Muscle contraction

chapter 11 AQA Biology

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Types of muscle

Cardiac muscle : found exclusivley round the heart

Smooth muscle : found in the walls of blood vessels and the gut

Skeletal muscle : makes up bulk of body muscle; attached to bone.

Individual muscles are made up of millions of tiny muscle fibres called myofibrils.

To overcome weakness between cells, mucles cells have become fused together into fibres which share a nuclei and cytoplasm, called sarcoplasm. Within the sarcoplasm is a large number of mitochondria and endoplasmic reticulum.

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Structure of Skeletal Muscle

Each muscle fibre is made up of myofibrils, which are made up of two types of protein filament:

  • actin ; which is thinner and consists of two strands twisted around one another.
  • myosin ; which is thicker and consists of long rod-shaped fibres with bulbous heads that project to the side.

myosin filaments appear striped due to their alternating light-coloured and dark-coloured bands. The light bands are called isotropic bands ( I-bands ). These appear lighter because the actin and myosin filaments do not overlap in this region. The dark bands are called anisotropic bands ( A-bands ). these appear darker because actin and myosin filaments overlap in this region.

At the center of each anisotropic band is a lighter coloured region called the H-zone. At the center of each isotropic band is a line called the Z-line. The distance between adjacent Z-lines is called a sarcomere. When a muscle contracts, these sarcomeres shorten and the pattern of light and dark bands changes.

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Types of Muscle Fibre

  • slow-twitch fibres : these contract more slowly and provide less powerful contractions over a longer period of time. They are adapted to endurance work. In humans they are more common in the calf muscle ( needed to remain contracted to keep us upright ). Adaptions such as a large store of myoglobin (stores oxygen), a supply of glycogen (source of metabolic energy), a rich supply of blood vessels & numerous mitochondria make slow twitch fibres well adapted for aerobic respiration.
  • Fast-twitch fibres : these contract more rapidly and produce powerful contractions but only for a short period. Theyre adapted to intense exercise such as weight lifting. As a result they are more common in muscles like the upper arm. Thicker, more numerous myosin filaments, high concentrations of enzymes involves in anaerobic respiration and a store of phosphocreatine which can produce ATP from ADP in anaerobic conditions make these fibres adapted to their role.
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Neuromuscular junctions

This is the point where a motor neurone meets a skeletal muscle fibre. There are many such junctions along the muscle as otherwise, it would take time for a wave of contraction to travel across the muscle, and the movement would be slow. All muscle fibres supplied by a single motor neurone act together as a single functional unit and are known as a motor unit. 

When a nerve impulse is recieved at the neuromuscular junction, the synaptic vesicles fuse with the presynaptic membrane and release their acetycholine. The acetylcholine diffuses to the postsynaptic membrane, altering its permeability to Na+ ions, which enter rapidly depolarising the membrane. 

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The sliding filament mechanism

When a muscle contracts, the following changes occur to a sarcomere:

  • the I-Band becomes narrower
  • The Z-lines move closer together or in other words, the sarcomere shortens
  • the H-Zone becomes narrower.

The A-band remains the same width. As the A band is determined by the length of the myosin filaments, it follows that the myosin filaments have not become shorter. This discounts the theory that muscle contraction is due to the filaments themselves shortening.

The sliding filament mechanism is the hypothesis that the acin and myosin filaments slide over one another and is supported by the changes seen in the band pattern on myofibrils. 

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The sliding filament mechanism part II

The bulbous heads of the myosin filaments form cross-bridges with the actin filaments. They do thi by attaching themselves to the binding sites on the actin filaments and then flexing in unison, pulling the actin filaments along the myosin filaments. They then become detached and using ATP as a source of energy return to their original angle and reattach further along the actin filaments.This process is repeated up to 100 times per second. 

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Muscle Stimulation

  • An action potential reaches many neuromuscular junctions simultaneously causing calcium ion channels to open and calcium to move into the synaptic knob
  • The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release their acetylchlorine into the synaptic cleft.
  • Acetylchlorine diffuses across the synaptic cleft and binds with receptors on the postsynaptic membrane, causing it to depolarise.
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Muscle Contraction

  • The action potential travels deep into the fibre through a system of tubulesthat branch throughout the cytoplasm of the muscle (sarcoplasm)
  • The tubules are in contact with the endoplasmic reticulum of the muscle which has actively absorbed calcium ions from the cytoplasm of the muscle
  • the action potential opens up the calcium ion channels on the endoplasmic reticulum and calcium ions flood into the muscle down a diffusion gradient
  • the calcium ions cause the tropomyosin molecules that were blocking the binging sites on the actin filament to pull away 
  • The ADP molecule attached to the myosin heads means they are now in a state to bind to the actin filament and form a cross-bridge
  • Once attached to the actin filament the myosin heads change their angle pulling the actin filament along as they do so and releasing a molecule of ADP
  • An ATP molecule attaches to each myosin head causing it to become detached from the actin filament
  • The calcium ions then activate the enzyme ATPase which hydrolyses the ATP to ADP. The hydrolysis of ATP to ADP provides the energy for the myosin head to return to its original position.
  • The myosin head, once more attached with an ADP molecule reattaches itself further along to actin filament and the process is repeated.
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Muscle Relaxation

  • When nervous stimulation ceases, calcium ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP
  • This reabsorption of the calcium ions allows tropomyosin to block the actin filament again
  • Myosin heads are now unable to bind to actin filaments and contraction ceases.
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Energy supply during muscle contraction

Energy is supplied from the hydrolysis of ATP to ADP & inorganic phosphate. The energy released is needed for 

    • The movement of the myosin heads
    • the reabsorption of calcium ions into the endoplasmic reticulum via active transport

Phosphocreatin is stored in muscle and kept as a reserve supply of phosphate which is available immediately to combine with ADP and form ATP when needed.  

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