Skeletal Muscle - Tension

1
Skeletal Muscle - Tension

• (2) active tension (muscle activation)
• Excitation-contraction coupling
• Ca2, ATP
• Muscle size (cross-sectional area)
• Muscle length (stretch)
• Rate coding (frequency modulation) motor unit
  recruitment
• Shortening velocity
• Temperature
• Fiber-type
• Reflexes

2
Skeletal Muscle Activation

• Motor Unit
• Each a-motoneuron
• Innervates 20-
• 2000 fibers
• Action potentials
• trigger Excitation-
• Contraction Coupling

3
Excitation-Contraction Coupling

• 1. Action potential conducted down the sarcolemma
• 2. Action potential conducted into muscle fiber
  by transverse tubules
• 3. Voltage sensors (L-type Ca channels) in
  t-tubules pass signal to sarcoplasmic reticulum
• 4. Ca channels (ryanodine receptors) release
  Ca stored in the sarcoplasmic reticulum (SR)
• 5. Cai increases in sarcoplasm
• 6. Ca binds with troponin C - induces
  conformational change and lifts tropomyosin off
  actin-myosin binding site
• 7. Myosin head (X-bridge) combines w/ actin
  molecule - pulling action contracts skeletal
  muscle sarcomere
• Like oars pulling in the water in crew (rowing)
• 8. X-bridge cycling continues in presence of
  Ca and ATP

4
Skeletal Muscle Activation

• Composition
• Skeletal muscle fibers

5
Activation of Skeletal Muscle
Attaches to cytoskeleton
6
Voltage sensors on the t-tubules that can act as
Ca2 channels sensitive to dihydropyridine
(DHPRs) communicate with Ca2 channels in the SR
sensitive to ryanodine (RyRs) in skeletal
muscle(a) through direct mechanical linkage(b)
through small Ca2 currents or sparks
(calcium-induced calcium release)
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Cross-Bridge Formation in Muscle Contraction
8
Contractile Apparatus

Myosin heads pull on actin binding sites Once
trononin C lifts tropo- mysosin off the binding
sites
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X-bridge cycling

Ca2 binds to TroponinC and causes tromomyosin
to roll off the binding sites for
actin-myosin binding
Power stroke
ATP allows myosin heads to detach from actin to
attach at new binding site (like rungs in a
ladder)
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Muscle cross-sectional area (size)

• The more fibers and the larger the area of the
  fibers, the greater the number of actin-myosin
  cross-bridges
• Maximal strength (for a given training state) is
  a direct function of muscle cross-sectional area
  (90 N/cm2 is max.)

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Muscle cross-sectional area (size)


Elbow Flexor Strength (kN)
2
Cross-Sectional Area (cm )
Gender difference due to muscle size, especially
in upper extremities
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Changing Skeletal Muscle Length and Tension
Development

• Stretching a muscle will increase the number of
  actin-myosin cross-bridges and thus active
  tension
• Peak ability to produce peak tension increases
  near the end of the ROM
• Ex. Extension of elbow for elbow flexors
• -stretch reflex
• Also, increases Ca release from SR
• Wind-up or preparation for swing, throw, jump.
  etc.

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Active Tension Length Curve
Active tension is greatest when the muscle near
resting length.
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Active Tension Length Curve
End of ROM

Cross-bridges crowded
Optimal length
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Active Tension Length Curve
Flex wrist --gt Shorten wrist finger flexor
muscles Tension production low

Extend wrist --gt lengthen wrist finger flexor
muscles Tension production high
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Length-Tension Relationship

• Active Passive Tension
• Max. tension _at_ 120 of resting length
• Muscle w/ hi fast-twitch reach peak tension at
  greater lengths than slow twitch
• Aging decreases length at which peak tension
  occurs
• elasticity

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Total Tension Length Curve
End of ROM