Muscle fibers description

Skeletal muscle fibers have dilferent mechanical and metabolic properties, and fiber classification schemes are based on these differences. All fibers within a muscle unit are similar (but not identical) with respect to twitch characteristics, as these are largely determined by the innervation, and all fibers in a muscle unit are innervated by the same a-MN. However, even within a muscle unit, there may be appreciable interfiber differences in metabolic profile. Consequently, the following classifications should be viewed as useful simplified categories rather than a literal description of populations of fibers. 

Muscles are described as running from a proximal origin to a distal insertion. While these attachments are frequently discrete, distributed attachments, and distinctly bifurcated attachments, are also common. Description of the subdomains of a muscle is largely by analogy to the whole body. The mass of muscle fibers can be referred to as the belly. In a muscle with distinctly divided origins, the separate origins are often referred to as heads, and in a muscle with distinctly divided insertions, each mass of fibers terminating on distinct tendons is often referred to as a separate belly. 

A detailed description of the morphogenesis of the muscle fiber system in the developing heart is not available but there is evidence of an organized myofiber pattern by day 12 in the fetal mouse heart that is similar to that seen at birth (day 20) [25], Abnormalities of cardiac muscle fiber patterns have been described in some disease conditions. In hypertrophic cardiomyopathy, which is often familial, there is substantial myofiber disarray, typically in the interventricular septum [10,26]. 

Histologically, separation and rarefaction of myofibrils occur with vacuolization of the muscle fiber. Ul-trastructural investigations have shown the vacuoles to be membrane-bound vesicles, some of which represent dilations of the sarcoplasmic reticulum. Although detailed descriptions of the various ultrastructural types of vacuoles that occur in periodic paralysis are available, the reasons for, and the mechanisms of, formation of these vacuoles remain obscure [34]. 

Knowledge of the instantaneous and local functional distribution f(y,t) throughout the LV wall is required for the calculation of various cardiac phenomena (Beyar and Sideman, 1984b). This entails accounting for the anatomic microscopic orientation of the muscles fibers which hold a direct relation to the distribution of the mechanical properties. A 3-D dynamic model f(r,6,cp) is needed for a detailed description of the geometry of the cardiac chambers, and to account for the distributed parameters within the myocardium in the spatial inhomogeneities associated with certain diseases, like myocardial infarction. 

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