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NMP-9.pptx

Sai Sailesh Kumar Goothy
Sai Sailesh Kumar Goothy
Sai Sailesh Kumar GoothyAssociate Professor, Department of Physiology, R D Gardi Medical College

Length-tension relationship Muscle metabolism Fatigue

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Length-Tension
Relationship, Muscle
Metabolism
Dr. Sai Sailesh Kumar G
Professor
Department of Physiology
NRIIMS
Email: dr.goothy@gmail.com
Explain
Isotonic contraction
Isometric contraction
Isokinetic contraction
Motor unit
Macro motor unit
Length-tension Relationship
Every muscle has an optimal length (lo) at which maximal force can be
achieved during a tetanic contraction beginning at that length.
Contractile Activity at an optimal length
At lo, when maximum tension can be developed,
the thin filaments optimally overlap the regions of the thick filaments
where the cross bridges are located.
At this length, a maximal number of cross bridges and actin molecules are
accessible to each other for cycles of binding and bending.
The central region of thick filaments, where the thin filaments do not overlap
at lo, lacks cross bridges; only myosin tails are found here.
NMP-9.pptx
Contractile Activity at Lengths Greater Than lo
At greater lengths, as when a muscle is passively stretched (point B), the thin filaments are pulled
out from between the thick filaments,
decreasing the number of actin sites available for cross-bridge binding—
that is, some of the actin sites and cross bridges no longer “match up,” so they “go unused.”
When less cross-bridge activity can occur, less tension can develop.
In fact, when the muscle is stretched to about 70% longer than its lo(point C) the thin filaments are
completely pulled out from between the thick filaments, preventing cross-bridge activity;
consequently,
 no contraction can occur
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NMP-9.pptx

  • 1. Length-Tension Relationship, Muscle Metabolism Dr. Sai Sailesh Kumar G Professor Department of Physiology NRIIMS Email: dr.goothy@gmail.com
  • 2. Explain Isotonic contraction Isometric contraction Isokinetic contraction Motor unit Macro motor unit
  • 3. Length-tension Relationship Every muscle has an optimal length (lo) at which maximal force can be achieved during a tetanic contraction beginning at that length.
  • 4. Contractile Activity at an optimal length At lo, when maximum tension can be developed, the thin filaments optimally overlap the regions of the thick filaments where the cross bridges are located. At this length, a maximal number of cross bridges and actin molecules are accessible to each other for cycles of binding and bending. The central region of thick filaments, where the thin filaments do not overlap at lo, lacks cross bridges; only myosin tails are found here.
  • 6. Contractile Activity at Lengths Greater Than lo At greater lengths, as when a muscle is passively stretched (point B), the thin filaments are pulled out from between the thick filaments, decreasing the number of actin sites available for cross-bridge binding— that is, some of the actin sites and cross bridges no longer “match up,” so they “go unused.” When less cross-bridge activity can occur, less tension can develop. In fact, when the muscle is stretched to about 70% longer than its lo(point C) the thin filaments are completely pulled out from between the thick filaments, preventing cross-bridge activity; consequently,  no contraction can occur
  • 7. Contractile Activity at Lengths Less Than lo If a muscle is shorter than lo before contraction (point D), less tension can be developed for three reasons: 1. The thin filaments from the opposite sides of the sarcomere overlap, which limits the opportunity for the cross bridges to interact with actin. 2. The ends of the thick filaments become forced against the Z lines, so further shortening is impeded
  • 8. Contractile Activity at Lengths Less Than lo 3. Besides these two mechanical factors, at muscle lengths less than 80% of lo, not as much Ca+2 is released during excitation-contraction coupling for reasons unknown Furthermore, by an unknown mechanism, the ability of Ca+2 to bind to troponin and pull the troponin–tropomyosin complex aside is reduced at shorter muscle lengths. Consequently, fewer actin sites are uncovered for participation in cross- bridge activity
  • 9. Skeletal muscle metabolism Four steps in the excitation, contraction, and relaxation processes require ATP:
  • 10. Skeletal muscle metabolism Splitting of ATP by myosin ATPase provides the energy for the power stroke of the cross-bridge Binding (but not splitting) of a fresh molecule of ATP to myosin lets the cross-bridge detach from the actin filament at the end of a power stroke Active transport of Ca+2 back into the lateral sacs of the SR during relaxation depends on energy derived from the breakdown of ATP
  • 11. Skeletal muscle metabolism The ATP-dependent Na+–K+ pump actively returns the ions (Na+ back out of the cell and K+ back into the cell) that moved during the generation of a contraction inducing action potential in the muscle cell
  • 12. ATP Because ATP is the only energy source that can be directly used for these activities, for contractile activity to continue, ATP must constantly be supplied.  Only limited stores of ATP are immediately available in muscle tissue, enough to power the first few seconds of exercise. However, three pathways supply additional ATP as needed during muscle contraction: (1) transfer of high-energy phosphate from creatine phosphate to ADP, (2) oxidative phosphorylation (the electron transport system and chemiosmosis), and  (3) glycolysis.
