Biology Reference
In-Depth Information
Table I
Selectivity coe
Y
cients of
two di
V
erent Ca
2
þ
-selective liquid
membranes
cients (log
CaM
Pot
)
Selectivity coe
Y
ETH129
a
Interfering ion (M)
ETH1001
K
þ
5.4
b
7.2
Na
þ
5.5
b
5.8
Mg
2
þ
4.9
b
6.7
NH
4
þ
5.0
c
3.6
H
þ
4.4
c
2.5
a
Ammann et al. (1987)
.
b
Lanter et al. (1982)
.
c
Ammann et al. (1975)
.
10
15
headstages
that are typically used, which help to decrease the bulk
movement of Ca
2
þ
between the medium and liquid membrane. Spatial resolution
is decreased due to di
Ω
usion of Ca
2
þ
in the bulk medium from nearby transport
V
usion of Ca
2
þ
from 10 to 20
m
m away will reach the CaSM in only
events. Di
V
20
and
80 ms, indicating that the sampled volume is much larger than the immedi-
ate dimensions of the CaSM tip. As these events are di
using from regions further
away, the local concentration change that they produce near the tip of the CaSM
will be much smaller (proportional to 1/r
V
2
) than the signals from events immedi-
ately in front of the CaSM. The decay in signal with distance is evident from
measurements of extracellular K
þ
gradients due to e
ux through single K
þ
channels (
Messerli et al., 2009
). The sampling domain of the CaSM is therefore
slightly larger than the surface area of the liquid membrane and decays rapidly
with increasing distance from the surface.
Z
D. Response Time
Self-referencing of CaSMs requires the use of CaSMs with relatively short
response times so that the CaSM can reach equilibrium in a short period of time
at its new position. The response time of CaSMs is governed by the ability to
provide charge to the sensing node. In an ideal measuring system, di
usion
through the unstirred layer at the surface of the electrode defines the response
time of the sensors when the liquid membrane is equilibrated with the salt of an ion
to which the electrode responds (
Bakker et al., 1997
). For ion-selective microelec-
trodes, this process may occur so quickly that the electronics of the system slow the
measured response (
Ammann, 1986
). Low input impedance of the amplifier and
parasitic capacitances in the circuit will draw more charge than an ideal system
therefore slowing the response time of the system. Amplifier input impedances of
V
