CARDIAC
ELECTROPHYSIOLOGY
Comprehensive Study Notes
Electrical Properties of the Heart
1
, I. Why the Heart Is Self-Excitatory — Electrophysiological
Background
Cell Types & Their Properties
• Contractile (working) cells — atrial & ventricular cardiomyocytes (99% of myocardium)
◦ Pacemaker / impulse-conducting cells (specialized conduction system)
◦ Fo/F-type K⁺ channels → pacemaker current (If)
◦ Low-threshold T-type Ca²⁺ channels (ICa,T)
• Pacemaker cells (SA node) — generate spontaneous APs — TRUE automaticity
◦ SA and AV nodes: dominant/midcardiac pacemakers
• Inert (latent) pacemakers: His bundle / Tawara branches / Purkinje fibres (~1% of myocardium)
Heart rate set by SA node (~60–80 bpm in myocardium).
⚠ Cardiomyocytes can generate 100–160 bpm; Purkinje fibres only ~15–25/min
Autorhythmicity
• cardiac cells are excitable (they have rest MP, when stimulated AP)
• SA node fires spontaneously → AP spreads: atria → AV node → His → Purkinje → ventricles
Key Ion Channels Behind Automaticity
If (Funny current) — HCN channels, activated by hyperpolarisation, carries Na ⁺/K ⁺ inward
ICa,T — T-type Ca²⁺ channel, low threshold, contributes to late diastolic depolarisation
ICa,L — L-type Ca²⁺ channel, responsible for AP upstroke in nodal cells
IK — Delayed rectifier K⁺, responsible for repolarisation
II. Action Potential in Cardiac Cells
Resting Membrane Potential & ECG Correlation
• Resting MP: ~−90 mV (ventricular myocytes) — stable, NOT spontaneously depolarising
• Triggered by adjacent AP (electrotonic spread) once threshold is exceeded
ECG Wave Underlying Electrical Event
P wave Atrial depolarisation (SA → AV node propagation)
QRS complex Ventricular depolarisation (rapid AP spread via His-Purkinje)
T wave Ventricular repolarisation
U wave Late repolarisation (Purkinje / papillary muscles)
2
ELECTROPHYSIOLOGY
Comprehensive Study Notes
Electrical Properties of the Heart
1
, I. Why the Heart Is Self-Excitatory — Electrophysiological
Background
Cell Types & Their Properties
• Contractile (working) cells — atrial & ventricular cardiomyocytes (99% of myocardium)
◦ Pacemaker / impulse-conducting cells (specialized conduction system)
◦ Fo/F-type K⁺ channels → pacemaker current (If)
◦ Low-threshold T-type Ca²⁺ channels (ICa,T)
• Pacemaker cells (SA node) — generate spontaneous APs — TRUE automaticity
◦ SA and AV nodes: dominant/midcardiac pacemakers
• Inert (latent) pacemakers: His bundle / Tawara branches / Purkinje fibres (~1% of myocardium)
Heart rate set by SA node (~60–80 bpm in myocardium).
⚠ Cardiomyocytes can generate 100–160 bpm; Purkinje fibres only ~15–25/min
Autorhythmicity
• cardiac cells are excitable (they have rest MP, when stimulated AP)
• SA node fires spontaneously → AP spreads: atria → AV node → His → Purkinje → ventricles
Key Ion Channels Behind Automaticity
If (Funny current) — HCN channels, activated by hyperpolarisation, carries Na ⁺/K ⁺ inward
ICa,T — T-type Ca²⁺ channel, low threshold, contributes to late diastolic depolarisation
ICa,L — L-type Ca²⁺ channel, responsible for AP upstroke in nodal cells
IK — Delayed rectifier K⁺, responsible for repolarisation
II. Action Potential in Cardiac Cells
Resting Membrane Potential & ECG Correlation
• Resting MP: ~−90 mV (ventricular myocytes) — stable, NOT spontaneously depolarising
• Triggered by adjacent AP (electrotonic spread) once threshold is exceeded
ECG Wave Underlying Electrical Event
P wave Atrial depolarisation (SA → AV node propagation)
QRS complex Ventricular depolarisation (rapid AP spread via His-Purkinje)
T wave Ventricular repolarisation
U wave Late repolarisation (Purkinje / papillary muscles)
2
