BIPN 100 Lecture Notes - Lecture 14: Sarcolemma, Myosin, Troponin C
13 Jun 2018
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BIPN 100 Lecture 14
5/4/2018
• T-tubules
o Are extensions of the sarcolemma that associate with the ends (terminal
cisternae) of the SR
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• Myosin
o Thick filament
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o Is two intertwined molecules together, with rigid tails that connect to a hinged
region that has two myosin heads
o The heads have sites for ATP to bind to→myosin ATPase
▪ Hydrolyzes ATP to allow for muscle movement
o Found on the myosin head: a myosin-actin binding site, where the cross-bridge is
made; the cross-bridge is responsible for generating the tension of a muscle
o The myosin heads are all towards the end of the filament in skeletal muscle (not
the same in smooth muscle)
o The power-stroke of the myosin head causes the muscle to contract and shorten
• Actin
o A globular protein
o Looks like two beaded necklaces that are intertwined
o Forms the thin filament
• Sarcomere
o Patterns of light/dark/dark/light
o Shortens when the filaments contract→the actin slides towards the middle with
the power-stroke of the cross-bridge with the myosin
o Sarcomeres make skeletal muscle striated
• Regulatory proteins
o Are tropomyosin and troponin C
o They regulate the cross-bridge formation
o When calcium binds to troponin C, the troponin C has a change in conformation
that makes tropomyosin move out of the way to allow myosin to bind with actin
o Tropomyosin partially blocks the myosin-actin binding site
▪ Troponin C moves to allow tropomyosin to bind
• Accessory proteins
o Are titin and nebulin
o Titin
▪ Is a big, elastic protein
▪ Shown as a spring
▪ It stabilizes the position of actin and myosin
▪ Titin provides elasticity and stabilizes myosin
o Nebulin
▪ Helps align actin
▪ Is an inelastic protein→maintains actin straight
• Muscle contraction: sliding filament theory
o When the muscle is at rest, there is a little bit of overlap between the actin and
myosin
o When the muscle contracts, there is hydrolysis of ATP that allows the cross-
bridge to form, and there is an influx of calcium, moving tropomyosin out of the
way, so that actin and myosin can bind to the troponin C complex
o The length of the sarcomere changes as actin slides towards the middle
o The tension is NOT caused by the shortening
▪ The tensions is caused by the cross-bridge formation
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