EXAM 3: BIO&251 (Chapter 10: Part 1)

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  1. Three Types of Muscles
    • Cardiac: Found in walls of the heart/pumps blood
    • Smooth: Walls of hollow organs/moves eggs, sperm, urine, food, hairs
    • Skeletal (most important): Attached to bone or fascia/many functions
  2. Skeletal Muscle Functions
    • 1. Produce skeletal movement
    • 2. Maintain body and posture positions
    • 3. Support soft tissues (floor of pelvic cavity)
    • 4. Guard entrances and exits (esophagus, external anal sphincter)
    • 5. Produce heat - maintain body temperature
    • 6. Store nutrient (breakdown release amino acids)
  3. Organization of Skeletal Muscles
    • Three different Connective Tissues present
    • 1. Epimysium (outside)
    • 2. Perimysium (around)
    • 3. Endomysium (inside)

    Image Upload 1

    Cell and fiber are the same thing
  4. Distinctive Features of Skeletal Muscles
    • Stain darkly (contains lots of proteins)
    • Long and Thin
    • Striated
    • Peripheral nuclei (doesn't interfere with fusing)
    • Multinucleated (due to myoblasts fusing together)
    • Do not branch
  5. Myoblasts, Myosatellite Cells
    • These are stem cells
    • Live in Endomysium
    • Function in muscle development (Hyperplasia=more cells, Hypertrophy=large existing cells/happens more often)
    • Myoblasts proliferate, differentiate, fuse into multinucleate muscle cells
    • Some remain as myosatellite (satellite) cells in skeletal muscle
  6. Organization of CT Layers
    • Epimysium surrounds entire muscle (DENSE IRREGULAR CT)
    • Perimysium defines bundles of muscle fibers (DENSE IRREGULAR CT)
    • Endomysium surrounds individual muscle fibers (AEROLAR CT, MYOSATELLITE CELLS [STEM])

    All three CT layers contribute to tendons and aponeuroses
  7. Muscle CT and Organization
    Image Upload 2
  8. Skeletal Muscle Fibers (Cells)
    • Muscle Cell=muscle fiber=myofiber
    • Sarcolemma (cell membrane): Sarco=flesh, lemma=husk
    • Sarcoplasm: muslce cell cytoplasm
    • Sarcoplasmic Reticulum: modified ER/wraps around contractile proteins to organize them/stores calcium (Higher concentration inside than outside)
    • T-Tubules: invaginations of cell membrane
    • Myofibrils: contain contractile proteins wrapped by SR
    • Sarcomeres: regular arrangement of myofilaments within myofibril
  9. Structure of Skeletal Muscle Fiber (Cell)
    • Image Upload 3
    • Resting Muscle: Not permeable to Calcium
  10. Sarcoplasmic Reticulum (SR)
    • Specialized smooth endoplasmic reticulum
    • Wraps around myofilaments (contractile proteins) to form myofibrils

    • Stores Calcium in cisternae
    • 1.Swellings called terminal cisternae
    • 2.Ca2+ is continuously pumped into SR from cytoplasm
    • 3. SR not very leaky to Ca2+ in resting muscle cell
  11. Calcium and Muscle Cells
    • Flux=PxΔC
    • SE contains calcium binding proteins, calsequestrian

    • As a result of pumping Ca2+ into the SR:
    • 1. [Ca2+] in SR about 40,000X higher than in the sarcoplasm
    • 2. Total [Ca2+] = free Ca2+ + Ca2+ bound to calsequestrian (About 40X more Ca2+ bound to calsequestrian than is in the form of free Ca2+

    During contraction process, SR membrane becomes leaky, Ca2+ rapidly enters sarcoplasm
  12. Transverse Tubules
    • Invaginations of the sarcolemma (cell membrane)
    • Carry action potential deep into cell
    • Contain extracellular fluid
  13. Myofibrils
    • SR wraps around myofilaments
    • Organizes them into myofibrils
  14. Myofilaments

    • Types:
    • 1. Thin Filament: Actin and other proteins
    • 2. Thick Filament: Myosin and Titin

    Arranged with myofibrils into sarcomeres
  15. Sarcomeres
    Smallest functional unit of muscle contraction

    • *Repeating units along the length of a muscle fiber
    • *Regular arrangement causes striated appearance

    • Dark=A bands
    • Light=I bands
  16. Sarcomere Structure
    • Image Upload 4
    • 1 Sarcomere extends from 1 Z Line to another Z Line
    • H Band: Bright Band
    • Titin: Green wiggly line
    • *Largest known protein and goes all the way through
  17. Sarcomere Structure (Part 2)
    • Image Upload 5
    • Actin: Thin Filament
    • Actinin: Makes Titin squiggly
  18. Sarcomere (Filament Arrangement 1)
    • Z Line
    • *Actinin Filaments: Anchors thin filaments between sarcomeres
    • *Titin: Attaches here to anchor myosin (largest known protein)

