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The Muscular System


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A&P Muscular System Notes!!

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The Muscular System

  1. 1. The Muscular System Chapters 9 and 10
  2. 2. Introduction <ul><li>The word muscles originated from the Latin word –mus , meaning “little mice.” </li></ul><ul><li>This was because the muscles resembled little mice scurrying under the skin when they contracted. </li></ul><ul><li>The muscles make up about 50% of the body mass. </li></ul><ul><li>Animals use muscles to convert the chemical energy of adenine triphospate (ATP) into mechanical work. </li></ul>
  3. 5. There are three types of muscle tissue: <ul><li>Cardiac (Heart) Muscle </li></ul><ul><li>Smooth Muscle </li></ul><ul><li>Skeletal Muscle </li></ul>
  4. 6. Cardiac (Heart) Muscle <ul><li>Make up the walls of the heart </li></ul><ul><li>Main function is to pump blood by beating at a steady pace </li></ul><ul><li>Is striated (striped) and is under involuntary control </li></ul>
  5. 7. Smooth Muscle <ul><li>Found in the walls of all the hollow organs of the body (excluding the heart) </li></ul><ul><li>Contractions force fluids throughout the body </li></ul><ul><li>Generally involuntary </li></ul>
  6. 8. Skeletal Muscle <ul><li>Muscle that is attached to the skeleton </li></ul><ul><li>Responsible for movement </li></ul><ul><li>Striated </li></ul><ul><li>Voluntary muscle </li></ul>
  7. 9. <ul><li>The main differences in muscle tissue are its cell structure, location, function, and control. </li></ul><ul><li>There are four muscle functions: </li></ul><ul><ul><li>Producing movement </li></ul></ul><ul><ul><li>Maintaining posture </li></ul></ul><ul><ul><li>Stabilizing joints </li></ul></ul><ul><ul><li>Generating heat </li></ul></ul>
  8. 10. Anatomy of Skeletal Muscle <ul><li>Skeletal muscle contains blood vessels, nerves, and connective tissue. </li></ul><ul><li>The whole skeletal muscle is covered in an “overcoat” called the epimysium, which is a dense connective tissue that surrounds the whole muscle. </li></ul><ul><li>Individual skeletal muscle fibers (myofibrils) are covered in endomysium – a thin sheath of connective tissue. </li></ul>
  9. 12. Anatomy (con’t) <ul><li>Each myofibril contains mitochondria, endoplasmic reticulum, and many nuclei. Because a myofibril is not a single cell, its parts are often given special names such as: </li></ul><ul><ul><li>Sarcolemma = plasma membrane </li></ul></ul><ul><ul><li>Sarcoplasmic reticulum = endoplasmic reticulum </li></ul></ul><ul><ul><li>Sarcosome = mitichondria </li></ul></ul><ul><ul><li>Sarcoplasm = cytoplasm </li></ul></ul>
  10. 13. Anatomy (con’t) <ul><li>Groups of myfibrils are arranged in “bundles” called fascicles. </li></ul><ul><li>Each fasicle is surrounded by a fibrous connective tissue called the perimysium. </li></ul><ul><li>All connective tissue coverings support cells, reinforce muscle, contribute to the elasticity, and provide entry and exit for blood vessels and nerves. </li></ul>
  11. 15. Anatomy (con’t) <ul><li>The activity of muscles depends on the nerve and blood supply. </li></ul><ul><li>Each muscle is supplied with 1 artery and 1 vein. </li></ul><ul><li>Because contraction use lots of energy, oxygen and nutrients are continuously delivered to the muscle. </li></ul>
  12. 16. Anatomy (con’t) <ul><li>Muscle fibers are made of thick and thin filaments </li></ul><ul><li>The are made chiefly of the proteins myosin, actin, troponin, and tropomyosin. </li></ul>
  13. 17. Skeletal Muscle Attachment <ul><li>Most muscles join joints and are attached to bones in two places. </li></ul><ul><li>A single skeletal muscle is attached at its: </li></ul><ul><ul><li>Origin – immovable bone </li></ul></ul><ul><ul><li>Insertion – movable bone </li></ul></ul>
