Ch 9 - Muscles and Muscle Tissue (1)

  1. muscle tissue:

    3 types
    nearly half of body's mass; Transforms chemical energy (ATP) to directed mechanical energy --> exerts force

    • 3 types:
    • skeletal; cardiac; smooth

    • prefixes for muscle tissue and muscle cells:
    • myo; mys; sarco
  2. skeletal muscles
    • organs attached to bones and skin
    • elongated cells called muscle fibers (single cell within muscle tissue)
    • striated
    • vountary
    • contract rapidly; tire easily; powerful
    • require nervous system stimulation for contraction
  3. cardiac muscle
    • only in heart; bulk of heart walls
    • striated
    • con contract without nervous system stimulation
    • involuntary
  4. smooth muscle
    • in walls of hollow organs (stomach, urinary bladder, airways)
    • not striated
    • con contract without nervous system stimulation
    • involuntary
  5. special characteristics of muscle tissue
    • Excitability (responsiveness or irritability): ability to receive and respond to stimuli
    • contractility: ability to shorten forcibly when stimulated
    • extensibility: ability to be stretched
    • elasticity: ability to recoil to resting length
  6. 4 important muscle functions + additional functions
    • 1) movement of bones or fluids (blood)
    • 2) maintaining posture and body position
    • 3) stabilizing joints
    • 4) heat generation (especially skeletal muscles)

    - additional functions= protects organs, forms valves, controls pupil size, causes goosebumps
  7. skeletal muscle anatomy
    • each muscle is served by one artery, one nerve, and one or more veins
    • (enter/exit near central part and branch through connective tissue sheaths; every skeletal muscle fiber supplied by nerve ending that controls its activity; huge nutrient and oxygen need; generates large amount of waste)

    • connective tissue sheaths contain support cells and reinforce the skeletal muscle
    • From external to internal:
    • Epimysium = dense irregular connective tissue surrounding entire muscle; may blend with fascia (sheet of fibrous connective tissue surrounding muscle)
    • Perimysium = fibrous connective tissue surrounding fascicles (groups of muscle fibers)
    • Endomysium = fine areolar connective tissue surrounding each muscle fiber (muscle cell)
  8. skeletal muscle: attachments
    • attach in at least 2 places:
    • -insertion = movable bone
    • -origin = immovable (less movable) bone

    • attachments direct or indirect:
    • - direct = epimysium fused to periosteum of bone or perichondrium of cartilage
    • - indirect = connective tissue wrappings extend beyond muscle as ropelike tendon (biceps brachii muscle) or sheet-like aponeurosis (palmaris longus muscle)
  9. myofibrils
    • densely packed, rodlike elements of skeletal and cardiac muscle cells
    • ~80% of cell volume
    • contain sarcomeres - contractile units (sarcomeres contain myofilaments)
    • exhibit striations - perfectly aligned repeating series of: dark A bands and light I bands
  10. striations

    (need to see diagram)
    • H zone: lighter region in midsection of dark A band where filaments do not overlap
    • M line: line of protein myomesin bisects H zone
    • Z disc (line): coin-shaped sheet of proteins on midline of light I band that anchors thin filaments and connects  to one another
    • Thick filaments: run entire length of an A band
    • Thin filaments: run length of I band and partway into A band
    • Sarcomere: region between two successive Z discs
  11. sarcomere

    (need to see diagram)
    • smallest contractile unit (functional unit) of muscle fiber
    • align along myofibril like boxcars of a train
    • contains A band with 1/2 I band at each end
    • composed of thick and thin myofilaments made of contractile proteins
  12. myofibril banding pattern

    (see diagram)
    orderly arrangement of actin and myosin myofilaments within sarcomere

