Electrical Activity of the Heart

Question 1

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

Answer 1

Cardiac muscle forms a functional syncytium
- Electrically connected via gap junctions
- Physically connected by desmosomes
- These form the intercalated discs

Cardiac muscle has a long action potential
- Long refractory period, so cannot exhibit tetanic contraction
- Ca2+ entry from outside cell can regulate contraction
~ Ca2+ release does not saturate the troponin, so regulation of Ca2+ release can be used to vary the strength of contraction

Some cells have unstable resting membrane potential and act as pacemakers

Question 1

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

Question 2

Describe initiation and spread of electrical activity throughout the heart

Answer 2

Sinoatrial node
- Pacemaker
- ~0.5 m/s

Annulus fibrosus
- Non-conducting

Atrioventricular node
- Delay box
- ~0.05 m/s

Bundle of His and Purkinje fibres
- Rapid conduction system
- ~5 m/s

Ensures coordinated contraction of the heart

Question 3

Correlate the various components of the electrocardioagram with the electrical events in the heart