• Explain how membrane potential varies with stimulus amplitude.

1. Choose tutorial "The Ca Action Potential"

2. Click IClamp

3. Click Na Channels only

4. Click Reset & Run

5. Keep Lines

6. Click Ca Channels Only

7. Vary the amplitude of stimulation ( increase amplitude in 0.1 nA increments; Stimulus Control )

1. Describe what happened

   • fires an action potential like trace first at $0.9nA$$0.9\ nA$

   

   8. Erase V/T plot and I/T plot

   9. Set stimulus to 1.0 nA

   10. Click Na Channels only, reset and run

   11. Open Sum of Currents Plot

   12. Click Ca Channels only, reset and run

2. Indicate which channel type opened first during the time course, and which type closed last.

   • with sodium channel density at zero ,

     • Open First = potassium

     • Closed Last = potassium

   

   13. Click Na and Ca Channels

   14. Set stimulus to 0.2 nA

   15. Increase GCa ( Ca chan density ) from 0.01 S/cm2 in 0.02 S/cm2 increments

3. Indicate which parts of the action potential time course changed the most with increases in GCa.

   • re-polarization phase of action potential changes the most

   • slight increase in the peak

   • lasts longer in time

   • calcium channels don't inactivate

   • we are recruiting more and more potassium channels , and eventually there is a large enough force to get a very sharp ( steep negative slope ) repolarization

   

   

   16. Click Na and Ca Channels

   17. Increase GNa from 0.01 S/cm2 in 0.02 S/cm2 increments

   18. Then from 0.2 S/cm2 in 0.2 S/cm2 increments

4. Indicate which parts of the action potential time course changed the most with increases in GNa.

   • increases peak

   • makes action potential occur earlier in time

   • depolarization slope = more steep

   

   19. Click Cardiac-like AP

5. Explain how the action potential time course differed compared with your earlier simulations.

   • much much longer , at around 23 milli seconds

   • also , potassium channels take much longer to close

   

   20. Vary GK

6. Describe what happens

   • decreasing potassium conductance , even just by $0.001S/c{m}^2$$0.001\ S/cm^2$ , makes action potential time almost infinite

   • increasing potassium conductance , makes action potential time shorter

   • if $g_K$$g_K$ is too low , there isn't enough potassium exiting the cell to have it repolarize