Expansion
Branching striated fibres with central nuclei and intercalated discs
Expansion
Identifying features
- Striated, like skeletal muscle
- Branching fibres, unlike skeletal muscle
- One or two central nuclei, in contrast to the peripheral nuclei of skeletal muscle
- Intercalated discs: dark transverse lines, unique to cardiac muscle
- Abundant mitochondria, occupying about a third of cell volume
- Lipofuscin around the nuclei, increasing with age
Intercalated discs contain three junction types, each with a distinct role:
- Desmosomes and fascia adherens: mechanical coupling, transmitting force
- Gap junctions: electrical coupling, allowing the myocardium to behave as a functional syncytium
Purkinje fibres are modified cardiac myocytes, appearing larger and paler with abundant glycogen and few myofibrils, located subendocardially.
Pathological appearances: hypertrophy shows enlarged fibres with boxcar nuclei; infarction shows coagulative necrosis with contraction bands after reperfusion; and amyloid appears as interstitial pink material with apple green birefringence on Congo red.
Cardiac muscle is a permanent cell population, which is why infarcted myocardium is replaced by collagenous scar.