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From what are muscles composed?
Hundreds of muscle fibres
From what are muscle fibres composed?
From what are microfibrils composed?
Repeating structures known as sarcomeres
Which two proteins are contained within sarcomeres?
- Actin (thin filaments)
- Myosin (thick filaments)
What is the I band?
- The light band observed when a muscle is micrographed
- Composed of actin only
What is the H zone?
- The dark band observed when a muscle is micrographed
- Composed of myosin only
What is the darkest band that can be observed when micrographing muscle fibres?
The region composed of overlapping myosin and actin filaments
What is stated in the sliding filament theory?
Muscle contraction is caused by the shortening of sarcomeres as a result of the actin filaments sliding over myosin filaments
What other changes take place in the sarcomere as a result of contraction?
- The H zone shortens
- The I band shortens
- The A band remains the same
Describe step 1 of muscle contraction
An extension if the myosin filament known as the myosin head bunds to the actin filament at a myosin binding site
What is formed between the myosin head and the myosin binding site?
An actinomyosin cross bridge
Describe step 2 of muscle contraction
The myosin head rotates and propels the actin filament between the myosin filaments
What is the name given to the rotation of the myosin head?
The power stroke
Describe step 3 of muscle contraction
- A molecule of ATP binds to the myosin head allowing it to detach from the actin filament
- The myosin head then returns to its original position
What is the name given to the movement of the myosin head back to its original position?
The recovery stroke
Describe step 4 of muscle contraction
The ATP molecule is hydrolysed to form ADP and Pi by an ATPase enzyme on the myosin head
Describe step 5 of muscle contraction
The myosin head then binds to another myosin binding site further along the actin filament and the process is repeated
Describe stage 1 of muscle contraction control
A motor neurone meets a muscle fibre at a neuromuscular junction
Describe stage 2 of muscle contraction control
- The action potential crosses the synapse identically to a cholinergic one
- The one difference is that the post synaptic membrane is the sarcolemma
What is the sarcolemma?
The membrane surrounding the muscle fibre
Describe stage 3 of muscle contraction control
The depolarisation of the sarcolemma causes calcium ions to flood out of internal stores
Describe stage 4 of muscle contraction control
Calcium ions bind to a protein called tropomyosin
Describe stage 5 of muscle contraction control
- The binding of tropomyosin and calcium ions causes it to move away from the myosin binding sites on the actin filament
- This allows myosin heads to bind to their binding sites and the sliding filament process to begin
Describe stage 6 of muscle contraction control
- The calcium ions also activate the ATPase enzyme on the myosin head
- This allows the ATP to by hydrolysed into ADP and Pi
- This hydrolysis releases the energy required for the power stroke to take place
From where do muscle fibres derive their energy?
Through the hydrolysis of ATP to ADP and Pi
What is the energy supplied used for?
- The movement of the myosin heads
- The active transport of calcium ions back into their stores
From where is energy sourced for immediate muscle contraction?
- The small amounts of ATP already present in muscles
- After this is used up more must be synthesised during respiration
What may cause muscles to begin respiring anaerobically?
Oxygen supply cannot keep up with demand during strenuous exercise
How do muscles synthesise ATP in anaerobic conditions
A molecule of phosphocreatine stored within the muscle joins the ADP and Pi without the need for respiration
How is the phosphocreatine store replenished
Using the phosphate from ATP once aerobic conditions return
What is the function of fast twitch muscle fibres?
To contract rapidly and powerfully for short periods of intense activity
What is the function of slow twitch muscle fibres?
To contract less powerfully over longer periods of time used to maintain posture and endurance
What is the appearance of fast twitch muscle fibres?
Thick due to numerous myosin filaments
What is the appearance of slow twitch muscle fibres?
Deep red due to rich blood supply and stores of the pigment myoglobin which stores oxygen
Where are fast twitch muscle fibres located?
In the major muscles used in strenuous exercise such as biceps
Where are slow twitch muscle fibres located?
In the calf muscles and those associated with everyday activity
What is the biochemical makeup of fast twitch muscle fibres?
- High concentration of enzymes involved in anaerobic respiration
- Phosphocreatine is also present
What is the biochemical makeup of slow twitch muscle fibres?
Numerous mitochondria and rich blood supply for aerobic respiration
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