Describe the conducting system of the heart. Explain the genesis of the pacemaker potential and the properties of cardiac muscle.

Components of the Conducting System

• The SA node is the pacemaker because it has the fastest rate of spontaneous depolarisation and therefore reaches threshold first — it overdrive-suppresses the rest
• Blood supply — SA node from the right coronary artery in 60%; AV node from the right coronary in 90%

Conduction Velocities

AV nodal delay

• Duration 0.09–0.13 second
• Cause — small fibre diameter, fewer gap junctions, low RMP (−60 mV)
• Significance — allows the atria to complete their contraction and empty into the ventricles before ventricular systole begins

Pacemaker Potential (sa Node)

• RMP is unstable — only −55 to −60 mV
• Slow spontaneous depolarisation is called the prepotential or pacemaker potential

If ("funny") current — slow Na+ influx → T-type Ca2+ channels open → Threshold (−40 mV) reached → L-type Ca2+ influx → upstroke → K+ efflux → repolarisation

• The SA node upstroke is due to Ca2+, not Na+ → slow, no overshoot
• Sympathetic → ↑ slope of prepotential → tachycardia
• Vagus → ↑ K+ permeability → hyperpolarisation → bradycardia

Ventricular Action Potential

• Duration 250–300 ms — far longer than in skeletal muscle
• The plateau prolongs the refractory period so that it lasts almost the whole contraction → tetanus is impossible → the heart must relax and refill between beats

Properties of Cardiac Muscle

• Rhythmicity (autorhythmicity) — beats without any nerve supply; the denervated and even the excised heart continues to beat
• Conductivity — spreads as a functional syncytium via intercalated discs
• Excitability — with a long refractory period
• Contractility — obeys the all-or-none law as a whole
• Staircase (Treppe) phenomenon — force increases over the first few beats
• Refractory period 250–300 ms — the longest in the body

Applied Aspects

• Heart block

– First degree — PR interval > 0.2 s

– Second degree — some P waves not conducted (Mobitz I and II)

– Third degree (complete) — atria and ventricles beat independently; ventricular rate 30–40/min → Stokes–Adams attacks

• Ectopic pacemaker — when a focus fires faster than the SA node, or when SA conduction is blocked
• Sick sinus syndrome → requires an artificial pacemaker
• Digitalis slows AV conduction → used to control ventricular rate in atrial fibrillation
• Hyperkalaemia → depresses conduction → widened QRS → cardiac arrest
• Wolff–Parkinson–White syndrome — an accessory bundle of Kent bypasses the AV node → short PR interval with a delta wave → re-entrant tachycardia
• Vagal manoeuvres (carotid massage, Valsalva) slow AV conduction and can terminate supraventricular tachycardia
• Artificial pacemaker — indicated in complete heart block and sick sinus syndrome