Membrane potentials Flashcards
Where is sodium highest?
Outside the cell
Where is potassium highest?
Inside the cell
What does the equilibrium potential depend on?
Concentration gradient and electrical potential difference
What is an eqn used to estimate equilibrium potential for a given ion?
Nernst eqn
Approx. concentration of sodium outside the cell?
140mM
Approx. concentration of sodium inside the cell?
14mM
Approx. concentration of potassium inside the cell?
120mM
Approx. concentration of potassium outside the cell?
4mM
Typical equilibrium potential for sodium
+65mV
Typical equilibrium potential for potassium
-95mV
For uncharged substances, what is considered for equilibrium potential?
ONLY concentration gradient
How would you calculate the net driving force?
Membrane potential - Equilibrium potential
If the driving force is negative what happens?
Cations will enter the cell and anions will leave
If the driving force is positive what happens?
Anions will enter the cell and cations will leave
If the driving force is zero what happens?
No net movement
Movement of ions between the ECF and ICF causes what?
Ionic current
Ionic current is almost synonymous with what?
Conductance
What 2 factors determine membrane potential?
Concentrations of ions across the cell membrane and permeabilities
What eqn gives relation between the membrane potential and conc./permeabilities of ions?
Goldman
What is an action potential?
Transient increase in sodium permeability; basic signal of the nervous system
Describe the action potential phases
- Resting membrane potential is around -70mv, depolarization starts to occur and once the sodium coming in hits the threshold an AP is generated
- Depolarizing phase
- Overshoot
- Repolarizing phase
- Undershoot/hyperpolarizing phase
What is the depolarizing phase?
When the membrane potential becomes less negative than resting membrane potential
What is the overshoot?
When the inside of the cell becomes positive in respect to the outside of the cell for a brief moment
What is the repolarizing phase?
Membrane potential returns to resting level
What is the undershoot/hyperpolarizing phase?
When repolarizing, the membrane potential becomes slightly more negative than the resting membrane potential
Describe the sodium channels
2 gates - activation gate and inactivation gate
The activation gate is closed at resting but opens upon depolarization
The inactivation gate is open at resting but slowly closes upon depolarization
The act of the inactivation gate closing slowly upon depolarization - thus a delayed decline in sodium permeability - is known as what?
Sodium channel inactivation
Describe the potassium channels
Just one gate, that is closed at resting and opens to let potassium out of the cell upon depolarization
Does the potassium gate open when the sodium activation gate does?
No, the potassium gate is slower to open
What is the absolute refractory period?
The time right after an AP has been generated that no amount of stimulus could cause another AP to be fired
- Inactivation gates are closed
What is the relative refractory period?
The time after the absolute refractory period where the cell is hyperpolarizing, and if the stimulus were large enough an AP could be generated, but the stimulus must be larger than usual
What is accomodation?
When cell is held at depolarized level for a sustained manner - occurs bc inactivation gates of sodium close slowly and remain closed which causes an increased potassium conductance out of the cell and results in a net reduction in cell excitability
Hyperkalemia
High potassium in ECF, can cause muscle spasms because the resting membrane potential is less negative = depolarized
Hypokalemia
Low potassium in ECF hyperpolarizes the cell which makes the resting membrane potential more negative and makes it harder to fire an AP causing muscle weakness
Hypokalemic periodic paralysis
Attacks occur suddenly and may be triggered after exercise, stress or a high carb meal - muscle weakness
What does lidocaine do?
Blocks sodium channels and action potentials do not occur
What properties of the axon determine the way APs propagate?
Time constant and length constant
What is tau?
The shorter the tau, the faster the speed of the impulse propagation - thus you want low membrane resistance and capacitance
What is lamda?
The longer the lamda, the less the loss of the signal will be and the farther down the axon the voltage will spread - thus you want a large diameter to lower the internal resistance
What properties does myelination give?
Increases the membrane resistance and lowers the membrane capacitance thus increasing the conduction velocity - nodes of ranvier and saltatory conduction
What happens due to multiple sclerosis?
Loss of myelin sheath, causes a decreased in lamda and membrane resistance and thus a loss in the signal, so failure to conduct APs
What are the symptoms of multiple sclerosis?
Weakness in LE, numbness, paresthesia, blurred vision and pain with eye movements
