Muscles Ch 9 continued

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excitation-contraction coupling

excitation- contraction coupling is the sequence of events by which transmission of an action potential along the sacrolemma leads to the sliding of myofilaments. The action potential is brief and ends well before any signs of contraction are obvious. The events of excitation-contraction coupling occur during the latent period, between action potential initiation and the beginning of mechanical activity. The electrical signal does not act directly on the myofilaments. Instead, it causes the rise of intracellular calcium ion concentration that allow the filaments to slide.

summary of different types of channels involved in initiating muscle contraction

so lets summarize hat has to happen from the nerve endings on the finally excite muscle cells. Essentially this process involves activation of 4 sets of ion channels:

1

the process is initiated when the nerve impulse reaches the neuron terminal and opens voltage-gated calcium channels in the axonal membrane. Calcium entry triggers release of ACh into the synaptic cleft

2

Released ACh binds to ACh receptors in the sarcolemma, opening chemically gated Na+-K+ channels. Greater influx of Na+ causes a local voltage change (the end plate potential)

3

Local depolarization opens voltage-gated Na+ channels in the neighboring region of the sarcolemma. This allows more sodium to enter, which further depolarizes the sarcolemma, resulting in Action potential generation and propagation.

4

Transmission of an AP along the T tubules changes the shape of voltage-sensitive proteins in the t tubules, which in turn stimulate SR calcium release channels to release calcium into the cytosol.

troponin's effect on tropomyosin

Cross bridge formation (attachment of myosin heads to actin) requires calcium. Lets look more closely at how calcium ions promote muscle cell contraction. When intracellular calcium levels are low, the muscle cell is relaxed and the active (myosin-binding) sites on actin are physically blocked by tropomyosin molecules. As calcium levels rise, the ions bind to regulatory sites on troponin. To active its group of 7 actin's, a troponin must bind 2 calcium ions, change shape, and then roll tropomyosin into the groove if the actin helix, away from the myosin-binding sites. In short, the tropomyosin "blockade" is removed when sufficient calcium is present. Once binding sites on actin are exposed, the events of the cross bridge cycle occur in rapid succession.

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Sliding of thin filaments continues as long as the calcium signal is adequate ATP and are present. When nerve impulses are delivered rapidly, intracellular calcium levels increase greatly due to successive "puffs" of rounds of calcium released from the SR. In such cases, the muscle cells do not completely relax between successive stimuli and contraction is stronger and more sustained (within limits) until nervous stimulation ceases. As the calcium pumps of the SR reclaim calcium ions from the cytosol and troponin again changes shape, actin's myosin-binding sites are again covered by tropomyosin. The contraction ends, and muscle fiber relaxes.

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when the cycle is back where it started, the myosin had is in its upright high-energy configuration, ready to take another "step" and attach to an actin site farther along the thin filament. This "walking" of the myosin heads along the adjacent thin filaments during muscle shortening is much like a centipede's gait. The thin filaments cannot slide backward as the cycle repeats again and again because some myosin heads are always in contact with actin. Contracting muscles routinely shorten by 30 to 35% of their total resting length, so each myosin cross bridge attaches and detaches many times during a single contraction. It is likely that only half of the myosin heads of a think filament are pulling at the same instant. The others are randomly seeking their next binding site.

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Except for the brief period following muscle cell excitation, calcium ion concentrations in the cytosol are kept almost undetectably low. There is a reason for this: ATP is the cells energy source, and its hydrolysis yields inorganic phosphate (Pi). Pi would combine with calcium ions to form hydroxyapatite crystals, the stony-hard salts found in bone matrix, if calcium ion concentrations were always high. Such calcified muscle cells would die.

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In its relaxed state, a muscle is soft and unimpressive, not what you would expect of a prime mover of the body. However, within a few milliseconds, it can contract to become a hard elastic structure with dynamic characteristics that intrigue not only biologists but engineers and physicists as well.
Before we consider muscle contraction on the organ level, let's note a few principles of muscle mechanics.

1

The principle's governing contraction of a single muscle fiber and of a skeletal muscle consisting of a large number of fibers are pretty much the same.

