Skeletal Muscle Force Generation, Energy Usage & Fibre Type Flashcards
What are sarcomeres ?
Highly organised, functional units of skeletal and cardiac muscle.
They have a striated appearance.
State the contractile proteins
Actin (thin filaments)
Myosin (thick filaments)
Z disc
Defines the boundary of each sarcomere - thousands of sarcomeres can make up a single muscle myofibre
I band
Mainly actin filaments
A band
Mainly myosin and overlapping actin filaments
Describe sarcomere shortening
A band remains constant
H zone and I band both shorten
State the role of ATP in muscle contraction
Membrane potential
Ca2+ gradient
Power stroke
Cross bridge dissociation
State the role of ATP in Membrane potential
Sodium/Potassium ATPase in sacrolemma maintains Na+ and K+ gradients, allowing production and propagation of action potentials.
State the role of ATP in Ca2+ gradient
Active transport of calcium ions into the sarcoplasmic reticulum - lowering [Ca2+]i
State the role of ATP in Power Stroke
Hydrolysis of ATP by myosin - ATPase energises the cross-bridge formation, enabling sarcomere shortening and contraction.
State the role of ATP in Cross bridge dissociation
Binding of ATP to myosin dissociates cross-bridges bound to actin.
How much ATP do muscles contain ?
4mM of ATP
which is enough for 2 seconds of contraction
State an energy source available for contraction
Stores of Creatine Phosphate in muscle provide enough energy for around 8 seconds of contraction.
Myosin + ATPase
Contraction
Ca2+ ATPase
Relaxation
Describe events during cross-bridge formation to generate sarcomere shortening
6 STEPS
- ATP binding
- ATP hydrolysis
- Cross-bridge formation
- Release of Pi from myosin
- Power stroke
- ADP release
How is cross bridge formation regulated ?
Regulation of cross-bridge formation is due to availability of myosin binding sites on actin, via [Ca2+]i and tropomyosin.
What is rigor mortis ?
Muscular stiffness that occurs after death - post mortem rigidity.
When does rigor mortis occur ?
Can begin around 4 hours after death, peaks at about 13 hours and lasts around 50 hours.
Describe how rigor mortis happens
Death
Loss of muscle cell integrity
Ca2+ leaks into the cytosol from the SR
Ca2+ binds to tropomyosin
Myosin binds to actin
Metabolism and ATP production ceases
No ATP present to break cross-bridge
Muscles become stiff
Proteolytic enzymes work within a few days
State the main different types of skeletal muscle fibre
Type IIB/IIX - Fast
Type IIA - Intermediate
Type I - Slow
State some properties of Type IIB/IIX skeletal muscle fibre
Fast
Glycolytic Anaerobic
Low in myoglobin oxygen
State some properties of Type IIA skeletal muscle fibre
Intermediate
Mixture of fast oxidative and glycolytic
State some properties of Type I skeletal muscle fibre
Slow
Oxidative Aerobic
High in Myoglobin oxygen
How can skeletal muscle fibres be classified ?
Based on mechanical and metabolic characteristics :
- Maximal velocities of contraction (fast/slow)
- Major pathway for generation of ATP (oxidative or glycolytic)
State the different physiological properties of skeletal muscle fibre types
Number of mitochondria
Amount of myoglobin
Blood vessels/capillaries
Stores of glycogen / glycolytic enzymes/ creatine phosphate
Size
Which physiological properties are greater in slow oxidative fibres ?
Number of mitochondria
Amount of myoglobin
Blood vessels/capillaries
Which physiological properties are greater in fast glycolytic fibres ?
Stores of glycogen / glycolytic enzymes/ creatine phosphate
Size
Describe Slow oxidative fibres (Type I)
Combine slow myosin-ATPase activity with high oxidative capacity
Describe Fast oxidative-glycolytic fibres (Type IIa)
Combine fast myosin-ATPase activity with high oxidative capacity and intermediate glycolytic capacity.
Describe Fast glycolytic fibres (Type IIb or Iix in human)
Combines fast myosin-ATPase activity with high glycolytic capacity
Where do skeletal muscles receive stimuli from ?
From a motor neurone
Each myofibre/muscle cell has its own neuromuscular junction
What does duration of contraction depend on ?
Fibre type composition of individual muscles
Ocular muscle
Extremely rapid contraction velocity
Gastrocnemius muscle
Moderately rapid contraction velocity
Soleus muscle
Relatively slow contraction velocity
How does force change ?
Force changes depending on recruitment of muscle fibres.
What is a motor unit ?
All fibres innervated by a single neurone are called a motor unit.
A single motor neurone innervates multiple muscle fibres.
Describe the muscle fibres of small muscles
Small muscles with fine control have fewer muscle fibres per neurone.
e.g. laryngeal muscles
Describe the muscle fibres of large muscles
Large muscles may have hundreds of fibres in a motor unit.
e.g. soleus
What does the force of contraction depend on ?
Number of motor neurons recruited
Frequency of action potentials
How are motor units recruited ?
In a progressive way from smallest (weakest) to large (strongest)
Describe small motor units
More excitable
Conduct action potentials more slowly
Typically Type I (slow) fibres
Describe large motor units
Less excitable
Conduct action potentials more rapidly
Typically Type II (fast) fibres
What is muscle tension ?
The force exerted by a contracting muscle
What is load ?
The force exerted by an object to be moved
What must muscle do in order to shorten ?
Overcome the force exerted by an object to be moved (the load).
Greater force required –> More motor units recruited
What is summation ?
As muscle twitch far exceeds duration of AP, it is possible to initiate a second AP before 1st contraction has subsided.
The 2nd twitch is stronger than the 1st due to higher [Ca2+]i
What is frequency summation ?
Multiple action potentials occurring close together
What is tetanus ?
Stimulation frequency is so high that individual contractions fuse.
What is isotonic contraction ?
Muscle shortening will occur of peak tension is greater than the load force.
What is isometric contraction ?
Muscle stimulation will increase tension, but no shortening will occur if load force is greater than muscle peak force.
Define the length tension relationship
Length tension is directly related to the overlap between actin and myosin within the sarcomere.
When is tension high ?
When actin and myosin overlap within the sarcomere
Zero tension
No actin/myosin overlap
Describe the load-velocity relationship
When loads are applied, contraction velocity decreases with increasing load.
Describe the load-velocity relationship of muscle contraction
When the load equals the maximum force or tension that a muscle can exert, velocity of contraction is zero (isometric contraction)
Describe full muscle contraction with no load
Rapid
around 100ms
