Flashcards in Cellular Physiology (Part 3) Deck (63)
What are the 3 characteristics of a muscle fiber?
- it functions as a single unit
- contains myofibrils
Myofibrils of a muscle fiber are surround by _____ and are invaginated by _______.
transverse tubules (T tubules)
What are the thick filaments composed of in a myofibril?
Describe the structure of myosin
It is comprised of 6 polypeptide chains (2 heavy and 4 light chains). The heavy chains coil around one another to form the "tail" of the myosin molecule. The 4 light chains and the N terminus of each heavy chain forms 2 globular "heads" which contain an actin-binding site, which is necessary for cross-bridge formation
What are the thin filaments composed of in a myofibril?
What is the difference between G-actin and F-actin?
When actin is in its globular protein form it is called G-actin.
When actin is twisted into an alpha-helix structure to form filamentous actin it is called F-actin
Which form of actin is present in thin filaments?
Describe the shape actin is in when the muscle is at rest?
The myosin-binding sites on the actin are covered by tropomyosin and troponin so that actin and myosin cannot interact
What is tropomyosin's function?
It blocks myosin-binding sites on actin during rest, and when muscle contraction occurs it is moved out of the way so the 2 can interact
What are the 3 globular proteins that form troponin?
- troponin T
- troponin I
- troponin C
What is the function of troponin T?
It attaches the troponin complex to tropomyosin
What is the function of troponin I?
It inhibits the interaction of actin and myosin by covering the myosin-binding sites (with tropomyosin)
What is the function of troponin C?
It is a Ca2+ binding protein that plays a central role in the initiation of contraction
What happens at troponin C when Ca2+ concentration is high?
Ca2+ binds to troponin C, producing a conformational change in the troponin complex which moves tropomyosin out of the way, permitting the binding of actin to the myosin heads
What is the basic contractile unit of skeletal muscle?
Describe the structure of a sarcomere
Each sarcomere contains a full A band and one half of 2 I bands on either side of the A band
What structure denotes one sarcomere from the next?
What are A bands?
They are bands located in the middle of the sarcomere that contain the thick (myosin) filaments and also an overlap of thick and thin (actin) filaments which are potential sites of cross-bridge formation
What are I bands?
They are bands located on either side of the A band that contain thin (actin) filaments, intermediate filamentous proteins, and Z discs.
*contain no thick filaments
What is the bare zone of a sarcomere?
The center of each sarcomere in which there is no thin filaments, thus there can be no overlap of thick and thin filaments or cross-bridge formation
What is the M line?
The line in the sarcomere that bisects the bare zone and contains darkly staining proteins that link the central portions of the thick filaments together
What proteins establish the architecture of the myofibrils, ensuring that the thick and thin filaments are aligned correctly and at proper distances with respect to each other
What do transverse cytoskeletal proteins do?
They link thick and thin filaments, forming a "scaffold" for the myofibrils and linking sarcomeres of adjacent myofibrils
What actin-binding protein anchors the entire myofibril arrangement to the cell membrane?
Patients with a defective or absent dystrophin protein have what pathology?
Longitudinal cytoskeletal proteins include what 2 proteins?
titin and nebulin
Describe the structure of titin
It is a large molecular weight protein that extends from the M lines to the Z discs through the thick filaments
Describe the structure of nebulin
This molecule extends from one end of the thin filament to the other serving as a "molecular ruler", setting the length of the thin filaments during their assembly
What anchors the thin filaments to the Z disc?