CARDIAC MUSCLE

Heart Diagram

Cardiac muscle

Thick and thin filaments like skeletal muscle organized in sarcomeresHave more mitochondriaLarger T tubulesSame mechanism of contraction

Cardiac muscle Found in the heartInvoluntary rhythmic contractionBranched, striated fibre with single nucleus and intercalated discs

Functions of the HeartGenerating blood pressureRouting bloodHeart separates pulmonary and systemic circulationsEnsuring one-way blood flowHeart valves ensure one-way flowRegulating blood supplyChanges in contraction rate and force match blood delivery to changing metabolic needs

Heart WallThree layers of tissueEpicardium: This serous membrane of smooth outer surface of heartMyocardium: Middle layer composed of cardiac muscle cell and responsibility for heart contractingEndocardium: Smooth inner surface of heart chambers

Valve function:

Cardiac MuscleFound only in heartStriatedEach cell usually has one nucleusHas intercalated disks and gap junctionsAutorhythmic cellsAction potentials of longer duration and longer refractory periodCa2+ regulates contraction

Cardiac MuscleElongated, branching cells containing 1-2 centrally located nucleiContains actin and myosin myofilaments Intercalated disks: Specialized cell-cell contactsDesmosomes hold cells together and gap junctions allow action potentialsElectrically, cardiac muscle behaves as single unit

Cardiac Muscle, HistologySingle nucleus25% mitochondriaAerobic respirationStriated (sarcomeres)Autorhythmic contraction(doesnt need nerve stimulus)Intercalated discsenable action potential totravel to neighboring cellsDifferent action potentialthan skeletal muscle

Intercalated disksCardiac myocytes are however interconnected by intercalated discs

Intercalated disks are areas of low electrical resistance because they have gap junctions that permit the flow of ions between cells.

Cardiac myocytes function as one mass (functional syncytium) , i.e. stimulation of one cell can lead to excitation and contraction of the whole mass of cells. (compare to skeletal muscle where the cells are discrete and electrically isolated from each other).

Most aspects of Cardiac muscle contraction are equivalent to skeletal muscle

(Action potential, Ca++, sliding filaments, ATP uses, etc.)

Cardiac Muscle Physiology

Resting Membrane Potential = -90mvThreshold = -75 mVAction Potential results in increased cytoplasmic Ca++Ca++ binds to troponin exposes active sites cross-bridges, etc...[same as skeletal muscle]

Cardiac Muscle Action Potential Slower A.P. (250-300 msec) Rapid Depolarization (Na+ in) Plateau phase, due to influx ofextra-cellular Ca++ slow Repolarization (K+ outthrough slow channels) AP causes release Ca++ fromSR; triggers contraction

Cardiac myocyte action potential:

Cardiac myocyte action potential:

AP-contraction relationship:AP in skeletal muscle is very short-livedAP is basically over before an increase in muscle tension can be measured.

AP in cardiac muscle is very long-livedAP has an extra component, which extends the duration.The contraction is almost over before the action potential has finished.

Two Calcium Sources1. Extracellular2. Sarcoplasmic Reticulum

Extracellular Ca++ (20%)influx causes plateau phaseAP also triggers release of Ca++

from sarcoplasmic reticulum (80%)Ca++ binds to troponin series of events to expose active sites

Relation between cardiac muscle action potential and contraction

Refractory Period

cell cant respond to another stimulus, b/c of charge, ion distribution, etc.Absolute refractory = cant re-stimulate Relative refractory = needs gtr. stimulusCardiac m. refractory period is much longer than in skeletal m.Cardiac m. contraction occurs during AP,within refractory period summation is not possibleImportance? Prevents re-stimulation of fibers Heart in tetany cannot pump bloodRefractory period absolute relative

Cardiac CycleHeart is two pumps that work together, right and left halfRepetitive contraction (systole) and relaxation (diastole) of heart chambersBlood moves through circulatory system from areas of higher to lower pressure.Contraction of heart produces the pressure

Blood Pressure Measurement

Pressure relationships:

Cardiac conducting system:

Pacemaker potential:

EKG:

Muscle ControlSkeletalSkeletalCardiacSmooth

Regulation of the HeartIntrinsic regulation: Results from normal functional characteristics, not on neural or hormonal regulation

Extrinsic regulation: Involves neural and hormonal controlParasympathetic stimulationSupplied by vagus nerve, decreases heart rate, acetylcholine secretedSympathetic stimulationSupplied by cardiac nerves, increases heart rate and force of contraction, epinephrine and norepinephrine released

Cardiac ArrhythmiasTachycardia: Heart rate in excess of 100bpmBradycardia: Heart rate less than 60 bpmSinus arrhythmia: Heart rate varies 5% during respiratory cycle and up to 30% during deep respirationPremature atrial contractions: Occasional shortened intervals between one contraction and succeeding, frequently occurs in healthy people

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