G2ii: Outline normal impulse generation in heart and conduction in the heart. Describe the features in a normal heart that prevent generation and conduction of arrhythmias

Electrical conducting system = most important control of cardiac cycle

Impulse Generation

  • Occurs in specialised PM cells
  • No true RMP
  • Spontaneous slow decay
  • Their membrane naturally leaky to Na+/Ca2+
  • Reaches threshold = -40mV
  • Slow Type L Ca2+ open
    • Slow inward Ca2+ movement
    • Slow upstroke (depol)
  • Spontaneous decay influenced by ANS
    • SYMP: ↓K out & ↑Na/Ca in
      • =↑ slope Ph4
      • =↓time to reach threshold
      • =Faster d/c rate
    • PARASYMP: ↑K out & ↓Na/Ca in
      • =↓ slope Ph4
      • =More time taken to reach threshold
      • =↓d/c rate
      • =more negative membrane potential ∴more time taken to reach threshold

Normal Conduction Pathway

  • SA Node = high intrinsic frequency = 100bpm
  • Suppresses automaticity at other loci
  • Basal vagal tone → ∴normal HR ~70bpm
  • Impulse travels via 3 internodal pathways (ant., middle, post.) to AV node
  • @AV node 13sec delay due to ↓ no. gap junctions
  • Travels to Bundle of His
  • Down R & L bundle branches
  • Divide into PURKINJE FIBRES
    • Small fibres project through myocardium
    • Rapid propagation of AP @400cm/sec
    • Papillary contracts before ventricles → prevents regurg. of blood via AV valves
    • IV septum (except base)
    • Endocardium
    • Epicardium
    • Last = Post basal epicardial & basal IV septum
  • Almost as soon as signal reaches Purkinje, it’s transmitted to entire ventricle mass for organised ventricular contraction

Features Preventing Abnormal Conduction

  • Conducting pathways
    • Conducting tissue has multiple IC discs & gap junctions
    • If upstream PM is blocked, down stream PM can take over
  • Fibrous skeleton & AV node
    • Fibrous skeleton prevents conduction between atria & ventricles
    • AV node conducts v slowly
    • Allows full atrial contraction
    • Prevents conduction of rapid rates >220bpm
    • Ensures one way conduction
  • Refractory period
    • The time taken from Ph 0 until next possible depolarisation of a myocyte
    • Cells which are not PM cells (have fast AP) have different degrees of refraction depending on no. of Na+ channels which have recovered from inactive state

Absolute refractory = no stimulus can excite myocyte

  • RRP = supramaximal stimulus will depolarise the cell & cause an AP (not all Na+ channels recovered yet)