  • 13. Creatine Phosphate Like ATP, creatine phosphate contains a high-energy phosphate group, which can be donated directly to ADP to form ATP This reaction, which is catalyzed by the muscle cell enzyme creatine kinase, is reversible; energy and phosphate from ATP can be transferred to creatine to form creatine phosphate
  • 15. Creatine Phosphate As energy reserves are built up in a resting muscle, the increased concentration of ATP favors the transfer of the high-energy phosphate group from ATP to form creatine phosphate By contrast, at the onset of contraction when myosin ATPase splits the meager reserves of ATP, the resultant fall in ATP favors the transfer of the high-energy phosphate group from stored creatine phosphate to form more ATP.
  • 16. Creatine Phosphate Thus, creatine phosphate is the first source for supplying additional ATP when exercise begins. Muscle ATP levels actually remain fairly constant early in contraction, but creatine phosphate stores become depleted.  In fact, short bursts of high-intensity contractile effort, such as high jumps, sprints, or weight lifting, are supported primarily by ATP derived at the expense of creatine phosphate. Creatine phosphate stores typically power 5 to 10 seconds of exercise before the stores run out
  • 17. Oxidative phosphorylation The multistep oxidative phosphorylation pathway produces ATP at a relatively slow rate when compared to the transfer of a high-energy phosphate from creatine phosphate to ADP or the process of glycolysis Oxidative phosphorylation takes place within the muscle mitochondria if sufficient O2 is present Although it provides a rich yield of 32 ATP molecules for each glucose molecule processed, oxidative phosphorylation is relatively slow because of the number of enzymatic steps involved.
  • 18. Oxidative phosphorylation During light exercise (such as walking) to moderate exercise (such as jogging or swimming), muscle cells can form enough ATP through oxidative phosphorylation to keep pace with the modest energy demands of the contractile machinery for prolonged periods.  To sustain ongoing oxidative phosphorylation, exercising muscles depends on the delivery of adequate O2 and nutrients to maintain their activity. Activity supported in this way is aerobic (“with O2”) or endurance-type exercise.
  • 19. Glycolysis During glycolysis, a glucose molecule is broken down into two pyruvate molecules, yielding two ATP molecules in the process. Pyruvate can be further degraded by oxidative phosphorylation to extract more energy.  However, glycolysis alone has two advantages over the oxidative phosphorylation pathway: (1) glycolysis can form ATP in the absence of O2 (operating anaerobically)
  • 20. Glycolysis However, glycolysis alone has two advantages over the oxidative phosphorylation pathway: (1) glycolysis can form ATP in the absence of O2 (operating anaerobically) it can proceed more rapidly than oxidative phosphorylation Activity that can be supported in this way is anaerobic or high-intensity exercise
  • 22. Fatigue Contractile activity in a particular skeletal muscle cannot be maintained at a high level indefinitely. Eventually, the tension in the muscle declines as fatigue sets in. There are two types of fatigue: muscle fatigue and central fatigue.