  14. 18. Attachment (con’t) <ul><li>Attachments may be direct  where the epimysium is fused to the periosteum of a bone (or cartilage) </li></ul><ul><li>Attachments may be indirect  where muscle’s wrappings extend beyond a muscle as a tendon, anchoring muscle to bone. </li></ul><ul><li>Indirect attachments are more common and durable. </li></ul>
  15. 19. Appearance of Skeletal Muscle: <ul><li>Striated appearance is created by a pattern of alternating dark A bands and light I bands. </li></ul><ul><li>Each band is bisected – the A band is bisected by the H zone and the I band is bisected by the Z line. </li></ul><ul><li>The H zone is further bisected by the M line </li></ul>
  16. 21. Appearance (con’t) <ul><li>All of the filaments between the Z lines are called the sarcomere. </li></ul><ul><li>Shortening of the sarcomere in a myofibril shortens the myofibril (muscle contraction). </li></ul><ul><li>Made of thick and thin filaments </li></ul>
  17. 24. Neuromuscular Junction <ul><li>Nerve impulses (action potentials) travel down the motor neurons of the sensory-somatic branch of the nervous system, causing the muscle fibers at the end to contract. </li></ul><ul><li>The junction of the end of the motor neuron and a muscle fiber is called a neuromuscular junction. </li></ul>
  18. 26. Neuromuscular Junction (con’t) <ul><li>The ends of the motor axons contain 1000’s of tiny sacs (vesicles) that are filled with acetylcholine (ACh). </li></ul><ul><li>When a nerve impulse reaches the end of the axon, it signals the release of ACh, which allows sodium ions to diffuse in. </li></ul><ul><li>The influx of sodium ions causes contractions. </li></ul>
  19. 28. Neuromuscular Junction (con’t) <ul><li>Acetycholinesterase, an enzyme, breaks down ACh, easing the contraction. </li></ul><ul><li>The resting potential of the muscle fiber is restored by an outflow of potassium ions. </li></ul>
  20. 30. Sliding-Filament Model <ul><li>Thick filaments contain globular subunits, called myosin heads. </li></ul><ul><li>The myosin heads have binding sites for actin molecules found on the thin filaments and for ATP. </li></ul><ul><li>Activation of a muscle fiber allows the myosin head to bind to actin at an active site. This is called “building a cross bridge.” </li></ul>
  21. 32. Sliding-Filament (con’t) <ul><li>Active site exposure depends on the calcium concentration in the cell. </li></ul><ul><ul><li>When Ca level is low  muscle is relaxed and active sites are blocked </li></ul></ul><ul><ul><li>When Ca level is high  muscle changes shaped and cross bridge is built (myosin head binds to actin) </li></ul></ul>
  22. 33. Sliding-Filament (con’t) <ul><li>The connection is made. </li></ul><ul><li>The thin filament pulls a short distance past the thick filament. </li></ul><ul><li>The connection is broken and the myosin head attaches further down, repeating the process. </li></ul><ul><li>As a result, the filaments are pulled past each other in a ratchetlike action, causing the muscle to shorten. </li></ul>
  23. 35. Sliding-Filament (con’t) <ul><li>During this contraction, the Z lines come closer together, the width of the I bands decreases, the width of the H zones decreases, but there is no change in the width of the A band. </li></ul><ul><li>This theory was studied and presented by Hugh Huxley in 1954. </li></ul>
  24. 36. Rigor Mortis <ul><li>Muscle stiffening after death is caused by the dying cells failing to rid themselves of Ca. </li></ul><ul><li>The excess Ca comes into muscle cells and promotes the binding of the myosin cross bridges. </li></ul><ul><li>This results in muscle tension. </li></ul><ul><li>Condition only lasts until the muscle proteins begin to break down. </li></ul>
