Myocardium or heart muscle is a special type of muscle (differ than striated and smooth) and it is the main tissue of the walls of the heart.
Heart muscle consists of individual heart muscle cells (cardiomyocytes) which are joined together by intercalated discs. It also contains collagen fibres and other substances that form the extracellular matrix.
Heart muscle has a "special" mode of contraction: action potential triggers the release of calcium ions from the cell's internal store, the sarcoplasmic reticulum.
A tissue is an ensemble of <u>similar cells and their extracellular matrix (structure) </u>from the same origin that together carry out a specific function.
