The motor unit

  • Created by: Megan66
  • Created on: 26-10-16 21:24
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  • The motor unit
    • Definitions
      • Motor unit - a motor neurone and its muscle fibres
      • Motor neurones- Nerve cells which transmit the brain's instructions as electrical impulses to the muscles
      • Neuromuscular junciton - where the motorneurone and the msclefibre meet
      • All or none law - where a sequence of impulses has to have sufficient intensity to stimulate all of the muscle fibres in a motor unit for them to contract or none of them contract
      • Wave summation - where there is a repeated nerve impulse with no time to relax as a smooth, sustained contraction occurs, rather than twitches
      • Tetanic contraction - A sustained muscle contraction caused by a series of fast repeating stimuli
      • Spatial summation - When the strength of a contraction changes by altering the number and size of the muscle's motor units
    • About the Motor unit
      • Muscle fibres are grouped into motor units.
      • A motor unit  consists of a motor neurone and its muscle fibres.
      • Only one type of muscle fibre can be found in one particular motor unit.
      • Muscle fibres work with the nervous system so that a contraction can occur.
      • The motor neurone transits the nerve impulse to the muscle fibre
      • Each motor neurone has branches that end in the neuromascular junction on the muscle fbre
    • Good to know
      • Motor = Movement
      • Each muscle is made up of many motor units and they vary in size.
      • A small muscle that is used for fine motor control, for example eye movement will have motor units that have only a few fibres per motor neurone.
      • A large muscle used for gross motor control such as the quadriceps when the leg is extended, will have motor units with a motor neurone feeding hundreds of fibres.
    • The all or none law
      • Once the motor neurone stimulates the muscle fibres, either all or none of them contract.
      • It is not possible for a motor to partially contract, this is called the all or none law.
      • A minimum amount of stimulation called the 'threshold' is required in start the contraction
      • If the sequence of impulses is equal to or more than the threshold then all the muscle fibres will contract if not then no muscle action will occur.
    • Slow twitch and fast twitch motor units
      • Motor units contain the same type of muscle fibre so they are either slow twitch or fast twitch motor units.
      • The brain will recruit slow twitch motor units for low intensity activity such as jogging or long-distance swimming.
      • if a greater force of contraction is needed, the brain will recruit fast twitch motor units for activities such as sprinting or power lifting.
    • How to increase the strength of contraction
      • Wave summation
        • The greater force of stimuli, the greater the tension developed by them muscle.
        • Where a repeated activation of a motor neurone stimulating a given muscle fibre results in a greater force of contraction
        • Each time the nerve impulse reaches the muscle cell, calcium is released.
        • Calcium needs to be present  for muscles to contract.
        • If there are repeated nerve impulses with no time to relax,calcium will build up within the muscle cell.
        • This produces a forceful, sustained, smooth contraction which is referred to as a tetanic contraction.
      • Spatial summation
        • This occurs when impulses are received at the same time at different places on the neurone which add up to fire the neurone.
        • It is the recruitment of additional and bigger motor units within a muscle to develop more force.


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