Skeletal Muscle 9... Skeletal Muscle Mechanics Muscle fibers into whole muscle Whole muscle tension:

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Text of Skeletal Muscle 9... Skeletal Muscle Mechanics Muscle fibers into whole muscle Whole muscle tension:

  • 1

    Skeletal Muscle Mechanics

    Muscle fibers into whole muscle

    Whole muscle tension: • Number of muscle fibers contracting • Tension developed by each fiber

    Motor units:

    Figure 8.15 Page 269

    Motor unit recruitment

    Also influenced by fiber type!

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    Whole muscle tension depends on: 1. Frequency of stimulation

    Figure 8.17 Page 271

    Contractile activity

    Action potentials

    Single twitch

    Twitch summation


    Stimulation ceases or fatigue begins

    Whole muscle tension depends on: 2. Length of fiber at onset of contraction

    Figure 8.18

    Page 272

    Whole muscle tension depends on: 3. Extent of fatigue 4. Thickness of fiber

    Duration of activity

    Amount of motor unit recruitment

    Fiber type

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    Types of Contractions

    1. Isotonic • Muscle tension remains constant as muscle

    changes length Concentric & eccentric contractions

    2. Isometric • Tension develops at constant length

    3. Isokinetic • Fixed movement

    Skeletal Muscle Metabolism

    Major requirement of contraction-relaxation coupling is ATP…

    1. Splitting of ATP by myosin ATPase

    2. Binding of ATP to myosin

    3. Active transport of Ca2+ back into sarcoplasmic reticulum

    Must be in constant supply (readily available)

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    Intramuscular ATP supplied by 3 means

    1. Creatine Phosphate

    2. Glycolysis

    3. Oxidative Phosphorylation

    Immediate energy source

    Concentrations will drive reaction • Resting muscle: ~ 5x the amount of CP than ATP

    Only 1 enzyme (rapid reaction)

    Limited supply • Short bursts, high-intensity exercise

    Creatine Phosphate:

    Creatine Phosphate

    ADP Creatine ATP+ + Creatine Kinase


    No O2 requirement (like CP) ~ anaerobic

    Continuous high-intensity exercise

    Breakdown of glucose or glycogen:

    2 ATPPyruvic acid

    Pyruvic acid

    Lactic acid

    Oxidative phosphorylation


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    Oxidative Phosphorylation:

    If energy requirement continues

    Multiple steps • Time • Pathway fueled primarily by glucose & fatty acids

    Occurs within the mitochondria (O2!!) • O2 comes from hemoglobin & myoglobin • Electron-transport chain

    Yields 36 ATP (glucose), ~ 128 ATP (Fat)

    Muscle fiber


    Figure 8.23 Page 278


    1. Muscular • Increased concentration of Pi (inorganic

    phosphate) • Accumulation of lactic acid (lactate) • Glycogen or glucose depletion

    2. Neural (Central & Peripheral) • Psychological

    3. O2 debt & nutrient depletion

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    Skeletal Muscle Fibers

    1. Slow-oxidative (Type I) • Mitochondrial density

    More resistant to fatigue

    2. Fast-oxidative (Type IIa) • Higher myosin ATPase activity

    3. Fast-glycolytic (Type IIb) • Higher myosin ATPase activity

    Endurance training

    Changes in fibers:

    Weight lifting


    Motor Control

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    Input to Motor Neurons

    1. Input from afferent neurons • Intervening interneurons (spinal reflexes)

    2. Input from primary motor cortex

    3. Input from brainstem

    Figure 8.24 Page 285

    Cortical level

    Subcortical level

    Brain stem level

    Spinal cord level


    Premotor and supplementary motor areas

    Sensory areas of cortex

    Primary motor cortex

    Basal nuclei Thalamus

    Brain stem nuclei (including reticular formation and vestibular nuclei)


    Afferent neuron terminals

    Motor neurons

    Peripheral receptors

    Muscle fibers

    Other peripheral events, such as visual input

    Sensory consequences of movement


    = Pathways conveying

    afferent input

    = Corticospinal motor system

    = Multineuronal

    motor system

    Muscle Receptors

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    Coordinated movement:

    Learned muscle behaviors • CNS & muscle input • Proprioception

    2 types of muscle receptors: 1. Muscle spindles

    • Stretch reflex (knee-jerk)

    2. Golgi tendon organs • Respond to tension changes

    Alpha motor neuron axon

    Gamma motor neuron axon

    Secondary (flower-spray) endings of afferent fibers

    Extrafusal (“ordinary”) muscle fibers


    Intrafusal (spindle) muscle fibers

    Contractile end portions of intrafusal fiber

    Noncontractile central portion of intrafusal fiber

    Primary (annulospiral) endings of afferent fibers

    Figure 8.25 Page 286

    Figure 8.26 (1) Page 287

    Extrafusal skeletal muscle fiber

    Intrafusal muscle spindle fiber

    Spinal cord

    Afferent input from sensory endings of muscle spindle fiber Alpha motor neuron output to regular skeletal-muscle fiber

    Stretch reflex pathway Gamma motor-neuron output to contractile end portions of spindle fiber

    Descending pathways coactivating alpha and gamma motor neurons

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    Relaxed muscle; spindle fiber sensitive to stretch of muscle

    Contracted muscle in hypothetical situation of no spindle coactivation; slackened spindle fiber not sensitive to stretch of muscle

    Contracted muscle in normal situation of spindle coactivation; contracted spindle fiber sensitive to stretch of muscle

    Figure 8.26 (2) Page 287

    Figure 8.27 Page 288

    Patellar tendon

    Extensor muscle of knee (quadriceps femoris)

    Muscle spindle

    Alpha motor neuron