Electrical Activity of the Heart

Question 1

List the sequence events occurring during excitation-contraction coupling in cardiac muscle

Answer 1

1. Acetylcholine activates nicotinic receptors, depolarising the cell to threshold and thus, causing the release of an AP
2. Ca2+ is released from the sarcoplasmic reticulum
3. Ca2+ binds to troponin and allows cross bridges between actin and myosin to form
4. The muscle contracts

Question 2

Compare the basis of the action potential in the pacemaker and non-pacemaker tissue

Answer 2

Pacemaker AP;

• AP: increase in PCa2+
• Pacemaker potential: gradual decrease in PK+, early increase in PNa+, late increase in PCa2+ (T-type)

Non-pacemaker AP;

• Resting membrane potential: high resting PK+
• Initial depolarisation: increase in PNa+
• Plateau: increase in PCa2+ (L-type) and decrease in PK+
• Repolarisation: decrease in PNa+ and increase in PK+

Question 3

What is another name for a pacemaker potential?

Answer 3

Pre-potential because it occurs before the AP.

Question 4

Describe modulators of electrical activity

Answer 4

• Ca2+ channel blockers: decrease FOC
• Hypocalaemia: decrease HR and FOC
• Cardiac glycosides: increase FOC
• Temperature: increase by 10bpm per degree
• Hypercalaemia: increase HR and FOC
• Hypo/hyperkalaemia: AF and heart block

Question 5

Correlate various components of the ECG with the electrical events of the heart

Answer 5

P wave: atrial depolarisation
PR interval: the time between atrial and ventricular depolarisation (0.12-0.2sec)
QRS complex: ventricular depolarisation (<0.12sec)
QT interval: ventricular depolarisation and repolarisation (max. 0.42sec)
T wave: ventricular repolarisation