Lecture: Muscles and Muscle Tissue 1

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julianne.elizabeth
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Lecture: Muscles and Muscle Tissue 1
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2012-03-27 13:54:02
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gross microscopic anatomy skeletal muscle
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lecture notes on muscles and muscle tissue
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  1. Name the 3 types of Muscle Tissue
    • Skeletal
    • Cardiac
    • Smooth
  2. Characteristics of Skeletal Muscle
    • -Attached to the bone and skin
    • -Striated
    • -Voluntary
    • -Powerful
  3. Characteristics of Cardiac Muscle
    • -Only in the heart
    • -Striated
    • -Involuntary
  4. Characteristics of Smooth Muscle
    • -Found in the walls of gollow organs
    • -Not striated
    • -Involuntary
  5. Special Characteristics of Musclce Tissue
    1. Excitability (responsiveness or irritability): ability to recieve and respond to stimuli

    2. Contractibility: ability to shorten when stimulated

    3. Extensibility: ability to be stretched

    4. Heat generation: Ability to recoil to resting point after being stretched
  6. Muscle Functions
    1. Movement of bones or fluid (blood, food, locomotion, manipulations, etc.)

    2. Maintaining Posture and body position

    3. Stabilizing Joints

    4. Heat Generation (Esp skeletal muscle)

    *also protects visceral organs
  7. What supplies each muscle?
    1 artery, 1 nerve, and 1 or more veins
  8. Connective Tissue Sheaths of Skeletal Muscle
    • Epimysium: dense regular connective tissue surrounding entire muscle

    Perimysium: Fibrous connective tissue surrounding the fascicles (groups of muscle fibers)


    Endomysium: fine areolar connective tissue surrounding each muscle
  9. Skeletal Muscle attachments
    1. Directly: epimysium of muscle is fuse to the periosteum of bone or perichondrium of cartilage

    2. Indirectly: connective tissue wrappings extend beyong the miscle as ropelike tendon or sheetlike aponeurosis that anchors it to the muscle

    *tendons are a more common connection because they are tougher and take up less space
  10. Microscopic Anatomy of a Skeletal Muscle
    -Cylindrical Cell 10 to 100 micrometers in diameter, up to 30 centimeters long

    -Multiple Peripheral Nuclei just deep to the sarcolemma (plasma membrane)

    -Glycosomes for glycogen storage, myoglobin storage, and 02 Storage (all contained in sarcoplasm)

    -Also contain myofibrils, sacoplasmic reticulum (SR) and T Tubules
  11. Myofibrils


    Densely packed, rodlike elements

    About 80% of cell volume

    Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands which contain Sarcomeres
  12. Sarcomere
    -Smallest contractile unit (functional unit) of the muscle fiber

    -The region of a myofibril between 2 successive Z discs

    -Composed of thick and thin myofilaments made of contractile proteins
  13. Features of the Sarcomere

    Thick Filaments: run the entire length of the A band

    Thin Filaments: run the length of the I band and partway into the A band

    Z-discs: coin shaped sheet of protiens that anchors the thin filaments and connects myofibrils to one another

    H zone: lighter mideregion where filaments do not overlap (thick only)

    M Line: line of pritein Myomesin that holds adjacent thick filaments together
  14. Ultrastructure of Thick Filament
    • -Composed of the protien myosin
    • -Myosin tails contain: 2 interwoven, heavy polypeptide chains
    • -Myosin heads contain:
    • * two smalller, light polypeptide chains that act as ccross bridges during contraction
    • *binding sites for actin of thin filaments
    • *binding sites for ATP
    • *ATPase enzymes (split ATP to generate energy for contraction)
  15. Ultrastructure of Thin Filaments
    • -Twisted double strand of fibrous protein F actin
    • -F actin consists of G (globular) actin subunits
    • -G actin bears active sites for myosin heads attachment during contraction
    • -Tropomyosin and troponin: regulatory proteins bound to actin
    • *tropomyosin are stiffening strands, troponin blocks the active sites for head attachment
  16. Sarcoplasmic Reticulum (SR)
    • -Network of smooth endoplasmic reticulum surrounding each myofibril like a holey sweater
    • -Pairs of terminal cisternae form perpendicular cross channels
    • -Functions in the regulation (and storage) of intercellular Ca2+ levels
  17. T Tubules
    • -Continuous with the sarcolemma
    • -Penetrate the cells interior at east A band - I band junction
    • -Associate with the paired terminal cisternae to form triads that encircle each Sarcomere
  18. Triad Relationships
    -T tubules conduct impulses deep into muscle fiber and each sarcomere (to signal release of Ca2+)

