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
What can influence plateau length in ventricular AP?
Magnitude of K+ current, e.g. sympathetic stimulation increases K+ current and reduces plateau length
What happens in stage 4 ventricular AP?
Background current, Em drifts towards RMP
Refractory period
Which phase of ventricular A refers to diastolic phase?
Phase 4
How are atrial Aps different to ventricular APs?
Additional K+ current
Repolarise from an early stage
Less Ca2+ influx so phase 2 less visible
Compare diastolic potential in phase 4 in ventricular and SAN cells
Less negative in SAN cells
Lack background K+ to stabilise Em
Why is there no plateau phase in SAN?
No open L type channel to oppose hyperpolarisation
What is the effective refractory period in cardiac cells, why is this beneficial?
Myocyte can’t be depolarised even if another stimulus applied
Allows cell to relax completely so time to fill with blood
What causes phase 3 in SAN cells?
Open Delayed rectifier K+ channel
Close Ca2+ L type channel
HCN funny current stimulated
How are subepicardial and subendocardial APs differnet?
Subepicardial: more K+ current, so repol more pronounced - shorter AP
The action potential in ventricular myocytes normally has an approximate duration of…
300 ms
Are DRK+ channels VG?
Yes
What mv does the ventricular AP reach in depolarisation?
Towards ~50mv (just under ENa)
How long does phase 0 (depol) last in SAN vs ventricle?
Less than 2 ms, in ventricular cells and 10/20 ms in SAN cells.
The NCX moves how many Na+ in and how many Ca2+ out?
3Na+ in 1Ca2+ out
