Membrane potential is created by a difference in electrical charge across the selectively permeable cell membrane, representing potential energy. This results from ion concentration gradients of sodium, potassium, chloride, and other ions between the extracellular fluid and intracellular fluid. At rest, the membrane potential is around -70mV due to higher potassium and chloride concentrations inside the cell and higher sodium concentration outside. The sodium-potassium pump maintains these gradients by actively transporting ions against their gradients. Graded potentials result from stimuli that do not exceed the threshold, while exceeding the threshold triggers an all-or-none action potential via voltage-gated sodium and potassium channels.

1. Membrane Potential
Any animal cell’s
phospholipid bi-layer
membrane and associated
structures
A difference in electrical
charge across (ECF – ICF)
this membrane, representing
potential energy.
Can also be called a transmembrane potential.

2.  Requires a selectively permeable membrane
• Due to membrane components
 Force involved is electrochemical
• “Electro” due to the charges of the ions on either side
of the membrane
• “chemical” due to the number and types of ions on
either side of the membrane
• Main components?
 Na+ & K+
 Cl-
 A- negatively charged anions
 H+ (proton gradient) – specialized use

3.  Ion Concentrations (millimoles/liter)
Ion ECF ICF Eion at 37 C Permeability
Na+ 150 15 +60mV .04
K+ 5 150 -90mV 1
Ca2+ 1 .0001 +122mV negligible
Cl- 108 10 -63mV negligible

4. So, if we take those numbers and look graphically at what
happens between Na+, K+ and A-…
ECF
ICF
Concentration
gradient
for
K
+
Electrical
gradient
for
K
+
Concentration
gradient
for
Na
+
Electrical
gradient
for
Na
+
A-
K+
EK = -90mV
Na+
ENa = +60mV
- - - - - - - - - - - - - - - - - -
+ + + + + + + + + + + + + + + + + + +
[K+]=5 mmole/L [Na+]=150 mmole/L
[K+]=150 mmole/L
[Na+]=15 mmole/L

6.  The cell membrane is about 40 times less permeable to
Na+ than K+, putting the resting potential closer to EK+
(which is -90mV)
 The equilibrium potentials of K+, Na+, Cl- and A- result in
a membrane potential of -70mV
• This determined by the Goldman-Hodgkin-Katz equation
This equation boils down to – the resting membrane
potential is calculated by the combined effects of
concentration gradients times membrane permeability for
each ion, and really just concerning Na and K.
Vm = 61 log
PK+[K+]o + PNa+[Na+]o
PK+[K+]i + PNa+[Na+]i

7. Here’s How it Works…
Vm = 61 log
PK+[K+]o + PNa+[Na+]o
PK+[K+]i + PNa+[Na+]i
PK+ = permeability for Potassium = 1
PNa+ = permeability for Sodium = .04
[K+]o = concentration of Potassium outside the cell = 5
[K+]i = concentraiton of Potassium inside the cell = 150
[Na+]o = concentration of Sodium outside the cell = 150
[Na+]i = concentration of Sodium inside the cell = 15
Vm = 61 log
1(5) + .04(150)
1(150) + .04(15)
= 61 log
5 + 6
150 + .6
= 61 log
11
150.6
Vm = 61(log of .073) = 61 (-1.37) = -69mV +1mV (for the
Na+/K+ pump effect) = -70mV

8.  Without energy, the membrane potential would
eventually be destroyed as
• K+ leaks out the cell due to membrane leakage
channels
 There are just more of the K+ leakage channels than Na+, giving
us the difference in membrane permeability
• Na+ leaks in due to membrane leakage channels
 Na+/K+ ATPase (Sodium-Potassium Pump)
restores the balance pumping Na+ out and K+
back in.

9.  Resting membrane potential
• Just described at -70mV
 Threshold membrane potential
• The electrical change that causes specialized channels to cycle
through open/close confirmations
• This occurs in mot excitable tissues at -55mV
 Action potentials
• This is a change in the membrane potential due to rapid influxes
and effluxes of ions (Na+ and K+)
• Causes adjacent cell membrane to undergo same rapid change
• Continues on to end of the membrane
• Used for communication
 Graded potentials
• Change in membrane potential that is variable based on the rate
of and location of stimuli on the membrane
• Used for integration

10. NERVE AND MUSCLE
VOLTAGE GATED CHANNELS
DEPOLARIZATION LESS THAN
THRESHOLD IS GRADED
DEPOLARIZATION BEYOND
THRESHOLD LEADS TO ACTION
POTENTIAL
ACTION POTENTIAL IS ALL OR NONE

11. CELL
BODY
DENDRITES
AXON
AXON
HILLOCK
AXON
TERMINALS

12. THE MEMBRANE USES VOLTAGE
GATED CHANNELS TO SWITCH FROM
A POTASSIUM DOMINATED TO A
SODIUM DOMINATED POTENTIAL
IT THEN INACTIVATES AND RETURNS
TO THE RESTING STATE
THE RESPONSE IS “ALL OR NONE”

13. FOR EACH CONCENTRATION
DIFFERENCE ACROSS THE
MEMBRANE THERE IS AN ELECTRIC
POTENTIAL DIFFERENCE WHICH
WILL PRODUCE EQUILIBRIUM.
AT EQUILIBRIUM NO
NET ION FLOW OCCURS

14. + -
CONCENTRATION
POTENTIAL
K+ K+
IN

15. Na+
Na+
+
-
CONCENTRATION
POTENTIAL
IN
OUT

16. AT REST THE POTASSIUM CHANNELS
ARE MORE OPEN AND THE
POTASSIUM IONS MAKE THE INSIDE
OF THE CELL NEGATIVE
THE SODIUM CHANNELS ARE MORE
CLOSED AND THE SODIUM MOVES
SLOWER

17.  DEPOLARIZATION EXCEEDS THRESHOLD
 SODIUM CHANNELS OPEN
 MEMBRANE POTENTIAL SHIFTS FROM
POTASSIUM CONTROLLED (-90 MV) TO
SODIUM CONTROLLED (+60 MV)
 AS MEMBRANE POTENTIAL REACHES THE
SODIUM POTENTIAL, THE SODIUM
CHANNELS CLOSE AND ARE INACTIVATED
 POTASSIUM CHANNELS OPEN TO
REPOLARIZE THE MEMBRANE

18.  THE MEMBRANE DEPOLARIZES AND THEN THE
MEMBRANE POTENTIAL APPROACHES THE
SODIUM EQUILIBRIUM POTENTIAL
 THIS RADICAL CHANGE IN MEMBRANE POTENTIAL
CAUSES THE SODIUM CHANNELS TO CLOSE
(INACTIVATION) AND THE POTASSIUM CHANNELS
TO OPEN REPOLARIZING THE MEMBRANE
 THERE IS A SLIGHT OVERSHOOT
(HYPERPOLARIZATION) DUE TO THE POTASSIUM
CHANNELS BEING MORE OPEN

19. A RECEPTOR’S RESPONSE TO A
STIMULUS IS GRADED
IF THRESHOLD IS EXCEEDED, THE
ACTION POTENTIAL RESULTING IS
ALL OR NONE