Force Generation, Energy Usage and Fibre Types Flashcards
Give a brief review of sarcomere structure
- Sarcomeres are highly organised within skeletal & cardiac muscle and give a striped appearance – striated muscle
- A sarcomere is the portion of a myofibril that lies between two successive Z disks
What contractile proteins are found in sarcomeres?
- Thin filaments (actin protein)
* Thick filaments (myosin protein)
What bands represent what contractile proteins in sarcomeres?
- I band (isotropic) represents actin (thin) filaments
* A band (anisotropic) represents myosin (thick) filaments
Provide a brief review of cross bridge cycling?
• Ca2+ modulates contraction through regulatory proteins rather than direct interaction with contractile proteins
• Without the presence of Ca2+ these regulatory proteins act to inhibit actin-myosin interactions
• Troponin C is key Ca2+ sensitive regulator
– 2 high affinity binding sites (help in binding troponin C to thin filament)
– 2 low affinity binding sites (binding of Ca2+ to these sites brings about conformational change in troponin complex)
• When [Ca2+]i rises and Ca2+ binds to TnC a conformational change of proteins occurs
• Troponin I moves away from actin filament
• Troponin T pushes tropomyosin away from actin binding site
• As long as Ca2+ is present, multiple cross-bridges cycles can occur and muscle shortens
• When [Ca2+]i falls, Ca2+ dissociates from TnC leading to a reversal of the conformational change
What are the roles of ATP in skeletal muscle contractions?
- Hydrolysis of ATP by the Na+/K+-ATPase in sarcolemma maintains Na+ and K+ gradients
- Hydrolysis of ATP by the Ca2+-ATPase in the sarcoplasmic reticulum provides the energy for the active transport of calcium ions into the reticulum
- Hydrolysis of ATP by myosin-ATPase energises the cross-bridge formation providing energy of force generation
- Binding of ATP to myosin dissociates cross-bridges bound to actin
What does the hydrolysis of ATP by the Na+/K+-ATPase in sarcolemma maintaining Na+ and K+ gradients allow?
production and propagation of action potential
What does hydrolysis of ATP by the Ca2+-ATPase in the sarcoplasmic reticulum providing the energy for the active transport of calcium ions into the reticulum, so lowering Ca2+ allow for?
Allows for relaxation to occur as calcium ions are removed from the sarcomere and sequestered in the sarcoplasmic reticulum
*
What does the binding of ATP to myosin dissociating the cross-bridges bound to actin allow for?
Allows for bridges to repeat cycle activity
What are the 5 stages of the cross-bridge cycle in skeletal muscle?
- Step 1 – ATP binding
- Step 2 – ATP hydrolysis
- Step 3 – Cross-bridge formation
- Step 4 – Release of Pi from myosin
- Step 5 – ADP release
What is the initial position of the myosin head in the cross-bridge cycle of skeletal muscle?
Initially myosin head attached to actin filament after “power stroke” from previous cycle – can remain in this state for an indefinitely long period, as occurs in rigor mortis
Describe the relationship between ADP-free myosin complex and ATP in the regulation of cross bridge cycle in skeletal muscle
- ADP-free myosin complex (attached state) would quickly bind ATP at the concentrations found in cells
- Muscle cells do not regulate cross- bridge formation by regulating [ATP]i
Describe the regulation of cross bridge cycle in skeletal muscle
- Muscle cells control cycle by regulating step 3 i.e. preventing cross-bridge cycling until tropomyosin moves out of way as a result of es in [Ca2+]I
- muscle cells need to resynthesize ATP from ADP
Describe the skeletal muscle energy metabolism (glycolysis and oxidative phosphorylation)
- Gives very rapid ATP formation at the onset of muscle contraction
- Oxidative phosphorylation supplies most ATP in “moderate” levels of activity (occurs in mitochondria)
- At higher intensity exercise, glycolysis dominates (occurs in cytosol)
Describe the different sources of ATP in “moderate” exercise over time
- From muscle glycogen (5-40min exercise)
- Blood glucose, and fatty acids (next 30 mins of exercise)
- Then fatty acids predominate
Expand on the gross action of glycolysis at higher intensity exercise where glycolysis dominates (occurs in cytosol)
- Small quantities of ATP are produced BUT at a higher rate
2. The source is either glucose or glycogen
How is muscle force determined in skeletal muscle?
• Muscle force can be determined by number of individual muscle fibres stimulated at a given time
Describe the process of muscle force generation
- Multiple-fibre summation (or spatial summation)
* Small motor neuron cell bodies will be recruited first then larger and larger cell bodies (motor units)
What 5 things does the amount of force generated in whole muscle force generation in skeletal muscle depend on?
– Number of active muscle fibres – Cross-sectional area of muscle – Initial resting length of muscle – Rate at which muscle shortens – Frequency of stimulation
In what order are motor units recruited in size principle in skeletal muscle?
• Motor units recruited in a progressive order from smallest unit (weakest) to largest unit (strongest)
Compared to large motor units, small motor units are:
– more excitable
– conduct APs more slowly
– excite fewer fibres that tend to be
Type I
Compared to small motor units, large motor units are:
– less excitable
– conduct APs more rapidly
– excite many fibres that tend to be
Type II
Do skeletal muscle fibres all have the same mechanical and metabolic characteristics?
Naw
How are skeletal muscle fibres classified?
They are classified based on:
– their maximal velocities of shortening (fast or slow)
– the major pathway they use to form ATP - oxidative or glycolytic
How do fast and slow skeletal muscle fibres differ?
• Fast and slow fibres contain forms of myosin that differ in the maximal rates at which they use ATP
