Pertaining to the muscular tissue of the heart (the myocardium).
- m. asthenia — myocardial weakness and decrease in the power of the heart's contraction, leading to reduction in cardiac reserve and ultimately to congestive heart failure.
- m. cells — 1. being surrounded by an intact sarcolemma prevents the cells forming a true syncytium; but do form a functional syncytium because of presence of gap junctions with low electrical resistance which allow the passage of ions and small molecules. — 2. myocardial cells are specialized smooth muscle cells with acquired features and properties similar to those of skeletal muscles.
- m. contractility — myocardium has an intrinsic periodic contractility which enables the denervated heart to continue beating; in the intact animal neurogenic control of contractility is paramount, inhibited by parasympathetic effects, stimulated by sympathetic effects.
- m. damage — any disease causing loss of muscular or nervous function of the heart. Includes myocarditis, ischemia, degeneration.
- m. depressant factor — substances liberated from the pancreas, intestine and liver during any type of circulatory shock. They reduce cardiac and reticuloendothelial activity and cause vasoconstriction. Called also MDF.
- m. dystrophy — degenerative change in heart muscle, e.g. that occurring as a result of nutritional deficiency of selenium or vitamin E. Death usually occurs suddenly, with blood-stained froth pouring from the nostrils.
- m. fiber — see myocardial cells (above).
- m. heart failure — heart failure due to myocardial inefficiency.
- m. hypertrophy — increase in size of myocardial muscle due to increase in size of individual myocardial cells, usually in response to an added afterload.
- m. infarction (MI) — necrosis of the cells of an area of the heart muscle (myocardium) occurring as a result of oxygen deprivation, which in turn is caused by obstruction to the blood supply; commonly referred to in humans as a ‘heart attack’. This is not a common disease in animals, but does occur, e.g. in horses, especially in the right atrium. It is a cause of myocardial weakness. See myocardial asthenia (above).
- m. inflammation — see myocarditis.
- m. injury — see myocardial damage (above).
- ischemic m. necrosis — localized or widespread necrosis due to interruption of the blood supply via the coronary arteries.
- m. lipofuscinosis — occurs in aged or cachectic cattle, also in healthy cattle; most common in Ayrshires; no apparent significance in terms of health; called also brown atrophy.
- m. necrosis — focal or massive and subsequent scarring caused by infection, e.g. Histophilus somni, the encephalomyocarditis virus, nutritional deficiency, e.g. vitamin E–selenium, poisoning by plants, e.g. Acacia georgina, farm chemicals, e.g. monensin, and ischemic necrosis due to general or local inadequacy of blood supply to the myocardium.
- m. tension — affects myocardial contractility but the importance of the effect in the living animal is obscure.
