VPB 121: Veterinary Physiology - II (2+1)

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.