Lect. 8 muscular tissues

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  • A motor unit is all the muscle cells controlled by one nerve cell. This diagram represents two motor units. Motor unit one illustrates two muscle cells controlled by one nerve cell. When the nerve sends a message it will cause both muscle cells to contract. Motor unit two has three muscle cells innervated by one nerve cell.
  • A Acetylcholine binds with receptors in the muscle membrane to allow sodium ions to enter the muscle.
  • The influx of sodium will create an action potential in the sarcolemma. Note: This is the same mechanism for generating action potentials for the nerve impulse. The action potential travels down a T tubule. As the action potential passes through the sarcoplamic reticulum it stimulates the release of calcium ions. Calcium binds with troponin to move tropomyosin and expose the binding sites. Myosin heads attach to the binding sites of the actin filament and create a power stroke. ATP detaches the myosin heads and energizes them for another contraction. The process will continue until the action potentials cease. Without action potentials the calcium ions will return to the sarcoplasmic reticulum.  
  • The actin filaments are moved by the heads of the myosin filaments. In step one the myosin head attaches to an actin filament to create a cross bridge. Step two shows that the attached myosin head bends to move the actin filament. The myosin head has expended energy to create this movement. This is a power stroke or working stroke. Step three shows that energy in the form of ATP will unhook the myosin head. In step 4 the myosin head is cocked and ready to attach to an actin filament to start another power stroke.
  • We do not have to see our muscles to be sure that they are performing their intended actions. Muscle sense also contributes to our ability to distinguish the shape of objects.
  • Lect. 8 muscular tissues

