Cardiac Action Potentials (Non - pacemaker) Flashcards
What is the resting membrane potential?
-90mV
What is there an initial rapid depolarisation of to?
+20mV followed by an initial, partial repolarisation of some 5-10mV
What does the very slow repolarisation produce?
Action Potential Plateau
What is the an action potential plateau?
Membrane potential remains close to 0mV for some 150-300ms
After this the membrane potentil repolarises rapidly returning to the resting potential
What does the plateau do?
It greatly prolongs the action potential
When are the cardiac cells absolutely refractory to stimulation?
The whole duration of the action potential
How long after stimulation in ventricular cells can a second action potential be generated?
Period up to 350ms after
What does the prolonged action potential protect against?
Protects against Pump failue caused by sustained contraction
What channel does depolarisation first open and what does this do?
Depolarisation first opens Na channels, increasing Na conductance.. The resulting positive feedback accounts for the initial rapid depolarisation phase.
Why does the Sodium conductance decline again?
Because of depolarisation-induced inactivation of Na channels
What does the inward Ca current maintain?
Keeps the membrane depolarised and thus maintains the Plateau in the action potential.
How does decreased K conductance following depolarisation affect the plateau?
Less outward K current than normal means it is easier for the inward Ca current to maintain depolarisation.
When does K conductance rise and what does this do?
After 200ms, K conductance rises, increasing the outward current. This K current repolarises the membrane. Repolarisation is assisted in the reduction in the opposing inward Ca current.
How do we know that there is more than one type of K channel in these cells and each type responds differently to voltage changes?
The fact that K conductance first decreases and then increases in response to the initial depolarisation.
