Cardiac Muscle, Dr. D. Penney
Cardiac Muscle - Contrasts with Skeletal and Smooth Muscle:
- 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.
- Lmax - sarcomere length 2.2 um; significant passive tension at Lmax which then increases rapidly beyond Lmax (Length-Tension Diagram). Narrow range of working lengths. Heart operates below Lmax, skeletal muscle at Lmax.
- 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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