Biomedical Engineering Reference
In-Depth Information
3.3.1 All Or None
When a stimulus of a small amplitude is applied to an active membrane, the membrane will act in the
same way as a passive membrane.We argued in Ch. 2 that at some combination of strength and duration
(recall Fig. 2.6) the membrane will reach a threshold,
V
th
, after which the membrane becomes nonlinear.
Furthermore, the Hodgkin-Huxely model correctly predicts that once the membrane is above threshold,
the Sodium current begins the positive feedback cycle that results in the upstroke of the action potential.
Therefore, once the Sodium current is activated, an action potential is inevitable. This abrupt
all or none
response of a neuron also highlights the importance of the strength-duration curve because any stimulus
above the curve will result in an action potential.
3.3.2 Refractory Periods
After
I
Na
has caused the upstroke of the action potential and repolarization has begun,
m
will remain
relatively high.The
h
-gate, however,will slowly drop to a value forcing
I
Na
to become small.But remember
that
τ
h
is large so any changes in
h
occur slowly. Therefore,
h
will remain small for some time while the
repolarization of the action potential is occurring. During this time, no additional change in
m
will
cause a second large influx of
I
Na
. The time when no second action potential is possible is called the
absolute refractory period
(see Fig. 3.8). Following the absolute refractory period is the
relative refractory
period
. During the relative refractory period a second action potential is possible, but requires a greater
depolarization (e.g., larger
I
stim
) than the first action potential.
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Figure 3.8:
Demonstration of action potential refractory period.
