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1 2 . e l e c t r O t H e r a P y t H e O ry
105 EN WIRELESS PROFESSIONAL
12.2.3 Summary
The pulse current that is able to produce excitation (action potential) and also offer the patient the
maximum amount of comfort can be called the optimum current.
This pulse must have the following characteristics:
1. Constant pulses of current, i.e. produced by a constant current generator.
2. Vertical establishment in order to be effective immediately and to reduce the
application time of the current.
3. Rectangular shape in order to apply the lowest possible electrical intensity.
4. Pulse duration that is equal to the chronaxy of the nerve structure requiring
stimulation is order to minimise electrical energy.
5. Compensated pulse with an electrical mean of zero in order to prevent side effects
linked to polarisation.
12.3 Basic concepts of excitation electrophysiology
12.3.1 introduction
Passing an electrical current through an excitable living tissue results in a change to the resting potential
(
𝑉𝑜
).
The changed resting potential is called the local potential (
𝑉
).
If the variation in the local potential is sufficiently intense and in the right direction, a state of instability is
reached and excitation, i.e. the action potential, occurs. The value that the local potential
𝑉
must reach so
that action potential appears is called the excitation threshold (
𝑆𝑜
).
The local potential
𝑉
, caused by electrical charges provided by the current passing through the excitable
tissue (comparable to a neuron) returns to its initial value
𝑉𝑜
when the current is stopped. Returning to
the resting conditions does not occur instantly but gradually, in the same way as discharging a capacitor.
The mathematical law for the return of
𝑉
to its initial rest value is:
-dV/dt= (V-Vo) /k (1)
Where
k
has time dimensions and is the excitation time constant. The excitation time constant
characterises the tendency of the local potential to return to its initial value at a particular speed when the
neuron is no longer subjected to the current.
While the current is being passed, the local potential
𝑉
does not increase instantly but exponentially, in the
same way as the charge of a capacitor, with
𝑘
as the time constant. This constant therefore defines the
tendency of the neuron to oppose or resist the variation in potential caused by electrical charges provided
by the stimulation current, which is identical to the charge of a capacitor.