2

the force exerted by a contracting muscle on an object is called muscle tension, and the opposing force exerted on the muscle by the weight to be moved is called the load

3

A contracting muscle does not always shorten and move the load. If muscle tension develops but the load is not moved, the contraction is called isometric ("same measure"), as when you try to lift a 2000 pounds car. If the muscle tension developed overcomes the load and muscle shortening occurs, the contraction is isotonic ("same tension"), as when you lift a 5 pound sack of sugar. Increasing muscle tension is measured for isometric contractions, whereas the amount of muscle shortening (distance in millimeters) is measured for isotonic contractions

4

A skeletal muscle contracts with varying force and for different periods of time in response to stimuli of varying frequencies and intensities. To understand how this occurs, we must look at the nerve- muscle functional unit called a motor unit.

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Each muscle is served by at least one motor nerve, and each motor nerve contains axons (fibrous extensions) of up to hundreds of motor neurons. As an axon enters a muscle, it branches into a number of terminals, each of which forms a neuromuscular junction with a single muscle fibers. A motor unit consists of a motor neuron and all the muscle fibers it supplies. When a motor neuron fires (transmits an action potential). all the muscle fibers it innervates contracts.

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The number of muscle fibers per motor unit may be a high as several hundred or as few as 4. Muscles that exert fine control (such as those controlling the fingers and eyes) have small motor units. By contrast, large, weight bearing muscles, whose movements are less precise (such as the hip muscles), have large motor units. The muscle fibers in a single motor unit are not clustered together but are spread throughout the muscle. As a result, stimulation of a single motor unit causes a weak contraction of the entire muscle.

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The muscle contraction is easily investigated in the laboratory using an isolated muscle. The muscle is attached to an apparatus that produces a myogram, a graphic recording of contractile activity. The line recording the activity is called a tracing.

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The response of a motor unit to a single action potential of its motor neuron is called a muscle twitch. The muscle fibers contract quickly and then relax. Every twitch myogram has 3 distinct phases.

1. Latent period

The latent period is the first few milliseconds following stimulation when excitation-contraction coupling is occurring. During this period, muscle tension is beginning to increase but no response is seen on the myogram.

2. Period of contraction

The period of contraction is when cross bridges are active, from the onset to the peak of tension development, and the myogram tracing rises to a peak. This period lasts 10-100 ms. If the tension (pull) becomes great enough to overcome the resistance of a load, the muscle shortens.

3. Period of relaxation

The period of contraction is followed by the period of relaxation. This final phase, lasting 10-100 ms, is initiated by reentry of calcium into the SR. Because contractile force is declining, muscle tension decreases to zero and the tracing returns to the baseline. If the muscle shortened during contraction, it now returns to its initial length. Notice that a muscle contracts faster than it relaxes, as revealed by the asymmetric nature of the myogram tracing.

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Twitch contractions of some muscles are rapid and brief, as with the muscles controlling eye movements. In contrast, the fibers of fleshy calf muscles (gastrocnemius and soleus) contract more slowly and remain contracted for much longer periods. These differences between muscles reflect metabolic properties of the myofibrils and enzyme variations.

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Muscle twitches- like those single, jerky contractions provoked in a laboratory - may result from certain neuromuscular problems, but this is not the way our muscles normally operate. Instead, healthy muscle contractions are relatively smooth and vary in strength as different demands are placed on them. These variations, needed for proper control of skeletal movement, are referred to as graded muscle responses. In general, muscle contraction can be graded in 2 ways: (1) by changing the frequency of stimulation and (2) by changing the strength of stimulation.

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The nervous system achieves greater muscular force by increasing the firing rate of motor neurons. For example, if 2 identical stimuli (electrical shock or nerve impulse) are delivered to a muscle in a rapid succession, the second twitch will appear to ride on the shoulders of the first. This phenomenon, called temporal or wave summation, occurs because the second contraction occurs before the muscle has completely relaxed. Because the muscle is already partially contracted when the next stimulus arrives and more calcium is being squirted into the cytosol to replace that being reclaimed by the SR, muscle tension produced during the second contraction causes more shortening than the first. In other words, the contractions are summed. (However, the refractory period is always honored. Thus, if a second stimulus is delivered before repolarization is complete, no wave summation occurs.

