<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> Cardiac muscle (heart muscle) is one of the three major types of muscle, the others being skeletal and smooth muscle . It is an involuntary, striated muscle that is found in the walls of the heart . This muscle tissue is known as myocardium, and forms a thick middle layer between the outer layer of the heart wall (the epicardium) and the inner layer (the endocardium). Myocardium is composed of individual heart muscle cells (cardiomyocytes) joined together by intercalated disks, encased by collagen fibres and other substances forming the extracellular matrix . </P> <P> Cardiac muscle contracts in a similar manner to skeletal muscle, albeit with some important differences . An electrical stimulation in the form of an action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum . The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation contraction coupling . </P> <P> Diseases of heart muscle are of major importance . These include conditions caused by a restricted blood supply to the muscle including angina pectoris and myocardial infarction, and other heart muscle disease known as cardiomyopathies . </P>

Describe the structure of the sarcoplasmic reticulum of cardiac muscle
find me the text answering this question