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cardiac physiology iv

Structure of the Heart. adult human heart = 300-350 gbuilt upon a

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cardiac physiology iv

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    1. Cardiac Physiology(IV) A. Rüçhan Akar Ankara University School of Medicine December- 2003

    2. Structure of the Heart adult human heart = 300-350 g built upon a “collagenous skeleton” located at atrioventricular junction (fibrotendinous ring) the ring isolates the atria electrically from the ventricles, except at the bundle of His

    4. Cardiac and Skeletal MusclesSimilarities Both- Striated muscle Both use proteins actin and myosin Both contract in response to an action potential on the sarcolemmal membrane

    5. Cardiac and Skeletal MusclesDifferences Skeletal muscle Neurogenic (motor neuron-end plate-acetylcholine) Insulated from each other Contracts in all or none fashion Short action potential Cardiac Muscle Myogenic (action potential originates within the muscle) Gap-junctions Action potential is longer

    6. Cardiac Muscle Fiberscontractile or conductile Contractile Action potential leads to vigorous force development and/or mechanical shortening Conductile initiation or propagation of action potentials

    7. Conduction SystemConductile Fibers Sinoatrial (SA) node 100-110/min Atrioventricular (AV) node 40-60/min AV bundle (Bundle of His) 20-40/min Left and right bundle branch Purkinje fibers (rapid conduction) 20-40/min

    10. Nodal Cells Smaller than contractile cells or Purkinje cells Low propagation velocity (0.05m/sec) Reduced density of gap junctions Lack fast Na channels

    11. Purkinje Cells larger than ordinary cardiac fibers and bundle fibers Conduct action potentials four times faster than a ventricular myocyte (4m/sec) may be binucleate few myofibrils vacuous cytoplasm (filled with glycogen) subendocardial location linked to cardiac fibers and bundle fibers by gap junctions and desmosomes

    12. Excitation and Contraction of Cardiac Myocyte

    13. Suggested reading;The Structure and Function of the Cardiac Myocyte: a Review of Fundamental Concepts Walker CA, Spinale FG. J Thorac Cardiovasc Surg 1999;118:375-82

    14. Cardiac Myocyte Myofiber: is a group of myocytes held together by surrounding collagen connective tissue excess collagen, may cause LV diastolic dysfunction (e.g. left ventricular hypertrophy)

    17. Cardiac Myocyte 10-20 mm in diameter 50-100 mm long single central nucleus the cell is branched, attached to adjacent cells in an end-to-end fashion (intercalated disc) desmosomes (proteoglycan glue) gap junction (region of close apposition)

    18. Gap Junctions low resistance connections small pores in the center of each gap junction allows ions and small peptides to flow from one cell to another action potential is propagated to adjacent muscle cells

    19. Theoretically,

    20. Sarcomere basic contractile unit within the myocyte refers to the unit from one Z band to the next resting length:1.8-2.4 mm composed of interdigitating filaments thick myosin protein thin actin protein

    22. Sarcolemma (sarco = flesh; lemma = thin husk) each cell is bounded by a complex cell membrane composed of a lipid bilayer hydrophilic heads hydrophobic tails impermeable to charged molecules (barrier for diffusion) contains membrane proteins, which include receptors, pumps and channels

    24. Transverse Tubular System(T-tubules) the sarcolemma of the myocyte invaginates to form an extensive tubular network extends the extracellular space into the interior of the cell transmit the electrical stimulus rapidly (well developed in ventricular myocytes but is scanty in atrial and Purkinje cells)

    25. Mitochondria generate the energy in the form of adenosine triphosphate (ATP) maintain the heart’s contractile function and the associated ion gradients

    26. Sarcoplasmic Reticulum (SR) a fine network spreading throughout the myocytes demarcated by its lipid bilayer close apposition to the T tubules junctional SR longitudinal SR

    28. Subsarcolemmal Cisternae Junctional SR the tubules of the SR expand into bulbous swellings contains a store of Ca2+ ions release calcium from the calcium release channel (ryanodine receptor) to initiate the contractile cycle

    29. Longitudinal or Network SR consists of ramifying tubules concerned with the uptake of calcium that initiates relaxation achieved by the ATP-requiring calcium pump (SERCA= sarcoendoplasmic reticulum Ca2+ -ATPase)

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