Biomedical Engineering Reference
In-Depth Information
ion channels are not generally uniformly distributed over the surface of cell
membranes. The question is unsettled, however, of whether or not the mem-
brane capacitance changes according to the density of ion channels in the
membrane. Thurbon et al
19
. suggests that non uniformities in membrane
capacitance are caused by ion channel proteins. Ion channel proteins could
theoretically alter membrane capacitance by changing both the thickness
and dielectric constant of the membrane. However, Gentet et al
24
. found no
major changes in membrane capacitance from 0.9 F per cm
2
in kidney cells
transfected with a plasmid that added glycine receptors to the membrane.
On the other hand, both theoretical work
22;23
and experimental work
20;21
found membrane capacitance to be dependent on the density of sodium or
potassium channels in squid axon and pituitary nerve terminals. In models
of the squid giant axon, Schmid
22
concluded that \The membrane capac-
ity at rest exhibits a bell-shaped dependence on the ion channel density."
The greatest changes in membrane capacitance are attributed to gating
currents, which are minute currents in the channel proteins themselves, as-
sociated with conformation changes between the open and closed states.
For the squid giant axon
45
, the change in membrane capacitance due to
sodium channel gating currents ranged up to 0.15 F per cm
2
, which is a
15% dierence. Kilic and Lindau
21
found the maximum capacitance change
around 0:10F in pituitary nerve endings, which is a 3% dierence in pi-
tuitary capacitance, however the average was lower at 0:03F, which is a
1% dierence. Thus, although the evidence is very limited, the maximum
possible capacitance change would appear to be 15%, and a more likely
value would be between 1% and 5%.
6.3. Clinical Signicance
The capacitance across cell membranes is just as vital and central of a
building block of living function as is protein synthesis, chemical recep-
tors, and DNA. Given its fundamental biomedical importance, alterations
in membrane capacitance are accordingly a factor in the pathogenesis of
a great variety of disorders, such as asbestos toxicity
46
, seizures
47
, metal
poisoning
48
, and hearing loss
49
. The heart beat itself, and its shape and
timing, are dependent on membrane capacitance
50
. Despite its universality
and importance in maintenance of a healthy state, membrane capacitance
remains dicult to measure and manipulate. This has forced researchers in
thousands of studies in the past 40 years to resort to mathematical models
that assume a uniform capacitance in the face of increasing evidence that
Search WWH ::
Custom Search