In comparison with skeletal muscle, where the cells are long (1-40 mm) and straight, cardiac muscle cells are extremely small (≤80 μm) and are extensively branched. There are also specialized intercellular junctions in cardiac muscle known as intercalated disks, which do not exist in skeletal muscle (Fig. la). In common with skeletal muscle, cardiac muscle is striated. That is to say, the intra­ cellular proteins that make up the contractile machinery are arranged in a highly organized fashion, and this is apparent as 'banding' of the cells. In common with all other types of muscle, shortening of the cell is brought about by different types of muscle filaments sliding past each other. The two major proteins that make up these filaments are actin and myosin (Fig. lb). It is the regular arrange­ ment of these filaments that gives striated muscle (both skeletal and cardiac) its

Microscopic structure of cardiac muscle

Specialization related to function

Innervation of cardiac muscle

Related topics

Cardiac muscle cells are small striated myocytes. They are highly branched and are heavily invested with mitochondria. The sarcoplasmic reticulum (see Topic I3) is not as extensive as in skeletal muscle. The intercalated disks between myocytes are not uniform; gap junctions exist so that electrical signals can pass easily from one cell to its neighbors.