Biomedical Engineering Reference
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X2HEOBASE
2HEOBASE
'PVHF
T M
#HRONAXIE
Figure 2.6: Strength-duration curve.
and solving for D
D =− τ m · ln 1
V t m
2 I rhe R m
(2.21)
V t m
but we know that I rhe R m =
so
ln 1
1
2
D
=−
τ m ·
(2.22)
T c = D =
0 . 693 τ m .
(2.23)
The time T c is known as chronaxie . Therefore, given a strength duration curve, and Eqs. (2.19) and (2.23)
one could find the passive membrane properties.
2.3.1 A Fluid Analogy
An alternative way to think about the passive membrane is as a cup with a hole in the bottom and a
line painted half way up the side. In the analogy, the volume of the cup is similar to the capacitance in
that it can store the quantity of interest (water instead of charge). The size of the hole is similar to the
resistance in that it tends to leak out the quantity of interest (water instead of charge). The line on the
cup represents the threshold voltage. As long as the water level remains below the threshold, the water
will leak from the hole at a constant rate. A stimulus (a flow rate of charges) in our analogy would be the
steady poring of water into the cup.
Using our analogy we can gain some intuition about charging and discharging in response to a
stimulus. For example, if water is poured in slower than the rate at which it leaks out of the hole, then the
threshold line will never be reached. This is analogous to being below I rhe . You could imagine, however,
 
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