Action Potentials I

Question 1

Ohms Law

Answer 1

Change in Vfinal = I * R (I is current and R is lumped resistance of ion channels… or input resistance).

Question 2

Threshold

Answer 2

NOT point when all Na channels open (they have been opening slowly in response to slight depolarization, thus causing more depol… POSITIVE FEEDBACK).
When Na and K currents are exactly equal and opposite.

Question 3

Peak of action potential

Answer 3

Pna > Pk, so Vm is very close to sodium equil potential

Question 4

Repolarization

Answer 4

Pna declines back to resting level and Pk increases (go from pos Vm back to negative).

Question 5

Repolarization and Pna

Answer 5

Pna decreases, this happens because the inactivation gate closes (it is slower than the activation gate) in response to depolarization.
Because it is slower you have an instant where Na rushes into the cell.

Question 6

Repolarization and Pka

Answer 6

Pka increases. Na channels start closing and it starts repolarizing, and so more K channels open.
Because Vm is so far from Ek the K ions rush out of the cell and Vm plummets back towards resting. (back from positive to neg).
The K channels also open in response to depolarization, but with a DELAY (timed to allow max height of AP and rapid repolarization)

Question 7

Vm undershoots with repolarization

Answer 7

Because of delay in shutting the K, get Vm moving closer to Ek when returning to resting. When they close Vm relaxes to resting.
K channels have NEGATIVE FEEDBACK (depolarization causes them to open and repolarization then causes them to close).

Question 8

K channel role

Answer 8

Allows faster repolarization. They give add’l pathways for K ions to leave the cell, pulling mem. potential back to resting faster than without channels.
Mem. would repolarize but slower.
This allows more AP’s.

Question 9

Refractory period

Answer 9

Can’t generate another Ap for a few millisecs.
Have absolute and relative refractory (relative could respond if stimulus is large enough).
Results primarily because Na inactivation gates require time to open after repolarization. Also higher potassium conductance because they are slow to close makes it hard for a stimulus to depolarize.

Question 10

Na/K pump and Ap’s

Answer 10

The pump will ultimately be used to restore proper ion balances (without it axons eventually fill up with Na and lose K). How quickly that happens depends on surface area.
Big axons have small SA compared to their volume, so they run down relatively slowly.