THE MUSCLE AND ITS CONTRACTION

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Chapter 2-2

• THE MUSCLE AND ITS CONTRACTION

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• the Neuromuscular Junction (NMJ)

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Skeletal Muscle Innervation
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Illustration of the Neuromuscular Junction (NMJ)
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New Ion Channel Players

• Voltage-gated Ca2 channel
• in presynaptic???? nerve terminal
• mediates neurotransmitter release
• Nicotinic Acetylcholine???? Receptor Channel
• in muscle neuromuscular junction
  (postsynaptic???? membrane, or end plate)
• mediates electrical transmission from nerve to
  muscle

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Nerve Terminal Ca2 channels

• Structurally similar to Na channels
• Functionally similar to Na channels except
• activation occurs at more positive potentials
• activation and inactivation much slower than Na
  channels

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Neuromuscular Transmission
Myelin
Axon
Axon Terminal
Skeletal Muscle
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Neuromuscular Transmission Step by Step
Depolarization??? of terminal opens Ca channels
Nerve action potential invades axon terminal
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Ca2 induces fusion?? of vesicles with
nerve terminal membrane.
ACh is released and Diffuses?? across synaptic
cleft.
Nerve terminal
Binding of ACh opens channel pore that
is permeable to Na and K.
K
K
K
K
K
Outside
Muscle membrane
Inside
ACh binds to its receptor on the postsynaptic
membrane
K
K
K
K
K
K
K
K
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End Plate Potential (EPP)
VNa
The movement of Na and K depolarizes muscle
membrane potential (EPP)
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Muscle Membrane Voltage (mV)
Threshold
Presynaptic terminal
-90 mV
VK
Time (msec)
Presynaptic AP
Outside
Muscle membrane
Inside
Voltage-gated Na Channels
ACh Receptor Channels
Inward Rectifier K Channels
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Structural Reality
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Neuromuscular Transmission

• Properties?? of neuromuscular junction
• 11 transmission A chemical transmission which
  is designed to assure that every presynaptic
  action potential results in a postsynaptic one
• An unidirectional??? process
• Has a time delay. 20nm/0.5-1ms
• Is easily affect by drugs and some factors
• The NMJ is a site of considerable clinical
  importance

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NMJ Diseases

• Myasthenia Gravis (?????)
• Autoimmunity to ACh receptor
• Fewer functional ACh receptors
• Low safety factor for NM transmission
• Lambert-Eaton syndrome(???-????? ,???????? )
• Autoimmunity directed against Ca2 channels
• Reduced ACh release
• Low safety factor for NM transmission

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Microstructure of Skeletal Muscle
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Skeletal Muscle

• Human body contains over 400 skeletal muscles
• 40-50 of total body weight
• Functions of skeletal muscle
• Force production for locomotion?? and breathing
• Force production for postural?? support
• Heat production during cold stress

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• Fascicles?? bundles, CT(connective tissue????)
  covering on each one
• Muscle fibers muscle cells

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Structure of Skeletal MuscleMicrostructure

• Sarcolemma (????)
• Transverse (T) tubule
• Longitudinal tubule (Sarcoplasmic reticulum, SR
  ???)
• Myofibrils (????)
• Actin ???? (thin filament)
• Troponin (????)
• Tropomyosin (?????)
• Myosin ???? (thick filament)

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Within the sarcoplasm
Triad (???)

• Transverse tubules
• Sarcoplasmic reticulum -Storage sites for calcium
• Terminal cisternae - Storage sites for calcium

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Microstructure of Skeletal Muscle (myofibril)
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Sarcomeres

• Sarcomere ??? bundle of alternating thick and
  thin filaments
• Sarcomeres join end to end to form myofibrils
• Thousands per fiber, depending on length of
  muscle
• Alternating thick and thin filaments create
  appearance of striations??

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(No Transcript)
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Myosin ????

• Myosin head is hinged??
• Bends and straightens during contraction

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Thick filaments (myosin)

• Bundle of myosin proteins shaped like
  double-headed golf clubs
• Myosin heads have two binding sites
• Actin binding site forms cross bridge
• Nucleotide??? binding site binds ATP (Myosin
  ATPase)
• Hydrolysis?? of ATP provides energy to generate
  power stroke

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Thin filaments
?????
????
????
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Thin filaments (actin)

• Backbone two strands of polymerized globular
  actin fibrous actin
• Each actin has myosin binding site
• Troponin
• Binds Ca2 regulates muscle contraction
• Tropomyosin
• Lies in groove of actin helix
• Blocks myosin binding
• sites in absence of Ca2

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• Thick filament Myosin (head and tail)
• Thin filament Actin, Tropomyosin, Troponin
  (calcium binding site)