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2010-09-22 09:21:43
muscle motor units

Motor Units
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  1. motor units
    corresponds to all of the muscle fibers innervated by a single alpha-motor neuron
  2. specialization of motor units
    some muscles (such as eye muscles) contain many tiny motor units for fine movement; other muscles (such as leg muscles) contain few large motor units for producing posture and propulsive motion
  3. innervation of muscle fibers
    an individual muscle fiber is generally innervated by one motor neuron; exceptions include during development and in pathological conditions
  4. mosaic pattern of muscle fiber organization
    muscle fibers belonging to a unit are intermingled with muscle fibers belonging to other units
  5. retrograde activation
    motoneuron activity can be monitored via retrograde activation (stimulation of muscle fiber and then recording motoneuron activity)
  6. twitch contractions
    result from a single motor neuron action potential; important features are amplitude and time-to-peak
  7. tetanic contractions
    result from a train of motor neuron action potentials; represent fusion of underlying twitch contractions
  8. relationship between speed and force of motor units
    slow contraction associated with smaller force of contraction
  9. motoneuron electrical properties
    rheobase current and afterhyperpolarization
  10. rheobase current
    provides an inverse measure of excitability; involves injection of current until an action potential results (more current necessary implies a lower excitability)
  11. afterhyperpolarization
    occurs after an action potential; duration is inversely proportional with action potential firing frequency
  12. relations between motoneuron properties
    inverse correlation between rheobase current and afterhyperpolarization
  13. relationship between firing frequency and force output
    positive correlation between firing frequency and force output
  14. types of motor units
    slow; fast fatigue resistant; fast fatiguable
  15. type 2B fibers
    correspond to fast fatiguable units
  16. type 2A fibers
    correspond to fast fatigue resistant units
  17. type 1 fibers
    correspond to slow units
  18. size principle
    units are recruited in order of increasing force from slow to fast fatiguable
  19. rate modulation
    process by which motor unit force is increased by increasing motor unit firing frequency
  20. motor unit recruitment
    accounts for 10-15% of motor unit force generation