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I'm being taught that: a muscle fibre spans the entire length of the muscle, from the originating tendon to the inserting tendon.
The question is, can a muscle fibre contract only partially? Say, if the fibre is 1m long, can only the first 10cm contract without the rest contracting?
Additonally, to help me understand better: When a neuron innervates a muscle fibre, does it do it only at one spot, or can a muscle fibre be innervated in several spots? Can several neurons innervate the same muscle fibre?
Skeletal Muscle Fiber Type: Influence on Contractile and Metabolic Properties
Copyright: © 2004 Juleen R. Zierath and John A. Hawley. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abbreviations: FT, fast-twitch FTa, aerobic FT fiber FTb, anaerobic FT fiber HIF-1α, Hypoxia Inducible Factor-1α MAPK, mitogen-activated protein kinase MEF2, myocyte enhancer factor 2 PGC-1, peroxisome proliferator γ coactivator 1 PPARδ, peroxisome proliferator-activated receptor δ ST, slow-twitch VO2max, maximal O2 uptake
Skeletal muscle demonstrates a remarkable plasticity, adapting to a variety of external stimuli (Booth and Thomason 1991 Chibalin et al. 2000 Hawley 2002 Flück and Hoppeler 2003), including habitual level of contractile activity (e.g., endurance exercise training), loading state (e.g., resistance exercise training), substrate availability (e.g., macronutrient supply), and the prevailing environmental conditions (e.g., thermal stress). This phenomenon of plasticity is common to all vertebrates (Schiaffino and Reggiani 1996). However, there exists a large variation in the magnitude of adaptability among species, and between individuals within a species. Such variability partly explains the marked differences in aspects of physical performance, such as endurance or strength, between individuals, as well as the relationship of skeletal muscle fiber type composition to certain chronic disease states, including obesity and insulin resistance.
In most mammals, skeletal muscle comprises about 55% of individual body mass and plays vital roles in locomotion, heat production during periods of cold stress, and overall metabolism (Figure 1). Thus, knowledge of the molecular and cellular events that regulate skeletal muscle plasticity can define the potential for adaptation in performance and metabolism, as well as lead to the discovery of novel genes and pathways in common clinical disease states.
Individual bundles of muscle fibers are called fascicles. The cell membrane surrounding the muscle cell is the sarcolemma, and beneath the sarcolemma lies the sarcoplasm, which contains the cellular proteins, organelles, and myofibrils. The myofibrils are composed of two major types of protein filaments: the thinner actin filament, and the thicker myosin filament. The arrangement of these two protein filaments gives skeletal muscle its striated appearance.
Skeletal Muscle Fiber Structure
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.
Cell membrane > Sarcolemma
Smooth endoplasmic reticulum > Sarcoplasmic reticulum
ATP and Muscle Contraction
For thin filaments to continue to slide past thick filaments during muscle contraction, myosin heads must pull the actin at the binding sites, detach, re-cock, attach to more binding sites, pull, detach, re-cock, etc. This repeated movement is known as the cross-bridge cycle. This motion of the myosin heads is similar to the oars when an individual rows a boat: The paddle of the oars (the myosin heads) pull, are lifted from the water (detach), repositioned (re-cocked) and then immersed again to pull (Figure 4). Each cycle requires energy, and the action of the myosin heads in the sarcomeres repetitively pulling on the thin filaments also requires energy, which is provided by ATP.
Figure 4. Skeletal Muscle Contraction. (a) The active site on actin is exposed as calcium binds to troponin. (b) The myosin head is attracted to actin, and myosin binds actin at its actin-binding site, forming the cross-bridge. (c) During the power stroke, the phosphate generated in the previous contraction cycle is released. This results in the myosin head pivoting toward the center of the sarcomere, after which the attached ADP and phosphate group are released. (d) A new molecule of ATP attaches to the myosin head, causing the cross-bridge to detach. (e) The myosin head hydrolyzes ATP to ADP and phosphate, which returns the myosin to the cocked position.
Cross-bridge formation occurs when the myosin head attaches to the actin while adenosine diphosphate (ADP) and inorganic phosphate (Pi) are still bound to myosin (Figure 4a,b). Pi is then released, causing myosin to form a stronger attachment to the actin, after which the myosin head moves toward the M-line, pulling the actin along with it. As actin is pulled, the filaments move approximately 10 nm toward the M-line. This movement is called the power stroke, as movement of the thin filament occurs at this step (Figure 4c). In the absence of ATP, the myosin head will not detach from actin.