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If the stimulus strength is held constant and the muscle is stimulated at an increasingly faster rate, the relaxation time between the twitches becomes shorter and shorter, the concentration of calcium in the cytosol higher and higher, and the degree of wave summation greater and greater, progressing to a sustained but quivering contraction referred to as unfused or incomplete tetanus..

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Finally, as the stimulation frequency continues to increase, muscle tension increases until a maximal tension is reached. At this point all evidence of muscle relaxation disappears and the contractions fuse into a smooth, sustained contraction plateau called fused or complete tetanus. In the real world, fused tetanus happens infrequently, for example, when someone shows superhuman strength by lifting a fallen tree limb off a companion.

Vigorous muscle activity cannot continue indefinitely. Prolonged tetanus inevitably leads to muscle fatigue, a situation in which the muscle is unable to contract and its tension drops to zero.

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Wave summation contributes to contractile force, but its primary function is to produce smooth, continuous muscle contractions by rapidly stimulating a specific number of muscle cells. The force of contraction is controlled more precisely by recruitment, also called multiple motor unit summation.

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In the laboratory, recruitment is achieved by delivering shocks of increasing voltage to the muscle, calling more and more muscle fibers into play. Stimuli, that produce no observable contractions are called subthreshold stimuli. The stimulus at which the first observable contraction occurs is called the threshold stimulus. Beyond this point, the muscle contracts more and more vigorously as the stimulus strength is increased. The maximal stimulus is the strongest stimulus that produces increased contractile force. It represents the point at which all the muscle motor units are recruited. Increasing the stimulus intensity beyond the maximal stimulus does not produce a stronger contraction. In the body, the same phenomenon is called by neural activation of an increasingly large number of motor units serving the muscle.

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The recruitment process is not random. Instead it is dictated by the size principle. In any muscle, motor units with the smallest muscle fibers are controlled by small, highly excitable motor neurons, and these motor units tend to be activated first. As motor units with larger and larger muscle fibers begin to be excited, contractile strength increases. The largest motor units containing large, coarse muscle fibers, have as much as 50 times the contractile force of the smallest ones. They are controlled by the largest, least excitable (highest-threshold) neurons and are activated only when the most powerful contraction is necessary.

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The size principle is important because it allows the increases in force during weak contractions (for example, those that maintain posture or slow movements) to occur in small steps, whereas gradations in muscle force are progressively greater when large amount of force are needed for vigorous activities such as jumping or running. This principle explains how the same hand that pats your cheek can deliver and stinging slap.

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Although all the motor units of a muscle may be recruited simultaneously to produce an exceptionally strong contraction, motor units are more commonly activated asynchronously in the body. At a given instant, some are in tetanus (usually unfused tetanus) while others are resting and recovering. This technique helps prolong a strong contraction by preventing or delaying fatigue. It also explains how weak contractions promoted by infrequent stimuli can remain smooth.

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Skeletal Muscles are described as voluntary, but even relaxed muscles are almost always slightly contracted, a phenomenon called muscle tone. Muscle tone is due to spinal reflexes that activate first one group of motor units and then another in response to activation of stretch receptors in the muscles. Muscle tone does not produce active movements, but it keeps the muscles firm, healthy, and ready to respond to stimulation. Skeletal muscle tone also helps stabilize joints and maintain posture.

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As noted earlier, there are 2 main categories of contractions- isotonic and isometric. In isotonic contractions, muscle length changes and moves the load. Once sufficient tension has developed to move the load, the tension remains relatively constant through the rest of the contractile period.

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Isotonic contractions come in 2 "flavors" -- concentric and eccentric. Concentric contractions are those in which the muscle shortens and does work, such as picking up a book or kicking a ball. These contractions are probably more familiar. However eccentric contractions, in which the muscle generates force as it lengthens, are equally important for coordination and purposeful movements.Eccentric contractions occur in your calf muscle, for example, as you walk up a steep hill. Eccentric contractions are about 50% more forceful than concentric ones at the same load and more often cause delayed-onset muscle soreness. (consider how your calf muscles feel the day after hiking up that hill.) Just why this is so is unclear, but it may be that the muscle stretching that occurs during such contractions causes microtears in the muscles.

