Heart –Electrical Properties

1. Heart –Electrical Properties Prof. K. Sivapalan

2. 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. Electrical Properties

3. 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. Electrical Properties

4. Structure of the heart. Electrical Properties

5. 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. Electrical Properties

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

7. 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. Electrical Properties

8. 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. Electrical Properties

9. 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]. Electrical Properties

10. 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. Electrical Properties

11. 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. Electrical Properties

12. 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. Electrical Properties

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

14. 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. Electrical Properties

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

16. Conducting system. Electrical Properties

17. Conduction of impulse. Electrical Properties

18. 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. Electrical Properties