This energy is expended as the myosin head moves through the power stroke; at the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the cross-bridge formed is still in place, and actin and myosin are bound together. ATP can then attach to myosin, which allows the cross-bridge cycle to start again and further muscle contraction can occur Figure 1. The movement of the myosin head back to its original position is called the recovery stroke.
Resting muscles store energy from ATP in the myosin heads while they wait for another contraction. Figure 1. With each contraction cycle, actin moves relative to myosin. When a muscle is in a resting state, actin and myosin are separated. To keep actin from binding to the active site on myosin, regulatory proteins block the molecular binding sites.
Tropomyosin blocks myosin binding sites on actin molecules, preventing cross-bridge formation and preventing contraction in a muscle without nervous input. Types of muscle : The body contains three types of muscle tissue: skeletal muscle, smooth muscle, and cardiac muscle, visualized here using light microscopy.
Visible striations in skeletal and cardiac muscle are visible, differentiating them from the more randomised appearance of smooth muscle. Skeletal muscles are composed of striated subunits called sarcomeres, which are composed of the myofilaments actin and myosin. Myocytes, sometimes called muscle fibers, form the bulk of muscle tissue.
They are bound together by perimysium, a sheath of connective tissue, into bundles called fascicles, which are in turn bundled together to form muscle tissue. Myocytes contain numerous specialized cellular structures which facilitate their contraction and therefore that of the muscle as a whole.
The highly specialized structure of myocytes has led to the creation of terminology which differentiates them from generic animal cells. Myocytes can be incredibly large, with diameters of up to micrometers and lengths of up to 30 centimeters. The sarcoplasm is rich with glycogen and myoglobin, which store the glucose and oxygen required for energy generation, and is almost completely filled with myofibrils, the long fibers composed of myofilaments that facilitate muscle contraction.
The sarcolemma of myocytes contains numerous invaginations pits called transverse tubules which are usually perpendicular to the length of the myocyte. Each myocyte contains multiple nuclei due to their derivation from multiple myoblasts, progenitor cells that give rise to myocytes. These myoblasts asre located to the periphery of the myocyte and flattened so as not to impact myocyte contraction.
Myocyte: Skeletal muscle cell : A skeletal muscle cell is surrounded by a plasma membrane called the sarcolemma with a cytoplasm called the sarcoplasm.
A muscle fiber is composed of many myofibrils, packaged into orderly units. Each myocyte can contain many thousands of myofibrils. Myofibrils run parallel to the myocyte and typically run for its entire length, attaching to the sarcolemma at either end.
Each myofibril is surrounded by the sarcoplasmic reticulum, which is closely associated with the transverse tubules. Myofibrils are composed of long myofilaments of actin, myosin, and other associated proteins.
These proteins are organized into regions termed sarcomeres, the functional contractile region of the myocyte. Within the sarcomere actin and myosin, myofilaments are interlaced with each other and slide over each other via the sliding filament model of contraction.
The regular organization of these sarcomeres gives skeletal and cardiac muscle their distinctive striated appearance. Sarcomere : The sarcomere is the functional contractile region of the myocyte, and defines the region of interaction between a set of thick and thin filaments. Myofibrils are composed of smaller structures called myofilaments. There are two main types of myofilaments: thick filaments and thin filaments. Thick filaments are composed primarily of myosin proteins, the tails of which bind together leaving the heads exposed to the interlaced thin filaments.
Thin filaments are composed of actin, tropomyosin, and troponin. The molecular model of contraction which describes the interaction between actin and myosin myofilaments is called the cross-bridge cycle.
In the sliding filament model, the thick and thin filaments pass each other, shortening the sarcomere. Movement often requires the contraction of a skeletal muscle, as can be observed when the bicep muscle in the arm contracts, drawing the forearm up towards the trunk. The sliding filament model describes the process used by muscles to contract. It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle.
To understand the sliding filament model requires an understanding of sarcomere structure. A sarcomere is defined as the segment between two neighbouring, parallel Z-lines. Z lines are composed of a mixture of actin myofilaments and molecules of the highly elastic protein titin crosslinked by alpha-actinin. Actin myofilaments attach directly to the Z-lines, whereas myosin myofilaments attach via titin molecules. Surrounding the Z-line is the I-band, the region where actin myofilaments are not superimposed by myosin myofilaments.
The I-band is spanned by the titin molecule connecting the Z-line with a myosin filament. The region between two neighboring, parallel I-bands is known as the A-band and contains the entire length of single myosin myofilaments. Within the A-band is a region known as the H-band, which is the region not superimposed by actin myofilaments.
Within the H-band is the M-line, which is composed of myosin myofilaments and titin molecules crosslinked by myomesin.
