05. Cardiac Electrical Activity

• sinoatrial node is the pacemaker that spontaneously generates action potentials
• atrioventricular node causes delay, also spontaneously generates action potentials but slower than the sinoatrial node
• pacemaker membrane potential = constant updrift towards threshold value

  • found in sinoatrial node
  • also found in atrioventricular node but more gradual slope so slower action potential generation
• pacemaker potential in sinoatrial node caused by inwards Na+ movement there through HCN channels and inwards Ca2+ movement

  • this outweighs outwards K+
  • once threshold reached voltage gated Ca2+ channels open
  • repolarisation occurs via voltage gated K+ channels
• sympathetic activity on the sinoatrial node:

  • noradrenaline acts on beta 1 NAdRs to increase heart rate via increased Na+ and Ca2+ influx
  • leads to steeper slope of pacemaker potential
  • positive chronotropic effect
• parasympathetic activity of sinoatrial node:

  • ACh from vagus nerve acts on M2 AChRs to decrease heart rate via an increase in K+ efflux
  • flatter slope
  • negative chronotropic effect
• vagal tone dominates over sino-atrial node so heart rte at rest is 60-70bpm compared to 80-100
• AV delay ensures atrial depolarisation, contraction and ejection before ventricles depolarise
• heart rate is driven by the fastest pacemaker, the sinoatrial node
• action potential of ventricular cardiac myocytes:

  1. rapid depolarisation - opening of voltage gated Na+ channels leads to influx of Na+
  2. partial rapid repolarisation - inactivation of Na+ and activation of VG gated K+ channels
  3. plateau - opening of voltage gated Ca2+ channels, inwards Ca2+ and outward K+ means balanced
  4. terminal repolarisation - outwards K+ due to delayed rectifier K+ channel opening
• absolute refractory period = no chance of action potential being triggered
• relative refractory period = can generate action potential but more difficult
• refractory period is determined by the number of available and recovered voltage gated Na+ channels
• Na+ channels recover faster at more negative membrane potentials - long action potential = longer refractory period
• cardiac myocytes have gap junctions allowing passage of positively charged ions if there is a charge gradient - can trigger action potential in an adjacent cell

  • determined by charge gradient between cells
  • conduction velocity is set by magnitude of depolarising current
  • can be modified by gap junction expression/function