21.8 The Cardiac Cycle

• • Nodal cells within the heart spontaneously depolarize to threshold, generating an action potential. These action potentials spread throughout the conducting system of the heart to the cardiac muscle cells, initiating a contraction and a cardiac cycle.
• The plasma membranes of nodal cells possess unique qualities that allow these cells to spontaneously depolarize to threshold
• Nodal cells possess intercellular junctions that electrically couple cells to one another, to conducting fibers, and to cardiac muscle cells
• When a nodal cell depolarizes, it generates an action potential
• The action potential travels through the conducting system of the heart and reaches all the cardiac muscle tissue, causing a contraction and a cardiac cycle, or a complete heartbeat
• Rapidly depolarizing cells are called pacemaker cells
• Pacemaker cells are found in the sinoatrial node (SA node), or cardiac pacemaker
• The SA node is located in the posterior wall of the right atrium, near the entrance of the superior vena cava
• Bradycardia indicates a slower than normal heart rate
• Tachycardia refers to a faster than normal heart rate
• The cells of the SA node are electrically connected to those of the larger atrioventricular node (AV node) through conducting fibers in the atrial walls
• SUMMARY

  • Nodal cells depolarize spontaneously and determine heart rate
  • Pacemaker cells found in sinoatrial (SA) node (cardiac pacemaker) normally establish rate of contraction
  • From SA node, stimulus travels over internodal pathways to the atrioventricular (AV) node, then to the AV bundle, which divides into a right and left bundle branch. From there Purkinje fibers convey the impulses to the ventricular myocardium
  • Cardiac cycle consists of periods of atrial and ventricular systole (contraction) and atrial and ventricular diastole