Cardiac Muscle, Dr. D. Penney

Cardiac Muscle - Contrasts with Skeletal and Smooth Muscle:

Morphological:

• Striated, like skeletal muscle; organized by sarcomeres and myofibrils containing same contractile proteins.

• Cells shorter than skeletal muscle (15 x 100 um; 5.5 L/W ratio); often binucleate and polyploid (4N, 8N, etc.).

• Cells continue to divide after actin & myosin synthesized; stop at or about birth; no myotubes formed.

• The reddest of muscles due to high myoglobin content; operates on aerobic basis and uses lactate (more goes in than comes out).

• More mitochondria than skeletal muscle; don't increase with training, just hypertrophy.

• No attachment to bone, tendons, etc. (connective tissue rings at the four valves).

• Less extensive sarcoplasmic reticulum (sr) and transverse tubular (t) systems.

• Intercellular collagen network, holds cells together and may store energy.

Functional:

• L_max - sarcomere length 2.2 um; significant passive tension at L_max which then increases rapidly beyond L_max (Length-Tension Diagram). Narrow range of working lengths. Heart operates below L_max, skeletal muscle at L_max.

• No inherent tone, like skeletal muscle; Ca⁺⁺ comes from sr, mitochondria and outside cell (t tubules).

• Innervation is Sympathetic and Parasympathetic, stimulatory and inhibitory:
  
  Sympathetic postganglionics - release norepi on atria, ventricles & conducting system.
  
  Parasympathetic postganglionics - release Ach on atria and conducting system.

  Automaticity, unlike skeletal muscle (pacemaker & follower cells); rate of beating controlled by the Intrinsic Pacemaker, the SA node.

  Shows (+/-) Inotropic Effects, unlike skeletal muscle, like smooth muscle; (+) effect increases total tension, velocity (V) and Vmax.

  No recruitment of fibers as in skeletal muscle; whole organ is functional syncytium.

  Slow contraction rate; twitch duration, l00 msec. (atrial) - 270 msec. (ventricular).

  Long refractory period, thus no tetanus (this and above, fit physiological function).

  Treppe (Staircase, Bowditch effect) - increased frequency increases force & velocity of shortening.

  Intercalated discs at end of cells:
  

  desmosomes - provide structural strength.
  

  gap junctions - action potential moves from cell to cell.

  Specialized conduction system; differentiated muscle cells (internodal tracts, AV junction & node, bundle of His, bundle branches & Purkinje fibers.

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