    • I Band=isotropic=light in color
    • *Thin Filament and titin
    • *Shrinks when muscle contracts
  19. Sarcomere (Filament Arrangement 2)
    • A band=anisotropic=dArk colored
    • *Where thick filament is present
    • *Does not shrink when muscle contracts

    • Contains:
    • 1. M Line
    • *Myomesin: stabilizes thick filaments
    • 2. H Band
    • *Thick Filament only
    • 3. Zone of overlap
    • *Both think and thick filaments
  20. Think and Thick Filaments
    • Image Upload 6
    • Head is going to use ATP to move and grab an Actin molecule
  21. Thin Filament (Part 1)
    • Image Upload 7
    • 1. F Actin
    • *Composed of 2 strands of G actin subunits
    • *Has active sites (myosin-binding sites)

    • 2. Nebulin
    • *Helps hold G actin subunits together into F actin molecule
  22. Thin Filament-the T-T Complex (Part 2)
    • Image Upload 8
    • 3. Tropomyosin
    • *Covers myosin-binding sites on actin in resting muscle

    • 4.Troponin
    • *Binds tropomyosin
    • *Binds to G-actin and holds tropomyosin in place
    • *Has Ca2+ binding site

    • 5. Dystrophin: Links actin to sarcolemma proteins
    • *Transmits force to membrane → tendons
  23. Thick Filaments
    • Image Upload 9
    • They are bundles of Myosin filaments around a Titin core
    • *Myosin molecules have elongated tail, globular head, and hinge region
    • **Heads have ATP-binding site
    • **Heads form cross-bridges with actin during contraction
  24. Sliding Filament Theory
    • Image Upload 10
    • The I band and the H Zone shrinks.
  25. Events Leading to Contraction
    • 1. Electrical signal (action potential) travels along axon of motor neuron to motor end plate (a.k.a neuromuscular junction)
    • 2. Ca2+ enters the bouton.
    • 3. Neurotransmitter (acteylcholine-ACh) is released into the synapse between nerve and muscle cells.
    • 4. ACh diffuses across synapse and binds to ACh receptors on muscle cell surface.
    • 5. Receptor changes shape and allows Na+ to move down its concentration gradient into muscle cell.
    • 6. Action potential spreads over muscle cell surface, including T-Tubules (Excitation).
    • 7. Sarcoplasmic reticulum releases Ca2+ into cytoplasm (Excitation/Contraction coupling).
    • 8. Ca2+ interacts with troponin.
    • 9. Troponin pulls tropomyosin.
    • 10. Myosin-binding sites are uncovered.
    • 11. Myosin heads grab and pull on Actin ("power stroke" of contraction).
    • 12. Myosin heads bind and split new ATP.
    • 13. Myosin heads release Actin and "recock" for another power stroke.

    Cross-bridges are formed at different times by individual myosin heads causing a smooth sliding of filaments.
  26. Skeletal Muscle Innervation
    • Graded Potential: Travels along membrane of the synapse.
    • Action Potential: Spreads across entire muscle of cell membrane.
  27. Excitation-Contraction Coupling
    This is the link between an action potential on the muscle cell membrane and the beginning of contraction.
  28. Relaxation
    • 1. Motor neuron quits firing → no new ACh release
    • 2. Acetylcholinesterase (AChE) in synapse is continually breaking down ACh

    • Result
    • *No new action potential on muscle cell
    • **No new Ca2+ release from SR
    • **Ca2+ continually pumped into SR
    • **[Ca2+] in cytoplasm drops
    • *T-T complex covers up myosin-binding sites
    • **No interaction between actin and myosin
    • **Muscle relaxes passively
  29. Rigor Mortis ("Stiffness of Death")
    • Myosin heads already cocked with ATP
    • Ca2+ leaks out of SR (ATP required to pump it back in)
    • Intracellular [Ca2+] rises
    • T-T complex binds Ca2+ and uncovers myosin-binding sites
    • Contraction occurs
    • ATP stores become depleted over time
    • No ATP available to release myosin from actin and recock heads
  30. Some Drug and Toxin Effects on Contraction
    • Inhibitory Effects: Block ACh release - Botulinum toxin/Blocks binding of ACh to receptor - Atropine (belladonna), curate
    • Excitatory Effects: Block acetylcholinesterase - Neostigmine, DFP (nerve gas)/Increase ACh release - some spider venoms/Mimic ACh - Nicotine
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EXAM 3: BIO&251 (Chapter 10: Part 1)
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