Myocardium

Myocardium is the muscular tissue of the heart.

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Relationship to other layers

The other tissues of the heart are:

• the endocardium (inner lining, effectively a specialized endothelium)
• the epicardium (a connective tissue layer around the heart with a serous surface. It may be considered as the inner (visceral) layer of the pericardium)

Composition

The myocardium is composed of specialized cardiac muscle cells with an ability not possessed by muscle tissue elsewhere in the body. Cardiac muscle, like other muscles, can contract, but it can also carry an action potential (i.e. conduct electricity), like the neurones that constitute nerves. Furthermore, some of the cells have the ability to generate an action potential, known as cardiac muscle automaticity.

The blood supply of the myocardium is by the coronary arteries. The myocardium is subject to two opposed electrical subsets of control. First order electrical control of the myocardium is derived from the sinoatrial node. Propagation of first order control from the sinoatrial node is closely tied to sympathetic discharge. Second order electrical control of the myocardium is closely tied to parasympathetic influence from the spinal vertebral ganglia and vagus nerves.

Pathology

Occlusion of the coronary arteries by atherosclerosis and/or thrombosis can lead to myocardial infarction.

Certain viruses lead to inflammation of the myocardium, or myocarditis.

Cardiomyopathies are inherent diseases of the myocardium, many of which are caused by genetic mutations.

Failure of the heart to contract properly (for various reasons) is often termed heart failure although the proper term for this condition is myocardial failure. Heart failure is a general term referring to overwhelming heart disease from many causes (e.g., myocardial failure, valvular heart disease, increased ventricular stiffness) resulting in the inability of the heart to maintain normal ventricular filling pressure (resulting in fluid retention, edema, pulmonary oedema, hepatomegaly) and/or reduced blood flow to the body either at rest or during exercise. Myocardial failure resulting in heart failure results in a shortened life expectancy and decreased quality of life.

Disease of myocardium

Noncompaction Cardiomyopathy, is when the Myocardium tissue fails to compact as the human embryo develops.

Disease of the myocardium can also be considered to be ischemic (poor blood supply to the heart muscle) or nonischemic, implying disease within the heart muscle itself.

1. Ischemic myocardial disease is well described and is amenable to vessel bypass, stents, applied growth factors and many other interventions.

a. Diabetes is perhaps the best known physiologic model for accelerated ischemic disease of the myocardium.

2. Nonischemic myocardial disease (see also nonischemic cardiomyopathy) is an entirely different entity in myocardial disease.

a. Chagasic heart failure (see also Carlos Chagas) is perhaps the best known physiologic model in myocardial autonomic insufficiency.

sarcomere (a, i, and h bands; z and m lines), myofilaments (thin filament/actin, thick filament/myosin, elastic filament/titin, nebulin), tropomyosin, troponin (T, C, I)

costamere (dystrophin, α,β-dystrobrevin, syncoilin, synemin/desmuslin, dysbindin, sarcoglycan, dystroglycan, sarcospan), desmin

neuromuscular junction, motor unit, muscle spindle, excitation-contraction coupling, sliding filament mechanism