Muscle Tissue 11/14

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Lab 8A, BIO 105

• Muscle Tissue 11/14
• Nearly half of body's mass
• Three types
• Skeletal
• Cardiac
• Smooth
• Differ in structure, location, function and
  activation
• Muscle terminology
• Sarcolemma muscle plasma membrane
• Sarcoplasm cytoplasm of a muscle cell
• Prefixes for muscle Myo, mys, and sarco

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• Skeletal muscles
• muscles attached to skin and bones and covers
  bones
• Responsible for locomotion and manipulation
• Elongated cells called muscle fibers
• Striated (striped)
• Voluntary (i.e., conscious control)
• Contract rapidly tire easily powerful
• Require nervous system stimulation

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• Cardiac muscle
• Only in heart bulk of heart walls
• Striated, but involuntary
• we cant control rate and pace of contraction
  however neural controls allow heart to speed up
  for short periods
• Can contract without nervous system stimulation
  rate set by the pacemaker of the heart (group of
  special cells)
• Intercalated discs are gap junctions and allow
  innervation to spread in coordinated fashion
• Muscle is highly resistant to fatigue

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• Smooth muscle
• spindle shaped (fusiform) cells 1 nucleus
• Not striated
• Involuntary
• Can contract without nervous system
  stimulationhormones, local chemical changes and
  Autonomic Nervous System can stimulate
  contraction
• Muscle contractions are slow and sustained
• Role is to force substances through body channels
• In walls of hollow organs, e.g., stomach, urinary
  bladder, and respiratory passageways

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• Skeletal Muscle
• Each muscle is an organ composed of muscle
  tissue, blood vessels, nerves and connective
  tissue
• Every skeletal muscle fiber supplied by nerve
  ending that controls its activity
• Huge nutrient and oxygen need generates large
  amount of waste

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• Connective tissue sheaths of skeletal muscle
• Support cells reinforce whole muscle
• Internal to external
• Endomysium areolar connective tissue surrounding
  each muscle fiber located immediately superior
  to sarcolemma
• Perimysium fibrous connective tissue surrounding
  groups of muscle fibers called fascicles
• Epimysium dense irregular connective tissue
  surrounding entire muscle blends with tendons

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• Muscle Attachments
• Attach in at least two places
• Insertion movable bone
• Origin immovable (less movable) bone
• Attachments
• fused to periosteum of bone or cartilage
• connective tissue wrappings extend beyond muscle
  as tendon or aponeurosis (sheetlike connective
  tissue layer where muscle attaches)
• Dont need to know terms direct or indirect

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• Microscopic Anatomy of A Skeletal Muscle Fiber
• Long, cylindrical cell
• long because embryonic cells fuse to produce each
  fiber
• Multiple nuclei just below the sarcolemma
• Contain Myofibrils
• Densely packed, rodlike, contractile elements
• Contain sarcomeres smallest contractile units
• Sarcomeres contain myofilaments
• Exhibit striations - perfectly aligned repeating
  series of dark A bands and light I bands
• When myofibrils move, they move as group, not as
  individual units

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• Sarcomere
• Smallest contractile or functional and structural
  unit of muscle fiber
• Region of myofibril between 2 consecutive
• Z discs
• Composed of thick (myosin) and thin (actin)
  myofilaments made of contractile proteins

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• Myofibril Banding Pattern
• Orderly arrangement of actin and myosin
  myofilaments within sarcomere
• Actin myofilaments thin filaments
• Anchored to Z discs
• Extend across I band and partway in A band
• Myosin myofilaments thick filaments
• Extend length of A band
• Connected at M line

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• Thick Filament--Myosin
• Myosin tails-2 twisted, interwoven, rodlike
  chains
• Myosin heads act as cross bridges during
  contraction
• Binding sites for actin filaments
• Binding sites for ATP
• ATPase enzymes

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• Thin Filament--Actin
• Twisted double strand of fibrous protein
• Contains sites for myosin head attachment during
  contraction
• Tropomyosin and troponin - proteins bound to
  actin
• Tropomyosin in a relaxed fiber blocks the myosin
  binding sites
• Twists around actin fibers and help stiffen and
  stabilize it
• Troponin binds calcium ions

