Biomedical Engineering Reference
In-Depth Information
membrane's threshold voltage. Note that this is the reverse of what happens with intracel-
lular stimulation, where excitation occurs at the anode.
Depending on the arrangement of the electrodes, three stimulation modes can be dis-
tinguished:
1.
Bipolar
. Both electrodes are close to the target tissue.
2.
Monopolar
(also called
unipolar
). One electrode, normally the cathode, is close to
the target tissue, and the other (anode) is remote from the target tissue, making its
size and exact placement irrelevant.
3.
Field stimulation
. Both electrodes are remote from the target tissue.
ciency of bipolar and monopolar stimulation is similar. However, the current
delivered in the monopolar mode often crosses through nontarget tissue on its way to the
anode (yes, the conventional direction for current is in the opposite direction, but you know
what we mean) and is sometimes capable of stimulating these nontarget excitable cells
undesirably. Field stimulation is the most ine
The e
cient method but is very commonly the pre-
ferred mode of current delivery in nonchronic applications since it allows tissues to be
stimulated using noninvasive skin-surface electrodes.
A stimulus must be of adequate intensity and duration to evoke a response. If it is too
short, even a strong pulse will not be e
ned as the
minimum strength of stimulus (expressed either in volts or in milliamperes) required for
activation of a target tissue for a given stimulus duration. When thresholds for several dura-
tions are put together on the same graph, a strength-duration curve is formed. The nice
thing about the strength-duration curve is that with one quick look one can determine
whether or not a stimulus will be e
ff
ective. The stimulation threshold is de
fi
ff
ective. Any stimulus that falls above the curve will
excite the target tissue.
As shown in the stylized strength-duration curve of Figure 7.4, stimulus current and
duration can be mutually traded off
ectiveness
of a stimulus is characterized by the product of current
I
and duration
t
, where delivered
charge
Q
ff
over a certain range. For a short pulse, the e
ff
It
. Hence if the amount of charge required to activate the target tissue is
Q
threshold
and the stimulus duration is
t
, the current
I
threshold
required to achieve activation
will be
I
threshold
Q
threshold
/
t
.
It would seem from this relationship that the strength-duration curve should show a
decline to near zero as stimulus duration is increased. However, the strength-duration
curve of real excitable tissue
flattens out with long stimulus durations, reaching an asymp-
tote called the
rheobase
. The root
rheo
means current and
base
means foundation; thus,
the rheobase is the foundation, or minimum, current (stimulus strength) that will produce
a response. When the stimulus strength is below the rheobase, stimulation is ine
fl
ff
ective
even when stimulus duration is very long.
The reason for the di
ff
erence between the actual behavior and that predicted by
Q
threshold
/
t
is that the latter assumes that the membrane is an ideal capacitor. This
is not the case, and the leakage resistance shows its e
I
threshold
ect during prolonged stimulation
(large values of
t
). The equation fails to predict the charge transfer across the cell mem-
brane because under these conditions, more membrane current is carried by the leakage
resistance and less is used to charge the membrane capacitance. Membrane potential thus
rises exponentially to a plateau during prolonged stimulation instead of increasing linearly
with time.
The strength-duration curve was characterized by Lapicque [1909] by the value of the
rheobase (in volts or milliamperes) and a second number called the
chronaxie
. The root
chron
means time and
axie
means axis. The chronaxie is measured along the time axis and
is de
ff
ned as the stimulus duration (in milliseconds) that yields excitation of the tissue when
stimulated at twice the rheobase strength. In the strength-duration curve of Figure 7.4, the
fi
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