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Biceps curls provide a simple example of how concentric and eccentric contractions work together in our everyday activities. When you flex your elbow to raise this textbook to your shoulder, the biceps muscle in your arm is contracting concentrically. When returning this book to the desktop, the isotonic contraction of the biceps is eccentric. Basically, eccentric contractions put the body in position to contract concentrically. Both jumping and throwing activities involve both types of contraction.

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In isometric contractions, tension may build to the muscle's peak tension-producing capacity, but the muscle neither shortens nor lengthens. Isometric contractions occur when a muscle attempts to move a load that is greater than the force (tension) the muscle is able to develop--- think of trying to lift a piano singlehandedly . Muscles contract isometrically when they act primarily to maintain upright posture or to hold joints in stationary positions while movements occur at other joints.

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Lets consider knee bends as an example. When the squat position is held for a few seconds, the quadriceps muscles of the anterior thigh contract isometrically to hold the knee in the flexed position. They also contract isometrically when we begin to rise to the upright position until their tension exceeds the load (weight of the upper body). At that point muscle shortening (concentric contraction) begins. So the quadriceps contractile sequence for a deep knee bend from start to finish is (1) flex knee (eccentric), (2) hold squat position (isometric), (3) extend knee (isometric, then concentric). Of course, this list does not even begin to consider the isometric contractions of the posterior thigh muscles or the trunk muscles that maintain a relatively erect trunk posture during the movement.

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Electrochemical and mechanical events occurring within a muscle are identical in both isotonic and isometric contractions. However, the result is different. In isotonic contractions, the thin filaments are sliding. In isometric contractions, the cross bridges are generating force but are not moving the thin filaments, so there is no change in the banding pattern from that of the resting state. (You could say that they are "spinning their wheels" on the same actin binding sites.)

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How does the body provide the energy needed for contraction? As a muscle contracts, ATP supplies the energy for cross bridge movement and detachment and for operation of the calcium pump in the SR. Surprisingly, muscles store very limited reserves of ATP - 4 to 6 seconds worth at most, just enough to get you going. Because ATP is the only energy source used directly for contractile activities, it must be regenerated as fast as it is broken down if contraction is to continue.

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Fortunately, after ATP is hydrolyzed to ADP and inorganic phosphate in muscle fibers, it is regenerated within a fraction of a second by one or more of the 3 pathways: (1) direct phosphorylation of ADP by creatine phosphate, (2) the anaerobic pathway called glycolysis, which converts glucose to lactic acid, and (3) aerobic respiration. All body cells use glycolysis and aerobic respiration to produce ATP.

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As we begin to exercise vigorously, the demand for ATP soars and the ATP stored in working muscles is consumed within a few twitches. Then creatine phosphate (CP), a unique high-energy molecule stored in muscles, is tapped to regenerate ATP while the metabolic pathways are adjusting to the suddenly higher demands for ATP. The result of coupling CP with ADP is almost instant transfer of energy and a phosphate group from CP to ADP to form ATP.

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Muscle cells store 2 to 3 times as much CP as ATP, and the CP-ADP reaction, catalyzed by the enzyme creatine kinase, is so efficient that the amount of ATP in muscle cells changes very little during the initial period of contraction.

Together, stored ATP and CP provide for maximum muscle power for 14-16 seconds-- long enough to energize a 100-meter dash (slightly longer if the activity is less vigorous). The coupled reaction is readily reversible, and to keep CP "on tap", CP reserves are replenished during periods of rest or inactivity.

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A stored ATP and CP are exhausted, more ATP is generated by breakdown (catabolism) of glucose obtained from the blood or of glycogen stored in the muscle. The initial phase of glucose breakdown is glycolysis. This pathway occurs in both the presence and the absence of oxygen, but because it does not use oxygen, it is an anaerobic pathway. During glycolysis, glucose is broken down to 2 pyruvic acid molecules releasing enough energy to form small amounts of ATP (2 ATP per glucose).

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Ordinarily, pyruvic acid produced during glycolysis then enters the mitochondria and reacts with oxygen to produce still more ATP in the oxygen-using pathway called aerobic respiration. But when muscles contract vigorously and contractile activity reaches about 70% of the maximum possible (for example, when you run 600 meters with maximal effort), the bulging muscles compress the blood vessels within them, impairing blood flow and oxygen delivery. Under these anaerobic conditions, most of the pyruvic acid produced during glycolysis is converted into lactic acid, and the overall process is referred to as anaerobic glycolysis. Thus, during oxygen deficit, lactic acid is the end product of cellular metabolism of glucose.

