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Motor units have an “all-or-none” response

Motor units have an “all-or-none” response. All of the fibers innervated by the motor neuron contract Or none of them do. 1974 Hennenman’s Size Principle. Smaller motor units are recruited first

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Motor units have an “all-or-none” response

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  1. Motor units have an “all-or-none” response All of the fibers innervated by the motor neuron contract Or none of them do

  2. 1974 Hennenman’s Size Principle • Smaller motor units are recruited first • The frequency of motor unit recruitment (use/firing) is directly related to the size and ease of triggering an action potential in the soma (neuron cell body) • smaller cell bodies (slow motor units) will be recruited first, and overall, most frequently

  3. Control of Movement • CNS: brain and spinal cord • PNS: all the nerves extending from the brain and spinal cord • afferent nerves: direction of AP is toward the spinal cord, usually involves sensory information

  4. efferent nerves: direction of AP is away from the spinal cord, usually involves motor information • reflexes are “all-or-none” contractions, monosynaptic

  5. Resting membrane potential: ~70 mV

  6. EPSP: excitatory post-synaptic potential • release of ACH increases post-synaptic permeability to Na+, potential rises but may not reach depolarizing threshold level

  7. IPSP: inhibitory post-synaptic potential • another neurotransmitter is released which decreases the Na+ permeability, reducing the chance for potential reaching the threshold

  8. Activity in neuron within CNS • Depends on the sum total of EPSPs and IPSPs • Occurring in a narrow range of time and space

  9. Muscle Spindles • Connective tissue capsules in the shape of footballs that are filled with lymph and specialized fibers, implanted between muscle fibers • Afferent and efferent innervations • Provide information about absolute length of muscle and rate of change in length of the fiber

  10. When muscle is contracted, activation of the  motor neurons occurs, these efferent -neurons cause contraction of the muscle fibers within the muscle spindle • this “takes up the slack” within the fibers of the capsule, allowing it to respond to further stretch

  11. Golgi Tendon Organs (GTOs): respond to tension • exert inhibitory effects on the agonist and facilitate the effects on the antagonist muscles • is possible to disinhibit GTOs (minimizing the effectiveness of the GTOs) with training

  12. allows athlete to push the limits of the tissue • wrist wrestling: fractures and ruptures muscles and tendons occur

  13. Myoplasticity • Capacity of skeletal muscle for adaptive change • Due to training, nutrition, endocrine milieu

  14. Hypertrophy • Increase in fiber size

  15. Hyperplasia • Increase in fiber number

  16. Gender differences • men typically have greater mass cross-sectional area than women • men are able to generate more absolute strength than women • not true when strength is expressed Str/BW or Str/FFW, gender differences disappear

  17. when examined for the ability to generate force and power (normalized for cross-sectional area) there are no gender differences

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