3. 1. Regarding Resting membrane Potential, what is
the most important Cation of the intracellular
fluid compartment is:
a) Calcium
b) Iron
c) Magnesium
d) Potassium
e) Sodium
4. 2. Regarding Resting membrane Potential, What is
the most important Cation of the Extra cellular
Fluid Compartment:
a) Calcium
b) Iron
c) Magnesium
d) Potassium
e) Sodium
5. 3. How much voltage is contributed by Na+-K- Pump
to the Resting membrane Potential of a neuronal
cell?
a) -4 mV
b) -61 mV
c) -86 mV
d) -90 mV
e) -94 mV
6. 4. What is the Nernst Potential for Na+ ion across
the cell membrane, while developing RMP:
a) +4 mV
b) +61 mV
c) +86 mV
d) +90 mV
e) +94 mV
7. 5. How much voltage is contributed by Na+-K- Leak
Channel to the Resting membrane Potential of a
Skeletal Muscle Cell?
a) -4 mV
b) -61 mV
c) -86 mV
d) -90 mV
e) -94 mV
8. Lecture Outlines…
At the end of the Lecture ,students will be able to know:
1) What is Membrane Potentials?
2) What is Diffusion Potential ?
3) What is Nernst Potential?
4) What is Resting Membrane Potential?
5) What are various determinants of RMP?
9. Membrane Potentials…
o Electrical potentials exist across membranes of all the body cells
o Nerve and muscle cells (excitable cells), have the unique ability to
generate and transmit electrical potential across its membrane…
known as membrane potential
o In glandular cells, macrophages,
and ciliated cells, local changes in
membrane potentials also activate
many of the cells’ functions
10. Diffusion Potential…
o “Diffusion Potential” Caused by an
ion Concentration Difference on
the Two Sides of the Membrane
For instance,
K+ concentration is great inside a nerve fiber
membrane but very low outside the membrane
o Because of concentration gradient from inside toward outside,
there is a strong tendency for extra numbers of K+ to diffuse
outward through the membrane… generating Diffusion Potential
11. o K+ carry positive electrical charges to the outside, thus creating
electropositivity outside and electronegativity inside the
membrane, because of negative anions that remain behind and
do not diffuse outward with the potassium
o Within a m/sec or so, potential difference across the membrane,
the diffusion potential, becomes great enough to block further
net K+ diffusion to the exterior, despite the high K+ concentration
gradient
o In normal mammalian nerve fiber, potential
difference required is about -94 mV, inside
the fiber membrane
Diffusion Potential caused by K-ions…
12. o High concentration of Na+ is present outside the membrane
and low Na+ inside. Assuming membrane to be highly
permeable to the Na+ but impermeable to all other ions
o Diffusion of the positively charged Na+ to the inside creates a
membrane potential of opposite polarity, with negativity
outside and positivity inside
o Within m/sec, membrane potential rises
high enough to block further net diffusion
of Na+ to the inside; used to be about 61 mV
positive inside the fiber
Diffusion Potential caused by Na-ions…
13. Thus, concentration difference of ions across a selectively
permeable membrane, under appropriate conditions, can
create a membrane potential
14. The Nernst Potential…
o Diffusion potential level across a membrane that exactly
opposes the net diffusion of a particular ion through the
membrane is called the Nernst potential for that ion
Magnitude of Nernst potential…
is determined by the ratio of the concentrations of
that specific ion on the two sides of the membrane
o Greater this ratio, greater will be the tendency for the ion to
diffuse in one direction, hence greater will be the Nernst
potential required to prevent additional net diffusion
15. Nernst equation, can be used to calculate the Nernst potential
for any univalent ion at normal body temperature of 98.6°F :
