BMS1052 Lecture Notes - Lecture 16: Myocyte, Skeletal Muscle, Myoglobin
30 May 2018
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Week 6. Control of movement 2
CONTROLLING CONTRACTION
• Eventual limit – tetanus. ~3-5 times greater than isometric twitch tension
• Why does force generated last for so long?
o Ach esterase: AP propagation along plasma membrane
o Not because of Ca2+ sequestration
• What determines fused vs unfused tetanus?
o Whether AP arrives during rising phase
o AP size; separation of AP
• What limits maximum force generation?
1. Takes time for cross bridge binding to occur
Troponin-tropomyosin can reblock actin before binding occurs. With lots of
Ca2+, all binding sites are available.
2. Passive muscle tension
With multiple summed twitches, passive tension can be more easily overcome
3. Amount of overlap of actin and myosin filaments
• Twitch contraction (Rise-time) of ~35 ms; twitch duration ~150 ms.
• So with AP separation 35-150 ms we get summation (unfused tetanus)
• But with AP separation <35 ms we get smooth summation (fused tetanus)
• Passive vs active tension:
Passive tension
Active tension
o Titin filaments
o Have spring like properties
-> when stretched fibre is released it tends
to return to an equilibrium length
o Provide passive elastic properties of relaxed
muscles
o Has an optimal length for generating
tension: dependent on overlap of actin and
myosin
-cannot shorten more than ~60%
o Maximum length: no overlap of actin and
myosin
o Minimum length limited by:
1. Thin filament overlap
2. Overlap of thick-filaments and Z-discs
• Types of contractions:
Isotonic contraction
o Muscle changes length while maintaining constant tension
o Sarcomeres change length
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Isometric contraction
o Muscle develops tension without changing length
o If cycling continues but myosin cross-bridges repeatedly attach and
detach from the same position, then we will get force generation with
no shortening
-no lengthening occurs, cross bridges rebind in same place
o Rotation in power stroke is absorbed by cross bridge
Concentric contraction
o Force generation and shorten the muscle
o Eg. bicep muscle is shortening while contracting during rising phase of
bicep curl
o Initiates movements (opposite of eccentric)
Eccentric contraction
o Stimulating muscle to maintain tension but lengthening it
o Eg. bicep muscle is lengthening while contracting during lowering
phase of bicep curl
o Slows or stops movement (opposite of concentric)
o Potentially more damaging but also good muscle training
• Time-course of force generation in twitch response to a single AP
Isometric contraction
Isotonic contraction
o Latent period/delay (few ms)
-> depends only on time until first cross
bridge attachment occurs
o Peak force at ~35ms (this is contraction
time) ->largest number of myosin cross
o Latent period/delay (15ms)
-> this time depends on sufficient cross-
bridges forming to counteract the slack in
the muscle, and any existing load or weight
on the muscle. If a heavier load is placed on
find more resources at oneclass.com
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Document Summary
Troponin-tropomyosin can reblock actin before binding occurs. Ca2+, all binding sites are available: passive muscle tension. Passive tension: titin filaments, have spring like properties. > when stretched fibre is released it tends to return to an equilibrium length. Active tension: has an optimal length for generating tension: dependent on overlap of actin and myosin. Cannot shorten more than ~60: provide passive elastic properties of relaxed, maximum length: no overlap of actin and muscles myosin, minimum length limited by, thin filament overlap, overlap of thick-filaments and z-discs, types of contractions: Isotonic contraction: muscle changes length while maintaining constant tension, sarcomeres change length. Isometric contraction: muscle develops tension without changing length. If cycling continues but myosin cross-bridges repeatedly attach and detach from the same position, then we will get force generation with no shortening. No lengthening occurs, cross bridges rebind in same place: rotation in power stroke is absorbed by cross bridge.
