Cardiac Muscle

Prof. K. Sivapalan

June 2013

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.

Cardiac muscle 2

June 2013

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.

Cardiac muscle 3

June 2013

Ionic basis of action potential.

• Depolarization – sodium influx.

• Plateau – calcium influx and potassium efflux.

• Repolarization – potassium efflux.

Na+.

Ca++

K+.

Cardiac muscle 4

June 2013

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.

Cardiac muscle 5

June 2013

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].

Cardiac muscle 6

June 2013

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.

Cardiac muscle 7

Cardiac muscle 8

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 9

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

June 2013

Conducting system.

• SA node.

• Inter nodal pathways & atrial musculature.

• AV node.

• Bundle of His.

• Bundle branches – Purkinje fibers.

• Cardiac muscles through intercalated discs.

Cardiac muscle 10

June 2013

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.

Cardiac muscle 11

June 2013

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.

Cardiac muscle 12