Molecular mechanisms of muscle contraction Flashcards
Describe the structure of a muscle fiber.
Multiple fibrils encased by a membrane (sarcolemma). Each fibril bundle (fiber) is surrounded by a sarcoplasmic reticulum (SR) and terminal cisterns. T-tubules open to the exterior of the fiber and combined with adjacent SRs/cisterns form a triad.
How is an AP transmitted to a fiber?
Depolarization of sarcolemma
How is an AP transmitted to the interior of muscles?
Voltage gated Na channels/T-tubules
After contraction, how is Ca2+ re-sequestered into the SR?
Active transport by the Ca2+-ATPase pump in the SR membrane.
What causes muscle relaxation?
Lowering of the sarcoplasmic free Ca2+
Describe the structure of myosin.
Rod-like tail (shaft of thick filament) with two globular heads (projecting crossbridges).
What makes up the thin filaments?
Primarily actin, a globular protein that associates to form a double-stranded helix.
In what fashion does myosin arrange itself?
Bipolar, antiparallel
How are actin and myosin bound in rigor versus relaxation?
In rigor the bridges bind tightly to the actin at an angle. In the relaxed state most of the bridges are detached from the actin and stick out at right angles to the filament axis.
What are the globular heads of myosin called? What do they do?
S-1. They contain the ATPase and have the ability to combine with actin.
What is polymerized actin called?
F-actin.
During contraction, muscle hydrolyzes ATP in amounts ______ proportional to the tension developed and the degree of tension is ________ on the number of crossbridges interacting.
directly proportional, dependent
Release of myosin head is associated with _____ binding.
ATP
What energetic status does the myosin head have when it binds to actin?
ADP Pi
What causes the myosin head to cock into place ready to bind to actin?
ATP hydrolysis to ADP + Pi
What is the rate-limiting step in the Lymn-Taylor cycle?
Dissociation of the products of hydrolysis (myosin from ADP+Pi)
To what regulatory protein does calcium bind?
Troponin
What are the 3 components of troponin and what do they do?
Tn-I inhibits the ATPase activity of actomyosin
Tn-C binds calcium to release inhibition
Tn-T links Tn-I and Tn-C to tropomyosin
On which filament do troponin and tropomyosin reside?
Actin
Tropomyosin covers ___ actin monomers in a ____ angstrom repeat.
7 actin monomers, 400 angstrom
How does tropomyosin block myosin binding?
Sterically blocks myosin binding sites on actin.
How does smooth muscle use calcium regulation?
Myosin-phosphorylation
How does actin speed up the “reaction”
Improves the dissociation of myosin, ADP+Pi complex
What happens to actin-myosin cycling in the absence of calcium?
Crossbridges stuck with ADP,Pi on it but no actin/myosin binding