One part of the myosin head attaches to the binding site on the actin, but the head has another binding site for ATP. ATP binding causes the myosin head to detach from the actin (Figure 4d). After this occurs, ATP is converted to ADP and Pi by the intrinsic ATPase activity of myosin. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position (Figure 4e). The myosin head is now in position for further movement.
When the myosin head is cocked, myosin is in a high-energy configuration. This energy is expended as the myosin head moves through the power stroke, and 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 formed cross-bridge is still in place, and actin and myosin are bound together. As long as ATP is available, it readily attaches to myosin, the cross-bridge cycle can recur, and muscle contraction can continue.
Note that each thick filament of roughly 300 myosin molecules has multiple myosin heads, and many cross-bridges form and break continuously during muscle contraction. Multiply this by all of the sarcomeres in one myofibril, all the myofibrils in one muscle fiber, and all of the muscle fibers in one skeletal muscle, and you can understand why so much energy (ATP) is needed to keep skeletal muscles working. In fact, it is the loss of ATP that results in the rigor mortis observed soon after someone dies. With no further ATP production possible, there is no ATP available for myosin heads to detach from the actin-binding sites, so the cross-bridges stay in place, causing the rigidity in the skeletal muscles.
BUOYANCY, LOCOMOTION, AND MOVEMENT IN FISHES | Undulatory Swimming
Regional Variation in Myotomal Muscle Properties
Myotomal muscle contractile properties vary between fiber types, with position on the body axis and among developmental stages. Force production and shortening by skeletal muscle are caused by myosin cross-bridge cycling. This requires adenosine triphosphate (ATP) energy and the binding and release of Ca 2+ from troponin C, a protein component of thick myosin filament. Muscle shortening velocities decrease in the order white > pink > red in concert with a decreasing myosin-ATPase content. Factors affecting the Ca 2+ occupancy of troponin-binding sites and the rates of cross-bridge attachment and detachment change the rates of muscle activation, shortening, and relaxation. Differences in the rates of activation, shortening, and relaxation can occur independently as they are influenced by separate muscle proteins.
Both red and white muscle fibers in a number of teleost species have slower rates of activation and/or relaxation moving along the body axis from anterior to posterior. These differences have been correlated with longitudinal changes in the expression of three muscle proteins: parvalbumin, troponin T, and the myosin light chain (MLC specifically MLC2 the regulatory light chain). Regional differences have been found in the relative amount of these proteins and/or in the relative proportions of alternate protein isoforms:
Binding of Ca 2+ to troponin C is an essential step intriggering cross-bridge cycling and muscle contraction. Parvalbumin binds free Ca 2+ in the myoplasm, competing with troponin C. Parvalbumin, therefore, influences muscle relaxation by reducing the concentration of free Ca 2+ in the myoplasm. High parvalbumin concentrations should be associated with rapid muscle relaxation. Parvalbumin content declines from anterior to posterior in trout (Oncorhynchus mykiss), sheepshead (Archosargus probatocephalus), and kingfish (Menticirrhus americanus) red band white muscle, and in cod (Gadus morhua) and largemouth bass (Micropterus salmoides) white muscle, likely being a factor in increased relaxation times moving from anterior to posterior in these species.
Troponin T is a component of muscle thin filaments (see also DESIGN AND PHYSIOLOGY OF THE HEART | Cardiac Excitation–Contraction Coupling: Calcium and the Contractile Element ). It is thought to affect the rate at which Ca 2+ dissociates from troponin C, and therefore the rate at which muscle relaxes. The relative proportions of two troponin T isoforms shift from anterior to posterior in cod (G. morhua) and largemouth bass (M. salmoides), with the dominant anterior form likely having faster kinetics than the alternate isoform.
MLC2 is a thick filament component that may modulate cross-bridge kinetics by controlling sensitivity to Ca 2+ . The amounts of the slow MLC2 isoform increase, moving from anterior to posterior in both the slow and fast muscle of rainbow trout (O. mykiss).
The list of regionally variable proteins that influence myotomal muscle contractile properties is likely to grow as more data become available. Fewer data are available for elasmobranchs, but similar regional differences in contractile properties have not yet been detected.
In addition to regional mechanical differences, there is also variation in the extent to which fish muscle fibers rely on either aerobic or anaerobic metabolic pathways to supply energy for contraction. The volume fraction of red fibers occupied by mitochondria is typically greater than 25% compared with less than 10% in white muscle. Accordingly, the activities of mitochondrial enzymes associated with aerobic metabolism, such as cytochrome oxidase and citrate synthase, are higher in red than in white muscle. Pink muscle shows an intermediate level of activity for these enzymes. White muscle relies primarily on anaerobic glycolysis for its energy supply, as indicated by higher levels of glycolytic enzymes, such as phosphofructokinase, than found in red or pink muscle.
