Muscle 2 Flashcards
What is the typical duration of a muscle action potential?
Around 2ms
How long does a muscle twitch last?
20-100ms
Why does a muscle twitch have the start delayed and what does this allow?
start delayed due to neuronal and muscle action potential – allows force to be maintained without having to fire action potentials continuously
What is and isn’t muscle contraction dependent on?
- Muscle contraction isn’t directly dependent on electrical activity of muscle membrane
- Muscle contraction is dependent on calcium – this takes longer than an action potential
If a second stimulus arrives before the muscle is relaxed what do we see?
If a second stimulus arrives before the muscle is relaxed we see summation and unfused tetanus with increased rate. The calcium concentrations will reach much higher levels
What is the force produced by muscle proportional to?
Calcium concentration
What do higher rates of AP lead to?
fused tetanus (not the disease – here it is the state at which a muscle is maximally contracted)
What is the smallest contractile unit and what is this?
• Smallest contractile unit of muscle is a motor unit
- Set of muscle fibres that are controlled by a single motor neurone
- The bigger the motor neurone cell body size the more fibres it will innovate
What is Henneman’s size principle?
- As a muscle is stimulated to contract by a neurone in the CNS motor units will become recruited in order of size
- Smaller neurones fired first – more likely to fire an action potentials at low stimuli than larger neurones
- As stimulus size is increased we recruit the larger motor neurones increasing contraction force
- By recruiting different motor neurones with different size motor units we can regulate the size of contraction and produce very small and very large contractions of a muscle
What are the different skeletal muscle types?
- Slow-twitch oxidative
- Fast-twitch glycolytic
- Fast-twitch oxidative
Give features of slow fibres (type I)
- Used for posture maintenance etc.
- Have myoglobin (red) as oxygen store
- Many mitochondria
Give features of Fast Fibres (Type IIa, IIb)
- Both have fast myosin isoform
- Fast Ca transient (high SERCA pump)
- Allows rapid shortening but at high energy cost as ATP is hydrolysed quickly
Give features of type IIa oxidative fibres
- Lots of mitochondria
- Pretty good blood supply
- Good glycogen stores
- Resist fatigue
Give features of type IIb glycolytic fibres
Lactate accumulation and acidosis can limit contraction
Give features of muscle fibre differentiation
- Depends heavily on neuronal input
- Plastic somewhat – can use training to influence distribution of type II fibres
- Differences in ATPase activity are because the different fibre types have different levels of myosin
Look at the table in notes to find the properties of different fibre types
Look at the table in notes to find the properties of different fibre types
What is Duchenne Muscular Dystrophy
- Duchenne muscular dystrophy is an X-linked disorder caused by a mutation in the dystrophin gene and it affects about 1:3500 male births
- The mutations cause skeletal muscle fibres to not be linked properly to the extracellular matrix
- Excess calcium enters the cells and the muscle fibres die and are replaced by fat or connective tissue.
- Patients with Duchenne experience progressive muscle weakness and have an average life expectancy of 25 to 30 years
- There is no treatment for Duchenne muscular dystrophy but it may be a candidate in the future for gene therapy