  • 23. Muscle Fatigue Muscle fatigue occurs when an exercising muscle can no longer respond to stimulation with the same degree of contractile activity.  Muscle fatigue is a defense mechanism that protects a muscle from reaching a point at which it can no longer produce ATP. An inability to produce ATP would result in rigor mortis
  • 24. Muscle Fatigue The local increase in inorganic phosphate from ATP breakdown is considered the primary cause of muscle fatigue. Increased levels of Pi reduce the strength of contraction by interfering with the power stroke of the myosin heads.  In addition, increased Pi appears to decrease the sensitivity of the regulatory proteins to Ca+2 and to decrease the amount of Ca+2 released from the lateral sacs
  • 25. Muscle Fatigue Inappropriate leakage of Ca+2 through the SR’s Ca+2 release channels are the latest factor implicated in muscle fatigue after long and intense exercise. Some of the leaked Ca+2 exits the cell and cannot be returned to the SR by the SERCA pump. This Ca+2 loss from the cell depletes the SR Ca+2 supply needed to sustain contractile activity, leading to weaker contractions
  • 26. Muscle Fatigue Depletion of glycogen energy reserves may also lead to muscle fatigue in exhausting exercise
  • 27. Central Fatigue Central fatigue occurs when the central nervous system (CNS) no longer adequately activates the motor neurons supplying the working muscles. The person slows down or stops exercising even though the muscles are still able to perform. The mechanisms involved in central fatigue is poorly understood
  • 29. Types of skeletal muscle fibers Classified by their biochemical capacities, there are three major types of muscle fibers 1. Slow-oxidative (type I) fibers 2. Fast-oxidative (type IIa) fibers 3. Fast-glycolytic (type IIx) fibers
  • 30. Types of skeletal muscle fibers As their names imply, the two main differences among these fiber types are their speed of contraction (slow or fast) and the type of enzymatic machinery they primarily use for ATP formation (oxidative or glycolytic).
  • 31. Fast Versus Slow Fibers Fast fibers have higher myosin ATPase (ATP-splitting) activity than slow fibers do. The higher the ATPase activity, the more rapidly ATP is split and the faster the rate at which energy is made available for cross-bridge cycling. The result is a fast twitch
  • 32. Fast Versus Slow Fibers The time to peak twitch tension for fast fibers is 15 to 40 msec compared to 50 to 100 msec for slow fibers
  • 34. Oxidative Versus Glycolytic Fibers Those with a greater capacity to form ATP are more resistant to fatigue. Because oxidative phosphorylation yields considerably more ATP from each nutrient molecule processed,  it does not readily deplete energy stores. Furthermore, it does not result in lactate accumulation. Oxidative types of muscle fibers are therefore more resistant to fatigue
  • 35. Oxidative Versus Glycolytic Fibers  The oxidative fibers, both slow and fast, contain an abundance of mitochondria, the organelles that house the enzymes involved in oxidative phosphorylation. Because adequate oxygenation is essential to support this pathway, these fibers are richly supplied with capillaries. Oxidative fibers also have high myoglobin content. Myoglobin not only helps support oxidative fibers’ O2dependency, but also gives them a red color. Accordingly, these muscle fibers are called red fibers
  • 36. Oxidative Versus Glycolytic Fibers  The fast fibers specialized for glycolysis contain few mitochondria but have a high content of glycolytic enzymes instead. Also, to supply the large amounts of glucose needed for glycolysis, they contain a lot of stored glycogen. Because the glycolytic fibers need relatively less O2 to function, they have only a meager capillary supply
  • 37. Oxidative Versus Glycolytic Fibers  The glycolytic fibers contain little myoglobin and therefore are pale in color, so they are sometimes called white fibers
  • 38. Example  The most readily observable comparison between red and white fibers is the dark and white meat in poultry;  muscles of the legs consist primarily of red fibers and the breast muscles consist primarily of white fibers
  • 41. Muscle hypertrophy  The actual size of the muscles can be increased by regular bouts of anaerobic, short- duration, high intensity resistance training, such as weight lifting. The resulting muscle enlargement comes primarily from an increase in diameter (hypertrophy) of the fast-glycolytic fibers called into play during such powerful contractions. Most fiber thickening results from increased synthesis of myosin and actin filaments, which permits a greater opportunity for cross-bridge interaction and consequently increases the muscle’s contractile strength
  • 42. Muscle Atrophy  if a muscle is not used, its actin and myosin content decreases, its fibers become smaller, and the muscle accordingly atrophies(decreases in mass) and becomes weaker.
  • 43. Disuse atrophy  occurs when a muscle is not used for a long period even though the nerve supply is intact, prolonged bed confinement
  • 44. Denervation atrophy  occurs after the nerve supply to a muscle is lost.  If the muscle is stimulated electrically until innervation can be reestablished, such as during the regeneration of a severed peripheral nerve, atrophy can be diminished but not entirely prevented.
  • 45. Age-related atrophy, or sarcopenia  occurs naturally with aging. Beginning at approximately 40 years of age, people progressively lose motor neurons,  particularly those that innervate the fast-glycolytic fiber types. Although age-related muscle atrophy is inevitable, resistance training exercise and proper diet can slow the rate of development of sarcopenia