    • - actin myofilaments = thin filaments
    • extend across I band and partway in A band; anchored to Z discs
    • - myosin myofilaments = thick filaments
    • extend length of A band; connected at M line
  13. ultrastructure of thick filament
    • composed of protein myosin
    • each composed of 2 heavy and 4 light polypeptide chains
    • -- myosin tails contain 2 interwoven, heavy long polypeptide chains
    • -- myosin heads contain 2 smaller, light polypeptide chains that act as cross bridges during contraction (binding sites for actin of thin filaments; binding sites for ATP; ATPase enzymes)
  14. ultrastructure of thin filament
    • twisted double strand of fibrous protein F actin
    • F actin (polypeptide chain) consists of G (globular) actin subunits
    • G actin (monomer) bears active sites for myosin head attachment during contraction
    • Tropomyosin and Troponin are regulatory proteins bound to actin
  15. structure of myofibril (fyi)
    • elastic filament: composed of protein titin; holds thick filaments in place; helps recoil after stretch; resists excessive stretching
    • dystophin: links thin filaments to proteins of sarcolemma
    • nebulin, myomesin, C proteins bind filaments of sarcomeres together; maintain alignment
  16. sarcoplasmic reticulum (SR)
    • network of smooth endoplasmic reticulum surrounding each myofibril (most run longitudinally)
    • pairs of terminal cisternae form perpendicular cross channels
    • functions in regulation of intracellular Ca2+ levels (stores and releases Ca2+)
  17. T tubules
    • continuations of sarcolemma
    • lumen continuous with extracellular space
    • increase muscle fiber's surface area
    • penetrate cell's interior at each A band - I band junction
    • associate with paired terminal cisterns to form triads that encircle each sarcomere
  18. triad relationships
    • T tubules conduct impulses deep into muscle fiber; every sarcomere
    • integral proteins protrude into intermembrane space from T tubule and SR cistern membranes - act as voltage sensors
    • SR foot proteins: gated channels that regulate Ca2+ release from SR cisterns
  19. sliding filament model of contraction

    (see diagram)
    • in relaxed state, thin and thick filaments overlap only at ends of A band
    • myosin heads bind to actin; sliding begins
    • cross bridges form and break several times, ratcheting thin filaments toward center of sarcomere (causes shortening of muscle fiber; pulls Z discs toward M line -- I bands shorten; Z discs closer; H zones disappear; A bands move closer- length stays same)
    • generation of force
    • does not necessarily cause shortening of fiber
    • shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening

    • sliding filament model of contraction:
    • - during contraction, thin filaments slide past thick filaments --> actin and myosin overlap more
    • - occurs when myosin heads bind to actin --> cross bridges
  20. physiology of skeletal muscle fibers: for skeletal muscle to contract
    • Activation (at neuromuscular junction)
    • must be nervous system stimulation -- voluntary
    • must generate action potential in sarcolemma

    • Excitation-contraction coupling
    • action potential propogated along sarcolemma
    • intracellular Ca2+ levels must rise briefly
  21. neuromuscular junction (NMJ)
    • situated midway along length of muscle fiber
    • axon terminal and muscle fiber separated by gel-filled space called synaptic cleft
    • synaptic vesicles of axon terminal contain neurotransmitter acetylcholine (ACh)
    • junctional folds of sarcolemma contain ACh receptors
    • NMJ includes axon terminals, synaptic cleft, junctional folds
  22. the nerve stimulus
    • skeletal muscles are stimulated by somatic motor neurons
    • axons of motor neurons travel from central nervous system via nerves to skeletal muscle
    • each axon (nerve fiber) forms several branches as it enters muscle
    • each axon ending forms neuromuscular junction with single muscle fiber
  23. events at neuromuscular junction - 6 steps
    • 1) AP arrives at axon terminal
    • 2) voltage-gated Ca2+ channels in axon terminal open. Ca2+ moves down electochemical gradient
    • 3)Ca2+ entry causes ACh (a neurotransmitter) to be released by exocytosis
    • 4) ACh diffuses across synaptic cleft to muscle -- binds to its receptors on the sarcolemma
    • 5) ACh binding to ion channels opens them (allows simultaneous passage of Na+ into muscle fiber and K+ out of muscle fiber. More Na+ ions enter than K+ ions exit, which produces local change in membrane potential called the end plate potential)
    • 6) Extracellular ACh is broken down (ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from junction)
Card Set
Ch 9 - Muscles and Muscle Tissue (1)
ch 9 part 1