Heart –Electrical Properties

Prof. K. Sivapalan

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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.

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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.

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Structure of the heart.

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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.

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Location of the heart.

• In the mediastinum.• Hanging on the large

vessels.• Lying on the

diaphragm.• Supported by fibrous

pericardium.

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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.

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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.

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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].

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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.

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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.

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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.

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Ionic basis of action potential.

• Depolarization – sodium influx.

• Plateau – calcium influx and potassium efflux.

• Repolarization – potassium efflux.

Na+.

Ca++

K+.

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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.

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Spread of impulse.

• SA node.• Inter nodal pathways

& atrial musculature.• AV node.• Bundle of His.• Bundle branches –

Purkinje fibers.• Cardiac muscles

through intercalated discs.

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Conducting system.

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Conduction of impulse.Tissue Conduction

rate. [M/S]Time taken-

[Sec.]

SA node 0.05

Atrial pathways. 1.0

AV node 0.05 0.1

Bundle of His. 1.0

Purkinje system 4.0 0.08

Ventricular muscle. 1.0

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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.