Veterinary Histology and Embryology

CARDIAC MUSCLE

• Myocardium or heart muscle is composed of an interlacing network of muscle fibres (cells). The muscle net is irregularly and incompletely divided into bundles and laminae which wind about the heart in the form of spirals. The fibres within a bundle are mostly parallel but the bundles themselves course in different directions so that a section of myocardium shows fibres cut in longitudinal, transverse and oblique planes.
• The muscle substance is composed of fibres or cells which show cross striations like skeletal muscle and are 9 to 20 microns in diameter, connected with each other by anastomosing branches. The myofibrils resemble that of skeletal muscle but the cross striations are closer together and not very distinct. These fibrillae are thicker and more closely packed together at the periphery of the fibre but may be so fine and sparse in the centre that there appears to be an axial core of sarcoplasm.
• In cross section the arrangement of cut ends of fibrillae frequently suggests short parallel bands or the spokes of a wheel Nuclei are in the interior near the central axis. They are oval and quite large. There is an accumulation of sacroplasm around the nucleus which contains mitochondria, fat droplets etc. A sarcolemma or plasmalemma is presented similar to that of the skeletal muscle.
• Cardiac muscle also shows intensely staining transverse bands, at certain levels and these are known as intercalated discs. These are 0.5 to 1 micron thick that is less than a cross striation or sarcomere. They are strongly refractile in fresh muscle, but deeply stained in fixed material. They run in a straight line across but frequently are irregular or broken into “step formations”. They are the cell membranes of adjacent cells meeting each other at cell junctions.

Connective tissue of cardiac muscle :

• A network of reticular fibres and fine collagenous fibres surrounds each muscle fibre.
• It corresponds to endomysium of skeletal muscle. Between the bundles of muscle fibres there are coarser collagenous fibres and elastic fibres. These regions corrspond to perimysium of skeletal muscle.

Blood vessels and nerve supply

• An extensive plexus of blood and lymph capillaries is found in the connective network surrounding each muscle fibre.
• Branches of sympathetic and parasympathetic (vagus) nerves terminate in fine free endings on the muscle fibres.

Conducting system

• Some of the cardiac muscle fibres are modified to form the impulse conducting system, whose function is to regulate successive a contractions of atria and ventricles. It extends from the right atrium into the ventricles and is known as the atrio-ventricular bundle of his. The AV bundle and its branches consist of modified cardiac muscle fibres called purkinje fibres.
• They are distinguished form cardiac muscle fibres by
  • the reduced number of myofibril which are restricted to the periphery of the fibre ,
  • greater amount of sarcoplasm ,
  • more rounded nuclei, which occur in groups of two or more innermost of the cells.