Muscle fiber

Typically, in order for motion to occur, several muscle sets must work together to perform even the simplest movements. The bicep is a two-muscle set the tricep is a three-muscle set. Each set works in tandem. Within each muscle group, muscle fibers obey the aU. or none principle, ie, aU. muscle fibers contract or none contract. Therefore, if the muscle fibers of a muscle group are stimulated enough by nerve impulses to contract, they contract to the maximum. 

Proteins can be broadly classified into fibrous and globular. Many fibrous proteins serve a stmctural role (11). CC-Keratin has been described. Fibroin, the primary protein in silk, has -sheets packed one on top of another. CoUagen, found in connective tissue, has a triple-hehcal stmcture. Other fibrous proteins have a motile function. Skeletal muscle fibers are made up of thick filaments consisting of the protein myosin, and thin filaments consisting of actin, troponin, and tropomyosin. Muscle contraction is achieved when these filaments sHde past each other. Microtubules and flagellin are proteins responsible for the motion of ciUa and bacterial dageUa. 

Muscle fibers contain myosin and actin which slide against each other during muscle contraction... 

The cells of the latter three types contain only a single nucleus and are called myocytes. The cells of skeletal muscle are long and multinucleate and are referred to as muscle fibers. At the microscopic level, skeletal muscle and cardiac muscle display alternating light and dark bands, and for this reason are often referred to as striated muscles. The different types of muscle cells vary widely in structure, size, and function. In addition, the times required for contractions and relaxations by various muscle types vary considerably. The fastest responses (on the order of milliseconds) are observed for fast-twitch skeletal... 

FIGURE 17.11 The structure of a skeletal muscle cell, showing the mauuer iu which t-tubules enable the sarcolemmal membrane to contact the ends of each myofibril iu the muscle fiber. The foot structure is shown iu the box. 

Szent-Gyorgyi further showed that the viscosity of an actomyosin solution was lowered by the addition of ATP, indicating that ATP decreases myosin s affinity for actin. Kinetic studies demonstrated that myosin ATPase activity was increased substantially by actin. (For this reason, Szent-Gyorgyi gave the name actin to the thin filament protein.) The ATPase turnover number of pure myosin is 0.05/sec. In the presence of actin, however, the turnover number increases to about 10/sec, a number more like that of intact muscle fibers. 

Molecular Tweezers of Light Take the Measure of a Muscle Fiber s Force... 

The trigger for all musele eontraetion is an increase in Ca eoneentration in the vicinity of the muscle fibers of skeletal muscle or the myocytes of cardiac and smooth muscle. In all these cases, this increase in Ca is due to the flow of Ca through calcium channels (Figure 17.24). A muscle contraction ends when the Ca concentration is reduced by specific calcium pumps (such as the SR Ca -ATPase, Chapter 10). The sarcoplasmic reticulum, t-tubule, and sarcolemmal membranes all contain Ca channels. As we shall see, the Ca channels of the SR function together with the t-tubules in a remarkable coupled process. 

FIGURE 17.24 Ca is the trigger signal for muscle contraction. Release of Ca" through voltage- or Ca" -sensitive channels activates contraction. Ca" pumps induce relaxation by reducing the concentration of Ca" available to the muscle fibers. 

The central canal (CC), radial canals (RC), and peripheral vestibules (PV) are indicated, (d) The relationship between the foot structures, t-tubule, terminal cisternae, and muscle fiber. (Photo courtesy of Sidney Fleischer, Vanderbilt University)... 

The structure of heart myocytes is different from that of skeletal muscle fibers. Heart myocytes are approximately 50 to 100 p,m long and 10 to 20 p,m in diameter. The t-tubules found in heart tissue have a fivefold larger diameter than those of skeletal muscle. The number of t-tubules found in cardiac muscle differs from species to species. Terminal cisternae of mammalian cardiac muscle can associate with other cellular elements to form dyads as well as triads. The association of terminal cisternae with the sarcolemma membrane in a dyad structure is called a peripheral coupling. The terminal cisternae may also form dyad structures with t-tubules that are called internal couplings (Figure 17.31). As with skeletal muscle, foot structures form the connection between the terminal cisternae and t-tubule membranes. 

Rigor ll a muscle condition in which muscle fibers, depleted of ATP and phosphocreatine, develop a state of extreme rigidity and cannot be easily extended. (In death, this state is called rigor mortis, the rigor of death.) From what you have learned about muscle contraction, explain the state of rigor in molecular terms. 

Fleischfarbe, /. flesh color, fleisch farbig, farben, a. flesh-colored. Fleisch-faser, /. muscle fiber, -faulnis, /. spoiling or putrefaction of meat, fleischfressend, p.a. meat-eating, carnivorous. Fleisch-gift, n. meat toxin, ptomaine, -guinmi,... 

Ridgway, E. B., and Ashley, C. C. (1967). Calcium transients in single muscle fibers. Biochem. Biophys. Res. Commun. 29 229-234. 

Muscle fiber remodeling increased proportion of l Weight gain... 

A skinned fiber is a muscle fiber, the sarcolemma of which has been mechanically removed or which is made freely permeable to small molecules, such as Ca2+, Mg2+, EGTA, ATP, soluble enzymes and others by a chemical agent (saponin, (3-escin or Staphylococcus a-toxin). The organization of the sarcoplasmic reticulum (SR) and myofibrils is kqrt as they are in the living muscle. 

The smooth muscle cell does not respond in an all-or-none manner, but instead its contractile state is a variable compromise between diverse regulatory influences. While a vertebrate skeletal muscle fiber is at complete rest unless activated by a motor nerve, regulation of the contractile activity of a smooth muscle cell is more complex. First, the smooth muscle cell typically receives input from many different kinds of nerve fibers. The various cell membrane receptors in turn activate different intracellular signal-transduction pathways which may affect (a) membrane channels, and hence, electrical activity (b) calcium storage or release or (c) the proteins of the contractile machinery. While each have their own biochemically specific ways, the actual mechanisms are for the most part known only in outline. 

In a nerve process or skeletal muscle fiber, the spread of activity is essentially only in one dimension, along the fiber. However, in smooth muscle the situation is rather more complex geometrically, and all three dimensions are involved. Action potentials conduct electrotonically just as they do in nerve fibers but they do so in three dimensions. In situ, regions supporting action potentials are not pointlike but tend to be large and the spread from them is more like a surface, approximating a plane. 

Skeletal muscle is made up of many muscle fibers (Figure 1) each of which is a multinucleated cell that was formed during development by the fusion of many cells (myoblasts). Skeletal muscle is formed from precursor myoblasts which arise... 

Figure 1. Muscle development. A skeletal muscle fiber is formed by the fusion of many single cells (myoblasts) into a multinucleated myotube. Myotubes then develop into the muscle fiber (see text). Sarcomeres form in longitudinal structures called myofibrils. The repeating structure of the sarcomere contains interdigitating thick and thin filaments.

All these experiments were carried out with actin and myosin in solution, either using moderate ionic strength and low actin concentrations (Lymn and Taylor, 1971) or using low ionic strength and high actin concentrations (Stem et al., 1979). However, in muscle both the actin concentration and the ionic strength are high. To confirm these models, these same experiments need to be carried out in muscle fibers. 

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