Biomedical Engineering Reference
In-Depth Information
3.3.3 Anode Break
An interesting phenomenon can occur if a hyperpolarizing current is applied to the membrane for a long
time. In this case,
m
is forced low and because the current is on for a long time,
h
is forced high. When
the stimulus is turned off,
m
will respond quickly and increase.
h
, on the other hand, will respond slowly
and stay at approximately 1. The combination of these two factors can cause
g
Na
m
3
h
to reach a level
high enough for
I
Na
to increase in magnitude. As in a normal activation, the increase in
I
Na
can lead
to the run-away process that causes the upstroke. The firing of an action potential using a long duration
hyperpolarizing current is known as
anode break
and is shown in Fig. 3.9.
ï
ï
ï
!0
ï
ï
Figure 3.9:
Demonstration of an action potential generated by anode break.
3.3.4 Accommodation
Another interesting phenomenon can be observed if the membrane is very slowly depolarized. A slow
depolarization can be accomplished by making
I
stim
a ramp with a small slope as in Fig. 3.10. If the
depolarization is slow enough, the
h
gate will have time to reach
h
(unlike in a normal depolarization).
Therefore, even though
m
increases,
h
will decrease such that
g
Na
m
3
h
will not be a large enough to
initiate the run-away
I
Na
current. The effect is that
V
m
can become higher than
V
t
m
without causing an
action potential to fire. This phenomenon is called
accommodation
because the
h
gate is impacting the
value of the threshold. The meaning of both anode break and accommodation is that what we typically
think of as the threshold voltage is not an exact number and depends on the path taken to cause the
run-away
I
Na
. Both are related to the time constant of
h
.
∞
