Cardiac muscle (histology)

Histologically, cardiac muscle consists of cells - fibres - of about 10micrometre diameter and 200micrometre length. Cells are branched and are joined to each other at their ends by intercalated discs. The discs provide a mechanical link between cells by their maculae adherens. Also, they provide an electrical link between cells by virtue of their gap junctions which permit the passage of small molecules. Therefore, cardiac muscle becomes a 'functional synctium' to permit the spread of excitation.

With the light microscope, cardiac muscle shows a number of cross-striations. These represent the Z discs which are the ordered array of actin filaments bordering two sarcomeres. Between each actin filament is an array of myosin filaments. The centre of the latter array is seen as a discrete M band on light microscopy.

The outer membrane of each myocyte is termed the sarcolemma. It invaginates to form an intracellular network that communicates with the extracellular fluid - the T-tubules. The tubules ramify inwards at the level of the Z-discs within each cell. They are more evident in the ventricles than the atria. Adjoining the T-tubules is a membranous system that runs longitudinally along the outside of the contractile elements. This is the sarcoplasmic reticulum. Its association with the T-tubule is not as highly ordered as it is in skeletal muscle; the junctions are between the T-tubule and one sarcolemmal membrane rather than two - dyads as opposed to triads. Numerous mitochondria are deep to the sarcolemma.

Each myocyte has one or two nuclei which are sited centrally. The region around the nucleus is filled with mitochondria, Golgi apparatus and glycogen. Also, atrial cells exhibit granules in the perinucleolar region which are thought to contain atrial natriuretic peptide.

Cardiac cells do not regenerate when lost; fibrosis occurs.