ESSAY 34- Characteristics of working myocardium. Excitation

• Myocardium


  The muscular layer of the heart which consists of cells called cardiac myocytes
• Myocardium


  • In RV it's made of 2 layers
  • In LV it's made of 3 layers
• Cardiac muscle


  Striated as a result of the arrangement of the actin and myosin filaments in the sarcomeres
• Cardiac muscle fibers


  • Smaller (about 15 micrometers) than most skeletal muscle fibers (10 - 100 micrometers)
• Myocardium


  • Consists of individual muscle cells with 1 - 2 centrally placed nuclei which branch, anastomose and are arranged in a linear array, each fiber is about 85 - 100 micrometers long
• Intercalated disk


  The junction between two cardiac muscle cells, which enables AP to spread to all cells, exciting them all at the same time
• Intercalated disk


  • Made up of three types of cell junctions: fascia adherents, desmosomes and gap junctions
• Cardiac muscle


  • More vascularized and has more abundant mitochondria than skeletal muscle (40% of volume vs. 2%)
  • Contains glycogen granules between the myofibrils
• Cardiac muscle


  Intrinsically rhythmic although it is regulated through nervous and hormonal mechanisms
• Characteristics of cardiomyocytes


  • Contractility- ability to contract
  • Excitability- ability to generate action potential
  • Conductivity- ability to spread electrical impulses through conductive system and myocardial cells
  • Automaticity- ability to generate AP without external stimulation
• Resting membrane potential of cardiac cells

  Between -85 and -90 mV, primarily determined by the ratio of intracellular-to-extracellular potassium
• Excitation-contraction coupling


  The series of events that link the action potential of the muscle cell membrane to muscular contraction
• Cardiac cell action potential

  1. Resting phase

  2. Rapid depolarisation

  3. Plateau

  4. Repolarisation
• Refractory period


  After an action potential, cardiac cells are unable to generate a new action potential for about 250ms
• Absolute refractory period


  Re-excitation cannot occur even with adequate stimulus
• Relative refractory period


  Re-excitation can occur if stimulates with stimulus of threshold or suprathreshold manner. Extrasystoles appear here
• Extrasystoles


  Additional heartbeats that occur outside the physiological heart rhythm, caused by a disturbance of the cardiac rhythm that occurs outside the sinus node
• Atrial fibrillation

  The most common type of abnormal heart rhythm, where the heart receives electrical signals that come from outside the SA node causing the atria to contract in a disorganized fashion
• Cardiac metabolism


  A series of chemical reactions leading to the conversion of substrates for energy production in the form of ATP to sustain cell function and allow contraction and also for the synthesis of building blocks to allow for growth, repair, and regeneration
• ATP is especially important for maintaining contraction of cardiomyocytes as well as calcium ions which are needed to initiate contraction
• Levels of ATP fall when there is not enough O2 and this will impair cardiac activity too