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Most of the lactic acid diffuses out of the muscles into the bloodstream and is gone from the muscle tissue within 30 minutes after exercise stops. Subsequently, the lactic acid is picked up by the liver, heart, or kidney cells, which can use it as an energy source. Additionally, liver cells can reconvert it to pyruvic acid or glucose and release it back into the bloodstream for muscle use, or convert it to glycogen for storage.

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The anaerobic pathway harvests only about 5% as much ATP from each glucose molecule as the aerobic pathway, but it produces ATP about 2 1/2 times faster. From this reason, when large amounts of ATP are needed for moderate periods (30-40 seconds) of strenuous muscle activity, glycolysis can provide most of the ATP needed as long as the required fuels and enzymes are available. Together, stored ATP and CP and the glycolysis-lactic acid pathway can support strenuous muscle activity for nearly a minute.

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Although anaerobic glycolysis readily fuels spurts of vigorous exercise, it has shortcomings. Huge amounts of glucose are used to produce relatively small harvests of ATP, and the accumulating lactic acid is partially responsible for muscle soreness during intense exercise.

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During rest and light to moderate exercise, even if prolonged, 95% of the ATP used for muscle activity comes from aerobic respiration. Aerobic respiration occurs in the mitochondria, requires oxygen, and involves a sequence of chemical reactions in which the bonds of fuel molecules are broken and the energy released is used to make ATP.

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During aerobic respiration, which includes glycolysis and the reactions that take place in the mitochondria, glucose is broken down entirely, yielding water, carbon dioxide, and large amounts of ATP as the final products

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The carbon dioxide released diffuses out of the muscle tissue into the blood and is removed from the body by the lungs. As exercise begins, muscle glycogen provides most of the fuel. Shortly thereafter, bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids are the major sources of fuels. After about 30 minutes, fatty acids because the major source of fuels. Aerobic respiration provides a high yield of ATP (about 32 ATP per glucose), but it is slow because its many steps and it requires continuous delivery of oxygen and nutrient fuels to keep it going.

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Which pathways predominate during exercise? As long as it has enough oxygen, a muscle cell will form ATP by the aerobic pathway. When ATP demands are within the capacity of the aerobic pathway, light to moderate muscular activity can continue for several hours in well-conditioned individuals. However, when exercise demands begin to exceed the ability of the muscle cells to carry out the necessary reactions quickly enough, glycolysis begins to contribute more and more of the total ATP generated. The length of time a muscle can continue to contract using aerobic pathways is called aerobic endurance, and the point at which muscle metabolism coverts to anaerobic glycolysis is called anaerobic threshold.

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Exercise physiologists have been able to estimate the relative importance of each energy producing system to athletic performance. Activities that require a surge of power but last only a few seconds, such as weight lifting, driving, and sprinting, rely entirely on ATP and CP stores. The more on and off or burstlike activities of tennis, soccer, and a 100 meter swim appear to be fueled almost entirely by anaerobic glycolysis.

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Prolonged activities such as marathon runs and jogging, where endurance rather than power is the goal, depend mainly on aerobic respiration. Levels of CP and ATP don't change much during prolonged exercise because ATP is generated at the same rate as it is used -- a "pay as you go" system. Compared to anaerobic energy production, aerobic generation of ATP is relatively slow, but the ATP harvest is enormous.

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Muscle fatigue is a state of physiological inability to contract even though the muscle still may be receiving stimuli. Although many factors appear to contribute to fatigue, its specific causes are not fully understood. Most experimental evidence indicates that fatigue is due to a problems at the neuromuscular junction. Availability of ATP declines during contraction, but normally it is unusual for a muscle to totally run out of ATP. So, ATP is not a fatigue-producing factor in moderate exercise. A total lack of ATP results in contractures, states of continuous contraction because the cross bridges are unable to detach (not unlike what happens in rigor mortis). Writers cramp is a familiar example of temporary contractures.