Titin molecules connect the Z-line with the M-line and provide a scaffold for myosin myofilaments. Their elasticity provides the underpinning of muscle contraction. Titin molecules are thought to play a key role as a molecular ruler maintaining parallel alignment within the sarcomere. Another protein, nebulin, is thought to perform a similar role for actin myofilaments. The molecular mechanism whereby myosin and acting myofilaments slide over each other is termed the cross-bridge cycle.
During muscle contraction, the heads of myosin myofilaments quickly bind and release in a ratcheting fashion, pulling themselves along the actin myofilament. At the level of the sliding filament model, expansion and contraction only occurs within the I and H-bands.
The myofilaments themselves do not contract or expand and so the A-band remains constant. The sarcomere and the sliding filament model of contraction : During contraction myosin ratchets along actin myofilaments compressing the I and H bands. During stretching this tension is release and the I and H bands expand.
The A-band remains constant throughout as the length of the myosin myofilaments does not change. The amount of force and movement generated generated by an individual sarcomere is small. However, when multiplied by the number of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle, the amount of force and movement generated is significant.
The breakdown of one glucose molecule produces two ATP and two molecules of pyruvic acid , which can be used in aerobic respiration or when oxygen levels are low, converted to lactic acid Figure b.
If oxygen is available, pyruvic acid is used in aerobic respiration. However, if oxygen is not available, pyruvic acid is converted to lactic acid , which may contribute to muscle fatigue.
This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. Glycolysis itself cannot be sustained for very long approximately 1 minute of muscle activity , but it is useful in facilitating short bursts of high-intensity output.
This is because glycolysis does not utilize glucose very efficiently, producing a net gain of two ATPs per molecule of glucose, and the end product of lactic acid, which may contribute to muscle fatigue as it accumulates. Aerobic respiration is the breakdown of glucose or other nutrients in the presence of oxygen O 2 to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria.
The inputs for aerobic respiration include glucose circulating in the bloodstream, pyruvic acid, and fatty acids. Aerobic respiration is much more efficient than anaerobic glycolysis, producing approximately 36 ATPs per molecule of glucose versus four from glycolysis. However, aerobic respiration cannot be sustained without a steady supply of O 2 to the skeletal muscle and is much slower Figure c.
To compensate, muscles store small amount of excess oxygen in proteins call myoglobin, allowing for more efficient muscle contractions and less fatigue.
Aerobic training also increases the efficiency of the circulatory system so that O 2 can be supplied to the muscles for longer periods of time. Muscle fatigue occurs when a muscle can no longer contract in response to signals from the nervous system. The exact causes of muscle fatigue are not fully known, although certain factors have been correlated with the decreased muscle contraction that occurs during fatigue.
ATP is needed for normal muscle contraction, and as ATP reserves are reduced, muscle function may decline. This may be more of a factor in brief, intense muscle output rather than sustained, lower intensity efforts. Lactic acid buildup may lower intracellular pH, affecting enzyme and protein activity. Intense muscle activity results in an oxygen debt , which is the amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction.
Oxygen is required to restore ATP and creatine phosphate levels, convert lactic acid to pyruvic acid, and, in the liver, to convert lactic acid into glucose or glycogen. Other systems used during exercise also require oxygen, and all of these combined processes result in the increased breathing rate that occurs after exercise.
Until the oxygen debt has been met, oxygen intake is elevated, even after exercise has stopped. Relaxing skeletal muscle fibers, and ultimately, the skeletal muscle, begins with the motor neuron, which stops releasing its chemical signal, ACh, into the synapse at the NMJ. Without the ability to form cross-bridges between the thin and thick filaments, the muscle fiber loses its tension and relaxes. The number of skeletal muscle fibers in a given muscle is genetically determined and does not change.
Muscle strength is directly related to the amount of myofibrils and sarcomeres within each fiber. Factors, such as hormones and stress and artificial anabolic steroids , acting on the muscle can increase the production of sarcomeres and myofibrils within the muscle fibers, a change called hypertrophy, which results in the increased mass and bulk in a skeletal muscle.
Likewise, decreased use of a skeletal muscle results in atrophy, where the number of sarcomeres and myofibrils disappear but not the number of muscle fibers. It is common for a limb in a cast to show atrophied muscles when the cast is removed, and certain diseases, such as polio, show atrophied muscles.
Duchenne muscular dystrophy DMD is a progressive weakening of the skeletal muscles. Over time, as muscle damage accumulates, muscle mass is lost, and greater functional impairments develop. DMD is an inherited disorder caused by an abnormal X chromosome. The skeletal muscle contractile machine is fueled by both calcium and ATP. Calcium ions activate the contractile machinery by binding to troponin C and relieving troponin-tropomyosin inhibition of actinomyosin interaction.
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