Skeletal Muscle

1
Skeletal Muscle
Myofilament Structure
Neuromuscular Junction
Excitation-Contraction Coupling
Sliding Filament Theory
All-or-None Principle
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Myofilament Structure
The following links outline the structure of
the thick and thin myofilaments. Note the
structure of the myosin (thick) filament with the
heads (crossbridges) for grabbing and pulling the
actin(thin) filament. The actin (thin)
filaments consisting of a helix of spherical
molecules with tropomyosin wrapped around and
troponin sitting on top.
http//www.brookscole.com/chemistry_d/templates/st
udent_resources/shared_resources/animations/muscle
s/muscles.html
http//www.mhhe.com/biosci/esp/2002_general/Esp/fo
lder_structure/su/m4/s11/sum4s11_9.htm
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Neuromuscular Junction
4
Neuromuscular Junction
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Neuromuscular Junction
(Excitation)
Action Potential The Signal for Muscular
Contraction

• The signal from the brain travels through the
  axon until it reaches the axon terminal at the
  motor end plate
• This signals the release of a neurotransmitter
  (acetylcholine) into the synaptic junction
• The binding of acetylcholine allows sodium ions
  to flow through the sarcolemma, causing
  depolarization of the membrane
• This depolarization spreads across the sarcolemma
  (action potential) and down the transverse
  tubules (to the interior of the muscle cell)

The following links will guide you through the
activity that takes place at the neuromuscular
junction.
Neuromuscular junction 1
Neuromuscular junction 2
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Excitation-Contraction Coupling
MOTOR NEURON
NEUROMUSCULAR JUNCTION
MUSCLE RELAXES!!!
1
MUSCLE CONTRACTED!!!
TRANSVERSE TUBULE
TRANSVERSE TUBULE
ATP
ADP Pi
POWER STROKE
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Excitation-Contraction Coupling
MOTOR NEURON
NEUROMUSCULAR JUNCTION
1
TRANSVERSE TUBULE
TRANSVERSE TUBULE
ATP
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SARCOPLASMIC RETICULUM
4
TROPONIN
TROPOMYOSIN
(THIN FILAMENT)
MUSCLE CONTRACTION
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STRONG BINDING SITES
ACTI N
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ATP
ADP Pi
MYOSIN CROSSBRIDGE
MUSCLE RELAXES
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MYOSIN (THICK FILAMENT)
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Sliding Filament Theory
(Contraction)
Sliding Filament Theory

• muscle cells or fibres contract by shortening of
  the myofibrils due to
• actin(thin filaments) sliding over the myosin
  (thick filaments)

• the sarcomere shortens as the z-lines move
  closer together

• the I-band gets closer smaller (dist. b/w
  sucessive thick filaments)

• the the H-band disappears (dist. b/w the ends of
  the thin filaments)

• the the A-band doesnt change (spans the thick
  filament)

http//highered.mcgraw-hill.com/sites/0072437316/s
tudent_view0/chapter42/animations.html
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Muscle Contraction Sliding Filament Theory
I, A band
H zone
I, A bands
NO H zone
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Muscle Contraction Enegry (ATP)
The energy for muscular contractions comes from
the breakdown of ATP by the enzyme myosin ATPase.
This enzyme is located on the head of the myosin
cross-bridge.
The breakdown of ATP to ADP and Pi releases
energy which allows the heads of the
cross-bridges to pull the actin molecules over
them, shortening the muscle.
http//www.sci.sdsu.edu/movies/actin_myosin.html
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Regulation of Excitation-Contraction Coupling
- myosin cross-bridges are always attached to
actin (unless moving from site to site)
- cross-bridges move from weak binding sites to
strong binding sites
- proteins tropomyosin and troponin restrict
access of cross-bridges to strong binding sites
- proteins tropomyosin and troponin
restrict(inhibit) access of cross-bridges to
strong binding sites
- calcium released from the sarcoplasmic
reticulum combines with troponin which then
causes the tropomyosin to move off the strong
binding sites
- myosin then binds with strong site to pull
actin across itself
- one cycle shortens muscle 1 of its resting
length, cycle will repeat until muscle is fully
contracted (up to 60 of resting length)
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All-or-none Principle
Muscle contractions require a stimulus to get
them started. Depending on the nerve impulse,
the magnitude or threshold of the stimulus can
vary. The greater the threshold stimuli, the
greater number of motor units that will be
recruited. e.g. Greater weight ? More motor
units recruited As long as the stimuli is of
threshold strength, the muscle fibre will
contract to its fullest extent. If the stimuli
is not of threshold strength, the fibre will not
contract. (no partial contractions of muscle
fibres) Muscles consist of many motor units.
Each of which reponds in an all-or-none fashion.
A whole muscle however can partially contract
depending on the number of motor units recruited.
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Motor Units