  25. 37. Coupling Excitation to Contraction <ul><li>Calcium ions link action potentials in a muscle fiber to cause contractions. </li></ul><ul><li>In resting muscle fibers, Ca ions are stored in the sarcoplasmic reticulum. </li></ul><ul><li>Spaced along the sarcolemma of the muscle fiber are pockets of membrane that form tubules of the “T system.” </li></ul><ul><li>The tubules of the T system terminate near the Ca-filled sacs of sarcoplasmic reticulum. </li></ul>
  26. 39. Coupling (con’t) <ul><li>Each action potential created at the neuromuscular junction sweeps quickly along the sarcolemma and is carried into the T system. </li></ul><ul><li>The arrival of the action potential at the ends of the T system triggers the release of Ca ions. </li></ul><ul><li>The Ca ions diffuse among the thick and thin filaments where it binds to troponin on the thin filaments. </li></ul>
  27. 41. Coupling (con’t) <ul><li>This turns on the interaction between actin and myosin and the sarcomere contracts. </li></ul><ul><li>Because of the speed of the action potential, the action potential arrives virtually simultaneously a the ends of all the tubules of the T system, ensuring that all sarcomeres contract in unison. </li></ul><ul><li>When the process is over, the Ca is pumped back into the sarcoplasmic reticulum using a Ca ATPase. </li></ul>
  28. 43. The Motor Unit <ul><li>Each muscle is served by a least one motor nerve, which contains hundreds of motor neuron axons. </li></ul><ul><li>Each neuron axon terminates in a neuromuscular junction with a single muscle fiber. </li></ul><ul><li>Nerve impulses passing down a single motor neuron will thus trigger contraction in all the muscle fibers at which the branches of that neuron terminate. </li></ul>
  29. 44. Motor Unit (con’t) <ul><li>The minimum unit of contraction is called the motor unit. (The motor neuron and muscle fibers supplied) </li></ul><ul><li>Most skeletal muscles are in a state of partial contraction called tonus. Tonus is maintained by the activation of a few motor units at all times even in resting muscle. As one set relaxes, another set takes over. </li></ul>
  30. 45. Muscle Tone <ul><li>Skeletal muscle are voluntary, but even relaxed, they are always in a slightly contracted state. </li></ul><ul><li>This is muscle tone. </li></ul><ul><li>Muscle tone stabilizes joints and maintains posture. </li></ul>
  31. 46. Isotonic & Isometric Contractions <ul><li>Two main categories of contractions: </li></ul><ul><li>1. Isotonic contraction </li></ul><ul><li>- Muscle changes in length </li></ul><ul><li>- Moves a load </li></ul><ul><li>2. Isometric contraction </li></ul><ul><li>- Tension continues to increase </li></ul><ul><li>- Muscle doesn’t shorten or lengthen </li></ul>
  32. 47. Fuel of Muscle Contractions
  33. 48. Fueling the Contraction <ul><li>ATP is the source of energy </li></ul><ul><li>It provides the energy for cross bridge movement and detachment and for the operation of the Ca pump. </li></ul><ul><li>Our ATP “pool” is only large enough to allow 4-6 seconds of contraction </li></ul><ul><li>Sources of energy comes from: creatine phosphate, glycogen, and cellular respiration. </li></ul>
  34. 49. Creatine Phosphate <ul><li>Phosphate group in creating phosphate is attached by a “high energy” bond link in ATP. </li></ul><ul><li>Creatine phosphate derives its high-energy phosphate from ATP and can donate it back to ADP to form ATP. </li></ul>
  35. 50. Glycogen (Anaerobic Respiration) <ul><li>Skeletal muscle fibers contain about 1% glycogen. </li></ul><ul><li>A breakdown of glycogen goes to the glycolytic pathway, forming lactic acid molecules. </li></ul><ul><li>Not much lactic acid is formed, but it’s enough to keep the muscles functioning if it fails to get proper oxygen. </li></ul>
