Heart wall is made up of cardiac muscles and hence called myocardium. A few bunches of unstriped muscles fibres are found in cardiac muscles. These fibres are branched and form networks. Each cardiac muscle of fibre is surrounded by sarcolemma.
Dark and lights bands are present in these fibres. Besides these bands thick transverse bands known as intercalated discs are found at regular intervals. It is because of these discs that the cardiac muscle is seen to be divided into cells. However, these discs are nor actually the ultimate boundaries of the cells. Cardiac muscles are involuntary, do nor exhaust and contract rhythmically. Heart beat is due to the rhythmical contraction of these fibres. They never undergo fatigue.