Cardiac Muscle Prof. K. Sivapalan

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

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. June 2013 Cardiac muscle

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

Sarcomere, filaments and fibrils. Z lines – center of actin filaments. M line – center of myosin filaments. A band – length of myosin filaments. Sarcomere is a unit of myofibrils between two Z lines. June 2013 Cardiac muscle

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]. June 2013 Cardiac muscle

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. June 2013 Cardiac muscle

Non-tetanization The muscle twitch lasts for about 300 ms. The refractory period extends until more than half of the relaxation period June 2013 Cardiac muscle

Initial Length and Force Initial length is proportional to the force of contraction Starling’s law Excessive stretch- reduction of force [as in skeletal muscle] June 2013 Cardiac muscle

Conducting system. SA node. Inter nodal pathways & atrial musculature. AV node. Bundle of His. Bundle branches – Purkinje fibers. Cardiac muscles through intercalated discs. June 2013 Cardiac muscle

Properties of Conducting System 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. Properties of Conducting System June 2013 Cardiac muscle

Innervation No motor end plates- nerves end in varicosities Sympathetics innervate the nodes and myocardium [noradrenaline] 10th cranial nerve, vagus, innervates SA and AV nodes of the heart [acetyl choline]. Stimulation causes chronotropic and ionotropic effects. June 2013 Cardiac muscle