Muscle area, cross-sectional

Also, smooth muscle has less actin and myosin than striated muscle has (about 5 times). Nevertheless, the maximum force developed by smooth muscle per cross-sectional area is larger than that of striated muscle Clearly, striated muscle is not optimized for force but rather for speed. 

Histopathological examination shows the typical corelike lesions in a high proportion of muscle fibers in older patients this may amount to 100%. Most typically the cores are large and centrally-placed, but multiple cores may occur in the same fiber cross section. Most older patients show a striking predominance of type 1 (slow twitch oxidative) fibers and virtually all fibers with cores are type 1. Sometimes younger family members have more normal proportions of type 1 and type 2 fibers but, again, the cores are confined to the type 1 fibers. It is well established that muscle fiber types can interconvert due to altered physiological demands, and it is likely that fibers with cores convert to a basically slow twitch-oxidative metabolism to compensate for the fact that up to 50% of their cross sectional area may be devoid of mitochondria. 

Strength training involves movement of muscles against an increasing load which is performed in sets and then repeated with fixed rest intervals. In response, the cross-sectional area of muscle increases due to an increase in the number of myofibrils within a fibre. 

Fig. 7.4 Top) Dil-positive stem cells (red) in the midmyocardium of the anterolateral wall. Middle) a-Smooth muscle actin staining with FITC green) showing cross-section of vessel wall. Bottom) Stained areas show colocalization yellow) of stem cells and smooth muscle cells, suggesting transformation of stem cells into smooth muscle cells. The vessel shown is in the myocardial interstitium. Arrows point to vessel media. Reprinted from [63]...

Although much of the focus has been on the DPC of striated muscle, it is likely that desmin attachments to dense plaques of smooth muscle play critical roles in regulating the transmission of contractile forces in this tissue as well. This is particularly relevant in light of the observed defects in smooth muscle of desmin-deficient mice, in which active force per cross-sectional area was reduced to 40% of controls of smooth muscle tissue (Sjuve et al, 1998). IFAP candidates for serving this linking function are plectin and other components of the actin-rich cortex, including calponin (which also plays a role in the cytoplasm of smooth muscle cell dense bodies see below), and the spectrin/ankyrin complex. 

Computed tomography (CT), as applied to nutrition research, was described by Kvist et ai. (198S). The technique involves exposure to X-rays for about 5 seconds, with a resulting cross-sectional picture of the abdomen called a "slice." The picture represents a "slice" of the body, where the slice is about 12 mm thick. The abdominal slice is taken at the level of one of the lumbar vertebra. The results take the form of a picture where areas of variable densities can be interpreted as being subcutaneous fat, visceral fa I, muscle, or bone. 

Smooth muscle fibers generate as much isometric force per cross-sectional area as skeletal muscle fibers with only 20% as much myosin (Murphy et al., 1974). There have been various possible explanations for this, including different mechanical properties of the myosin itself. Experiments with the in vitro motility assay provide insight into this possibility. 

Smooth and skeletal muscle myosins have important functional differences with respect to their motor activities and their regulation. The differences in motor properties are evident in the behavior of smooth and skeletal muscle myosins in an in vitro motifity assay. Purified smooth muscle myosin propels actin filaments at one tenth the velocity of skeletal muscle myosin and produces an average of 3-4 times more force per unit time period than skeletal muscle myosin, as measured by a micro-needle assay (Warshaw et al 1990, Van Buren et al 1994). These differences in the functional properties of smooth and skeletal muscle myosins at the molecular level parallel differences in the functional properties of smooth and skeletal muscle tissues. Smooth muscle tissues produce the same isometric force per cross-sectional area as skeletal muscle, but contain only one fifth as much myosin (Murphy et al 1974). In addition, the maximal shortening velocities of smooth muscle tissues are 1-2 orders of magnitude slower than those of skeletal muscles (Murphy et al 1997). 

MHC Content and Fiber CSA The cross-sectional area (CSA) of a muscle fiber relates to the number of crossbridges that can form in parallel (MHC content per halfsarcomere) and thus, to the force that a fiber can generate. Sarcomeres contain two symmetric functional elements, each generating force in opposing directions thus, the... 

Maximum muicle stress Maximum active stress, or specific tension, varies somewhat among fiber types and species (Table 48.2) around a generally accepted average of 250 kPa. This specific tension can be determined in any system in which it is possible to measure force and estimate the area of contractile material. Given muscle PCSA, maximum force produced by a muscle can be predicted by multiplying this PCSA by specific tension (Table 48.2). Specific tension can also be calculated for isolated muscle fibers or motor units in which estimates of cross-sectional area have been made. 

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