    -Integral Protiens protrude into the intermembrane space from T tubule and Sr cisternae membranes

    -T tubule protiens: voltage sensors

    -SR foot protiens: grated channels that regulate Ca2+ release from SR cisternae
  19. Contraction
    Contraction: The generation of force

    -Does not necessarily cause shortening of the fiber

    -Shortening occurs when tension generated by the cross bridges on the thin filaments exceeds forces opposing shortening
  20. Sliding Filament Model of Contraction
    -In the relaxed state, thin and thick filaments overlap only slightly

    -During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line

    • -As H Zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens
  21. Requirements for Skeletal Muscle Contraction
    1. Activation: neural stimulation at a neuromuscular junction

    • 2. Excitation-contraction coupling:
    • -generation and propagation of an action potential along the sarcolemma
    • -Final trigger: a brief rise in intracellular Ca2+ levels
  22. The Neuromuscular junction
    -skeletal muscles are stimulated by somatic motor neurons

    -Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles

    -Each axon forms several branches as it enters the muscle

    • -each axon ending forms a neuromuscular junction with a single fiber
  23. Neuromuscular junction
    -Situated midway along the length of the muscle fiber

    -Axon terminal and muscle fiber are separated by a gel-filled space called the synaptic cleft

    -Synaptic vesicles of axon terminal contain the neurotransmitter acetycholine (ACh)

    • -Junctional folds od the sarcolemma contain ACh receptors
  24. Events at the Neuromuscular Junction
    -Nerve impulses arrive at axon terminal

    -ACh is released and binds with receptors on the sarcolemma

    -Electrical events lead to the generation of an action potential
  25. Destruction of Acetylcholine (ACh)
    ACh effects are quickly terminated by the enzyme acetylcholinesterase

    Prevents continued muscle fiber contraction in the absence of additional stimulation
  26. Events in Generation of Action Potential
    • 1. Local depolarization (end plate potential)
    • 2. Generation and Propogation of an action potential
    • 3. Repolarization
  27. Events in Generation of Action Potential
    Local Polarization (end plate potential)
    -ACh binding opens chemically (ligand) gated ion channels

    -Simultaneous diffusion of Na+ (inward) and K+ (outward)

    -More Na+ diffuses, so the interior of the carolemma becomes less negative

    • -Local depolarization-end plate potential
  28. Events in Generation of Action Potential
    Generation and Propagation of an action potential
    -end plate potential spreads to adjecent membrane areas like a ripple in a pond

    -Voltage gated Na+ channels open

    -Na+ influx decreases the membrane voltage toward a critical threshold

    -Local depolarization wave continues to spread, changing the permeability of the sarcolemma

    • -Voltage-regulated Na+ channels open in the adjacent patch, causing it to depolarize to threshold
  29. Events in Generation of Action Potential
    Repolarization
    -Na+ channels close and voltage-gated K+ channels open

    -K+ efflux (flow out) rapidly restores the resting polarity

    -Fiber cannot be stimulated and is in a refractory period until repolarization is complete

    • -Ionic conditions of the resting state are restored by the Na+- K+ pump
  30. Excitation-Contraction (E-C) Coupling
    -Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of myofilaments

    • -Latent period
    • *Time when E-C coupling events occur
    • *time between AP initiation and the beginning of contraction
  31. Events of E-C Coupling
    -AP is propagated along sarcomere to T tubules

    • -Voltage-sensitive proteins stimulate Ca2+ release from SR
    • * Ca2+ is necessary for contraction

    • *Calcium binds to troponin and removes the blocking action of tropomyosin, allowing myosin heads to connect to actin and the sliding contraction to occur
  32. Role of Ca2+ in Contraction
    At LOW Intracellular Ca2+ concentration
    -Tropomyosin blocks the active sites on actin

    -Myosin heads cannot attach to actin

    -Muscle fiber relaxes
  33. Role of Ca2+ in Contraction
    At HIGHER Intracellular Ca2+ concentration
    -Ca2+ binds to troponin

    -Troponin changes shape and moves tropomyosin away from active sites

    -Events of the cross bridge cycle occur

    -When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends
  34. Cross Bridge Cycle
    -Continues as long as the Ca2+ signal and adequate ATP are present

    1. Cross bridge formation: high energy myosin head attaches to hin filament

    2. Working (power) stroke: myosin head povots and pulls this filament toward M line

    3. Cross bridge detachment: ATP attaches to myosin head and the cross bridge detaches

    • 4. "Cocking" of the myosin head: energy from hydrolysis of ATP cocks the myosin head into the high-energy state

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