heart –electrical properties
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Heart –Electrical Properties. Prof. K. Sivapalan. 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. Design of the circulatory system. - PowerPoint PPT PresentationTRANSCRIPT
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.