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• Sarcoplasmic Reticulum (SR)
• Network of smooth endoplasmic reticulum
  surrounding each myofibril
• Pairs of terminal cisternae form perpendicular
  cross channels throughout SR
• Functions in regulation of intracellular calcium
  levels
• Stores and releases Ca2

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• T Tubules
• Continuations of sarcolemma--pushes deeply into
  interior of cell
• Increase muscle fiber's surface area
• Conduct nerve impulses to deepest areas of muscle
  fiber
• T tubules run between cross channels called
  terminal cisternae (cisterns) of the sarcoplasmic
  reticulum (smooth endoplasmic reticulum)
• Impulses signal calcium to be released from
  adjacent terminal cisternae
• Are associated with paired terminal cisterns to
  form triads that circle each sarcomere
• triads are where the terminal cisterns border a T
  tubule

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• Physiology of Skeletal Muscle Fibers
• For skeletal muscle to contract
• Activation (at neuromuscular junction)
• Requires nervous system stimulation
• AND an electric current or action potential along
  sarcolemma
• Intracellular Ca2 levels must rise (final
  requirement for contraction to begin)

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• Nerve Stimulus and the Neuromuscular Junction
• Skeletal muscles stimulated by motor neurons of
  voluntary nervous system
• Axons of motor neurons travel via nerves to
  skeletal muscle cells
• Each axon forms several branches as it enters
  muscle
• Each axon ending or branch, forms a neuromuscular
  junction with a single muscle fiber

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• Neuromuscular Junction (NMJ) is where axon and
  muscle fiber meet.
• Axon terminal and muscle fiber separated by space
  called synaptic cleft
• Synaptic vesicles from axon terminal contain
  neurotransmitter acetylcholine (ACh)
• Sarcolemma in NMJ contain ACh receptors
• ACh diffuses across synaptic cleft and attaches
  to ACh receptors in sarcolemma
• ACh binding triggers electrical events to
  generate an action potential

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• Action potential causes changes in properties of
  cell membrane channels
• Ca2 channels open ? Ca2 moves into nerve axon
  ?causing release of ACh into synaptic cleft
• ACh diffuses across cleft?sarcolemma? initiates
  action potential in muscle

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• Contraction of Skeletal Muscles
• Contraction produces muscle tension, force
  exerted on load or object to be moved
• Refers to activation of sliding filaments and
  forming of cross bridges between actin and myosin
• Contraction ends when cross bridges are
  deactivated because of lack of nerve stimulation
  or not enough calcium present

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• Sliding Filament Model of Contraction
• In relaxed state, thin and thick filaments
  overlap only at ends of A band
• Sliding filament model of contraction
• Upon muscle stimulation, thin filaments slide
  past thick filaments ? actin and myosin overlap
  to a greater degree
• When myosin heads bind to actin ? cross bridges
  form and sliding begins
• Cross bridges form and break several times,
  moving thin filaments toward center of sarcomere
• Causes shortening of muscle fiber

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• Motor Unit The Nerve-Muscle Functional Unit
• Each muscle is served by at least one motor nerve
• A motor nerve contains axons of many motor
  neurons
• Axons branch into terminals, each of which form a
  NMJ with single muscle fiber
• Motor unit motor neuron and all muscle fibers
  it supplies
• For fine controleach motor neuron supplies a
  smaller number of fibers
• For large, weight bearing muscles, each motor
  neuron supplies a lot of muscle fibers
• Muscles response to single threshold stimulus
  called a Muscle Twitch

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• LABWORK
• 1. Identify and describe the three kinds of
  muscle tissue (3 microscope slides).
• 2. Identify and describe all components of a
  muscle, including connective tissue wrappings
  around each part (models and muscle cross section
  slide).
• 3. Identify and describe microstructure of
  skeletal
• muscle cells, and basics involved in contraction
• mechanism (models and neuromuscular junction
  slide).
• 4. Explain the concepts covered about contraction
  and muscle physiology