Cardiac muscles excitation contraction

CARDIAC MUSCLESCARDIAC MUSCLES

EXCITATION-CONTRACTION EXCITATION-CONTRACTION COUPLING MECHANISMCOUPLING MECHANISM

GAP JUNCTIONSGAP JUNCTIONS Proteinaceous tubes that connect Proteinaceous tubes that connect

cellscells Permit ions to flow from one cell to Permit ions to flow from one cell to

another without having to pass from another without having to pass from plasma membrane of the cellplasma membrane of the cell

Dissolved substances such as glucose, Dissolved substances such as glucose, ions can pass through gap junctionsions can pass through gap junctions

SYNCYTIUMSYNCYTIUM

Action potential can pass from one direction Action potential can pass from one direction to anotherto another

InterconnectionsInterconnections

Two kinds of syncytiumsTwo kinds of syncytiums Atrial syncytiumAtrial syncytium Constitute the walls of 2 atriaConstitute the walls of 2 atria

Ventricular syncytiumVentricular syncytium Constitute the walls of 2 ventriclesConstitute the walls of 2 ventricles

BARRIERS TO CONDACTIONBARRIERS TO CONDACTION Separation of atria from ventricles by Separation of atria from ventricles by

means of fibrous band tissuemeans of fibrous band tissue High electrical resistanceHigh electrical resistance Propogation of myocardial action Propogation of myocardial action

potential from atria to ventriclespotential from atria to ventricles By specialized conductive system By specialized conductive system

called AV node or AV bundlecalled AV node or AV bundle

IMPORTANT SEQUENCE OF IMPORTANT SEQUENCE OF TWO SYNCYTIUM STRUCTURESTWO SYNCYTIUM STRUCTURES

The separate structures allow the The separate structures allow the atria to contract in time slightly atria to contract in time slightly before the ventriclesbefore the ventricles

A condition central to cardiac A condition central to cardiac pumping effectivenesspumping effectiveness

ACTION POTENTIAL IN ACTION POTENTIAL IN CARDIAC MUSCLESCARDIAC MUSCLES

Resting membrane potentialResting membrane potential -85mv to -90mv for normal cardiac -85mv to -90mv for normal cardiac

musclesmuscles -90 to -100mv for specialized fibers -90 to -100mv for specialized fibers

e.g bundle of his, AV nodal fiberse.g bundle of his, AV nodal fibers Magnitude for ventricular muscles Magnitude for ventricular muscles

about 105mv i.e going from about -about 105mv i.e going from about -85mv to +20mv 85mv to +20mv

IONS AND MYOCARDIAL IONS AND MYOCARDIAL ACTION POTENTIALACTION POTENTIAL

Abrupt change in membrane potentialAbrupt change in membrane potential Corresponds to initial spikeCorresponds to initial spike Depolarization duration=about 0.2 s for Depolarization duration=about 0.2 s for

atrial muscle and 0.3 sec for ventricular atrial muscle and 0.3 sec for ventricular musclesmuscles

Plateau phasePlateau phase Extends the duration of action potential Extends the duration of action potential

significantly 3-15 times more as compared significantly 3-15 times more as compared to skeletal musclesto skeletal muscles

IONS AND MYOCARDIAL ACTION IONS AND MYOCARDIAL ACTION POTENTIALPOTENTIAL

Sudden opening of fast sodium Sudden opening of fast sodium channelschannels

Allow to enter sodium ions from Allow to enter sodium ions from extracellular fluidextracellular fluid

Fast channelsFast channels Duration of openingDuration of opening Few thousands of secFew thousands of sec Abrupt closureAbrupt closure

SLOW RESPONSE DUE TO SLOW RESPONSE DUE TO ACTIVATION OF SLOW CALCIUM ACTIVATION OF SLOW CALCIUM

CHANNELSCHANNELS Fast sodium channelsFast sodium channels Slow calcium channelsSlow calcium channels Also called calcium-sodium channelsAlso called calcium-sodium channels Slower to openSlower to open Remain open for several tenths of secRemain open for several tenths of sec Prolonged period of contractionProlonged period of contraction Formation of plateauFormation of plateau

RECOVERY OF RESTING RECOVERY OF RESTING MEMBRANE POTENTIALMEMBRANE POTENTIAL

Inactivation of sodium-calcium Inactivation of sodium-calcium channelschannels

Membrane permeability of potassium Membrane permeability of potassium increasesincreases

Potassium efflux results in Potassium efflux results in repolarization increasesrepolarization increases

MYOCARDIAL ACTION MYOCARDIAL ACTION POTENTIALPOTENTIAL

PLATEAU FORMATIONPLATEAU FORMATION

ACTION POTENTIAL SPREADACTION POTENTIAL SPREAD Over the surfaceOver the surface To interior of fiber along transverse To interior of fiber along transverse

tubulestubules


The mechanism by which the action The mechanism by which the action potential causes the myofibrils of potential causes the myofibrils of muscles to contractmuscles to contract

Sarcoplasmic reticulum as a source Sarcoplasmic reticulum as a source of calcium ions in skeletal musclesof calcium ions in skeletal muscles

Diffusion of large quantity of extra Diffusion of large quantity of extra calcium ions from T-tubulescalcium ions from T-tubules

No contraction of cardiac musclesNo contraction of cardiac muscles


Less developed sarcoplasmic reticulumLess developed sarcoplasmic reticulum T-tubules have diameter 5 times great T-tubules have diameter 5 times great

as compared to skeletal musclesas compared to skeletal muscles 25 times great volume25 times great volume A large quantity of mucopolysaccharidesA large quantity of mucopolysaccharides Electronegativity chargedElectronegativity charged Binding with abundant source of calcium Binding with abundant source of calcium

ionsions


Concentration of calcium ions in Concentration of calcium ions in extracellular fluidextracellular fluid

Opening of T-tubules pass directly Opening of T-tubules pass directly through cell membrane into through cell membrane into extracellular spacesextracellular spaces


Permission of percolation of calcium Permission of percolation of calcium ions from extracellular fluidions from extracellular fluid

Dependence of contraction on Dependence of contraction on calcium ions from extracellular fluidcalcium ions from extracellular fluid

Comparison to skeletal musclesComparison to skeletal muscles Cessation of contraction Cessation of contraction Pump systemPump system T-tubules extracellular fluidT-tubules extracellular fluid

OVERALL VIEW OF CONTRACTION OVERALL VIEW OF CONTRACTION OF SKELETAL AND CARDIAC OF SKELETAL AND CARDIAC

MUSCLESMUSCLES SKELETAL MUSCLESSKELETAL MUSCLES Highly developed Highly developed

sarcoplasmic sarcoplasmic reticulumreticulum

Less developed T-Less developed T-systemsystem

No electronegativityNo electronegativity Extracellular fluid is Extracellular fluid is

not effieciennot effiecien

CARDIAC MUSCLESCARDIAC MUSCLES Less developed Less developed

sarcoplasmic sarcoplasmic reticulumreticulum

Highly developed T-Highly developed T-systemsystem

ElectronegativityElectronegativity Extracellular fluid is Extracellular fluid is

effiecient and major effiecient and major source of calcium ionssource of calcium ions

Cardiac muscles excitation contraction

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