Energy For Contraction
The energy for muscle contraction comes from ATP molecules in the muscle fiber. Recall that ATP is a product of cellular respiration. However, there is only a small amount of ATP in each muscle fiber. Once it is used up, more ATP must be formed in order for additional contractions to occur.
While a muscle fiber is relaxed it uses cellular respiration to release energy from nutrients and transfers that energy to the high-energy phosphate bonds of ATP. Once there are sufficient amounts of ATP available in the muscle fiber, the high-energy phosphate is transferred to creatine to form creatine phosphate (CP), which serves as a storage form of readily available energy. The resulting ADP is then reconverted to ATP using cellular respiration.
Muscle contraction quickly reduces ATP levels, resulting in the high-energy phosphate group being transferred back from the creatine phosphate to the ADP, forming ATP, which can then be used to power additional contractions.
There is four to six times more creatine phosphate than ATP in a muscle fiber so it is an important source for immediate ATP formation without waiting for the slower process of cellular respiration. However, it can also be depleted in under 10 seconds in a muscle that is contracting repeatedly.
Oxygen And Cellular Respiration
Cellular respiration is the process of breaking down glucose in two steps: (1) anaerobic respiration in the cytosol and (2) aerobic respiration in the mitochondria. Due to the need of a constant supply for glucose to generate ATP, muscle fibers store large amounts of glucose as muscle glycogen. Whether or not a muscle fiber uses just anaerobic respiration or also includes aerobic respiration depends on the availability of oxygen. During periods of strenuous exercise such as weight lifting, muscle fibers will employ mostly anaerobic respiration because the respiratory and cardiovascular systems cannot provide oxygen to muscle fibers quickly enough to maintain aerobic respiration. The muscle fibers will break down glycogen to glucose and glucose to pyruvic acid, in a process called glycolysis, forming only a small amount of ATP per molecule of glucose.
Since anaerobic respiration is not favorable in muscle fibers, muscle tissue is adapted to facilitate aerobic respiration. Muscle tissue possesses a large number of blood vessels and obtains large amounts of oxygen from the blood via hemoglobin, the red pigment in red blood cells. Muscle fibers also have a similar pigment, myoglobin, which stores oxygen within the sarcoplasm and helps transfer oxygen to the mitochondria. In the same manner that creatine phosphate stores extra energy in times of muscle inactivity, some of the oxygen carried to muscle fibers is transferred from hemoglobin to myoglobin and stored for later use during periods of muscle activity. This function of myoglobin reduces the muscle fiber’s dependence on oxygen carried to it by the blood at the onset of exercise. During inactivity or light to moderate physical activity (e.g. endurance training), muscle fibers receive sufficient oxygen to carry on the aerobic respiration. This process involves the breakdown of pyruvic acid produced in glycolysis, or other organic nutrients, into carbon dioxide and water. In contrast to anaerobic respiration, aerobic respiration provides a large amount of ATP per molecule of glucose.
Excess Post-Exercise Oxygen Consumption (Epoc)
When a muscle fiber utilizes anaerobic respiration, such as during strenuous exercise, it accumulates lactic acid and depletes its ATP, CP, and oxygen stores. To restore resting conditions within a muscle fiber after activity ceases, respiratory and heart rates remain elevated to support excess post-exercise oxygen consumption or EPOC (formerly oxygen debt). EPOC is the amount of oxygen required to replenish myoglobin and to produce the ATP needed for the metabolism of the lactic acid in the liver, heart, and skeletal muscles and the restoration of ATP and creatine phosphate in the muscle fibers.
If a muscle is stimulated to contract for a long period, its contractions will gradually decrease until it no longer responds to stimulation. This condition is called fatigue. Although the exact mechanism is not known, several factors seem to be responsible for muscle fatigue. The most likely cause of fatigue in long term muscle activity is a lack of available nutrients, such as muscle glycogen and fatty acids, to utilize for ATP production.
