Muscle contraction

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  • Created by: r98
  • Created on: 30-03-16 12:08
What are the three types of muscle in the body?
Cardiac muscle, smooth muscle, skeletal muscle.
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What type of muscle makes up the bulk of body muscle in vertebrates? What's it attached to? What type of control is it under?
Skeletal muscle, it's attached to bone and acts under voluntary, conscious control.
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What are individual muscles made up of?
Millions of tiny muscle fibres called myofibrils.
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What would be the outcome if muscle were made of individual cells joined end to end?
It wouldn't be able to perform its function of muscle contraction efficiently, because the junction between adjacent cells would be a point of weakness, so would reduce the overall strength of the muscle.
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What is the actual structure of muscles?
The seperate cells are fused together into muscle fibres, these share nuclei and cytoplasm, called sarcoplasm.
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Where is sarcoplasm mostly found?
Around the circumference of the fibre.
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What is found within the sarcoplasm?
A large concentration of mitochondria and endoplasmic reticulum.
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What 2 types of protein filament make up myofibrils?
Actin & myosin.
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What's the structure of actin?
It's thinner and consists of two strsnds twisted around one another.
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What's the structure of myosin?
It's thicker and consists of long rod-shaped fibres with bulbous heads that project to the side.
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Why do myofibrils appear striped?
Due to their alternating light-coloured & dark-coloured bands.
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What are the light-coloured bands called?
Isotropic bands (I-bands).
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Why do these regions appear lighter?
Because the actin and myosin filaments do not overlap.
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What are the dark-coloured bands called?
Anisotropic bands (A-bands).
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Why do these regions appear darker?
Because the actin and myosin filaments overlap.
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What & where is the H-zone?
It's the lighter-coloured region at the centre of each anisotropic band.
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What is at the centre of each isotropic band?
A line called the Z-line.
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What is a sarcomere?
The distance between adjacent Z-lines.
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What happens to the sarcomeres when a muscle contracts?
They shorten and the pattern of light and dark bands changes.
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What are the names of two other important proteins found in muscle?
Tropomyosin & troponin.
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What does tropomyosin do?
It forms a fibrous strand around the actin filament.
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What is the globular protein, troponin, involved in?
Muscle contraction.
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What are the two types of muscle fibre?
Slow-twitch fibres and fast-twitch fibres.
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What are the contractions of slow-twitch fibres like?
Slow-twitch fibres contract more slowly and provide less powerful contraction over a longer period.
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What are slow-twitch fibres adapted for?
Endurance work.
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Why & how are slow-twitch fibres adapted for aerobic respiration?
To avoid a build-up of lactic acid. Adaptions: a large store of myoglobin, a supply of glycogen, a rich supply of blood vessels, and numerous mitochondria to produce ATP.
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What is myoglobin and where's it found?
It's a bright red molecule that stores oxygen, found in slow-twitch fibres.
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Why do slow-twitch fibres have a supply of glycogen?
It provides a source of metabolic energy.
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What do the supply of blood vessels deliver to slow-twitch fibres?
Oxygen and glucose.
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What are the contractions of fast-twitch fibres like? What does this make them adapted to?
Rapid and powerful, but only for a short period. They're adapted to intense exercise.
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How are fast-twitch fibres adapted to their role?
They have: thicker and more numerous myosin filaments, a high concentration of enzymes (for anaerobic respiration), & a store of phospjocreatine.
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Why do fast-twitch fibres have a store of phosphocreatine?
Because phosphocreatine can generate ATP from ADP in anaerobic conditions, so providing energy for muscle contraction.
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What is a neuromuscular junction?
The point where a motor neurone meets a skeletal muscle fibre.
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Why are there many neuromuscular junctions along a muscle?
To ensure that contraction of a muscle is rapid and powerful when it's simultaneously stimulated by action potentials.
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What is a motor unit?
When all the muscle fibres supplied by a single motor neurone act together as a single functional unit.
