Lecture: Muscles and Muscle Tissue 2

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Lecture: Muscles and Muscle Tissue 2
2012-03-27 19:58:20
muscle mechanics twitch change stimulus metabolism tone force velocity

second portion of notes for A &P lecture
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  1. Review principles for Muscle Mechanics
    1. Same principles apply to contraction of a single fiber and a whole muscle

    2. Contraction produces tension, the force exerted on teh load or object to be moved (muscle tension)

    • 3. Contraction does not always shorten a muscle
    • *Isometric Contraction: no shortening; muscle tension increases but does not exceed load
    • *Isotonic Contraction: muscle shortens because muscle tension exceeds the load

    4. Force and duration of contraction vary in response to stimuli of different frequencies and intensities
  2. Motor Unit: The Nerve-Muscle functional unit
    • motor unit: a motor neuron and all (four to several hundred) muscle fibers it supplies
  3. Motor Unit:
    • -Small motor units in muscles that control fine movements (fingers, eyes)
    • -Large motor units in large weight bearing muscles (thighs, hips)

    -muscle fibers from a motor unit are spread throughout muscle so that a single motor unit causes a weak contraction of the entire muscle

    -motor units in a muscle ususally contract asynchronously; helps prevent fatigue
  4. Muscle Twitch
    -Response of a muscle to a single, brief threshhold stimulus

    -Simplest contraction observable in the lab (recorded by a myogram)

    • Three phases of a Twitch
    • 1. Latent period: events of E-C coupling
    • 2. Period of Contraction: cross bridge formation; tension increases
    • 3. Period of Relaxation: Ca2+ reentry into the SR; tension declines to zero
  5. Muscle Twitch Comparisons
    Different strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles
  6. Graded Muscular Responses
    • -variations in the degree of muscle contraction
    • -required for proper control of skeletal movement

    • Responses are graded by:
    • 1. changing the frequency of the stimulation
    • 2. changing the strength of the stimulation
  7. Changes in response to stimulus FREQUENCY
    • -A single stimulus results in a single contractile response-a muscle twitch
    • -increase in frequency of stimulus (muscle does not have time to relax between stimuli)

    • -Ca2+ release stimulates further contraction--> temporal (wave) summation
    • -Further increase in stimulus frequency--> unfused (incomplete) tetanus

    • -If stimuli are given quickly enough, fused (complete) tetany occurs
    • *pretty rare, like if someone is lifting up a car

    • twict, wave, unfused tetanus, fused tetany
  8. Treppe
    • When frequency and strength are constant
  9. Response to change in stimulus STRENGTH
    Threshold stimulus: stimulus strength at which that first observable muscle contraction occurs

    -Muscle contracts more vigorously as stiimulus strength is increased above threshold

    -Contraction force is precisely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action

  10. Size Princple
    Size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases

  11. Muscle Tone
    -Constant, slightly contracted state of all muscles

    -Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles

    -Keeps muscles firm, healthy, and ready to respond (helps posture and stabilize joints)
  12. Isotonic Contractions
    -muscle changes in length and moves the load

    • -Isotonic contraction are either concentric or eccentric
    • 1. Concentric: the muscle shortend and does the work
    • 2. Eccentric: the muscle contracts as it lengthens (like your calf muscle when you walk uphill)
  13. Isometric Contractions
    -The load is greater than the tension the muscle is able to develop

    -Tension increases to the muscle's capacity, but the muscle neither shortens nor lengthens

    *object is not lifted or something is being held in position
  14. Muscle Metabolism: Energy for contraction
    -ATP is the only source used directly for contractile activities

    -Available stores of ATP are used within 4-6 seconds

    • -ATP is regenerated by
    • 1. Directy phosphorylation of ADP by creatine phosphate (CP)---> it gives enough ATP for 15-16 seconds and is replenished when resting
    • 2. Anaerobic Pathways
    • 3. Aerobic respiration
  15. Anaerobic Pathway
    -At 70% of maximum contractile activity:

    • *bulging muscles compress blood bessels
    • *oxygen delivery is impaired
    • * pyruvic acid is converted into lactic acid

    • Lactic acid:
    • *diffuses into the bloodstream
    • *used as fuel by the liver, kidneys, and heart
    • *converted back into pyruvic acid by the liver
  16. Aerobic Pathway
    -Produces 95% of ATP during rest and light to moderate exercise

    -Fuels: stored glycogen, then bloodstream glucose, pyruvic acid, and free fatty acuds

    -occurs int he mitochondria

  17. Muscle Fatigue
    -Physiological inability to contract

    • Occurs when:
    • *Ionic imbalances (K+, Ca2+, etc) interfere with E-C coupling
    • *Prolonged exercise damages the SR and interferes with Ca2+ regulation and release

    -Total lack of ATP is rare, during states of continuous contraction, and causes contractures (continuous contractions) like ragamortis
  18. Oxygen Deficit
    Extra O2 needed after exercise for:

    • -The replenishment of:
    • * oxygen reserves
    • * glycogen stores
    • *ATP and CP reserves

    -Conversion of lactic acid to pyruvic acid, glucose, and glycogen
  19. heat production during Muscle Activity
    About 40% of the energy released in muscle activity is useful as work

    The other 60% is given off as heat

    -dangerous heat levels are prevented by radiation of heat from the skin and sweating
  20. Force of Muscle Contraction
    The force of the conraction is affected by:

    *number of muscle fibers stimulated (Recruitment)

    *Relative size of the fibers --hypertrophy of cells increases strength (increase size by regular resistence exercise)

    *Frequency of Stimulation- higher frequency allows time for more effective transfer of tension to noncontractile components (summation of contractions)

    • *Length-Tension Relationship: muscles contract most strongly when muscle fibers are 80-120% of their normal resting length (when cross bridges can happen)
  21. VELOCITY and Duration of the contraction
    -How fast they can contract and how long it takes for it to fatigue

    • Influenced by:
    • 1. Muscle fiber type
    • 2. Load
    • 3. Recruitment
  22. Muscle Fiber Types
    Classified according to 2 different characteristics:

    • 1. Speed of contraction: slow or fast, according to
    • *speed at which myosin ATPases split ATP
    • *Patter of electrical activity of the motor neurons

    • 2. Metabolic oathways for ATP synthesis:
    • *oxidative fibers--use aerobic pathways
    • *Glycolytic fibers--use anaerbic glycolysis

    • Split into 3 types:
    • 1. Slow oxidative fibers (SO)
    • 2. Fast oxidative fibers (FO)
    • 3. Fast Glycolytic fibers (FG)

  23. Influence of Load
    larger the load = the larger the latent period, smaller the contraction, and shorter duration of contration
  24. Influence of recruitment
    Recruitment--> faster contraction and longer duration of contraction
  25. Effects of AEROBIC exercise
    Endurance exercise

    • leads to increased:
    • 1. Muscle capillaries
    • 2. number of mitochondria
    • 3. Myoglobin synthesis

    results in greater endurance, strength, and resistance to fatigue

    May convert fast glycolytic fibers into fast oxidative fibers
  26. Effects of Resistance Exercise
    Typically Anaerobic

    • Results in:
    • 1. Hypertrophy (due to increase in fiber size)
    • 2. Increased mitochondria, myofilaments, glycogen stores, and connective tissue
  27. The Overload principle
    Forcing a muscle to work hard promotes increased muscle strength and endurance

    Muscles adapt to increasing demands

    Muscles can be overloaded to produce further gains