Cardiac Physiology IV

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Cardiac Physiology(IV)

• A. Rüçhan Akar
• Ankara University
• School of Medicine
• December- 2003

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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

JR Levick, 1995 An Introduction to Cardiovascular
Physiology Butterworth-Heinemann
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Fibrous skeleton of the heart
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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

Sperelakis N, Kurachi Y, Terzic A, Cohen
MV. Heart Physiology and Pathophysiology Academic
Press, 2001
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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

Sperelakis N, Kurachi Y, Terzic A, Cohen
MV. Heart Physiology and Pathophysiology Academic
Press, 2001
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

Sperelakis N, Kurachi Y, Terzic A, Cohen
MV. Heart Physiology and Pathophysiology Academic
Press, 2001
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

Specialised cardiac muscle cells
Noble, D. Clarendon, Oxford, 1978
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AV node (node of Tawara) irregularly arranged
branching fibers
Bundle of His unbranched fibers
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endothelium endocardium
Purkinje fibers
Ventricular myocardium
Purkinje fibers
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Nodal Cells

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

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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

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Excitation and Contraction of Cardiac Myocyte
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Suggested readingThe Structure and Function of
the Cardiac Myocyte a Review of Fundamental
Concepts

• Walker CA, Spinale FG.
• J Thorac Cardiovasc Surg 1999118375-82

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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)

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endothelium endocardium
Purkinje fibers
Ventricular myocardium
Purkinje fibers
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(No Transcript)
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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)

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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

Heart behaves as a single motor unit
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Theoretically, an ion inside an SA nodal cell
could travel throughout the heart via the gap
junctions
Sperelakis N, Kurachi Y, Terzic A, Cohen
MV. Heart Physiology and Pathophysiology Academic
Press, 2001
20
Sarcomere

• basic contractile unit within the myocyte
• refers to the unit from one Z band to the next
• resting length1.8-2.4 mm
• composed of interdigitating filaments
• thick myosin protein
• thin actin protein

Walker CA, Spinale FG J Thorac Cardiovasc Surg
1999, 118375-82
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T T tubules mit mitochondria g glycogen
contractile unit sarcomere Z line the actin
filaments are attached I band of actin
filaments, titin and Z line A band of
actin-myosin overlap H clear central zone
containing only myosin
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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

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Sarcolemma contains a number of ion channels and
pumps that contribute to overall Ca 2 levels
within the myocyte
Ferrari R, Opie LH, 1992 Atlas of the
myocardium Raven Press Ltd.
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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)

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Mitochondria

• generate the energy in the form of adenosine
  triphosphate (ATP)
• maintain the hearts contractile function and the
  associated ion gradients

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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

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JSR junctional SR LSR longitudinal SR
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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

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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)