G1iii: Structure and functional significance of the excitatory and conductive elements of heart

  • Pacemaker = system that sends an electrical impulse to set the heart rhythm
    • PM cells are specialised myocytes found in the conductive tissue of the heart
  • Myocytes = UNIQUE → display:
    • AUTOMATICITY = intrinsic ability to generate a heartbeat in absence of external influences
    • RHYTHMICITY = generate heartbeat at regular intervals

SA Node

  • Wall of RA, lateral to opening of SVC
  • Almost nil contractile filaments
  • Fibres connect direct with atrial muscle fibres ∴AP spreads directing to muscle wall

SA node rhythmicity = 100bpm

  • High intrinsic rhythmicity suppresses automaticity at other loci
  • Basal vagal tone ∴normally ~70bpm

→ impulse travels through BACHMANN’S BUNDLE to LA

→ via internodal pathways: ant, middle, post to AV Node (spont rhythm 50bpm)

AV node

  • Located post wall RA at opening of Coronary Sinus
  • 13 sec delay due to ↓no gap junctions (↑resistance to conduction)
  • Allows
    1. One way conduction
    2. Full atrial contraction before commencement of ventricular contraction

Travels to Bundle of His & down R & L Bundle Branches

 Bundle branches terminate in Purkinje Fibres

  • Fibres are millions of small fibres projecting through myocardium
  • They are the largest myocytes + allow rapid propagation of AP
  • Almost as soon as signal reaches P. Fibres, it’s transmitted to entire ventricle wall
  • ∴organised contraction of ventricles

Papillary m. contract before ventricles, to prevent regurg. of blood via AV valves

  1. IV septum (except basal portion) first to be excited
  2. Then endocardium
  3. Then epicardium
  4. Then last portion is posterior basal epicardial & basal IV septum