Cardiac Muscles

Cardiac muscle shows a mix of features found in both striated and non-striated muscle. Its sarcolemma is not distinctly visible, so although each fibre is actually uninucleate, the tissue can appear multi-nucleate under the microscope. Adjacent cardiac muscle fibres join together, giving the whole tissue a branched appearance. The points where fibres join are marked by transverse thickenings of the sarcolemma called intercalated discs.

These intercalated-disc junctions allow the cardiac muscle fibres in a region to contract together as a single unit, which helps a stimulus travel quickly through the heart. Cardiac muscle is striated but involuntary — it shares the banded appearance of skeletal muscle but, like smooth muscle, cannot be consciously controlled. Cardiac muscle forms the myocardium, the muscular wall of the heart. …