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Several ionic imbalances contribute to muscle fatigue. As action potentials are transmitted, potassium is lost from the muscle cells, and the Na+-K+ pumps are inadequate to reverse the ionic imbalances quickly, so K+ accumulates in the fluids of the T tubules. This ionic change disturbs the membrane potential of the muscle cells and halts calcium release from the SR. theoretically, in short-duration exercise, an accumulation of inorganic phosphate (Pi) from CP and ATP breakdown may interfere with calcium release from the SR or alternatively with the release of Pi from myosin and thus hamper myosin's power strokes. Lactic acid has long been assumed to be a major cause of fatigue, but it seems to be more important in provoking central (psychological) fatigue (in which the muscles are still willing to "go" but we feel too tired to continue the activity) than physiological fatigue. Excessive intracellular accumulation of lactic acid (which causes the muscles to ache) raises the concentration of H+ and alters contractile proteins; however, pH is normally regulated within normal limits in all but the greatest degree of exertion. Additionally, lactic acid has recently been shown to counteract high K+ levels which do lead to muscle fatigue.

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In general, intense exercise of short duration produces fatigue rapidly via ionic disturbances that alter E-C coupling, but recovery is also rapid. In contrast to short-duration exercise, the slow-developing fatigue of prolonged low-intensity exercise may require several hours for complete recovery. It appears that this type of exercise damages the SR, interfering with calcium regulation and release, and therefore with muscle activation.

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Whether or not fatigue occurs, vigorous exercise causes a muscle's chemistry to change dramatically. For a muscle to return to its resting state, its oxygen reserves must be replenished, the accumulated lactic acid must be reconverted to pyruvic acid, glycogen stores must be replaced, and ATP and creatine phosphate reserves must be resynthesized. Additionally, the liver must convert any lactic acid persisting in blood to glucose or glycogen. During anaerobic muscle contraction, all of these oxygen-requiring activities occur more slowly and are (at least partially) deferred until oxygen is again available. For this reason, we say an oxygen deficit is incurred, which must be repaid. Oxygen deficit is defined as the extra amount of oxygen that the body must take in for these restorative processes. It represents the difference between the amount actually used. All anaerobic sources of ATP used during muscle activity contribute to this deficit.

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Only about 40% of the enery released during muscle contraction is converted to useful work (still, this percentage is significantly higher than that of many mechanical devices). The rest is given off as heat, which has to be dealt with if the body homeostasis is to be maintained. When you exercise vigorously, you start to feel hot as your blood is warmed by the liberated heat. Like a car's cooling system that dissipates heat, heat buildup in the body is prevented from reaching dangerous levels by several homeostatic processes, including sweating and radiation of heat from skin surface. Shivering represents the opposite side of homeostatic balance, in which muscle contractions are used to produce more heat.

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the force of muscle contraction is affected by (1) the number of muscle fibers stimulated, (2) the relative size of the fibers, (3) the frequency of stimulation, and (4) the degree of muscle stretch.

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As already discussed, the more motor units that are recruited the greater the muscle force.

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The bulkier the muscle (the greater its cross-sectional area), the more tension it can develop and the great its strength, but there is more to it than this. As noted earlier, the large fibers of large motor units are very effective in producing the most powerful movements. Regular resistance exercise increases muscle force by causing muscle cells to hypertrophy or increase in size.

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As a muscle begins to contract, the force generated by the cross bridges--- the internal tension--stretches the connective tissue sheaths (noncontractile components) . These in turn become taut and transfer their tension, called the external tension, to the load (muscle insertion) and when the contraction ends, recoil of the noncontractile components helps to return the muscle to its resting length.

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Time is required to take up slack and stretch the noncontractile components, and while this is happening, the internal tension is already declining. So, in brief twitch contractions, the external tension is always less than the internal tension. However, when a muscle is stimulated rapidly, contractions are summed, becoming stronger and move vigorous and ultimately producing tetanus. During tetanic contractions more time is available to stretch the noncontractile components, and external tension approaches, the internal tension. So, the more rapidly a muscle is stimulated, the greater the force it exerts.

