Muscle III Flashcards
Name the two major structures inside the muscle fiber aside from the myofibrils.
The T-tubules and the sarcoplasmic reticulum.
Describe the structure of the T-tubules.
They are holes in the outer membrane of the muscle fiber that are continuous with the outside of the cell.
Describe the structure of the sarcoplasmic reticulum.
It is a network of compartments on the inside of the cell and that are flush with the T-tubules.
What are the two key proteins involved in excitation-contraction coupling?
Ryanodine receptors and DHP receptors.
Describe what DHP receptors are and where they are found.
They are calcium ion channels that are located in the T-tubule membrane and touch the DHP receptors. They open when the T-tubule membrane gets depolarized.
Describe what Ryanodine receptors are and where they are located.
They are found in the membrane of the sarcoplasmic reticulum and touch the DHP receptors. They are also Ca2+ ion channels.
What is inside the sarcoplasmic reticulum? How does this differ from the contents of the general cytoplasm?
There is an extremely high concentration of Ca2+ ions. This is different from the cytoplasm of the muscle fiber, which has a very low concentration of Ca2+.
Describe the first part of the excitation-contracting coupling process (up to ion release).
When an action potential is fired after neuromuscular transmission and the depolarization of the endplate, the action potential spreads throughout the fiber and depolarizes the membrane, including the membrane of the T-tubules since they’re flush with the membrane. When the T-tubulus get depolarized, this activates the voltage-gated Ca2+ DHP receptors and causes a change in conformation. This pushes on the Ryanodine receptor and causes it to change conformation as well. This causes Ca2+ to flow out of the SR and into the cytoplasm of the tubule.
How does the timing work in terms of activation of different Ryanodine and DHP receptors? How does this affect muscle activity?
The excitation-contraction coupling process happens simultaneously across the whole fiber, allowing the whole muscle to contract at the same time.
Name the two proteins involved in excitation-contraction coupling AFTER calcium ion release.
Troponin and tropomyosin.
Describe the location and appearance of tropomyosin in the muscle fiber.
It is a long, filamentous protein that lines and wraps around the thin actin filaments.
Describe the location and appearance of troponin in the muscle fiber.
It is a small globular protein that is dotted along thin actin filaments at regular intervals. It binds Ca2+.
Describe the function of tropomyosin when the muscle is relaxed.
The headgroups of myosin are blocked from binding to the actin filaments by tropomyosin, as it covers the binding sites.
Describe the process of excitation-contracting coupling AFTER the release of ions.
After Ca2+ is released into the cytoplasm, it binds to troponin, which has a Ca2+ binding site. It then changes its shape and pushes away the tropomyosin. This allows the myosin headgroups to now bind to the actin and cause the muscle to contract (assuming there is sufficient ATP).
What is a twitch?
The contraction of a muscle fiber in response to a single action potential.
