SKELETAL MUSCLE: SLIDING FILAMENT THEORY

Question 1

Describe the sliding filament theory

Answer 1

Sliding filament theory: filaments within I-band slide between A-filaments as the muscle changes length. cross-bridges that attach between the actin and myosin filaments act as independent force generators that produce a contractile force that act to pull the ends of the sarcomere (Z-lines) together

• Within each sarcomere there are many cross-bridges that act in parallel to each other, so that their forces sum
• Note that the cross-bridges cycle, and they do not produce force for most of their cycle. However, with many crossbridges available within the sarcomere, there will always be some cross-bridges that can generate force, so a continuous force and smooth contraction can occur.

Question 2

Describe the main ingredients for skeletal muscle contraction (4)

Answer 2

High myoplasmic [Ca2+]
Supply of ATP
Actin and myosin
Tropomyosin and troponin

Question 3

Describe the cross-bridge cycle

Answer 3

refer to diagram from class.

Question 4

True or false: all cross-bridges undergo the power stroke phase simultaneously

Answer 4

false; only a fraction of cross-bridges will undergo a power stroke at any given time

Question 5

At what point does the cross-bridge cycle stop in relaxed living muscle?

Answer 5

between the dissociation of phosphate + partial phosphate hydrolysis and the binding stage (refer to diagram)

Question 6

At what point does the cross-bridge cycle stop in rigor mortis? Why?

Answer 6

between the release of ADP + Pi and the ATP binding phase (because there is absence of ATP)

Question 7

Define: temporal summation

Answer 7

effects of impulses received at the same place can add up if the impulses are received in close temporal succession

Question 8

Define: unfused tetanus

Answer 8

when the muscle fibers do not completely relax before the next stimulus because they are being stimulated at a fast rate

Question 9

Define: fused tetanus

Answer 9

there’s no relaxation period between muscle contractions. Your muscle contractions completely fuse to create one continuous muscle contraction

Question 10

What two chains makeup the molecular structure of myosin? What are each of their purposes

Answer 10

light chains: essential and regulatory
heavy chains: binds actin

Question 11

List the different MHC isoforms (4); describe them briefly

Answer 11

MHC-I: slow
MHC-IIa: fast oxidative
MHC-IIb: fast fatigueable
MHC-IIx: mixed

Question 12

Differentiate between fast twitch and slow twitch muscles via the following properties

• nerve properties:
  
  • cell diameter
  • conduction velocity
  • excitability
• muscle cell properties:
  
  • number of fibers
  • fiber diameter
  • force unit
  • metabolic profile
  • contraction velocity
  • fatiguability

Answer 12

fast twitch (IIa and IIb)
- nerve properties:
- cell diameter: large
- conduction velocity: very fast
- excitability: low
- muscle cell properties
- number of fibers: many
- fiber diameter: large
- force unit: high
- metabolic profile: fast glycolytic (IIb) and fast oxidative (IIa)
- contraction velocity: fast
- fatiguability: high

Slow twitch (I): endurance
- nerve properties:
- cell diameter: small
- conduction velocity: fast
- excitability: high
- muscle cell properties
- number of fibers: few
- fiber diameter: moderate
- force unit: low
- metabolic profile: oxidative
- contraction velocity: moderate
- fatiguability: low

Question 13

Differentiate between isometric and isotonic contractions

Answer 13

Isometric contractions: muscle contraction where a constant length is maintained.

Isotonic contractions: muscle contraction where the active muscle is shortened

Question 14

How can researchers measure cross-bridge force

Answer 14

• Polystyrene beads are attached to each end of an actin filament
• Optical tweezers (finely focused laser beam), traps the bead at a specific point and physically moves it
• Two tweezers are used to suspend the actin filament above a cover-glass
• Attached to the cover-glass is a silicone bead with myosin molecules attached
• Isotonic experiments can be conducted where tension/force is held constant with the tweezers. Then researchers measuredisplacement of polystyrene bead away from the center of the trap

Question 15

Define: size principle

Answer 15

Size principle: under all known conditions of muscle use, motor neurons are always recruited from small to large which allows for precision, energy efficiency, and minimizes fatigue.
- small excitations fire small motoneurons
- Larger excitations cause small motor neurons to increase firing rate + larger motor neurons start to fire