Heart –Electrical Properties Prof. K. Sivapalan

Electrical Properties Introduction. Function of the blood is transport of substances. Function of the heart and vessels are: Keeping blood flowing. Delivering more blood to needy tissue. 2013 Electrical Properties

Design of the circulatory system. All blood goes to lungs in pulmonary circulation The blood flows to all other organs [including heart] in systemic circulation. 2013 Electrical Properties

Electrical Properties Structure of the heart. 2013 Electrical Properties

Components of the pumping system. Collectors- atria. Pumps- intermittent pump – ventricles. Regulators of flow - valves. Rhythm control – conducting system. Adjustments – by autonomic nerves and hormones. 2013 Electrical Properties

Electrical Properties Location of the heart. In the mediastinum. Hanging on the large vessels. Lying on the diaphragm. Supported by fibrous pericardium. 2013 Electrical Properties

Properties of cardiac muscle. Branching cells. Separated by intercalated discs – tight junctions with pores permeable to ions. [electrical continuity] Functional syncytium. Striations – similar to skeletal muscles. 2013 Electrical Properties

Sarcomere, filaments and fibrils. Z lines – centre of actin filaments. M line – centre of myosin filaments. A band – length of myosin filaments. Sarcomere is a unit of myofibrils between two Z lines. 2013 Electrical Properties

Myofibrils and T tubular system. Myofibrils - bundle of actin + myosin [Yellow] Mitochondria [blue]. Sarcoplasmic reticulum + T tubules [pink] at Z line. Intercalated discs at Z line [light blue]. Central nucleus [purple]. 2013 Electrical Properties

Excitation contraction coupling. Action potential spreads across intercalated discs. Spreads along T tubules [Z line] to Terminal cistern. Calcium released from cistern and influx from ECF. Actin myosin binding and sliding. Removal of Calcium results in relaxation. 2013 Electrical Properties

Electrical Properties Contraction. Actin and myosin do not overlap in a relaxed muscle. Calcium binding to Troponin C initiates sliding. Contraction can not reduce length to zero. In heart, there will be residual blood after maximal contraction. 2013 Electrical Properties

Electrical properties of cardiac muscle. Resting membrane potential – 85 – 95 mV. Depolarized to +20 mV. Rising phase – 2 m sec. Plateau – 0.15-0.2 sec in atrium and 0.3 in ventricles. Refractory period – 0.3 sec. 2013 Electrical Properties

Ionic basis of action potential. Depolarization – sodium influx. Plateau – calcium influx and potassium efflux. Repolarization – potassium efflux. Na+. Ca++ K+. 2013 Electrical Properties

Origin of Cardiac Impulse. Pacemaker – junctional tissue. Pacemaker potential – after each impulse declines to firing level. Rate of action potential depends on the slope of the prepotential. It is due to reduction of K+ efflux (↑ by Ach) and then increase in Ca++ influx (↑ by NA). Ca++ T (transient) channels complete prepotential and L (long lasting) action potentials [no sodium] in nodal tissues. SA node – 120/min, AV node – 45/min, Purkinje system – 35/min. First area to reach threshold will be the pace maker. Origin of Cardiac Impulse. 2013 Electrical Properties

Electrical Properties Spread of impulse. SA node. Inter nodal pathways & atrial musculature. AV node. Bundle of His. Bundle branches – Purkinje fibers. Cardiac muscles through intercalated discs. 2013 Electrical Properties

Spread of impulse in the heart. SA node to AV node. Nodal delay. Septum – left to right. Apex and wall – from inside outwards. Towards base. Examination – ECG. 2013 Electrical Properties