Electrical activity of the heart

Question 1

Types of cardiac action potential

Answer 1

1. SA node
2. Atrial muscle
3. AV node
4. Purkinje fibres
5. Ventricular muscle

Question 2

How are the atrial and ventricular muscle A.Ps similiar?

Answer 2

Both have an absolute resting potential.

Both have a fast depolarisation, little spike, plateau and repolarisation

Question 3

Why is the plateau critical for the sustained contraction of the atria and ventricles?

Answer 3

It protects the heart, you do not want summation in the heart- need defined contraction and relaxation

Question 4

What are the phases of the cardiac action potential?

Answer 4

0= upstroke of the A.P -50mv nodal, -70mv muscle, Ica (T channel and then L channel)-slow, Ina- fast
1=rapid repolarisation (ventricular muscle), due to outward, shortlived K+ current
2=Plateau phase- depends on entry of calcium through L type channels- repol delayed
3=Repolarisation of AP- Ik, inactivation of long type Ca
4=electrical diastolic phase- -60mV in SA and AV nodal cells- If, If in PFs and ‘no’ current in atrial and ventricular muscle

Question 5

Difference between parasympa and sympa control of pacemaker cells

Answer 5

Vagal nerve= causes hyperpolarisation and decreases the pre-potential slope
Sympa control= increases pre-potential slope and increases the firing rate

Question 6

What are the 4 currents?

Answer 6

1. Na+ current- rapid depolarisation in A&V muscle and PFs
2. Ca2+ current- rapid depolarisation in SA&AV nodal cells
3. K+ current- repolarisation phase
4. Pacemaker current- SA&AV nodal cells and PFs, hyperpolarisation induced

Question 7

What causes the threshold potential to be reached in pacemaker cells?

Answer 7

Pacemaker potential- slow, positive increase in voltage across the membrane- from the end of one action potential to the start of the next