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Title: The Cardiac Conduction System 1- Introduction: The inherent and rhythmical electrical activity in the heart is responsible for its continuous beating. The source of this beating is a group of highly specialized cells called the autorhythmic cells. a- These cells are self-excitable b- They repeatedly generate action potentials that trigger heart contractions c- These cells will continue to beat even when removed from the body – this makes heart transplants possible – even after all nerves have been cut! d- Less than 1% of the heart cells set the rhythm e- Cells that are not autorhythmic (do not set the pace) are contractile fibers, which cause the heart to contract f- The autorhythmic cells act as the hearts pacemaker. They set the contraction for the entire heart. They form the conduction system. 1- The conduction system is the route that delivers the action potentials throughout the entire heart muscle. The conduction system ensures that the chambers of the heart are stimulated and contract in a coordinated manner. II- The conduction System A- SA Node B- AV Node C- AV Bundle D- R/L Bundle Branches E- Purkinje Fibers F- Diagram G- The SA Node initiates action potentials about 100 times per minute (faster than the autorhythmic fibers) For this reason the SA node is considered the pacemaker of the heart and sets the rate at which it beats. 1- Impulses from the autonomic nervous system and various hormones modify the strength and timing of the beats but do not change the rhythm. III – Electrocardiography – Transmission of action potentials through the conduction system generates an electric current. This current can be detected by electrodes placed on the outside of the body. This recording is called an electrocardiogram or EKG 1-The EKG is a series of up and down waves produced by the cardiac muscle fibers during each beat 2- We are used to seeing what is called a Single Lead II (righty arm to left) 3- Diagram A- P wave B- QRS Complex C- T-Wave 4- Variation in the size and duration of these waves can be used to diagnose a variety of cardiac muscle and conduction system abnormalities 5- Correlation of EKG waves with heart activity a- Systole b- Diastole 6- Cardiac output