Electrical properties of the Heart Flashcards
What is the RP of a cardiac myocyte?
-90mV (NB threshold potential is -70mV and this is where the iNa begins)
What is phase 0 of the cardiac action potential?
Rapid depolarization of the membrane due to an increase in sodium permeability.
Thus the iNa
membrane potential reaches nernst of sodium.
They will rapidly inactivate (only slowly reactivate)
What is phase 1 of the cardiac action potential?
Early repolarization due to a transient outward potassium current (ito) and the iCl (assuming an inward chloride)
What is phase 2 of the cardiac action potential?
Plateau phase
Na channels are inactive–> REFRACTORY
Inflow and outflow currents balanced
Inwards (iCa): L-type calcium channels and calcium release from SR; Also the sodium calcium exchager (3 sodium in one calcium out)=depolarizing
Outwards (iK1): outwards potassium flow=repolarizing
What is phase 3 of the cardiac action potential?
Repolarization
the iK is switched on (are delayed) and the iK1 is reactivated, speed up the repolarization.
What background pumps are on?
- Calcium pump, ouward
- Na/Ca exchanger, 3 Na in and one Ca out, repolarizing
- Na/K ATPase 3Na out and 2 K in, repolarising, sets membrane potential in cells.
What is different in cells with a slow response?
The phase 0 is a slow upstroke due to a slow inward calcium current (iCa)
found in SA and AV nodes
What is the absolute refractory period?
phase 0 to roughly 3, the time where the membrane can not be re-excited
What is the relative refractory period?
in phase 3 before 4, where a larger than normal stimulus can cause another action potential.
What determines the cardiac resting potential?
the iK1 and is phase 4
What is the supernormal period?
part at start of hase 4 where a smaller than normal stimulus can stimulate an AP
What are the parts of a pacemaker AP?
An outward potassium current, iK will decrease
The inward currents increase:
if, which cause sodium to enter the cell, and channels open when cell has repolarised
iCa, a slow inward calcium current which contributes a small amount and may continue into diastole.
How can you alter pacemaker discharge?
alter slope, or rate of depolarization
alter threshold potential
alter max diastolic potential
How does PNS affect the SA node
Ach will increase K poermeability by causing the iK.Ach
This will hyperpolarise the cell and decrease the rate of depolarisation.
How does SNS stimulate pacemaker cells
Noradrenaline will increase pacemaker slope
