CVS – Excitation

Master answer

Normal Impulse Generation

Occurs in the sinoatrial node

• Innervated by (predominantly right) vagal nerve and sympathetic nerves
• Upslope of phase 4 due to “Funny” Na current (inward flow of Na)
  • Parasympathetic (vagal) stimulation causes decrease (more negative) in the minima and shallower upward gradient of phase 4, delaying depolarization
  • Sympathetic stimulation causes an increase (more positive) in the minima and steeper upward gradient of phase 4, hastening depolarization
• Action potential generated when threshold potential reached (~-50-55mV) → Opening of voltage gated Ca channels → Ca influx
• Cell begins repolarization when Ca channels close and K channels open à K efflux

Normal Conduction

• SA node (5cm/sec) → Internodal fibres and Bachmann’s plexus (100cm/sec) → AV node (5cm/sec) → Bundle of His (100cm/sec) → Purkinje fibres (400cm/sec)
• Occurs due to fast wave action potentials (slow wave in AV node)

Ventricular AP:

• Phase 0 (Fast upstroke)
  • Occurs due to opening of voltage gated Na channels (Na influx)
    • Overshoot to +20mV
• Phase 1 (Early repolarization)
  • Inactivation of Na channels and opening of K channels (K efflux) → Start of absolute refractory period
• Phase 2
  • Opening of voltage gated Ca channels (Ca influx) balances K efflux
• Phase 3
  • Closure of Ca channels → Repolarization
  • Na channels transition to closed state (absolute refractory period transitions to relative refractory period)
• Phase 4
  • Resting membrane potential (-80mV~-90mV) maintained by Na/K ATPase

Features which Prevent Arrhythmias / Conduction Abnormalities

Preventing the generation of arrhythmias

• SA nodal rate has faster intrinsic firing rate than other areas (~30bpm) causing
  override inhibition of other pacemakers
  

Preventing the conduction of arrhythmias

• AV node prevents the conduction of impulses >220bpm
• Normal myocyte action potential has absolute (where no stimulus can initiate a new
  action potential) and relative (where a greater than normal stimulus is required for initiation of a new action potential) refractory periods
• Fibrous cardiac skeleton prevents conduction of action potentials other than through
  the specialized conduction pathways

Sakurai 2016

Answer by Sakurai