Ionic Basis Of Action Potential Flashcards
Concentration of K+ outside cell:
5 mM
Concentration of Na+ outside cell:
140 mM
Concentration of K+ inside cell:
140mM
Concentration of Na+ inside cell:
10 mM
What would happen to a cell that is only permeable to potassium?
Potassium equilibrium potential would be negative as positive K+ would leave cell
What would happen if a cell is only permeable to sodium?
Sodium equilibrium potential would be positive as positive NA+ enter cell
What are all animal cells at rest more permeable to?
More permeable to K+ than Na+
How does resting membrane potential come about?
Very small number of K+ diffuse out down conc. gradient
Charge separation occurs as K+ ions not accompanied by anions
Interior electrical potential of cell becomes negative compared to extracellular solution
How can you record resting membrane potentials?
Axon in saline solution
Add KCl filled microelectrodes (one in solution, one in axon)
Use a current stimulator and oscilloscope
Ohm’s law:
V = IR
Why is resting potential closer to K+ equilibrium potential?
As cell more permeable to K+
More K+ leave than Na+ entering
What is depolarisation?
Movement of membrane potential to more positive value
What is repolarisation?
Movement of membrane potential to more negative value
What’s hyperpolarisation?
More negative than normal
What happens when there is an injection of current in inexcitable cell?
Resting membrane potential depolarises slightly
How are excitable membranes different to inexcitable membranes?
Excitable membranes have voltage-sensitive Na+ channels
What happens when current is injected in excitable membrane?
Threshold potential is reached which causes conformational change to Na+ voltage-sensitive channels (they open)
Rapid depolarisation and action potential generated
WHat happens after rapid depolarisation in excitable membrane?
Na+ channels quickly close again - causes repolarisation
Hyperpolarisation results
Return to resting potential
What must happen for action potential to be generated?
Initial depolarisation must reach critical threshold
Depolarisation peaks around +30 mV (all-or-none response)
Repolarisation back to resting level
How does membrane repolarisation?
Voltage sensitive K+ channels activated by membrane potential at threshold - open slower than Na+ channels
Why is there hyperpolarisation?
As existing K+ voltage-sensitive channels still open
Ratio of permeability at rest:
Na+ permeability : K+ permeability
1 : 50
What is the upstroke (depolarisation) caused by?
Rapid increase in Na+ permeability (x600)
What is repolarisation associated with?
Delayed 10 fold increase in K+ permeability
What is voltage-gated sodium channel like at resting potential?
Closed but capable of opening
-70 mV
What is voltage-gated sodium channel like from threshold to peak potential?
Open (activated)
Rapid opening triggered by threshold potential
-50mV to +30 mV
What is voltage-gated sodium channel like from peak to resting potential?
Closed and not capable of opening (inactivated)
+30 mV to -70 mV
When do voltage-gated sodium channels become capable of opening again after repolarisation?
At resting potential -70 mV
When are voltage-gated potassium channels closed?
At resting potential
Delayed opening triggered at threshold + remains closed to peak potential
-70 mV to +30 mV
When are voltage-gated potassium channels open?
From peak potential through to after hyperpolarisation
+30 mV to -80 mV
Why is this an example of positive feedback?
Na+ channels open -> increased Na+ permeability -> increased flow of Na+ into cell -> membrane depolarisation