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The optimal operating length for muscle fibers is the length at which they can generate maximum force. Within a sacromere, the ideal length-tension relationship occurs when a muscle is slightly stretched and the thin and thick filaments overlap optimally, because this relationship permits sliding along nearly the entire length of the thin filaments. If a muscle fiber is stretched so much that the filaments do not overlap, the myosin heads have nothing to attach to and cannot generate tension. Alternatively, if the sacromeres are so compressed and cramped that the Z discs abut the thick myofilaments, and the thin filaments touch and interfere with one another, little or no further shortening can occur.

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Identical relationships exist in a whole muscle. If you stretch a muscle to various extents and then stimulate it tetanically, the active tension and muscle can generate varies with length. A severely stretched muscle (say one over 180% of its optimal length) cannot develop tension. Like wise, at 75% of a muscles resting length, force generation (or shortening) is limited because the actin myofilaments in its sarcomeres overlap and the thick filaments run into the Z discs, restricting further shortening.

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In the body, skeletal muscles are maintained near their optimal operating length by the way they are attached to bones. The joints normally prevent bone movements that would stretch attached muscles beyond their optimal range.

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Muscles vary in how fast they can contract and in how long they can continue to contract before they fatigue. These characteristics are influenced by muscle fiber type, load and recruitment.

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There are several ways of classifying muscle fibers, but learning about these classes is easier if you initially pay attention to just two major function characteristics: speed of contraction and the major pathways for forming ATP.

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On the basis of speed of shortening, or contraction, there are slow fibers and fast fibers. The difference in their speed reflects how fast their myosin ATPases split ATP, and on the pattern of electrical activity of their motor neurons. Duration of contraction also varies with fiber type and depends on how quickly calcium is moved from the cytosol into the SR.

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The cells that rely mostly on the oxygen-using aerobic pathways for ATP generation are oxidative fibers, and those that rely more on anaerobic glycolysis are glycolytic fibers.

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Using these 2 criteria, we can classify skeletal muscle cells as being slow oxidative (SO) fibers, fast oxidative (FO) fibers, or fast glycolytic (FG) fibers.

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Slow oxidative fibers
1. contract relatively slowly because its myosin ATPases are slow (a criterion)

2. Depends on oxygen delivery and aerobic pathways (high oxidative capacity- a criterion)

3. Is fatigue resistant and has high endurance (typical of fibers that depend on aerobic metabolism)

4. Is thin (a large amount of cytoplasm impedes diffusion of O2 and nutrients from the blood)

5. Has relatively little power (a thin cell can contain only a limited number of myofibrils)

6. Has many mitochondria (actual sites of oxygen use)

7. Has a rich capillary supply (the better to deliver blood-borne O2)

8. Is red (its color stems from an abundant supply of myoglobin, muscle's oxygen-binding pigment that stores O2 reserves in the cell and aids diffusion of O2 through the cell.)

Add these functions together and you have muscle fibers best suited to endurance-type activities.

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Fast glycolytic fibers:

1. contract rapidly due to the activity of fast myosin ATPases

2. Does not use oxygen

3. Depends on plentiful glycogen reserves for fuel rather than on blood-delivered nutrients

4. Tires quickly because glycogen reserves are short-lived and lactic acid accumulates quickly, making it a fatigable fiber

5. Has a large diameter, indicating that plentiful contractile myofilaments that allow it to contract powerfully before it "poops out"

6. Has few mitochondria, little myoglobin and low capillary density (and so is white), and is a much thicker cell (because it doesn't depend on continuous oxygen and nutrient diffusion from the blood.

For these reasons, the fast glycolytic fibers are best suited for short-term, rapid, intense movements (moving furniture across the room, for example)

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Finally, consider the less common intermediate muscle fiber types, called fast oxidative fibers. They have many characteristics (fiber diameter and power, for example) intermediate between the other 2 types. Like fast glycolytic fibers, they contract quickly, but like slow oxidative fibers, they are oxygen dependent and have a rich supply of myoglobin and capillaries.

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Some muscles have a predominance of one fiber type, but most contain a minute of fiber types, which give them a range of contractile speeds and fatigue resistance. For example, a calf muscle can propel us in a sprint (using its white fast glycolytic fibers) or a long-distance race (making good use of its slow and fast oxidative fibers.) But, as might be expected, all muscle fibers in a particular motor unit are of the same type.

