Cardiac muscle

CARDIAC MUSCLE

  • Involuntary, single centrally located nucleus, numerous mitochondria. The T-tubular of mammalian cardiac muscles are larger (esp. in Ventricles) than those of skeletal muscles and are located at the Z lines rather than at the A – I band junction as in skeletal muscles fibers. While the groups of skeletal muscle fibers an arranged in a parallel fashion, those of cardiac muscles branch freely with other fibers to form 2 separate net works.
    • Muscular walls and septum of atria
    • Muscular walls of septum of ventricles
  • When a single fiber of either network is stimulated all the fibers in the network become stimulated as well. Thus each network contracts as a functional unit. Each fiber in a network is separated from the next fiber by an irregular transverses thickening of sarocolima called intercalated disc. Thus cardiac muscle can act as a Syncytium (which provides more rapid spread of impulses between fibers).
  • Under normal conditions cardiac muscles tissue contracts & relaxes rapidly continuously and rhythmically with out stopping.
    • AP spreads cell to cell through “Gap Junctions”, and in this way to entire heart (in skeletal muscles each fiber must be separately stimulated by a motor neuron).
    • T – tubules poorly developed in atrial musculature. In verticals, they are much larger in diameter as compared to skeletal muscle.
    • Location: They lie over Z-lines sarcoplam reti is not well developed;
    • No lateral cisternae – So no Triads (ie., have diads).
    • AP = 100 – 250 ms duration (Skeletal muscles = 2-3 ms duration.
    • Catechols – increase the Ca2+ release which causes force and frequency increase
    • Purkinje fibres specialized cardiac fibers for impulse conduction, part of the pace maker
    • Have extensive Ca++ - Na+ exchange system.

Rhythmicity

  • Repetitive discharge of impulses normally occurs in the pace maker cells of SA node, smooth muscle and many neurones of CNS in which the resting membrane potential is only - 55 to - 60 mV which is not enough to close the Na+ and Na+- Ca++ channels. This cause inward flow of Na+ and Ca++ creates still less negativity inside these cells and generates spontaneous and cyclic action potential. More over their cell membranes are naturally leaky to Na+ to generate the action potential at a faster rate.
Last modified: Saturday, 3 December 2011, 5:41 AM