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What is the sequence of events that follows, after a nerve impulse is received at the neuromuscular junction?
The synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine, this diffuses to the postsynaptic membrane, altering its permeability to Na+ ions, which enter rapidly, depolarising the membrane.
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How is it ensured that the muscle is not over-stimulated?
Acetylcholinesterase breaks down acetylcholine.
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What's the name of the process that involves the actin and myosin filaments sliding past one another?
The sliding filament mechanism.
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What chages occur to a sarcomere, when a muscle contracts?
The I-band becomes narrower, The Z-lines move closer together (the sarcomere shortens), and the H-zone becomes narrower.
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What remains the same width in a contracted muscle?
The A-band. As its width is determined by the length of the myosin filaments, it follows that the myosin filaments have not become shorter.
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What are the three main proteins involved in the sliding filament mechanism?
Myosin, actin and tropomyosin.
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What are the two types of protein that make up myosin?
A fibrous protein arranged into a filament (the tail) & a globular protein formed into two bulbous structures at one end (the head).
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What type of protein is actin?
Globular.
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What happens to the action potential when it reaches the postsynaptic membrane?
The action potential travels deep into the fibre through a system of tubules (T-tubules) that branch through the sarcoplasm.
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What are the T-tubules in contact with?
The sarcoplasmic reticulum, which has actively absorbed Ca2+ ions from the cytoplasm of muscle.
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How does the action potential affect the endoplasmic reticulum?
It opens the calcium ion channels on the endoplasmic reticulum and Ca2+ ions flood into the sarcoplasm down a diffusion gradient.
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How do the calcium ions affect tropomyosin?
They cause the tropomyosin molecules, that were blocking the binding sites on the actin filament, to pull away.
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What happens when tropomyosin molecules are no longer blocking the binding sites on the actin filament?
The ADP molecule attached to the myosin heads means they're now in a state to bind to the actin filament and form a cross-bridge.
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What happens to the myosin heads once they're attached to the actin filament?
They change their angle and pull the actin filament along as they do so and release a molecule of ADP.
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What happens to the myosin head after it has moved the actin filament?
A molecule of ATP attaches to it, causing it to become detached from the actin filament.
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What enzyme is activated by the calcium ions? What does this enzyme do? How does this affect myosin?
ATPase, which hydrolyses ATP to ADP (and Pi), this provides enough energy for the myosin head to return to its original position.
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The cycle of the myosin head attaching to the actin filament will be repeated as long as what continues?
Nervous stimulation.
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How does muscle relaxation occur?
When nervous stimulation ceases, Ca2+ ions are actively transported back into the endoplasmic reticulum, using energy from hydrolysis of ATP. Reabsorption of Ca2+ ions allows tropomyosin to block the actin filament again.
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What supplies the energy for muscle contraction?
The hydrolysis of ATP to ADP and inorganic phosphate.
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What is the energy, released from the hydrolysis of ATP, needed for?
The movement of myosin heads & the reabsorption of Ca2+ ions into the endoplasmic reticulum (by active transport).
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How is most ATP regenerated? What is required for this process?
It's regenerated from ADP during respiration of pyruvate in the mitochondria, this requires oxygen.
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What happens to the oxygen supply in a very active muscle? What then is used to generate ATP?
It's used up rapidly and it takes time for the blood supply to replenish it. So phosphocreatine (a chemical) is used to rapidly generate ATP anaerobically.
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What does phosphocreatine act as a reserve of?
It acts as a reserve of phosphate.
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How is the store of phosphocreatine replenished?
Using phosphate from ATP when the muscle is relaxed.
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What type of muscle makes up the bulk of body muscle in vertebrates? What's it attached to? What type of control is it under?

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Skeletal muscle, it's attached to bone and acts under voluntary, conscious control.

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Card 4

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What would be the outcome if muscle were made of individual cells joined end to end?

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Card 5

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