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Membrane_Potentials_2021-46887.ppt
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
5.
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
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
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