Muscle Contraction

Contents:

  • Microscopic structure of skeletal muscle
  • Types of muscle fibre
  • Neuromuscular junctions
  • Contraction of skeletal muscle 
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  • Created by: Emilie
  • Created on: 17-06-15 15:26
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Muscle Contraction
Structure of skeletal muscle
There are three types of muscle in the body:
Cardiac muscle which is found exclusively in the heart
Smooth muscle
which is found in the walls of blood vessels and the gut
Skeletal muscleis attached to bone and acts under voluntary, conscious control.
Individual muscles are made up of millions of tiny muscle fibres known as myofibrils which are
bundled together to form progressively larger units.
If muscle were made of individual cells joined end to end it would not be able to perform the
function of contraction very efficiently. This is partly because the junction between adjacent cells
would be a point of weakness that would reduce the overall strength of the muscle. To overcome
this, muscles have a different structure. The separate cells are fused together to form muscle
fibres which share nuceli and cytoplasm, called sarcoplasm , which is mostly found around the
circumference of the fibre. Within the sarcoplasm is a large concentration of mitochondria and
endoplasmic reticulum (synthesis of proteins, glycoproteins and carbohydrates).
Microscopic structure of skeletal muscle
Myofibrils 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.
Myofibrils appear striped due to their alternating light and dark coloured bands. The isotropic (I)
bands appear lighter because the actin and myosin filaments do not overlap in this region. The
anisotropic (A) bands appear d arker because the actin and mysosin filaments do overlap in this
region.

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At the centre of each A band is a lighter coloured region
called the H zone and at the centre of each I 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.
Two other important proteins are found in muscle:
Tropomyosin , which forms a fibrous strand around the actin filament
A globular protein (troponin) involved in muscle contraction.…read more

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The acetylcholine diffuses to the
postsynaptic membrane, altering its permeability to sodium ions, which enter rapidly, depolarising
the membrane.
The acetylcholine is broken down by acetylcholinesterase to ensure that the muscle is not
over-stimulated. The resulting choline and ethanoic acid (acetyl) diffuse back into the neurone,
where they are recombined to form acetylcholine using energy provided by the mitochondria
found there.…read more

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The myosin then reattaches itself further along the actin filament and the cycle is repeated as
long as nervous stimulation of the muscle continues.
Relaxation:
When nervous stimulation ceases, calcium ions are actively transported back into the
sarcoplasmic 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.…read more

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