Muscle Structure and Function

Question 1

3 types of muscle:

Answer 1

1. Striated (Skeletal, Muscle cells are myofibres or muscle fibres, with many parallel myofibrils in the cytoplasm)
2. Cardiac (Heart Muscle, Differs from skeletal by: branching fibres, central nuclei, intercalated discs)
3. Smooth (Blood Vessels, Intestine, Skin)

Question 2

Organisation is muscle cells:

Answer 2

Myofibre (A cell) –> Myofibrils –> Sarcomeres (Repeat units of Myofibrils)

Sarcomeres:
A Band = Anisotropic (directionally dependent)
I Band = Isotropic (uniformity in all orientations)
Thick Filaments = Myosin
Thin filaments = Actin + troponin + tropomysoin

Hexagonal orientation of filaments.

Question 3

Actin filaments pointed-end is embedded into the Z-band.

TRUE OR FALSE

Answer 3

FALSE

Actin filaments BARBED-end is embedded into the Z-band.

Question 4

Myosin proteins all have:

Answer 4

• A highly conserved head domain
• A motor domain or catalytic domain
• A unique, cargo-binding tail domain
  16 Known classes of myosin motor proteins are known.
  Class II sarcomeric is involved in skeletal muscle contraction.

Question 5

Sarcomeric Myosin II

Answer 5

Head Domain: Important for binding to thin filaments. (Motor)

Hinge Domain: Important for movement.

Tail Domain: Tail forms an alpha-helix, and then a coiled-coil with another myosin monomer (i.e. tail is a coiled coil of 2 alpha helices). Coiled tail domain is required for arrangement in the thick filament, and specifies function. Different length tails means different length displacement on power stroke.

Note: Muscle contraction involves the shortening of the I-band but not the A-band.

Question 6

Myosin S1:

Answer 6

The S1 fragment myosin head.
it consists of the:
- Globular motor domain (containing the ATP- and actin-binding site)
- Lever arm (which amplifies small conformational changes in the motor domain into larger motions by which myosin moves actin).

Question 7

Which Myosin is used as a model for studying Myosin function?

Answer 7

Myosin V:

• Found in many tissues; abundant in brain and nervous system
• Involved in vesicle transport
• Head domain of myosin V is about twice as long as that of muscle myosin
• Myosin V heads move processively along actin filaments

Question 8

Muscle Force Generation Requires Which Element? How does this element go on to generate muscle force?

Answer 8

Ca2+

• Calcium enters rapidly through T tubules and sarcoplasmic reticulum
• Troponin changes its conformation, allowing tropomyosin to shift its position on actin filament when calcium is present, permitting the interaction with myosin.

Question 9

How can the position of myofilaments be determined?

Answer 9

1. The Z-Disc/Z-Line:
   - Actin filaments attach to the Z-disc with CapZ and alpha-actinin accessory proteins.
   - Tropomodulin and CapZ are thin filament capping proteins in muscle.
2. Molecular Rulers:
   - Titin and Nebulin regulate the length and spacing of thick and thin filaments.
   - Titin is the largest known protein, a single molecule extends from the Z-Disc to the M-Band
   - Nebulin is coincident with the location of the troponin -tropomyosin complexes associated with the thin filaments.