Electrical and mechanical properties of heart

• Cardiac muscle


  • Ability to excite on its own
  • Contracts regularly
  • Whole chamber contracts almost simultaneously (functional syncytium & conducting system)
  • Does not get fatigued
  • Does not undergo prolonged, sustained contraction (tetanization)
  • Able to alter its function (e.g. rate, force)
• Phases of action potential of ventricular muscle fiber

  1. Rapid depolarization (Phase 0)

  2. Initial mild but rapid repolarization (Phase 1)

  3. Plateau (Phase 2)

  4. Rapid repolarization (Phase 3)

  5. Resting membrane potential (Phase 4)

  
• Pacemaker potential


  Gradual rise in potential between each two heartbeats due to inward-leaking sodium and calcium ions, until threshold voltage is reached to trigger action potential
• Resting membrane potential (RMP) of cardiac muscle is -90 mV

  
• Cardiac muscle has a long refractory period of 250-300 ms
• Automaticity


  Heart beats on its own without external stimulus
• Rhythmicity


  Heart beats at regular intervals
• Autorhythmicity


  Automaticity which is rhythmic - heart beats on its own without external stimulus at regular intervals
• Excitability


  Ability of cardiac muscle to respond to a stimulus of adequate strength & duration by generating an action potential
• Conductivity


  Property by which excitation/impulse is conducted through the cardiac tissues

  
• Conducting system of the heart

  • Sinus node
  • Internodal pathways
  • Atrioventricular (AV) node
  • AV bundle
  • Left and right bundle branches of Purkinje fibers

  
• AV nodal delay


  Caused by slow conduction in transitional fibres and AV nodal fibres, and few gap junctions connecting successive fibres
• Significance of AV nodal delay is to allow atria to empty before ventricles contract
• Purkinje system ensures synchronous excitation of the ventricular muscle
• Contractile response


  Ability of cardiac muscle to actively generate force to shorten and thicken to do work when stimulus applied
• All or none law

  Either full response or no response at any constant conditions
• Length-tension relationship


  The force generated by cardiac muscle depends on its contractility and initial length
• Frank-Starling law of the heart

  The force of contraction is directly proportional to the initial length (end diastolic volume) of the cardiac muscle fiber