Cardiac muscle

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Cardiac Muscle is composed of smaller interconnection cells with single nucleus per cell instead of long multinucleate cells in skeletal muscle. Interconnection which appears as dark lines under microscope is known as intercalated discs. These interconnections make the cardiac muscle cells to form single functioning unit called myocardium. Some cardiac muscle cells generate electric impulses which spread across the gap junctions from cell to cell by itself this enables cell contractions in the myocardium[1].

Cardiac muscle fibers are electrically coupled to each other and consequently excitation of one cardiac muscle fiber triggers a series of action potentials throughout all of the muscle fibers in the cardiac muscle, hence allowing cardiac muscle to contract as one entity, much like single-unit smooth muscle cells. The strength of the cardiac muscle is further enhanced by the fact that action potentials are maintained in cardiac muscles cells considerably longer than in skeletal muscle fibers; cardiac muscle cells remain depolarized for several hunder milliseconds whilst a nerve or skeletal muscle fiber is depolarizaed for several milliseconds. The significantly longer depolarization span in cardiac muscles induces a longer refractory period which inhibits "circus movements" of constant re-excitation around the wall of the heart [2].

Cardiac muscles are able to undergo muscle hypertrophy both as a result of increased physiological demand and as a result of some disease processes [3].

Cardiac Muscle cells are striated, just as in skeletal muscle and are found only in the heart. They work on a rhythmn set by a group of pacemaker cells locted in the right atrium of the heart. This self-automated contraction property is called being 'myogenic'.

The pacemaker releases an electrical impulse approximately 70 times per minute, which causes the heart to contract. The signals are sent via the involuntary nerves from the cardia control centre loacted in the medulla of the brain.

The rate of impulses can be increased by the cardiac accelerator nerve, or by the hormone adrenalin.

The rate of impulses can be decreased by the cardiac depressor nerve [4].

References

  1. Raven, P.H. and Johnson, G.B. (1999) Biology(5th ed.) P915 WCB/McGraw-Hill
  2. Moffet, Moffett, Schauf(1993)Human Physiology, 2nd Edition, St. Louis, p313-314
  3. Stevens A. et al. (2005), Human Histology, Third Edition, Philadelphia, Elsevier Limited
  4. http://www.bbc.co.uk/science/humanbody/body/factfiles/skeletalsmoothandcardiac/heart_beat.shtml fckLR1/12/2011
 
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