Muscle structure and filaments

Question 1

Each muscle fiber is multinucleate and behaves as a single unit. It contains what?

Answer 1

Bundles of myofibrils, surrounded by SR and invaginated by transverse tubules (T tubules)

Question 2

Each myofibril contains

Answer 2

Interdigitating thick and thin filaments arranged longitudinally in sarcomeres

Question 3

A sarcomere runs from ____ to ____

Answer 3

Z line to Z line

• Memory aid:*
• Zarcomerez

Question 4

Thick filament

1. Represented by what in microscopy?
2. They contain what?

Answer 4

1. A band in the center of the sarcomere
2. Myosin

• Memory aid:*
• A! Myosin is thick!

Question 5

Myosin

1. Biochem component
2. Structural component

Answer 5

1. Six polypeptide chains, including one pair of heavy chains and two pairs of light chains
2. Two “heads” attached to a single “tail”

Question 6

Action of myosin heads

Answer 6

Bind ATP and actin, and are involved in cross-bridge formation

Question 7

Thin filaments

1. Represented by what in microscopy
2. Components

Answer 7

1. present in the I bands and interdigitate with the thick filaments in a portion of the A band
2. Contain actin, tropomyosin, and troponin

Question 8

Regulatory protein that permits cross-bridge formation when it binds Ca²⁺

Answer 8

Troponin

Question 9

Troponin is a complex of three globular proteins. These are:

Answer 9

• Troponin T (“T” for tropomyosin)
  
  • attaches the troponin complex to tropomyosin
• Troponin I (“I” for inhibition)
  
  • inhibits the interaction of actin and myosin
• Troponin C (“C” for Ca²⁺)
  
  • the Ca²⁺-binding protein that, when bound to Ca²⁺, permits the interaction of actin and myosin

Question 10

T tubules

1. Function
2. Where are they located?
3. Contains what?

Answer 10

1. Extensive tubular network, open to the extracellular space, that carry the depolarization from the sarcolemmal membrane to the cell interior
2. Junctions of A bands and I bands
3. Voltage-sensitive protein called the dihydropyridine receptor; depolarization causes a conformational change in the dihydropyridine receptor

Question 11

Internal tubular structure that is the site of Ca2+ storage and release for excitation–contraction coupling

Answer 11

Sarcoplasmic reticulum

Question 12

Parts of the SR

Answer 12

• Terminal cisternae
  
  • Make intimate contact with the T tubules in a triad arrangement
• Ca²⁺-ATPase (Ca²⁺ pump)
  
  • Transports Ca²⁺ from intracellular fluid into the SR interior, keeping intracellular [Ca²⁺] low
• Calsequestrin
  
  • Calcium-binding protein of the SR; helps hold calcium in the cisterna of the SR
• Ryanodine receptor
  
  • Responsible for the release of Ca²⁺ from sarco/endoplasmic reticulum