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Lecture B

Contraction of Skeletal Muscle

  • muscle fiber contraction is “all or none”

  • within a skeletal muscle, not all fibers may be stimulated during the same interval

  • different combinations of muscle fiber contractions may give differing responses

  • graded responses - different degrees of skeletal muscle shortening

    • can be produced by changing

      • the frequency of muscle stimulation

      • the number of muscle cells being stimulated at one time

Types of Graded Responses

  • twitch

    • single, brief contraction

    • not a normal muscle function

  • tetanus (summing of contractions)

    • one contraction is immediately followed by another

    • the muscle does not completely return to a resting state

    • the effects are added

  • unfused (incomplete) tetanus

    • some relaxation occurs between contractions

    • the results are summed

  • fused (complete) tetanus

    • no evidence of relaxation before the following contractions

    • the result is a sustained muscle contraction

Muscle Response to Strong Stimuli

  • muscle force depends upon the number of fibers stimulated

  • more fibers contracting results in greater muscle tension

  • muscles can continue to contract unless they run out of energy

Energy for Muscle Contraction

  • initially, muscles use stored ATP for energy

    • ATP bonds are broken to release energy

    • only 4-6 seconds worth of ATP is stored by muscles

  • after this initial time, other pathways must be utilized to produce ATP

  • Direct phosphorylation of ADP by creatine phosphate (CP)

    • muscle cells store CP

      • CP is a high-energy molecule

    • after ATP is depleted, ADP is left

    • CP transfers energy to ADP, to regenerate ATP

    • CP supplies are exhausted in less than 15 seconds

  • Aerobic respiration

    • glucose is broken down into carbon dioxide and water, releasing energy (ATP)

    • this is a slower reaction that requires continuous oxygen

    • a series of metabolic pathways occur in the mitochondria

  • Anaerobic glycolysis and lactic acid formation

    • a reaction that breaks down glucose without oxygen

    • glucose is broken down to pyruvic acid to produce some ATP

    • pyruvic acid is converted to lactic acid

  • this reaction is not as efficient, but is fast

    • huge amounts of glucose are needed

    • lactic acid produces muscle fatigue

Muscle Fatigue and Oxygen Deficit

  • when a muscle is fatigued, it is unable to contract even with a stimulus

  • common cause for muscle fatigue is oxygen debt

    • oxygen must be “repaid” to tissue to remove oxygen deficit

    • oxygen is required to get rid of accumulated lactic acid

  • increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Types of Muscle Contractions

  • isotonic contractions

    • myofilaments can slide past each other during contractions

    • the muscle shortens and movement occurs

  • isometric contractions

    • tension in the muscles increases

    • the muscle is unable to shorten or produce movement

Muscle Tone

  • some fibers are contracted even in a relaxed muscle

  • different fibers contract at different times to provide muscle tone

  • the process of stimulating various fibers is under involuntary control

Effect of Exercise on Muscles

  • exercise increases muscle size, strength, and endurance

    • aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue *isotonic

      • makes body metabolism more efficient

      • improves digestion, coordination

    • resistance (isometric) exercise (weight lifting) increases muscle size and strength

phosphorylation - reattaching a phosphate to the ADP to recreate ATP

Lecture B

Contraction of Skeletal Muscle

  • muscle fiber contraction is “all or none”

  • within a skeletal muscle, not all fibers may be stimulated during the same interval

  • different combinations of muscle fiber contractions may give differing responses

  • graded responses - different degrees of skeletal muscle shortening

    • can be produced by changing

      • the frequency of muscle stimulation

      • the number of muscle cells being stimulated at one time

Types of Graded Responses

  • twitch

    • single, brief contraction

    • not a normal muscle function

  • tetanus (summing of contractions)

    • one contraction is immediately followed by another

    • the muscle does not completely return to a resting state

    • the effects are added

  • unfused (incomplete) tetanus

    • some relaxation occurs between contractions

    • the results are summed

  • fused (complete) tetanus

    • no evidence of relaxation before the following contractions

    • the result is a sustained muscle contraction

Muscle Response to Strong Stimuli

  • muscle force depends upon the number of fibers stimulated

  • more fibers contracting results in greater muscle tension

  • muscles can continue to contract unless they run out of energy

Energy for Muscle Contraction

  • initially, muscles use stored ATP for energy

    • ATP bonds are broken to release energy

    • only 4-6 seconds worth of ATP is stored by muscles

  • after this initial time, other pathways must be utilized to produce ATP

  • Direct phosphorylation of ADP by creatine phosphate (CP)

    • muscle cells store CP

      • CP is a high-energy molecule

    • after ATP is depleted, ADP is left

    • CP transfers energy to ADP, to regenerate ATP

    • CP supplies are exhausted in less than 15 seconds

  • Aerobic respiration

    • glucose is broken down into carbon dioxide and water, releasing energy (ATP)

    • this is a slower reaction that requires continuous oxygen

    • a series of metabolic pathways occur in the mitochondria

  • Anaerobic glycolysis and lactic acid formation

    • a reaction that breaks down glucose without oxygen

    • glucose is broken down to pyruvic acid to produce some ATP

    • pyruvic acid is converted to lactic acid

  • this reaction is not as efficient, but is fast

    • huge amounts of glucose are needed

    • lactic acid produces muscle fatigue

Muscle Fatigue and Oxygen Deficit

  • when a muscle is fatigued, it is unable to contract even with a stimulus

  • common cause for muscle fatigue is oxygen debt

    • oxygen must be “repaid” to tissue to remove oxygen deficit

    • oxygen is required to get rid of accumulated lactic acid

  • increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Types of Muscle Contractions

  • isotonic contractions

    • myofilaments can slide past each other during contractions

    • the muscle shortens and movement occurs

  • isometric contractions

    • tension in the muscles increases

    • the muscle is unable to shorten or produce movement

Muscle Tone

  • some fibers are contracted even in a relaxed muscle

  • different fibers contract at different times to provide muscle tone

  • the process of stimulating various fibers is under involuntary control

Effect of Exercise on Muscles

  • exercise increases muscle size, strength, and endurance

    • aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue *isotonic

      • makes body metabolism more efficient

      • improves digestion, coordination

    • resistance (isometric) exercise (weight lifting) increases muscle size and strength

phosphorylation - reattaching a phosphate to the ADP to recreate ATP

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