TAILIEUCHUNG - Lecture Human anatomy and physiology - Chapter 18: The cardiovascular system: The heart (part b)

After completing this chapter, students will be able to: Describe the structural and functional properties of cardiac muscle, and explain how it differs from skeletal muscle; briefly describe the events of cardiac muscle cell contraction; name the components of the conduction system of the heart, and trace the conduction pathway;. | The Cardiovascular System: Part B: The Heart: Depolarization of the heart is rhythmic and spontaneous About 1% of cardiac cells have automaticity— (are self-excitable) Gap junctions ensure the heart contracts as a unit Long absolute refractory period (250 ms) Intrinsic cardiac conduction system A network of noncontractile (autorhythmic) cells that initiate and distribute impulses to coordinate the depolarization and contraction of the heart Sinoatrial (SA) node (pacemaker) Generates impulses about 75 times/minute (sinus rhythm) Depolarizes faster than any other part of the myocardium Atrioventricular (AV) node Smaller diameter fibers; fewer gap junctions Delays impulses approximately second Depolarizes 50 times per minute in absence of SA node input Atrioventricular (AV) bundle (bundle of His) Only electrical connection between the atria and ventricles Right and left bundle branches Two pathways in the interventricular septum that carry the impulses toward the apex of the heart Purkinje fibers Complete the pathway into the apex and ventricular walls AV bundle and Purkinje fibers depolarize only 30 times per minute in absence of AV node input Figure (a) Anatomy of the intrinsic conduction system showing the sequence of electrical excitation Internodal pathway Superior vena cava Right atrium Left atrium Purkinje fibers Inter- ventricular septum 1 The sinoatrial (SA) node (pacemaker) generates impulses. 2 The impulses pause ( s) at the atrioventricular (AV) node. The atrioventricular (AV) bundle connects the atria to the ventricles. 4 The bundle branches conduct the impulses through the interventricular septum. 3 The Purkinje fibers depolarize the contractile cells of both ventricles. 5 Defects in the intrinsic conduction system may result in Arrhythmias: irregular heart rhythms Uncoordination of atrial and ventricular contractions Fibrillation: rapid, irregular contractions; useless for pumping blood Heartbeat is modified by the Autonomic Nervous System

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