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