Sliding Filament Theory

Question 1

What sarcomere regions move

Answer 1

I band
H zone

Question 2

How does the sliding filament model explain cross bridges

Answer 2

Filaments within I band slide between the A band filaments as muscle changes length
Cross bridges act as independent force generators (generated in parallel) that produce contractile force
Forces sum for total muscle force

Question 3

How can a continuous force and smooth contraction occur

Answer 3

The cross bridge cycle does not always produce force
There will always be some xb that can generate force
Not all myosin heads bind at once- steric hindrance, would be jerky movement

Question 4

Main ingredients for skeletal muscle contraction, which one is limiting factor

Answer 4

High myoplasmic Ca -limiting factor
Supply of ATP- always abundant
Actin and myosin
Tropomyosin and troponin

Question 5

Molecular structure of myosin

Answer 5

MHC- myosin heavy chain (binds actin)
MLC- myosin light chain (essential and regulatory)

Question 6

Explain steps of cross bridge cycle

Answer 6

ATTACHED STATE (rigor: myosin locked onto actin (need ATP to release))
1- ATP binds to myosin head, causing the dissociation of the actin-myosin complex
RELEASED STATE
2- ATP is hydrolyzed by enzymatic region of myosin, causing myosin heads to return to their resting conformation
COCKED STATE
3- a high force cross bridge forms and the myosin head binds to a new position on actin (Ca available throughout but this is dependent stage)
CROSS BRIDGE STATE
4- P is released, ADP still attached. Myosin heads change conformation, resulting in the power stroke. Filaments slide past each other.
POWER STROKE STATE
5- ADP release

Question 7

Draw the detailed cross bridge cycle

Answer 7

Pg 6 DRAW IT

Question 8

What is sarcomere force

Answer 8

The sum of all the xbs that develop force

Question 9

How many myosin heads per thick filament, 1 myosin head to how many actin molecules

Answer 9

600 myosin heads
1 myosin head for 8 actin molecules

Question 10

% and force # of myosin heads bound to actin during max contraction

Answer 10

40%
5-150N

Question 11

What is unfused tetanus

Answer 11

Slight relaxation between contractions

Question 12

What is fused tetanus

Answer 12

No relaxation during contraction
Tension falls off, can not maintain indefinitely

Question 13

What are different types of fibres

Answer 13

Type 1- slow oxidative
Type 2B- fast glycolytic
Type 2A- fast oxidative

Question 14

Name fast twitch fibres

Answer 14

Type 2A (FO) and type 2B (FG)
Powerful bursts

Question 15

Name slow twitch fibres

Answer 15

Type1 (SO)
Endurance

Question 16

What is a mixed muscle fibre

Answer 16

Both slow and fast twitch
Ex. Gastrocnemius

Question 17

How can we have diff speed of fibres

Answer 17

• Different MHC (myosin heavy chains) isoforms
• Leads to differences in ATPase activity and kinetics
• motor neuron decides if motor unit is fast or slow

Question 18

What is isometric contraction

Answer 18

Constant length
Measure force produced

Question 19

What is isotonic contraction

Answer 19

Constant load
Measure length

Question 20

What is passive force

Answer 20

• force that resists stretch as a muscle contracts, increases as stretch increases
• due to elastic tissue, parallel or series elastic components (ex. Titin)
• muscles get stiffer as they distend (more passive force)

Question 21

What does the active force length curve describe

Answer 21

Active curve = Total tension - passive tension
Described by sliding filament model
Curve drops off since no passive force to keep total force constant

Question 22

Experiment: measuring actin displacement and cross-bridge force

Answer 22

*Isotonic experiment (force held constant with tweezers)
* Measure displacement of polystyrene bead away from centre of trap
- distance moved = 11nm per xb
- Force = 5pN per xb
- actin filaments attached at each end to polystyrene bead
- optical tweezers = finely focused laser beam
- optical tweezers trap the bead at a specific point
- two tweezers used to suspend actin filament above glass
- attached to glass is silicone bead with myosin molecule