Ionic Basis of the Action Potential

Question 1

Learning objectives

Answer 1

Define Na+ and K+ equilibrium potential, depolarisation, repolarisation and hyperpolarisation
Compare the response of electrically excitable and inexcitable cells to depolarisation
Describe the main characteristics of an action potential
Describe the relationship between the changes in membrane permeability and membrane voltage
Explain the role of voltage-gated and time-dependent Na and K channels in the action potential

Question 2

Are equilibrium potentials measured inside or outside the cell?

Answer 2

inside the cell

Question 3

What are the equilibrium potentials if the cell was only permeable to potassium? What about sodium?

Answer 3

Potassium: negative equilibrium potential
Sodium: positive equilibrium potential

Question 4

What is polarization and depolarization?

Answer 4

Depolarisation: Resting membrane potential becomes more positive, less negative, repolarisation is the opposite

After depolarization, the membrane potential always repolarized

Question 5

What is hyperpolarization and what can cause it?

Answer 5

Hyperpolarisation is when the resting membrane potential becomes even more negative. You can do this by opening up more potassium channels

Question 6

What directions are the concentration, chemical and electric gradient for potassium and sodium

Answer 6

Sodium
Concentration: Into the cell
Chemical: Into the cell
Electrical: Out of the cell

Potassium: Out of the cell
Chemical: Out of the cell
Electrical: Into the cell

Question 7

What is the resting membrane potential?

Answer 7

Resting Membrane Potential is the voltage difference across the cell membrane when the cell is at rest

Question 8

What is the potassium and sodium equilibrium potential?

Answer 8

The voltage of the cell membrane after the chemical and electrical gradient balances out

Question 9

Compare the response of electrically excitable and in-excitable cells to depolarization

Answer 9

Unexcitable cells: A depolarization event would occur with the membrane potential become closer to 0 but won’t activate any channels nor will create an action potential

Excitable calls: A depolarization event will occur and will activate sodium and potassium channels and create an action potential

Question 10

What are the characteristics of an action potential?

Answer 10

The initial depolarization must reach a certain threshold, once the threshold has been met
Once the threshold has been met, the depolarization upstroke starts quickly
Depolarization peaks at around +30 mV
Potential re-polarizes and undershoots the resting membrane potential and then stabilizes back to the resting membrane potential

Question 11

What are the permeability change of sodium and potassium during an action potential?

Answer 11

Starts off with some permeability of potassium at resting membrane potential

once the membrane potential threshold has been met, sodium channels open up and sodium permeability increases very quickly. After depolarization, permeability of sodium decreases very quickly and a delayed increase of potassium permeability occurs

Question 12

Explain the process and structure of the voltage-gated sodium channel

Answer 12

Structure: Integral protein with an activation gate covering the pore and an ball like structure inactivation gate

Function: Once depolarized, the activation gate open and the pore lets sodium in, increasing sodium permeability. After depolarization, the inactivation gate closes the channel, unable to be reopened. After it reaches about -70 mV, it returns back into its regular form

Question 13

Explain the process and structure of the voltage-gated potassium channel

Answer 13

Structure: Integral protein with an activation gate but no inactivation gate

Function:
Once it reaches positive membrane potential, the channel opens and lets in potassium to increase potassium permeability and closes again after sodium channels close up