800 V) produce
very high current densities close to the electrodes, which activate the pain receptors. In con-

µ s in duration) high-voltage pulses (

B

I

Coil

Local
depolarization

E

E

Axon membrane

Figure 7.23 Excitable tissue can be stimulated by strong, time-varying magnetic fields. A coil is
placed over the tissue to be stimulated and a capacitor bank is discharged into the coil. Because of
Faraday's law of induction, the time-varying current I(t) in the coil generates a time-varying magnetic
field B which induces eddy currents in the tissue, causing an electric field E that leads to stimulation.