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Motor Neurons

Dendrite. Soma (body). Axon. Motor Neurons. receives and integrates information. transmits information. Motor Unit. A motor unit is composed of a motor neuron and all of the muscle fibers it innervates. It is the smallest functional unit of muscular shortening. Motor Unit (cont).

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Motor Neurons

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  1. Dendrite Soma (body) Axon Motor Neurons receives and integrates information transmits information

  2. Motor Unit A motor unit is composed of a motor neuron and all of the muscle fibers it innervates It is the smallest functional unit of muscular shortening

  3. Motor Unit (cont) • each muscle has many motor units (m.u.) • # of fibers in a m.u. is dependent on the precision of movement required of that muscle (average: 100-200 fibers per m.u.) • more precision is obtained with more neurons • 100 to 2000 motor neurons per muscle • # of m.u.’s in a muscle decreases in the elderly

  4. Precision of 2 Muscles 1st muscle 2nd muscle 10,000 fibers 10,000 fibers 100 motor neurons 200 motor neurons 100 motor units 200 motor units 100 fibers/mu 50 fibers/mu less precision more precision

  5. Muscle Number of Number of Mean Muscle MU’s Number of Fibers Fibers Per MU Platysma 27,100 1,100 25 Brachioradialias >129,200 330 >410 First Lumbrical 10,000 100 110 Tibialias 250,000 450 600 Anterior Gastrocnemius 1,120,000 580 2,000 (medial head)

  6. Neuromuscular Control • a motor nerve action potential stimulates the release of acetylcholine (ACh) from the nerve ending • ACh binds to the muscle fiber which causes depolarization and results in the release of calcium ions from the sarcoplasmic reticulum (5 ms) • the calcium ions permit the actin-myosin interaction, which produces force • the contraction stops when the calcium ions are removed by a pumping action (100 ms)

  7. EMG

  8. Electromechanical Delay • electromechanical delay - stimulation begins before force is developed • it is thought that this is the time necessary to “take up the slack in the SEC”

  9. Control of Tension • excitation of each motor unit is an all-or-nothing event • increased tension can be accomplished by: • increasing the # of stimulated motor units (recruitment) • increasing the stimulation rate of the active motor units (rate coding)

  10. Stimulation vs Activation threshold Voltage Voltage NOTHING ALL

  11. Recruitment • each motor unit has a stimulation threshold at which it will begin to produce force • small motor units have a lower threshold than large motor unit, therefore they are recruited first (size principle)

  12. Rate Coding tetanus (C) - sustained maximal tension due to high frequency stimulation summation (B) - the overall effect of added stimuli

  13. Sensory Receptors • Sensory neurons provide feedback on the characteristics of the muscle or other tissues. 2 neuromuscular proprioceptors: MUSCLE SPINDLES & GOLGI TENDON ORGANS

  14. Muscle Spindles • location: • interspersed throughout muscle belly • responds to: • muscle length • muscle velocity • causes: • autogenic facilitation • reciprocal inhibition

  15. Stretch Reflex • The muscle spindle is responsible for the stretch reflex. • As a muscle is rapidly stretched, the muscle spindle responds by facilitation of the same muscle and inhibition of the antagonistic muscle. • This reflex can be seen in the patellar tendon tap.

  16. Golgi Tendon Organ • location: • near the muscle-tendon junction • responds to: • muscle tension • causes: • autogenic inhibition • antagonistic facilitation Muscle Fibers GTO tendon

  17. GOLGI TENDON ORGAN

  18. “My Little GTO” • possibly the critical determinant to maximal lifting levels in weight training • may also be responsible for uncoordinated responses in untrained individuals • response is adapted through training

  19. BALLISTIC STATIC activate muscle spindles which elicits a stretch reflex if static position achieved slowly then can minimize muscle spindle response if held for sufficiently long period (~30s) then can elicit GTO response may result in tearing a muscle STATIC BETTER THAN BALLISTIC

  20. ACTIVE STRETCH Spindle response: minimal if performed slowly GTO response: active stretch of hip extensors causes GTO to relax hip extensors and to activate the hip flexors motive force: actions of the hip flexors consequences: no negatives -- limited ROM limits possibility of injury and exercise antagonists

  21. PASSIVE STRETCH Spindle response: minimal if performed slowly GTO response: passive stretch of hip extensors causes GTO to relax hip extensors motive force: external force consequences: no direct control of ROM thus may exceed physiological limits and induce muscle damage

  22. Stretching • Proprioceptive Neuromuscular Facilitation • PNF • alternating contraction - relaxation of agonist & antagonist muscles • takes advantage of the response of the proprioceptors • e.g. hamstrings • passive static stretch of hams - relax • active maximal concentric action of hams - relax • repeat

  23. Plyometric Training Plyometric training consists of exercises that rapidly stretch a muscle followed immediately by a contraction. They improve power output in the muscle by: Neurological Influences: rapidly stretching of the muscle, which excites the motoneurons via the stretch reflex. Structural Influences: involving elastic energy from the stretch-shortening cycle.

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