Assuming, potential in the ECF remains at zero potential, and
the Nernst potential is the potential inside the membrane
16. o Nernst potential will be positive if negative ion is diffusing from
inside to outside, and it is negative if the ion diffusing is positive
o So, when the concentration of positive K+ ions on the inside is
10 times that on the outside, the log of 10 is 1, so that the
Nernst potential calculates to be –61 mV inside the membrane
&
If the concentration of positive Na+ ions on the outside is 10
times that on the inside, the log of 10 is 1, so that the Nernst
potential calculates to be +61 mV inside the membrane
17. Diffusion Potential….
When the Membrane Is Permeable to Several Different Ions
When a membrane is permeable to several different ions, the
diffusion potential that develops depends on three factors:
o Polarity of electrical charge of each ion
o Permeability of the membrane to each ion
o Concentrations of respective ions across the membrane
18. Goldman equation, gives the calculated membrane potential
on the inside of the membrane when two univalent positive
Na+ and K+, and one univalent negative Cl– ion, are involved
20. Resting membrane potential of the cell, that is present across
the membrane when it is at rest, i.e.; it is not excited, used to
be about –90 mV in the large nerve fibers and Skeletal muscle
21. Role of Na+-K+ Pump
o All cell membranes of the body have a
powerful Na+-K+ pump that continually
pumps 3 Na+ to the outside of the cell
And 2 K+ to the inside
o Na+-K+ pump is an electrogenic pump…
pumping more +ve charges to the outside than to the inside,
leaving a net deficit of +ve ions on the inside; causing a -ve
potential inside the cell membrane
22. o Na+-K+ Pump causes large concentration
gradients for Na+ and K+ across the resting
nerve membrane
o The ratios of these two respective ions
from the inside to the outside are:
23. Role of Na-K “leak” channel
A channel protein in the nerve membrane through which Potassium
and Sodium-ions can leak is present, called Na+-K+ “leak” channels
o Leak channels are about 100 times
more permeable to K+ than to Na+
o This differential in permeability is
exceedingly important in determining
level of the normal RMP
25. Contribution of the K+ Diffusion Potential
o Assuming that membrane is only permeable to only K+,
o Because of the high ratio of K+ inside to outside, 35:1,
the Nernst potential corresponding to this ratio is –94 mV
o Logarithm of 35 is 1.54, and this times –61 mV is –94 mV
Hence, if K+ were the only factor causing RMP, the resting potential
inside the fiber would be equal to –94 mV
26. Contribution of the Na+ Diffusion Potential
o Assuming that membrane is only permeable only to Na+ this
time, causing diffusion of Na+ through the Na+-K+ leak channels
o Ratio of Na+ from inside to outside the membrane is 0.1, which
gives a calculated Nernst potential of the membrane of +61 mV
27. o Na+-K+ Leak channel is about 100 times more permeable to K+
than to Na+, hence it is logical that diffusion of K+ contributes
far more to membrane potential than does the diffusion of Na+
o Using Goldman equation, net potential
created by Na+-K+ Leak channel is
–86 mV on the inside of membrane
28. Contribution of the Na+-K+ Pump
o Na+-K+ Pump provide an additional contribution to the RMP by
continuous pumping of 3 Na+ to the outside for each 2 K+
pumped to the inside of the membrane
o With a continual loss of positive charges; Na+-K+ Pump creates
an additional degree of negativity, about –4 mV on the inside
o So, the net membrane potential with all these factors
operative at the same time is about –90 milliVolts
29. In summary, the diffusion potentials alone caused by K+ and
Na+ diffusion would give a membrane potential of about –86
millivolts, almost all of this being determined by K+ diffusion
Then, an additional –4 mV is contributed by continuously
acting electrogenic Na+-K+ Pump, giving a net RMP of –90 mV
31. 1. Regarding Resting membrane Potential, what is
the most important Cation of the intracellular
fluid compartment is:
a) Calcium
b) Iron
c) Magnesium
d) Potassium
e) Sodium
32. 1. Regarding Resting membrane Potential, what is
the most important Cation of the intracellular
fluid compartment is:
a) Calcium
b) Iron
c) Magnesium
d) Potassium
e) Sodium
33. 2. Regarding Resting membrane Potential, What is
the most important Cation of the Extra cellular
Fluid Compartment:
a) Calcium
b) Iron
c) Magnesium
d) Potassium
e) Sodium
34. 2. Regarding Resting membrane Potential, What is
the most important Cation of the Extra cellular
Fluid Compartment:
a) Calcium
b) Iron
c) Magnesium
d) Potassium
e) Sodium
35. 3. How much voltage is contributed by Na+-K- Pump
to the Resting membrane Potential of a neuronal
cell?
a) -4 mV
b) -61 mV
c) -86 mV
d) -90 mV
e) -94 mV
36. 3. How much voltage is contributed by Na+-K- Pump
to the Resting membrane Potential of a neuronal
cell?
