Sliding Filament Theory Flashcards
What sarcomere regions move
I band
H zone
How does the sliding filament model explain cross bridges
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
How can a continuous force and smooth contraction occur
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
Main ingredients for skeletal muscle contraction, which one is limiting factor
High myoplasmic Ca -limiting factor
Supply of ATP- always abundant
Actin and myosin
Tropomyosin and troponin
Molecular structure of myosin
MHC- myosin heavy chain (binds actin)
MLC- myosin light chain (essential and regulatory)
Explain steps of cross bridge cycle
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
Draw the detailed cross bridge cycle
Pg 6 DRAW IT
What is sarcomere force
The sum of all the xbs that develop force
How many myosin heads per thick filament, 1 myosin head to how many actin molecules
600 myosin heads
1 myosin head for 8 actin molecules
% and force # of myosin heads bound to actin during max contraction
40%
5-150N
What is unfused tetanus
Slight relaxation between contractions
What is fused tetanus
No relaxation during contraction
Tension falls off, can not maintain indefinitely
What are different types of fibres
Type 1- slow oxidative
Type 2B- fast glycolytic
Type 2A- fast oxidative
Name fast twitch fibres
Type 2A (FO) and type 2B (FG)
Powerful bursts
Name slow twitch fibres
Type1 (SO)
Endurance
What is a mixed muscle fibre
Both slow and fast twitch
Ex. Gastrocnemius
How can we have diff speed of fibres
- Different MHC (myosin heavy chains) isoforms
- Leads to differences in ATPase activity and kinetics
- motor neuron decides if motor unit is fast or slow
What is isometric contraction
Constant length
Measure force produced
What is isotonic contraction
Constant load
Measure length
What is passive force
- 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)
What does the active force length curve describe
Active curve = Total tension - passive tension
Described by sliding filament model
Curve drops off since no passive force to keep total force constant
Experiment: measuring actin displacement and cross-bridge force
*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