Effects of Exercise On Muscles
Exercise has a profound effect on skeletal muscles. Strength training, which involves resistance exercise such as weight lifting, causes a muscle fiber to be repetitively stimulated to maximum contraction. Over time, the repetitive stimulation produces hypertrophy-an increase in muscle fiber size and strength. The number of muscle fibers cannot be increased after birth. Instead, hypertrophy results from an increase in the number of myofibrils in muscle fibers, which increases the diameter and strength of the muscle fibers and of the whole muscle itself. In comparison, lack of repetitive stimulation to maximum force causes muscular atrophy, which is the reduction in muscle size and strength due to loss of myofibrils. Atrophy can be caused by damage to the nerve stimulating the muscle or lack of use, such as when a limb is in a cast. Aerobic exercise, or endurance training, does not produce hypertrophy. Instead it enhances the efficiency of aerobic respiration in muscle fibers by increasing (1) the number of mitochondria, (2) the efficiency of obtaining oxygen from the blood, and (3) the concentration of myoglobin.
Heat production by muscular activity is an important mechanism in maintaining a normal body temperature. Muscles are active organs that form a large proportion of the body weight. Heat produced by muscles results from cellular respiration and other chemical reactions within the muscle fibers. Recall that 60% of the energy released by cellular respiration is heat energy. Muscle generates so much heat that exercise leads to an increase in body temperature that requires sweating to help remove heat from the body. On the other hand, the major response to a decrease in body temperature is shivering, which is involuntary muscle contractions.
How does the propagated electrical impulse spreading along the TT system produce Ca 2+ release? The Schneider and Chandler hypothesis that the excitatory signal passes from the TT to the SR membrane by way of charges moving in the TT membrane connecting with the junctional feet of the SR Ca 2+ release channel initiated the path to answer this question. Yet, it is still unknown how this movement of charge, originating in Cav1.1, transfers a signal across to the RyR1 (feet) to trigger Ca 2+ release. This question is particularly fascinating because of the as yet unknown molecular structure-function relationship between these components in two different membrane systems, the TT (Cav1.1 voltage sensors) and the SR (RyR1 Ca 2+ release channels). Cryo-EM has revealed amazing details of the structure of the Cav1.1 (in a closed configuration) and of the RyR1 (in closed and ligand-induced open conformations). The next generation of high-resolution cryo-EM, together with electrophysiological assays using chimeric constructs or site-directed mutagenesis, may provide a more comprehensive molecular picture of the interaction between Cav1.1 and RyR1 in their respective membranes.
Myasthenia Gravis and Myasthenic Syndromes
Angela Vincent MA (MB, BS, MSc Lond.), FRCPath, FMedSci , in Neurobiology of Disease , 2007
Neuromyotonia (NMT), or Isaacs's syndrome, is a syndrome of spontaneous and continuous muscle fiber contraction . The clinical features include muscle stiffness, cramps, myokymia (visible undulation of the muscle), pseudomyotonia (slow relaxation after contraction) and weakness, most prominent in the limbs and trunk. Increased sweating is common. Myokymia characteristically continues during sleep and even during general anesthesia, indicating its peripheral origin. The diagnosis largely rests on a combination of clinical and electromyographic findings of spontaneous motor unit discharges that occur in distinctive doublets, triplets, or longer runs. These neuromyotonic discharges have a high intraburst frequency and usually occur at irregular intervals of 1 to 30 seconds. Some patients have sensory symptoms, and central nervous system symptoms such as insomnia, hallucinations, delusions, and personality change are not infrequent. Cramp fasciculation syndrome, formerly thought to be a different disorder, may represent part of the same spectrum [ 33, 34 ]. These conditions usually begin between 25 and 60 years.
