Electrical

• CO: the volume of blood that is squeezed per minute
• Contractile Cells contain

  • Contractile proteins (actin, myosin, troponin, tropomyosin)
  • Sarcoplasmic reticulum
  • Generate the force that pushes blood out of the heart
• The heart is innervated by nerves influencing heart rate, contractility, and cardiac output. The nervous system doesn't generate cardiac impulses but influences impulse rate, contractility, and cardiac output
• Rhythmical Excitation system of the Heart

  1. Responsible for generating rhythmical electrical impulses to cause rhythmical contraction of the heart muscle

  2. Functions in conducting impulses rapidly through the heart
• Excitation Sequence

  SA node fires to the Bachmann's bundle causing the left atrium to contract, then impulses propagate to the AV node where they are delayed. Action potential proceeds towards the interventricular septum, then reaches the AV bundle or bundle of His. The right bundle spreads the impulse across the right ventricle, and the left bundle across the left ventricle. The action potential then travels to Purkinje fibers and smaller units within the ventricles
• The heart is endowed with a special system for generating rhythm and pace
• SV: the volume of blood that is squeezed for one beat
• Types of myocardium cells

  • Nodal Cells
  • Contractile Cells
• The heart has many different cells that can generate action potentials by themselves
• Specialized Excitatory and Conductive system of the Heart

  • Sinus (sinoatrial) node
  • AV (atrioventricular) node
  • Bachmann's Bundle
  • Purkinje fibers
  • Left Bundle branch
  • Right Bundle branch
  • Internodal pathways
  • A-V bundle
  • Ventricular syncytium
• The heart has the ability to intrinsically depolarize itself and doesn’t depend on the central nervous system for that
• Automaticity


  The intrinsic ability of the heart to spontaneously depolarize itself and send action potentials to trigger the heart muscle to contract
• SA node


  Specialized ellipsoid strip of cardiac muscle, known as the pacemaker, directly linked to the
• SA node


  Specialized ellipsoid strip of cardiac muscle that is flat and small in size, directly linked to the right atrium
• Cardiac impulse from SA node to atrial muscle

  Spreads throughout the atrial muscle by two routes: Ordinary Atrial muscle fibers and Anterior, middle, and posterior conducting bundles (internodal bundles)
• Action potential travels towards specialized conducting fibers known as Purkinje fibers


  Link to smaller units within the ventricles
• The impulse after leaving SA node takes 0.03 sec to reach the AV node
• Slow Conduction in penetration A-V bundle fibers is caused by diminished number of gap junctions
• Delay of 0.09 sec in A-V node allows time for the atria to empty their blood into the ventricles before ventricular contraction begins, increasing the efficiency of pumping action of the heart
• Nodal cells contain
• Sinus rhythm of 60-80 beats per minute is generated without any assistance from the central nervous system
• SA node

  • Establishes sinus rhythm, located on the superior posterolateral wall of the right atrium, responsible for regulating the heart rate
• The AV node exhibits smaller fiber diameter, leading to a reduction in conduction velocity
• The A-V bundle allows only forward conduction from atria to ventricles, preventing re-entry of cardiac impulse from ventricles to atria
• The AV node has significantly fewer gap junctions compared to other nodal cells
• The total time for the cardiac impulse to travel from the bundle branches to the terminations of Purkinje fibers averages only 0.03 sec
• Nodal cells

  • Contain numerous gap junctions which enhance the speed of action potentials and facilitate the transfer of ions from one cell to another
• One-Way Conduction Through A-V Bundle
• Rapid conduction through Purkinje fibers ensures different parts of ventricles are excited almost simultaneously, increasing the efficiency of the heart as a pump
• Transmission of the Cardiac Impulse in the Ventricular Muscle
• Right and left bundle branches split from the AV bundle and are present on the respective sides of the ventricular septum
• The A-V bundle exhibits the inability of action potentials to travel backward from the ventricles to the atria
• Conduction velocity is influenced by the diameter of the fibers

  Larger diameters promote faster conduction, and smaller diameters result in slower conduction
• Mechanism of Sinus Nodal Rhythmicity
• Ventricular contraction


  1. Allows the atria to empty their blood into the ventricles before ventricular contraction begins

  2. Ventricles effectively collect the blood from the atria and push it into the aorta and pulmonary circulation

  3. Increasing the efficiency of pumping action of the heart
• Atrial muscle is separated from ventricular muscle by a continuous fibrous barrier acting as an insulator to prevent the passage of cardiac impulse between them except at AV bundle
• Ventricular contraction begins 0.1 - 0.2 sec after the contraction of atria
• The resting membrane potential of the SA node is destabilized by leaky Na+ channels allowing gradual entry of Na+ ions and activated Ca+2 channels
• Conduction velocity varies in myocardial tissues, being slowest in the AV node and fastest in the Purkinje fibers
• Cardiac muscle has 3 types of membrane ion channels - Slow Ca+2 channels, Fast Na+ channels, K+ channels - important in causing voltage changes of action potentials