Cardiac action potential (grilling session) Flashcards
What is the resting membrane potential of SAN cells?
There isn’t one, it is constantly drifting
What ion carries the depolarising current in SAN cells?
Ca2+ ions
Describe the role of fast Na+ currents in the SAN action potential
There are no fast Na+ channels, so they have no role
Word that best describes the speed of an SAN action potential, and why?
Slow - because Ca2+ currents are relatively slow
What are the phases of an SAN action potential? Outline them
Phase 4 - pacemaker potential or spontaneous depolarisation up to threshold potential of -40 to -30 mV
Phase 0 - depolarisation phase or upstroke to about +10 mV
Phase 3 - repolarisation to -60 mV… cycle then spontaneously repeats
Why does the membrane potential change anyway?
Movement of charged ions across membrane through ion channels that open and close at different times over the course of the action potential changing the electrical conductance of said ions.
Ion flow generates a current that changes membrane potential
Which ion channel is opened after full repolarisation of the SAN cell, and what current does it generate?
What phase does it initiate?
Non-selective cation channel that conducts SLOW, inward movement of cations (largely Na+)
Generates the funny current causing spontaneous depolarisation, thus initiating phase 4
What is necessary for spontaneous depolarisation to occur in SAN cells?
Hyperpolarisation at the end of phase 3, as this activates the generation of the funny current
SAN cells: as membrane potential drifts upwards in phase 4, what other ion channel(s) is/are activated in order to reach threshold potential?
at -50mV, TTCC activated increasing Ca2+ conduction into the cell, further depolarising the cell
at -40mV, LTCC activated further increasing Ca2+ conduction and depolarising to threshold potential
What channel(s) is/are responsible for the upstroke of the SAN action potential?
LTCC Ca2+ current primarily responsible, though rate of depolarisation made slower by the facts that:
– it is a relatively slow current
– TTCC and funny current channel close
Once the peak of the SAN action potential has been reached, what causes the downstroke?
Opening of outward K+ current channels, which are slow to open, that hyperpolarise the cell
AND
LTCC inactivation and closure
Word that best describes the speed of a non-pacemaker action potential, and why?
Fast, because they contain fast inward Na+ current channels
What is the resting membrane potential of non-pacemaker cells?
-90mV
What are the phases of non-pacemaker cell action potentials? Outline each of them
Phase 4 - resting membrane potential Phase 0 - upstroke Phase 1 - peak followed by partial repolarisation Phase 2 - plateau Phase 3 - repolarisation
What three ions are implicated in the action potential or non-pacemaker cells?
Ca2+, K+ and Na+
What maintains the resting membrane potential seen in phase 4 in non-pacemaker cells?
The K1 inward rectifier current, preventing excessive hyperpolarisation, and potassium leak channels - potassium conductance is large
What is the threshold potential of non-pacemaker cells?
-70mV
What causes initial depolarisation of non-pacemaker cells?
Action potential propagation from adjacent cell - thus, influx of Na+ through fast Na+ channels
What causes the upstroke of phase 0 in non-pacemaker cells
Positive feedback mechanism of VGNaCs opening
Opening of the LTCC
Closure of outward directed (and inward directed) potassium channels
What causes the small, transient repolarisation after the upstroke? Why is the repolarisation subsequently postponed?
Opening of a transient outward potassium channel that generates the IKto current
LTCCs still opening, carrying Ca2+ into the cell, so when the Ikto current diminishes, it opposes repolarisation
What sustains the plateau of the action potential?
Long lasting Ca2+ current, and slow opening of outward rectifying K+ channels
How is the repolarisation elicited?
LTCCs inactivate
Activation of Ikr current channel which is a slow outward rectifier, and Iks current channel
