07 Electrical Properties of the Heart

Question 1

What is the duration of action potential in the ventricular cells and why

Answer 1

500 ms (skeletal is 1.5 ms)
This is to ensure effective pumping of the heart

Question 2

What causes absolute refractory period

Answer 2

Sodium channel inactivation

Question 3

List the phases of ventricular action potential and indicate what channels are responsible for each phase

Answer 3

Upstroke (Ik channel)
Early repolarisation 
Plateau (Ica/L type Ca2+ channel)
Repolarisation
Resting membrane potential

Question 4

Compare the electrical properties between atria and ventricular cells

Answer 4

Atria

1) do not have Ik1 channels therefore there is unstable resting membrane potential
2) T type Ca2+ channel which recruit more negative potential
3) upstroke relies on Ca2+

Question 5

How does sympathetic innervation cause increase in heart rate

Answer 5

It decreases the time for resting membrane potential to reach threshold (steeper resting membrane)

Question 6

What is the location of SA node

Answer 6

Just below epicardium at the junction between SVC and right atrium

Question 7

What does cardiac impulse propagation rely on

Answer 7

Both passive spread of current and the existence of a threshold

Question 8

What determines the spread of excitatory current

Answer 8

Gap junction