1 2 . e l e c t r O t H e r a P y t H e O ry
106
EN
WIRELESS PROFESSIONAL
It must be stated that
𝑘
does not depend on the shape and qualities of the stimulation current; it is a
feature of the neuron itself, which expresses the time factor of its tendency to return the membrane
potential to the resting value.
The critical value that the local potential
𝑉
must reach to trigger excitation, i.e. the excitation threshold So,
is only a constant value if the pulse duration is extremely short. If, however, the current lasts longer, the
threshold increases (
𝑆
). This phenomenon is demonstrated by the well-known fact that a current which
increases slowly must reach a higher value in order to produce stimulation than a current which increases
quickly.
The increase in the excitation threshold is known as accommodation. Accommodation is an increase in
the threshold (
𝑆
) which is the result of the change in the local potential caused by the electrical charges
provided by the current passing through the neuron.
The increase in the threshold does not occur instantly but gradually and at a particular speed. A second
time factor (
𝜆
) is therefore involved in the process of electrical excitation, which defines the rate at which
the threshold changes (
𝑆
).
When the local potential
𝑉
is returned to its resting potential
𝑉𝑜
,
𝑆
returns exponentially to its initial value.
So with
𝜆
as the time constant according the mathematical law:
ds/dt= (S - So) /λ (2)
This equation is for S what equation (1) is for
𝑉
, with
𝜆
replacing
𝑘
.
The electrical charges provided by the current passing through the neuron change the membrane
potential. They produce a local potential
𝑉
and this causes the threshold
𝑆
to increase. Excitation occurs
if a sufficient quantity of electrical charges is provided to allow the local potential to catch up with the
threshold value, i.e. when
𝑉
=
𝑆
(Fig. 1).
ØV = V-Vo = Vm ax (1-e
-t/k
)
Excitation state -
action potential
Stimulation current is passed
through
V
and
S
increase
Resting State
Fig. 1