Cardiac Muscle Tissue
Introduction to the Cardiac Muscle Tissue:
Cardiac muscle cells (cardiocytes or cardiac myocytes) make up the myocardium portion of the heart wall.
They are relatively short, branched fibers that measure approximately 10 to 10 micrometers in diameter and 50 to 100 micrometers in length.
Typically each cardiac myocyte contains a single nucleus, which is centrally positioned.
Thick and thin myofilaments are present and organized into myofibrils.
Their overlapping arrangements creates alternating dark (A) and light (I) bands or striations, similar to those seen in skeletal muscle tissue.
Sarcoplasmic reticulum tubules surround the myofibrils. However, they are not well organised and do not have terminal cisternae. T-tubules are also present but run along the Z-discs (instead of the myofilament overlap zones).
The mitochondria in the cardiac myocytes are large and numerous. They supply the ATP for repeated contractions of the heart.
Unlike other types of muscle tissue, cardiac myocytes are joined end to end by intercalated discs.
These complex, highly convoluted couplings contain both anchoring junctions and electrical junctions.
Forming the anchoring juntions are fascia adherens and desmosomes, which attach the adjacent myocytes.
The electrical junctions are composed of connexion protein channels, which usually occur in clusters referred to as gap junctions.
Connexion proteins span the distance between adjacent plasma membranes and ions can travel through the channel pores.
The ion movement allows action potentials to pass directly from cell to cell. This property makes the entire myocardium act like a single cell (or funtional syncytium).