Biomedical Engineering Reference
In-Depth Information
TABLE 12.1 Approximate Intracellular and Extracellular Concentrations of the Important Ions across a
Squid Giant Axon, Ratio of Permeabilities, and Nernst Potentials
Ion
Cytoplasm (mM)
Extracellular Fluid (mM)
Ratio of Permeabilities
Nernst Potential (mV)
K
þ
400
20
1
74
Na
þ
50
440
0.04
55
Cl
52
560
0.45
60
P
K
:P
Na
:P
Cl
—and not absolute. Data were recorded at 6.3
C, resulting in KT/q
Note: Permeabilities are relative—that is,
approximately equal to 25.3 mV.
TABLE 12.2 Approximate Intracellular and Extracellular Concentrations of the Important Ions across a
Frog Skeletal Muscle, Ratio of Permeabilities, and Nernst Potentials
Ion
Cytoplasm (mM)
Extracellular Fluid (mM)
Ratio of Permeabilities
Nernst Potential (mV)
K
þ
140
2.5
1.0
-105
Na
þ
13
110
0.019
56
Cl
3
90
0.381
-89
Note: Data were recorded at room temperature, resulting in KT/q approximately equal to 26 mV.
permeability to one ion is exceptionally high, as compared with the other ions, then
V
m
pre-
dicted by the Goldman equation is very close to the Nernst equation for that ion.
Tables 12.1 and 12.2 contain the important ions across the cell membrane, the ratio of
permeabilities, and Nernst potentials for the squid giant axon and frog skeletal muscle.
The squid giant axon is extensively reported and used in experiments due to its large size,
lack of myelination, and ease of use. In general, the intracellular and extracellular concen-
tration of ions in vertebrate neurons is approximately three to four times less than the squid
giant axon.
EXAMPLE PROBLEM 12.2
Calculate
V
m
for the squid giant axon at 6.3
C.
Solution
Using Equation (12.33) and the data in Table 12.1 gives
1
20
þ
0
:
04
440
þ
0
:
45
52
V
m
¼
25
:
3
ln
mV
¼
60
mV
1
400
þ
0
:
04
50
þ
0
:
45
560
12.4.5 Ion Pumps
At rest, separation of charge and ionic concentrations across the cell membrane must be
maintained, or
changes. That is, the flow of charge into the cell must be balanced by the
flow of charge out of the cell. For
V
m
Na
þ
, the concentration and electric gradient creates a force
