## Layers of the Heart Wall **Key Point:** The myocardium is the thick muscular layer of the heart composed of cardiac muscle fibers (cardiomyocytes) arranged in a spiral pattern. It is responsible for the contractile function of the heart and is supplied by the coronary arteries. ### Three-Layer Structure of the Heart Wall | Layer | Composition | Function | Blood Supply | | --- | --- | --- | --- | | **Epicardium** | Visceral pericardium; thin connective tissue with fat and vessels | Protective outer layer | Coronary arteries run here | | **Myocardium** | Cardiac muscle fibers (cardiomyocytes) in spiral arrangement | **Contraction** | Coronary arteries penetrate deeply | | **Endocardium** | Simple squamous epithelium + connective tissue | Lines cardiac chambers; smooth surface for blood flow | Supplied by diffusion from ventricular blood | **High-Yield:** The myocardium is the only layer with contractile function. Myocardial infarction occurs when coronary artery occlusion deprives the myocardium of oxygen, leading to necrosis of muscle fibers. ### Coronary Artery Penetration Coronary arteries run epicardially (on the surface) and then penetrate into the myocardium, supplying the full thickness of the ventricular wall. The subendocardial region is the last to be perfused and the first to become ischemic in low-flow states. **Clinical Pearl:** In transmural MI, the entire myocardial thickness is infarcted. In subendocardial (non-transmural) MI, only the inner layers are affected, often due to demand ischemia or hypotension. **Mnemonic:** **EME** — **E**picardium (outer), **M**yocardium (muscle/contraction), **E**ndocardium (inner lining). 
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