16 - Controlling Contractions Flashcards
What are the 2 aspects to delay in axon conduction velocity?
- conduction/latency
- delay in NMJ
Conduction/latency
Time taken for AP to travel from point of stimulation to NMJ
Delay in NMJ
Process by which ACh vesicles fuse w/ membrane, release and bind to post-synaptic receptors
What’s the motor end plate?
Post-synaptic region of NMJ
Why is Ca2+ only briefly available in the intracellular space of muscle cell?
When Ca2+ floods out of SR, it’s simultaneously being actively pumped back into SR by Ca2+-ATPase
What has to be available for cross-bridge cycling to occur?
Ca2+ and ATP
When does cross-bridge cycling stop?
When all Ca2+ pumped back into SR
What does Ca2+ limit?
Duration of cross-bridge cycling
What does ATP limit?
How quickly cross-bridge cycling occurs (more ATP = faster)
Sliding filament theory
Myosin heads marching along actin, pulling Z discs towards centre of sarcomere
Latent period
Before force generation
Contraction period
Force always increasing and Ca2+ binding to troponin, revealing actin binding sites
Relaxation period
All Ca2+ pumped back into SR
What are 2 reasons for latent period?
- takes time for AP to be conducted to muscle
- additional latent period after release of ACh reflects Ca2+ release from SR
What limits maximum force generation?
- takes time for cross-bridge binding to occur
- passive muscle tension
If 2nd stimulation occurs during contraction period…
Force will smoothly summate but if arrives during relaxation period, unfused tetanus occurs
Titin filaments
3rd protein filament integrated into sarcomeres
Somewhat elastic and accounts for passive muscle tension
Optimal length
Dependent on optimal overlap of actin and myosin
Maximum length
No overlap of actin and myosin
Minimum length limited by
Thin filament overlap
overlap of this filaments and Z discs
Isotonic contractions
Muscle changes length while maintaining constant tension
Isometric contractions
Muscle develops tension without changing length
Time-course of force generation in isometric contraction
- force generation begins from time of first cross-bridge attachment
- total duration depends on sequestration of Ca2+ by SR
Time-course of force generation in isotonic contraction
- evidence of shortening delayed until enough cross-bridges have attached to counteract load
- shortening ends once load returns to resting position
Concentric muscle contraction
Bicep muscle shortening while contracting
Eccentric muscle contraction
Bicep muscle lengthening while contracting
Concentric
After power stroke, myosin head progressively binds further along actin, leading to shortening
Eccentric
External force applied to muscle means it lengthens even as power strokes occur
How are types of skeletal muscle fibre defined?
- Max shortening velocity
2. ATP pathway
Max shortening velocity
Type 1 - slow; low ATPase activity
Type 2 - fast; high ATPase activity
What does ATPase activity affect?
Max rate of cross bridge cycling
ATP pathway
A) oxidative - depend on oxygen (numerous mitochondria, good blood supply, lots of myoglobin, red)
B) glycolysis - large glycogen stores(thicker, greater tension development, white)
What are the 3 fibre types?
1a - slow oxidative
2a - fast oxidative
2b - fast glycolytic
T/F muscle fibres in single muscle unit are different types
False - same type
If experimentally change type of AP stimulation, can change type of…
Muscle fibre that it appears to be
Type 1 fibres have motor units that
Have smaller cell bodies and smaller diameter axons
Produce steady, low freq activity
Type II fibres have motor units that
Have larger cell bodies and larger diameter axons
Produce occasional high freq bursts
T/F small units are recruited last
False - recruited first
Forcing slow motor neurons to innervate fast-twitch muscles causes a change in…
Muscle fibre phenotype
What have chronic low freq stimulation experiments shown?
Pattern and type of APs coming in to a fibre determine fibre’s type