a) -4 mV
b) -61 mV
c) -86 mV
d) -90 mV
e) -94 mV
37. 4. What is the Nernst Potential for Na+ ion across
the cell membrane, while developing RMP:
a) +4 mV
b) +61 mV
c) +86 mV
d) +90 mV
e) +94 mV
38. 4. What is the Nernst Potential for Na+ ion across
the cell membrane, while developing RMP:
a) +4 mV
b) +61 mV
c) +86 mV
d) +90 mV
e) +94 mV
39. 5. How much voltage is contributed by Na+-K- Leak
Channel to the Resting membrane Potential of a
Skeletal Muscle Cell?
a) -4 mV
b) -61 mV
c) -86 mV
d) -90 mV
e) -94 mV
40. 5. How much voltage is contributed by Na+-K- Leak
Channel to the Resting membrane Potential of a
Skeletal Muscle Cell?
a) -4 mV
b) -61 mV
c) -86 mV
d) -90 mV
e) -94 mV
Editor's Notes
Electrical potentials exist across the membranes of
virtually all cells of the body. In addition, some cells,
such as nerve and muscle cells, are capable of gen-
erating rapidly changing electrochemical impulses
at their membranes, and these impulses are used to
transmit signals along the nerve or muscle mem-
branes. In still other types of cells, such as glandular
cells, macrophages, and ciliated cells, local changes
in membrane potentials also activate many of the cells’ functions.
Electrical potentials exist across the membranes of
virtually all cells of the body. In addition, some cells,
such as nerve and muscle cells, are capable of gen-
erating rapidly changing electrochemical impulses
at their membranes, and these impulses are used to
transmit signals along the nerve or muscle mem-
branes. In still other types of cells, such as glandular
cells, macrophages, and ciliated cells, local changes
in membrane potentials also activate many of the cells’ functions.
K-ions carry positive electrical charges to the outside, thus creating electropositivity outside the membrane and electronegativity inside because of negative anions that remain behind and do not diffuse outward with the potassium.
Within a millisecond or so, the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient.
In the normal mammalian nerve fiber, the potential difference required is about 94 millivolts, with negativity inside the fiber membrane.
High concentration of sodium ions outside the membrane and low sodium inside. These ions are also positively charged. This time, the membrane is highly permeable to the sodium ions but impermeable to all other ions. Diffusion of the positively charged sodium ions to the inside creates a membrane potential of opposite polarity, with negativity outside and positivity inside.
Thus, concentration difference of ions across a selectively permeable membrane, under appropriate conditions, can create a membrane potential. In later sections of this chapter, we show that many of the rapid changes in membrane potentials observed during nerve and muscle impulse transmission result from the occurrence of such rapidly changing diffusion potentials.
The Nernst Potential
The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the Nernst potential for that ion.
The magnitude of this Nernst potential is determined by the ratio of the concentrations of that specific ion on the two sides of the membrane.
The resting membrane potential of large nerve fibers when not transmitting nerve signals is about –90 millivolts. That is, the potential inside the fiber is 90 millivolts more negative than the potential in the extra-cellular fluid on the outside of the fiber. In the next few paragraphs, we explain all the factors that determine the level of this resting potential, but before doing so, we must describe the transport properties of the resting nerve membrane for sodium and potassium.
All cell membranes of the body have a powerful Na+-K+ that continually pumps sodium ions to the outside of the cell and potassium ions to the inside, as illustrated on the left-hand side in Figure 5–4. Further, note that this is an electrogenic pump because more positive charges are pumped to the outside than to the inside (three Na+ ions to the outside for each two K+ ions to the inside), leaving a net deficit of positive ions on the inside; this causes a negative potential inside the cell membrane.
A channel protein in the nerve membrane through which K+ and Na+ can leak, called a K+-Na+ “leak” channel.
The emphasis is on potassium leakage
because, on average, the channels are
far more permeable to potassium than
to sodium, normally about 100 times
as permeable.
This differential in permeability is
exceedingly important in determining
the level of the normal resting membrane potential.
In Figure
5–5A, we make the assumption that the only move-
ment of ions through the membrane is diffusion of
potassium ions, as demonstrated by the open channels
between the potassium symbols (K+
) inside and
outside the membrane. Because of the high ratio of
potassium ions inside to outside, 35:1, the Nernst
