Unsourced material may electrical activity of the heart pdf challenged and removed. This process allows blood to be pumped throughout the body.
Dysfunction of the conduction system can cause irregular, fast, or slow heart rhythms. In a functional syncytium, electrical impulses propagate freely between cells in every direction, so that the myocardium functions as a single contractile unit. This property allows rapid, synchronous depolarization of the myocardium. While advantageous under normal circumstances, this property can be detrimental, as it has potential to allow the propagation of incorrect electrical signals. Embryologic evidence of generation of the cardiac conduction system illuminates the respective roles of this specialized set of cells. Cardiac muscle has some similarities to neurons and skeletal muscle, as well as important unique properties.
SA node cells, most important, the spontaneous depolarizations necessary for the SA node’s pacemaker activity. Coordinated contraction of ventricular cells. Ventricular contraction begins at the apex of the heart, progressing upwards to eject blood into the great arteries. Contraction that squeezes blood towards the exit is more efficient than a simple squeeze from all directions.
Depolarization propagates through cardiac muscle very rapidly. Cells of the ventricles contract nearly simultaneously. The action potentials of cardiac muscle are unusually sustained. This prevents premature relaxation, maintaining initial contraction until the entire myocardium has had time to depolarize and contract.
After contracting, the heart must relax to fill up again. Note that the red lines represent the depolarization wave, not bloodflow. The AV node functions as a critical delay in the conduction system. The bundle of His splits into two branches in the interventricular septum: the left bundle branch and the right bundle branch.
The left bundle branch is short, splitting into the left anterior fascicle and the left posterior fascicle. The left posterior fascicle is relatively short and broad, with dual blood supply, making it particularly resistant to ischemic damage. The left posterior fascicle transmits impulses to the papillary muscles, leading to mitral valve closure. As the left posterior fascicle is shorter and broader than the right, impulses reach the papillary muscles just prior to depolarization, and therefore contraction, of the left ventricle myocardium.
This allows pre-tensioning of the chordae tendinae, increasing the resistance to flow through the mitral valve during left ventricular contraction. This mechanism works in the same manner as pre-tensioning of car seatbelts. It is the restoring of the resting state. In the ECG, repolarization includes the J point, ST-segment, and T- and U-waves.