8.9. Layers of the Heart
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The heart is composed of three layers:
Endocardium - this is the innermost layer of endothelial cells that lines the chambers.
Myocardium - this is the middle layer. It is the thickest layer by far and is composed of cardiac myocytes. Each ‘heart cell’ is linked to its neighbours by intercalated discs (which use gap junctions to transmit action potentials across the cells). This means the heart is a ‘syncytium’, i.e. the heart muscle acts as one organ due to the coordinated electrical activity spreading through the intercalated discs.
Epicardium - this is the outermost layer that is often considered as being the visceral pericardium.