Muscle fibers

• Cardiac muscle cells: striated, short, branched, fat, interconnected

  • one central nuclei
  • so many mitochondria making it more resistant to fatigue(25-35)
  • Rest of volume composed of sarcomeres
  • Z discs, A bands, and I bands all present
  • T tubules are wider but less in numbers
  • SR is simpler, there are no triads
• Intercalated discs: connecting junctions between cells that contain:

  • Desmosomes: hold cells together during contractions
  • Gap junctions: allow ions to pass from cell to cell; electrically couple adjacent cells
  • Allows heart to be a functional syncytium: a single coordinated unit
• • Intercellular space between cells has connective tissue matrix (endomysium) 
  • Contains numerous capillaries
  • Connects cardiac muscle to cardiac skeleton, giving cells something to pull against
• • Similarities with skeletal muscle
  • depolarization action potential activates a muscle contraction
  • Depolarization wave travels down T tubules; causes sarcoplasmic reticulum (SR) to release Ca2+
  • Excitation-contraction coupling occurs
  • Ca2+ binds troponin causing filaments to slide
• Some Muscle cells are self-excitable: myocytes

  Contractile cells: responsible for contraction

  Pacemaker cells: noncontractile cells that spontaneously depolarize

  • will start up the whole heart
  • doesn't need brain involvement like skeletal muscles
• Heart contracts as a unit:

  • the entire heart(cardiomyocytes) contracts or it doesn't
  • This ensures that the heart is pumping properly
  • skeletal muscles contract independently
• extracellular Ca2+ causes intracellular Ca2+ to rush in:

  • depolarization causes the slow channels of Ca2+ to open and then SR releases Ca2+
  • skeletal doesn't use extracellular Ca2+
• Tetanic contractions do not exist in the heart:

  • absolute refractory period is as long as the contraction
  • multiple contractions at the same time is prevented
  • would not allow the heart to relax and refill and do its job
• Heart is strictly aerobic:

  • relies heavily on oxygen, so many mitochondria, cannot function without oxygen
  • skeletal muscle can go through other processes
  • both tissues can use other fuels and cardiac is more adaptable but it must have oxygen