Pediatric Neurology Part III
Norma Beatriz Romero , Nigel F. Clarke , in Handbook of Clinical Neurology , 2013
Myosin storage myopathy
This myopathy was first named hyaline body myopathy but myosin storage myopathy (MSM) is now the preferred name. It is defined by the presence of subsarcolemmal hyaline bodies in type 1 (slow twitch) muscle fibers which appear as regions that are pale on hematoxylin and eosin, pale green on Gomori trichrome stain, and which are devoid of oxidative enzyme activity. The inclusions are largely composed of amorphous myosin deposits that immunostain intensely with antibodies against slow myosin, but rarely other proteins ( Tajsharghi et al., 2003 Goebel and Laing, 2009 ). On EM, the hyaline deposits appear as amorphous granular material of uniform consistency that form lakes without surrounding membranes, which are sometimes traversed by sarcomeres. Most families have dominant (heterozygous) mutations in the MYH7 gene that encodes the main myosin isoform in both type 1 (slow twitch) muscle fibers and cardiac muscle and in some families, a single child is affected by a de novo dominant mutation ( Tajsharghi et al., 2003 Bohlega et al., 2004 ). One MYH7 mutation likely causes recessive MSM ( Tajsharghi et al., 2007b ). To date, all MYH7 MSM mutations alter single amino acids in the tail region of myosin. There is a wide clinical spectrum of severity that appears only partly explained by different mutations ( Goebel and Laing, 2009 ) since clinical symptoms and course may vary within a given family ( Bohlega et al., 2003 ). Most patients have slowly progressive generalized weakness that begins in childhood but they may not present until adulthood. The most severely affected patients have congenital or progressive childhood weakness, scoliosis, and contractures, and they can lose ambulation and need ventilatory support in early adulthood ( Bohlega et al., 2003 Dye et al., 2006 ). Common clinical features include scapuloperoneal or limb-girdle weakness, foot drop, calf hypertrophy, scoliosis, and respiratory failure. Cardiomyopathy and arrhythmias may be associated with some MYH7 MSM mutations ( Tajsharghi et al., 2007b, c ). CK levels are usually normal or mildly elevated. Mutations in the MHY7 gene also cause cardiomyopathy (hypertrophic and dilated) ( Oldfors, 2007 ) and Laing distal myopathy ( Meredith et al., 2004 ).
A tetanic contraction (also called tetanized state, tetanus, or physiologic tetanus, the latter to differentiate from the disease called tetanus) is a sustained muscle contraction  evoked when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate.   During this state, a motor unit has been maximally stimulated by its motor neuron and remains that way for some time. This occurs when a muscle's motor unit is stimulated by multiple impulses at a sufficiently high frequency. Each stimulus causes a twitch. If stimuli are delivered slowly enough, the tension in the muscle will relax between successive twitches. If stimuli are delivered at high frequency, the twitches will overlap, resulting in tetanic contraction. A tetanic contraction can be either unfused (incomplete) or fused (complete).  An unfused tetanus is when the muscle fibers do not completely relax before the next stimulus because they are being stimulated at a fast rate however there is a partial relaxation of the muscle fibers between the twitches.  Fused tetanus is when there is no relaxation of the muscle fibers between stimuli and it occurs during a high rate of stimulation.  A fused tetanic contraction is the strongest single-unit twitch in contraction.  When tetanized, the contracting tension in the muscle remains constant in a steady state. This is the maximal possible contraction.  During tetanic contractions, muscles can shorten, lengthen or remain constant length. 
Tetanic contraction is usually normal (such as when holding up a heavy box). Muscles often exhibit some level of tetanic activity, leading to muscle tone, in order to maintain posture  for example, in a crouching position, some muscles require sustained contraction to hold the position. Tetanic contraction can exist in a variety of states, including isotonic and isometric forms—for example, lifting a heavy box off the floor is isotonic, but holding it at the elevated position is isometric. Isotonic contractions place muscles in a constant tension but the muscle length changes, while isometric contractions hold a constant muscle length.
Voluntary sustained contraction is a normal (physiologic) process (as in the crouching or box-holding examples), but involuntary sustained contraction exists on a spectrum from physiologic to disordered (pathologic). Muscle tone is a healthy form of involuntary sustained partial contraction. In comparison with tetanic contraction in an isometric state (such as holding up a heavy box for several minutes), it differs only in the percentage of motor units participating at any moment and the frequency of neural signals but the low percentage and low frequency in healthy tone are the key factors defining it as healthy (and not tetanic). Involuntary sustained contraction of a hypertonic type, however, is a pathologic process. On the mild part of the spectrum, cramps, spasms, and even tetany are often temporary and nonsevere. On the moderate to severe parts of the spectrum are dystonia, trismus, pathologic tetanus, and other movement disorders featuring involuntary sustained strong contractions of skeletal muscle.
We thank Drs. M. Ikura, M.J. Lohse, RY. Tsien, and M. Brini for their generous gifts of plasmids and antibodies, and Drs. M. Mongillo, M. Zaccolo, D. Sandonà, E. Damiani, and A. Margreth for fruitful discussions.
R. Rudolf was supported by grants from the University of Padua and the Deutsche Forschungsgemeinschaft (RU 923/1-1). The following grants to T. Pozzan also partially supported this work: Italian Telethon (grant 1226), Italian Association for Cancer Research, Italian Ministry of Universities (Special Projects Prin RBNE01ERXR-001 and 20033054449-001, and FIRB 2002053318-005), Italian Ministry of Health, and the CARIPARO foundation.