Arrhythmias Part 1: myocytes and electrical control of the heart

What are the learning outcomes of this lecture? (3 points)

1. Describe the physiology of myocytes and explain the myogenic origin of the heartbeat

2. Explain the sliding filament theory and excitation-contraction coupling

3. Explain how the ECG relates to the stages of the cardiac cycle

What are key characteristics of the myocardium and myocytes? (4 po

. Myocardium resembles skeletal more than smooth muscle

2. Myocytes have high mitochondrial density due to high energy demands

3. Myocytes are connected by 'gap junctions' enabling electrical conduction

4. Myocytes maintain intracellular/extracellular ion gradients essential for excitability

What are typical ionic concentrations in and out of myocytes? (6 points)

1. Extracellular: K+ = 4 mM, Na+ = 140 mM, Ca2+ = 1.5 mM

2. Intracellular: K+ = 120 mM, Na+ = 10 mM, Ca2+ = 100 nM

What transport mechanisms are used in myocyte ionic regulation? (3 points)

. Na+/K+ Pump: 3 Na+ out, 2 K+ in

2. Ca2+ ATPase: actively pumps Ca2+ out

3. Na+/Ca2+ exchanger: 3 Na+ in for 1 Ca2+ out

What is the conduction pathway of electrical impulses in the heart? (9 points)

1. Sinoatrial (SA) node

2. Atrioventricular (AV) node

3. Bundle of His

4. Left bundle branch

5. Right bundle branch

6. Left anterior fascicle

7. Left posterior fascicle

8. Ventricular septum

9. Left and right ventricles

What is the origin and nature of electrical activity in myocytes? (3 poin

1. Electrical signals are generated within the heart (myogenic)

2. Most myocytes can display spontaneous activity

3. Heart rate is primarily driven by SA node (the pacemaker)

Q: What is the sliding filament theory of muscle contraction? (4 points)

1. Involves actin and myosin filaments sliding over one another

2. Tropomyosin and troponin regulate the binding of actin and myosin

3. Calcium ions bind to troponin to initiate contraction

4. Myosin heads pull actin filaments inward during contraction

What is excitation-contraction coupling in cardiac muscle? (3 points)

1. Action potentials trigger calcium influx

2. Calcium binds to troponin initiating filament interaction

3. This leads to muscle contraction

Q: How does the ECG relate to cardiac electrical activity? (3 points)

1. Measures voltage changes due to membrane depolarisation

2. Electrical currents flow through the body and are detected on skin

3. Represents different phases of the cardiac cycle