Properties of Properties of Cardiac Cardiac MuscleMuscle

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ObjectivesObjectives Define the terms; Rhythmicity, Excitability, Conductivity

and Contractility. Describe cardiac syncytium. Outline the normal pathway of the cardiac impulse. Describe the excitation-contraction coupling in cardiac

muscles and compare it to excitation-contraction coupling in skeletal muscles.

Compare and contrast action potential in sinoatrial node and ventricular muscle.

Explain the significance of the plateau and refractory period in ventricular muscle action potential.

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Heart Composed of Three Heart Composed of Three LayersLayers

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Properties of Cardiac Properties of Cardiac Muscle FibersMuscle Fibers

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Exhibit branching Adjacent cardiac cells are joined end

to end by specialized structures known as intercalated discs

Within intercalated discs there are two types of junctions Desmosomes Gap junctions..allow action

potential to spread from one cell to adjacent cells.

Heart function as syncytiumwhen one cardiac cell undergoes an action potential, the electrical impulse spreads to all other cells that are joined by gap junctions so they become excited and contract as a single functional syncytium.

Atrial syncytium and ventricular syncytium

Histological Properties of Histological Properties of Cardiac Muscle FibersCardiac Muscle Fibers

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THE CARDIAC MUSCLETHE CARDIAC MUSCLE

Contractile muscle fibres (myocardium 99%) Atrial muscle fibres & Ventricular muscle fibres

- Both contract same as in sk. Muscle - Duration of contraction much longer Excitatory & conductive muscle fibres (autorhythmic

1%) - Few contractile fibrils (v.weak contraction) - Exhibit either automatic rhythmic discharge(AP) OR Conduction of the AP through heart

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Properties of Cardiac Muscle Properties of Cardiac Muscle FibersFibers1. Autorhythmicity: The ability to initiate a heart

beat continuously and regularly without external stimulation

2. Excitability: The ability to respond to a stimulus of adequate strength and duration (i.e. threshold or more) by generating a propagated action potential

3. Conductivity: The ability to conduct excitation through the cardiac tissue

4. Contractility: The ability to contract in response to stimulation

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1. Autorhythmicity1. Autorhythmicity

myogenic myogenic (independent of nerve supply)(independent of nerve supply)

due to the due to the specialized excitatory & conductive systemspecialized excitatory & conductive system of the of the heartheart

intrinsic ability of self-excitationintrinsic ability of self-excitation (waves of depolarization)(waves of depolarization) cardiac impulsescardiac impulses

Definition: the ability of the heart to initiate its beat continuously and regularly without external stimulation

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Have two important functions1. Act as a pacemaker (set the rhythm of electrical excitation)2. Form the conductive system (network of

specialized cardiac muscle fibers that provide a path for each cycle of cardiac excitation to progress through the heart)

Autorythmic fibersAutorythmic fibers

Forms 1% of the cardiac muscle fibers

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Sinoatrial node (SA node)Specialized region in right atrial wall near opening of superior vena cava.

Atrioventricular node (AV node)Small bundle of pecializedcardiac cells located at base of right atrium near septum

Bundle of His (atrioventricular bundle)Cells originate at AV node and enters interventricular septumDivides to form right and left bundle branches which travel down septum, curve around tip of ventricular chambers, travel back toward atria along outer walls

Purkinje fibersSmall, terminal fibers that extend from bundle of His and spread throughout ventricular myocardium

Locations of autorythmic cellsLocations of autorythmic cells

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Mechanism of Mechanism of AutorythmicityAutorythmicity Autorythmic cells do not

have stable resting membrane potential (RMP)

Natural leakiness to Na & Ca spontaneous and gradual depolarization

Unstable resting membrane potential (= pacemaker potential)

Gradual depolarization reaches threshold (-40 mv) spontaneous AP generation

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Rate of generation of AP at different sites of the heartRate of generation of AP at different sites of the heart

SITESITE RATERATE

(Times/min)(Times/min)

SA nodeSA node 100100

AV nodeAV node 40 - 6040 - 60

AV bundle, bundle AV bundle, bundle branches,& Purkinje branches,& Purkinje fibresfibres

20 - 3520 - 35

SA node acts as heart pacemaker because it has the fastest rate of generating action potentialNerve impulses from autonomic nervous system and hormones modify the timing and strength of each heart beat but do not establish the fundamental rhythm.