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Although everyone's muscles contain mixtures of the 3 fiber types, some people have relatively more of one kind. These differences are genetically initiated, but are modified by exercise and no doubt determine athletic capabilities, such as endurance vs strength, to a large extent. For example, muscles of marathon runners have a high percentage of slow oxidative fibers (about 80%), while those of sprinters contain a higher percentage (about 60%) of fast oxidative and glycolytic fibers. Interconversion between the "fast" fiber types occurs as a result of specific exercise regimes.

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Because muscles are attached to bones, they are always pitted against some resistance, or load, when they contract. As you might expect, they contract fastest when there is no added load on them. A greater load results in longer latent period, a slower contraction, and a shorter duration of contraction. If the load exceeds the muscle's maximum tension, the speed of shortening is zero and the contraction is isometric.

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Just as many hands on a project can get a job done more quickly and also can keep working longer, the more motor units that are contracting, the faster and more prolonged the contraction.

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The amount of work a muscle does is reflected in changes in the muscle itself. When used actively or strenuously, muscles may increase in size or strength or become more efficient and fatigue resistant. Muscle inactivity, on the other hand, always lead to muscle weakness and wasting.

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Aerobic, or endurance exercise such a swimming, jogging, fast walking, and biking results in several recognizable changes in skeletal muscles. There is an increase in the number of capillaries surrounding the muscle fibers, and in the number of mitochondria within them, and the fibers synthesize more myoglobin. these changes occur in all fiber types, but are most dramatic in slow oxidative fibers, which depend primarily on aerobic pathways. The changes result in more efficient muscle metabolism and in greater endurance, strength, and resistance to fatigue. Additionally, regular endurance exercise may convert fast glycolytic fibers into fast oxidative fibers.

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The moderately weak but sustained muscle activity required for endurance exercise does not promote significant skeletal muscle hypertrophy, even though the exercise may go on for hours. Muscle hypertrophy, illustrated by the bulging biceps and chest muscles of a professional weight lifter, results mainly from high intensity resistance exercise (typically under anaerobic conditions) such a weight lifting or isometric exercise, in which the muscles are pitted against high-resistance or immovable forces. Here strength, not stamina, is important, and a few minutes every day is sufficient to allow a proverbial weakling to put on 50% more muscle within a year.

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The increased muscle bulk largely reflects increases in the size of individual muscle fibers (particularly the fast glycolytic variety) rather than an increased number of muscle fibers. [However, some of the increased muscle size may result either from longitudinal splitting or tearing of the fibers and subsequent growth of these "split" cells, or form the proliferation and fusion of satellite cells. The controversy is still raging]. Vigorously stressed muscle fibers contain more mitochondria, form more myofilaments and myofibrils, and store more glycogen. The amount of connective tissue between the cells also increases. Collectively these changes promote significant increases in muscle strength and size. Fast oxidative fibers can be shifted to fast glycolytic fibers in response to resistance activities. However, if the specific exercise routine is discontinued the fibers previously converted are reconverted to their original metabolic properties.

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Resistance training can produce magnificent bulging muscles, but if done unwisely, some muscle may develop more than other. Because muscles work in antagonistic pairs or groups, opposing muscles must be equally strong to work together smoothly. When muscle training is not balanced, individuals can become muscle-bound, which means that they lack flexibility, have a generally awkward stance, and are unable to make full use of their muscles.

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Whatever the activity, exercise gains adhere to the overload principle. Forcing a muscle to work hard promotes increased muscle strength and endurance, and as muscles adapt to the increased demands, they must be overloaded even more to produce further gains. However, a heavy-workout day should be followed by one of rest or an easy workout to allow the muscles to recover and repair themselves. Doing too much too soon, or ignoring the warning signs of muscle or joint pain, increases the risk of overuse injuries that may prevent future sports activities or even lead to lifetime disability.

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Endurance and resistant exercises produce different patterns of muscular response, so it is important to know what your exercise goals are. Lifting weights will not improve your endurance for a triathlon. By the same token, jogging will do little to improve you muscle definition or to enhance your strength for moving furniture. A program that alternates aerobic activities with anaerobic ones provides the best program for optimal health.

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