  36. 51. Cellular Respiration (Aerobic Respiration) <ul><li>Cellular respiration not only is required to meet the ATP needs of a muscle engaged in prolonged activity, but is also required afterwards to enable the body to resynthesize glycogen from the lactic acid produced earlier. </li></ul><ul><li>The body must repay its oxygen debt. </li></ul><ul><li>95% of the ATP used for muscle activity comes from aerobic respiration. </li></ul>
  37. 53. Muscle Fatigue <ul><li>When ATP production fails to keep pace with ATP use, muscles contract less and less effectively. </li></ul><ul><li>Ultimately muscle fatigue set is and the muscle stops contracting, even though it may still be receiving stimuli. </li></ul><ul><li>Muscle fatigue is the state of physiological inability to contract. </li></ul><ul><li>Muscle fatigue is caused by an ATP deficit, not total absence. </li></ul>
  38. 54. Fatigue (con’t) <ul><li>When no ATP is present, contractures set in. Contractures are the states of continuous contraction, resulting from the cross bridges inability to detach. </li></ul><ul><li>Example – writer’s cramp </li></ul>
  39. 55. Fatigue (con’t) <ul><li>Excessive accumulation of lactic acid can also cause muscle fatigue by causing the muscle pH to drop. </li></ul><ul><li>As action potentials are transmitted, K is lost from the muscle cells, and lots of Na enters. </li></ul><ul><li>If ATP is available, then, this shuts down the K ions, balancing the Na and K ions. </li></ul><ul><li>Without ATP, though, the muscle cells become unresponsive and muscles ache. </li></ul>
  40. 56. Cardiac Muscle <ul><li>Cardiac, or heart muscle, resembles skeletal muscle in some ways. It is striated and has sliding filaments of actin and myosin. </li></ul><ul><li>The myofibrils of each cell are branched. </li></ul><ul><li>Branches interlock to keep the fibers from ripping when the heart pumps. </li></ul><ul><li>The action potential comes from the heart itself. </li></ul>
  41. 57. Cardiac (con’t) <ul><li>Fibers contract in a synchronous wave, sweeping blood from the heart. Any interference caused fibrillation. </li></ul><ul><li>When the oxygen supply is limited, damage is caused to that area. (Heart attack) </li></ul>
  42. 58. Smooth Muscle <ul><li>Has no striations </li></ul><ul><li>Doesn’t depend on motor neurons to be stimulated </li></ul><ul><li>Contractions are slower and are sustained for much longer periods. </li></ul>
  43. 59. Muscle Diseases
  44. 60. Muscular Dystrophies (MD) <ul><li>Myosin, actin, tropomyosin, and troponin make up over ¾ of the protein in muscle. </li></ul><ul><li>Mutations in the genes coding for these proteins and the ~2 dozen others cause defective proteins to be produced. </li></ul><ul><li>Among the most common muscular dystrophies are those caused by the gene for dystrophin. </li></ul>
  45. 61. Duchenne Muscular Dystrophy (DMD) <ul><li>No dystrophin is synthesized, resulting in a severe form of the disease. </li></ul><ul><li>Sex-linked recessive trait </li></ul><ul><li>Passed from mother </li></ul><ul><li>Exclusively males </li></ul><ul><li>1/3,500 births </li></ul><ul><li>Lose all muscle control </li></ul>
  46. 62. Becker Muscular Dystrophy (BMD) <ul><li>A shortened dystrophin is synthesized, </li></ul><ul><li>resulting in a milder form of the disease. </li></ul>
  47. 63. Myasthenia Gravis <ul><li>Autoimmune disorder affecting neuro-muscular junctions, causing action potentials and contractions to cease. </li></ul><ul><li>Drugs used to compensate for loss of muscle control. </li></ul>
  48. 64. Cardiac Myopathies <ul><li>Mutations in the genes coding for proteins within the cardiac muscle are altered. </li></ul><ul><li>This can cause the wall of the heart to weaken and become enlarged. </li></ul><ul><li>Severity depends on mutation. </li></ul><ul><li>Mutations can be very serious resulting in heart failure in seemingly healthy and active young adults. </li></ul>