Biomedical Engineering Reference
In-Depth Information
Fig. 2.4
Cell membrane
representation in terms of
an equivalent electric circuit
showing capacitive effects. A
parallel capacitor and resistor
combination in schematic
form represents the equivalent
electrical circuit
V
I
ion
C
m
R
E
its equilibrium state. Epithelial cells, photoreceptors, etc. fall into the non-excitable
cell category.
Different research findings report that non-excitable cells are found not to gen-
erate all-or-none action potentials in response to depolarizing stimuli, due to a lack
of voltage-gated Na
+
or Ca
2
+
channels [
3
,
5
,
7
,
22
]. Consequently, membrane
potential changes are proposed to influence the localized (intracellular) concen-
tration of Ca
2
+
Ca
2
+
]
i
) responses, mainly by altering the driving force for
Ca
2
+
entry through ligand-gated or second messenger-operated channels. However,
it was reported [
16
] that during stimulation of a non-excitable cell (e.g. metabotropic
purinoceptors), membrane depolarization evokes an increase in Ca
2
+
concentration
in the interior cellular regions, primarily due to the release of Ca
2
+
from intracel-
lular stores. Although depolarization in non-excitable cells was found to result in
a decrease in
ions (
[
Ca
2
+
]
i
during acti-
vation of mast cells, lymphocytes, and related cell lines [
6
,
14
,
21
]. The results of
this research suggest that the electrogenic influences in non-excitable cells may also
originate from various organ-specific mechanisms.
On the other hand, in excitable cells, a strong externally applied current causes
the membrane potentials to undergo a large excursion, called an 'action potential',
before eventually returning to rest. Most neurons, cardiac cells, smooth and skeletal
muscle cells, secretory cells, etc., fall into the excitable cell category. In this section
of the chapter, we mainly address the aspects of electrical signal propagation and
its mathematical modeling in excitable cells. This is a century-old problem, which
Ca
2
+
]
i
, hyperpolarization causes an increase of
[
[
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