Skeletal muscle

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  • Created on: 05-04-15 12:12


-Muscles are made up of millions of tiny fibres called myofibrils

-These fibres are grouped into progressively larger bundles for added strength

-The muscle cells have also fused together, so that they now share a cytoplasm (sarcoplasm) and a nucleus, again for added strength

-There are two main proteins in myofibrils, actin and myosin

-Actin is thinner

-Myosin is thicker with a bulbous heads

-Other important proteins include tropomyosin (blocks the binding sites on the actin filament) and troponin (wraps around the actin filament)

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Light and dark bands

-Myofibrils have alternating light and dark bands

Light bands:

  • I-bands
  • Centre of each I-band is called the Z-line
  • It appears light because here myosin and actin filaments do not overlap

Dark bands:

  • A-bands
  • Centre of each A-band is called the H-zone
  • It appears dark because here myosin and actin filaments overlap
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Slow twitch and fast twitch fibres

-There are two types of muscle fibre, slow twitch and fast twitch, each has different adaptations

Slow twitch:

  • Adapted to aeorbic respiration
  • Has plentiful mitochondria 
  • Rich supply of blood vessels
  • Store of myoglobin
  • Supply of glycogen 

Fast twitch:

  • Adapted to short bursts of anerobic respiration
  • High concentration of enzymes used in anaerobic respiration
  • Store of phosphocreatine
  • More myosin filaments 
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Muscle stimulation

-Neuromuscular junctions work exactly like synapses

1. An action potential reaches a neuromuscular junction

2. The action potential travels through the sarcoplasm via T-tubules

3. The action potential then opens the calcium ion channels on the sarcoplasmic reticulum, causing calcium ions to flood into the sarcoplasm

4. The calcium ions causes tropomyosin to move away from the binding sites on the actin filament

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Sliding filament theory

1. Once the tropomyosin has moved away from the binding sites on the actin filament and ADP molecule attaches to the myosin head, causing it to get into the cocked position

2. The myosin head attaches to the actin filament

3. The myosin heads then change their angle, pulling the actin filament along

4. An ATP molecule then attaches to the myosin head, this causes the myosin head to detach from the binding sites on the actin

5. ATPase then hydrolyses ATP to ADP and inorganic phosphate

This process is a cycle and so can continue if there action potentials keep being received

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Muscle relaxation

-Muscle relaxation is simple, and occurs when nervous stimulation ceases

-The calcium ions are moved, by active transport back into the sarcoplasmic reticulum 

-Tropomyosin then moves to block the binding sites on the actin filament again

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