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Non-SA nodal tissues are Non-SA nodal tissues are latent pacemakers latent pacemakers that can that can take over (at a slower rate), should the normal take over (at a slower rate), should the normal pacemaker (SA node )failpacemaker (SA node )fail

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2. Excitability2. Excitability

Definition: The ability of cardiac muscle to respond to a stimulus of adequate strength & duration by generating an AP

AP initiated by SA nodetravels along conductive

pathway excites atrial & ventricular muscle fibres

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Action potential in Action potential in contractile fiberscontractile fibers

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Refractory periodRefractory period LongLong refractory period (250 refractory period (250

msec) compared to skeletal msec) compared to skeletal muscle (3msec)muscle (3msec)

During this period During this period membrane membrane is refractory to further is refractory to further stimulationstimulation until contraction until contraction is over.is over.

It lasts longer than muscle It lasts longer than muscle contraction, contraction, prevents tetanusprevents tetanus

Gives time to heart to relax Gives time to heart to relax after each contraction, after each contraction, prevent fatigueprevent fatigue

It It allows timeallows time for the heart for the heart chambers to fill during chambers to fill during diastole before next diastole before next contractioncontraction

AP in skeletal muscle : 1-5 msecAP in cardiac muscle :200 -300 msec17

3. Contractility3. ContractilityDefinition: ability of cardiac muscle to contract in

response to stimulation

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Excitation-Contraction Excitation-Contraction Coupling in Cardiac Coupling in Cardiac Contractile CellsContractile Cells

Similar to that in Similar to that in skeletal musclesskeletal muscles

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4. Conductivity4. ConductivityDefinition:Definition: property by which excitation is conducted property by which excitation is conducted

through the cardiac tissuethrough the cardiac tissue

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Criteria for spread of excitation & Criteria for spread of excitation & efficient cardiac functionefficient cardiac function

1. Atrial excitation and contraction should be complete before onset of ventricular contraction

- ensures complete filling of the ventricles during diastole

2. Excitation of cardiac muscle fibres should be coordinated ensure each heart chamber contracts as a unit accomplish efficient pumping

- smooth uniform contraction essential to squeeze out blood

3. Pair of atria & pair of ventricles should be functionally co-ordinated both members contract simultaneously

- permits synchronized pumping of blood into pulmonary & systemic circulation

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Tissue Conduction rate (m/s)

Atrial muscle

0.3

Atrial pathways

1

AV node 0.05

Bundle of His

1

Purkinje system

4

Ventricular muscle

0.3-0.5

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Cardiac impulse originates at SA nodeCardiac impulse originates at SA node Action potential spreads throughout right and left atriaAction potential spreads throughout right and left atria Impulse passes from atria into ventricles through AV Impulse passes from atria into ventricles through AV

node (only point of electrical contact between chambers)node (only point of electrical contact between chambers) Action potential briefly delayed at AV node (ensures Action potential briefly delayed at AV node (ensures

atrial contraction precedes ventricular contraction to atrial contraction precedes ventricular contraction to allow complete ventricular filling)allow complete ventricular filling)

Impulse travels rapidly down interventricular septum by Impulse travels rapidly down interventricular septum by means of bundle of Hismeans of bundle of His

Impulse rapidly disperses throughout myocardium by Impulse rapidly disperses throughout myocardium by means of Purkinje fibersmeans of Purkinje fibers

Rest of ventricular cells activated by cell-to-cell spread of Rest of ventricular cells activated by cell-to-cell spread of impulse through gap junctionsimpulse through gap junctions

Spread of Cardiac ExcitationSpread of Cardiac Excitation

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