Heart Action Potential

Question 1

5 phases of cardiac action potential

Answer 1

0- threshold, depolarisation and overshoot

1- early repolarisation of membrane

2. plateau
3. repolarisation
4. absolute refractory
5. resting membrane

Question 2

Explain phase 0

Answer 2

membrane potential reaches a threshold of -60 to -65mV

this suddenly increases the electrical conductance of the sarcolemma, as the sodium channels open resulting positive influx of positive charge

positive potential of +20/30 mV reached which is called an overshoot

Question 3

Explain phase 1

Answer 3

early repolarisation is partial, due to transient outward currents.

Current mainly carried by voltage gated potassium channels

when the membrane potential reaches -20mV from 0 mV, it becomes relatively stable for a long period 200-400ms, known as plateau

Question 4

Role of partial repolarisation

Answer 4

Enhances electrochemical gradient for calcium entry

influences action potential duration

Question 5

Explain phase 2 + benefit

Answer 5

myocardial action potentials last 100 times longer than nerve or skeletal muscle action potentials

inward calcium current driven by electrochemical gradient, carried by voltage gated L type calcium channels

total conductance of activated calcium channels is quite low, however enough to counterbalance the repolarising effect of the small outward potassium flow throughout the action potential

outward potassium flow is reduced during plateau due to the potassium conductance falling upon depolarisation

Question 6

What is the phenomenon that causes decreased potassium conductance during plateau?

Answer 6

Inward rectification- caused by the obstruction of the inner mouth of the channel by intracellular magnesium ions and polyamines during depolarisation

Question 7

Experiment to prove the existence of two inward currents?

Answer 7

Add TTX- blocks sodium channels, abolishes the initial spike of the action potential but not the plateau

Question 8

Final phase of plateau explained

Answer 8

calcium channels become progressively deactivated

inward current of sodium maintains late plateau via the Na/Ca exchanger

Question 9

Explain repolarisation

Answer 9

inward rectifier conductance restored as more calcium channels shut causing more potassium ones to open