Tendons contractility

Two major kinds of proteins are tough fibrous proteins, which compose hair, tendons, muscles, feathers, and silk, and spherical or oblong-shaped globular proteins, such as hanoglobin in blood or the proteins, which comprise enzymes. Proteins serve many functions. These include nutrient proteins, such as casein in milk structural proteins, such as collagen in tendons contractile proteins, such as those in muscles and regulatory proteins, such as insulin, which regulate biochemical processes. 

The compliance in series with the active force. Force exerted by the activated elements must be transmitted or borne by whatever structural elements are in series with them. In skeletal muscle there is clearly a tendon in series but not so with smooth muscle. In smooth muscle, the total length of contractile apparatus is broken up into individual cells with intercalating extracellular connective structures. In addition, the portions of the crossbridges in series with the pulling site must also be stretched before force can rise to isometric levels. Taken together, the... 

Contractile properties in rodents can be measured either in vitro in a dissected muscle or in vivo in an intact preparation with an anesthetized animal (e.g., (19-20)). Measurements made under isometric conditions are perhaps most common and use the most straightforward setup. The addition of servomotors for dynamic control of muscle length allows simulation of dynamic conditions (eccentric, isotonic, etc.) that may be modified by disease or other processes (22, 23). For in vitro studies, the muscle is anchored by ligating the tendon (origin) to a support, for in vivo studies, the bone (femur) is clamped to prevent movement. The other tendon (insertion) is then coupled to a force transducer. In both cases, a recording electrode is also placed in contact with the muscle to record the compound action potential, and a stimulating electrode is used to stimulate the nerve or the muscle, as described below. 

Storage Structural Contractile Ferritin Collagen (tendons), keratin (hair) Actin, myosin in muscle tissue... 

The sarcolemma consists of a plasma membrane and an onter coat made np of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fibre this outer coat of the sarcolemma fuses with a tendon fibre, and the tendon fibres in turn collect into bundles to form the muscle tendons that then insert into bones. The membrane is designed to receive and conduct stimuli, is extensible and encloses the contractile substance of a muscle fibre. The sarcolemma is attached to the cytoskeleton on its cytoplasmic surface. It invaginates into the c)doplasm, forming membranous tubules called transverse tubules sarcoplasmic reticulum (enlarged smooth endoplasmic reticulum) lies either side of the transverse tubules. The transverse tubules and sarcoplasmic reticulum transmit altered membrane permeability down the tubules and into the muscle. 

The force generated by a muscle is proportional to the contraction time. The longer the contraction time, the greater the force development up to the point of maximum tension. Slower contraction leads to greater force production because more time is allowed for the tension produced in contractile elements to be transferred through the noncontractile components to the tendon. This is the force-time relationship. Tension in the tendon will reach the maximum tension developed by the contractile tissues only if the active contraction process is of adequate (even up to 300 msec) duration [Sukop and Nelson, 1974). 

FIGURE 24.6 Structure of the musculotendonal model (based on the Hill viscoelastic model). Abbreviations CE, contractile element SE, series elastic element PE, parallel viscoelastic element TE, tendonal elastic element , length M, muscle T, tendon. 

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