Cardiac action potential (grilling session)

Question 1

What is the resting membrane potential of SAN cells?

Answer 1

There isn’t one, it is constantly drifting

Question 2

What ion carries the depolarising current in SAN cells?

Answer 2

Ca2+ ions

Question 3

Describe the role of fast Na+ currents in the SAN action potential

Answer 3

There are no fast Na+ channels, so they have no role

Question 4

Word that best describes the speed of an SAN action potential, and why?

Answer 4

Slow - because Ca2+ currents are relatively slow

Question 5

What are the phases of an SAN action potential? Outline them

Answer 5

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

Question 6

Why does the membrane potential change anyway?

Answer 6

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

Question 7

Which ion channel is opened after full repolarisation of the SAN cell, and what current does it generate?

What phase does it initiate?

Answer 7

Non-selective cation channel that conducts SLOW, inward movement of cations (largely Na+)

Generates the funny current causing spontaneous depolarisation, thus initiating phase 4

Question 8

What is necessary for spontaneous depolarisation to occur in SAN cells?

Answer 8

Hyperpolarisation at the end of phase 3, as this activates the generation of the funny current

Question 9

SAN cells: as membrane potential drifts upwards in phase 4, what other ion channel(s) is/are activated in order to reach threshold potential?

Answer 9

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

Question 10

What channel(s) is/are responsible for the upstroke of the SAN action potential?

Answer 10

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

Question 11

Once the peak of the SAN action potential has been reached, what causes the downstroke?

Answer 11

Opening of outward K+ current channels, which are slow to open, that hyperpolarise the cell

AND

LTCC inactivation and closure

Question 12

Word that best describes the speed of a non-pacemaker action potential, and why?

Answer 12

Fast, because they contain fast inward Na+ current channels

Question 13

What is the resting membrane potential of non-pacemaker cells?

Answer 13

-90mV

Question 14

What are the phases of non-pacemaker cell action potentials? Outline each of them

Answer 14

Phase 4 - resting membrane potential
Phase 0 - upstroke
Phase 1 - peak followed by partial repolarisation
Phase 2 - plateau
Phase 3 - repolarisation

Question 15

What three ions are implicated in the action potential or non-pacemaker cells?

Answer 15

Ca2+, K+ and Na+

Question 16

What maintains the resting membrane potential seen in phase 4 in non-pacemaker cells?

Answer 16

The K1 inward rectifier current, preventing excessive hyperpolarisation, and potassium leak channels - potassium conductance is large

Question 17

What is the threshold potential of non-pacemaker cells?

Answer 17

-70mV

Question 18

What causes initial depolarisation of non-pacemaker cells?

Answer 18

Action potential propagation from adjacent cell - thus, influx of Na+ through fast Na+ channels

Question 19

What causes the upstroke of phase 0 in non-pacemaker cells

Answer 19

Positive feedback mechanism of VGNaCs opening

Opening of the LTCC

Closure of outward directed (and inward directed) potassium channels

Question 20

What causes the small, transient repolarisation after the upstroke? Why is the repolarisation subsequently postponed?

Answer 20

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

Question 21

What sustains the plateau of the action potential?

Answer 21

Long lasting Ca2+ current, and slow opening of outward rectifying K+ channels

Question 22

How is the repolarisation elicited?

Answer 22

LTCCs inactivate

Activation of Ikr current channel which is a slow outward rectifier, and Iks current channel