    1. 1. Ms. O.HARA M.S. Muscular TissuesMuscular Tissues
    2. 2. Contractile cells generate motile forces through contraction. Contractile proteins are MYOSIN and ACTIN 1.Myoepithelial cells* - in secretory glands function to expel secretions from glandular acini. 1.Pericytes* - surround blood vessels. 2.Myofibroblasts* - have a contractile function as well as a role in secretion of extracellular matrix.  inconspicuous in normal tissues becomes dominant cell during scar formation in tissue repair. Specialized Contractile cells - single cell contractile units
    3. 3. Sweat gland Sweat gland Blood vessel Myofibroblast Specialized Contractile cells
    4. 4. Muscle cells  myofibers / myocytes contractile cells that function by forming multicellular contractile units. 3 types of muscles cells – based on structure, contractile properties, and control mechanisms 1.Skeletal muscle – for movement of skeleton, eye globe, and tongue 2.Smooth muscle - visceral muscle (blood vessels, GIT, uterus and urinary bladder) 3.Cardiac muscle - rhythmic contractions of heart.
    5. 5. Skeletal Muscle Referred as voluntary muscle. The primary function - to move the skeleton. The muscle contractions required for movement also produce heat, which contributes to the maintenance of a constant body temperature. Special terminology: Plasma membrane – sarcolemma Cytoplasm – sarcoplasm Endoplasmic reticulum – sarcoplasmic reticulum
    6. 6. Epimysium Perimysium Endomysium • A muscle is composed of FASCICLES of fibers, wrapped by CT- EPIMYSIUM. • Each fascicle is composed of MYOFIBERS, the cells of skeletal muscle. • Each fascicle is wrapped by CT – PERIMYSIUM • Each myofiber is wrapped by CT – ENDOMYSIUM SKELETAL MUSCLES are subdivided by CT
    7. 7. Connective Tissue wrappings Epimysium - dense, irreg. CT; collagen; binds the fascicles into a single muscle. Perimysium – less dense, irreg. CT; collagen; surrounding individual fasciculi through which larger vessels and nerves run. Endomysium - barely visible; delicate support; mainly of reticulin fibres and a small amount of collagen surrounding each muscle fiber. conveys numerous small blood vessels, lymphatics and nerves throughout the muscle.
    8. 8. Skeletal muscle fibers (muscle cells/myofiber) Long Cylindrical multinucleated cells with peripheral nuclei Multiple nuclei – due to the fusion of muscle cell precursor myoblasts during the embryonic development.
    9. 9. Each myofiber contains MYOFIBRILS that extend the length of the fiber • Orderly arrays of filamentous subunits
    10. 10. Myofibrils are made of MYOFILAMENTS formed by the contractile proteins • Actin (thin) and Myosin (thick)
    11. 11. IMPORTANT role of CT in muscle - to mechanically transmit the forces generated by contracting muscle cells, because in most instances, individual muscle cells do not extend from one end of a muscle to the other. Two orientations when cut: - transverse and longitudinal muscle fibers
    12. 12. I A Z N Mf S
    13. 13. Skeletal muscles are “striated” muscles due to distinct cross-striation patterns. In LM, light I bands and dark A bands are in each muscle fiber. Light bands are I bands ; formed by thin actin filaments Dark bands are A bands ; located in the middle of sarcomere. A bands are formed by overlapping actin and myosin filaments.
    14. 14. I Band is bisected by Z-line that divide each myofibril into numerous contractile units called SARCOMERE (2 Z-lines). Each A band is bisected by the lighter H band and the darker M line Actin THIN Myosin THICK
    15. 15. • As the muscle contracts  the sarcomere shortens  the Z lines are drawn closer together -- the thick and thin filaments slide past each other. • I and H bands shorten, while A bands stay the same. • SLIDING FILAMENT THEORY
    16. 16. Muscles are arranged around the skeleton so as to bring about a variety of movements. The two general types of arrangements -opposing antagonists • muscles that have opposing or opposite functions. - cooperative synergists • muscles with the same function or those that work together to perform a particular function. Antagonists
    17. 17. The role of the brain The contraction of skeletal muscles depends on the brain. (voluntary or conscious control) The nerve impulses for movement come from the frontal lobes of the cerebrum. Motor areas of the frontal lobes generate electrochemical impulses that travel along motor nerves to muscle fibers, causing the muscle fibers to contract. Frontal lobe
    18. 18. Motor Unit - Muscles are innervated by large motor nerves; supplying electrical impulse into the muscle and groups of muscle fibers (neuromuscular junctions). Motor unit - muscle cells controlled by one nerve cell.
    19. 19. Each muscle fiber has its own motor nerve ending; the neuromuscular junction is where the motor neuron terminates on the muscle fiber
    20. 20. Neuromuscular Junction
    21. 21. Surrounding each sarcomere in a repeating fashion are the tubules of sacroplasmic reticulum and mitochondria T tubule is surrounded on each side by the expanded terminal cisternae of the SR and form triads Calcium ions are stored in the sarcoplasmic reticulum (SR).
    22. 22. Myofilament components 1. ACTIN – Thin filament - composed of two chains of G-actin (globular) that forms F-actin (filamentous) arranged in helix.
    23. 23. • Tropomyosin - stabilizes the troponin and attaches to actin filament. • Troponin complex (TnT, TnC, TnI) – initially covers the F-actin during muscle rest • TnT – attaches to tropomyosin • TnC – (+) receptor for calcium ions • TnI – inhibits contraction
    24. 24. a. The heavy chains of myosin molecules form the core of a thick filament. b. Structure of a myosin molecule. 2. MYOSIN – Thick filament
    25. 25. H Band
    26. 26. Skeletal Muscle Contraction Before the arrival of the nerve impulse to the muscle, the muscle is relaxed and the calcium ions are stored in the cisternae of the sarcoplasmic reticulum. In summary, a nerve impulse causes depolarization of a muscle fiber, and this electrical change enables the myosin filaments to pull the actin filaments toward the center of the sarcomere, making the sarcomere shorter.
    27. 27. Nerve impulse reaches neuromuscular junction.
    28. 28. Acetylcholine / Ach (neurotransmitter) is released from motor neuron. Ach binds with receptors in the muscle membrane opens Na+ Channel to allow sodium to enter.
    29. 29. Sodium influx causes the sarcolemma to depolarize, becoming negative outside and positive inside. This will generate an action potential in the sarcolemma.
    30. 30. Action potential is propagated along the entire length of the sarcolemma and transmitted deep to every myofiber by the network of the T tubules. The T tubules bring the action potential to the interior of the muscle cell.
    31. 31. Cisternae of the SR release calcium ions into the individual sarcomeres and the overlapping thick and thin myofilaments of the myofiber. At each triad, the action potential is transmitted from the T tubules to the sarcoplasmic reticulum (SR) membrane and stimulates the release of calcium ions.
    32. 32. Calcium binds with troponin to move the troponin, tropomyosin complex Binding sites in the actin filament are exposed Myosin head attach to binding sites and create a power stroke ATP detaches myosin heads and energizes them for another contaction When action potentials cease the muscle stop contracting
    33. 33. SLIDING FILAMENT THEORY Initiated once calcium ions is released from the sarcoplasmic reticulum. Calcium ions activate binding between actin and myosin that results in their sliding past each other and muscle contraction. When the stimulus subsides and the membrane is no longer stimulated, calcium ions are actively transported back into and stored in the cisternae of the sarcoplasmic reticulum, causing muscle relaxation.
    34. 34. Muscle sense (proprioception) is the brain’s ability to know where our muscles are and what they are doing, without our having to consciously look at them. Stretch receptors (proprioceptors or muscle spindles)
    35. 35. Stretch receptors / muscle spindles Detect stretching of muscles and generate impulses, which enable the brain to create a mental picture to know where the muscles are and how they are positioned. Conscious muscle sense is felt by the parietal lobes. Unconscious muscle sense is used by the cerebellum to coordinate voluntary movements.
    36. 36. References Junquiera LC, Carneiro J. 2005. BASIC HISTOLOGY : TEXT AND ATLAS 11th Edition. McGraw-Hill’s ACCESS MEDICINE. Young B. 2009. WHEATER’S FUNCTIONAL HISTOLOGY. 5TH Edition. UK: Churchill Livingstone. Distributor: Phils: C & E Publishing, Inc.
    37. 37. Assignment 1. Explain the consequence of TETANUS AND BOTULISM in skeletal muscle contraction.  What is BOTOX and how does it work? 1. Describe MYASTHENIA GRAVIS and give its consequence to the process of muscular contraction.

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