The cardiac action potential Flashcards
What is the SAN and what is its specialised role?
Group of cells in the right atrium, specialised cells because they provide a myogenic impulse due to their unstable resting membrane potential.
Why do the SAN have an unstable resting membrane potential?
No background K+ current to stabilize Em means constant depolarization at slow rate until threshold for AP reached
Describe the resting membrane potential of SAN
Unstable RMP decays slowly over time, this is called the pacemaker potential and rises from an initial value of -50 to -70mv to a threshold of -40 to -55mv.
How is the hearts myogenic contraction controlled?
Rhythmically generate spontaneous APs by Ca2+ clock (cyclical Ca2+ release form SR) mechanism and voltage clock (cyclic de/activation of membrane ion channels) which synchronize the depolarisation
What occurs in phase 4 of the SAN action potential?
Pacemaker potential: progressive depolarization from -65mV
If activation (when Em gets more +ve than -50mV in phase 3 repolarisation)
Na+ influx through non selective channels (HCN-4).
What occurs when Em of SAN reaches -40mv (phase 0)?
Activation of L-type calcium channels due to the pacemaker potential reaching threshold, leads to the majority of the depolarisation.
Ca2+ ions move into the SAN cells,
Why is the depolarisation slower in SAN than ventricular myocyte?
SAN L-type calcium ion channels are slower to open than Na+ ion channels therefore resulting in a slower depolarisation wave than ventricular myocytes.
What occurs at phase 3 of the ventricular action potential?
Repolarisation, delayed rectifier K+ channels open, K+ efflux.
Deactivation of k+ efflux at end of phase 3.
What does the slope of the pacemaker potential determine?
Frequency of AP and heart rate
What causes depolarisation in ventricular myocytes?
Opening of Na+ channels when threshold reached caused by voltage change from neighboring cell.
What is the threshold for Na+ influx in a ventricular myocyte?
~-70mv
What occurs in phase 1 of the ventricular action potential?
Early repolarisation of the membrane, governed by the transient outward current of K+ ions towards 0mv
Why is early repolarisation important?
Mkes the potential slightly more negative, enhancing the electrochemical gradient for Ca2+ entry in the next phase
Influences the length of the action potential to allow different cardiac cells to have varying early repolarisation lengths so the contraction happens progressively across the heart to allow efficient pumping, from the outermost cells to inner cells.
What occurs in phase 2 (the plateau phase)?
Delayed rectifier K+ channels allow potassium to continue to leave the cell, but a long lasting inward current of Ca2+ counteracts the efflux of K+ from repolarising the membrane.
Calcium ions move in via L-type calcium ion channels down their electrochemical gradient.
Why doesn’t Em reach ENa in phase 0 ventricular AP?
Outward K+ current
