ENERGY SOURCES FOR MUSCLE CONTRACTION

Objectives

1. Energy used2. Energy produced3. Oxygen debt4. Muscle fiber types5. Muscle fatigue

ATP

• Muscle cells convert chemical energy to mechanical energy.

• ATP is the energy source used for this conversion.

• The ATP pool in skeletal muscle is small and capable of supporting only a few contractions if not replenished.

Creatine Phosphate

• Immediate high-energy source for replenishing the ATP supply

• The enzyme creatine phosphokinase (CPK) catalyzes the reaction

Carbohydrates

• Muscle cells contain glycogen,• Glycogen provide glucose for oxidative

phosphorylation and glycolysis

Fatty Acids and Triglycerides

• Fatty acids represent an important source of energy for muscle cells during prolonged exercise.

• Muscle cells contain fatty acids but can also take up fatty acids from blood.

• In addition, muscle cells can store triglycerides, which can be hydrolyzed when needed to produce fatty acids

Metabolic Adaptations Allow Contraction toContinue With an Inadequate Oxygen Supply

• Glycolytic (anaerobic) metabolism can provide energy for sudden, rapid, and forceful contractions of some muscles.

• In most muscles, especially under conditions of rest or moderate exercise, the supply of oxygen is adequate for aerobic metabolism (fed by fatty acids and by the end products of glycolysis) to supply the energy needs of the contractile system.

Muscle tissue has two sources of oxygen

(1) oxygen that diffuses into muscle fibers from the blood and (2) oxygen released by myoglobin within muscle fibers.

Depletion of aerobic energy capacity

This results from two effects: (1) the oxygen debt and (2) depletion of the glycogen stores of the muscles

Oxygen debt• The body normally contains about 2 liters of

stored oxygen that can be used for aerobic metabolism even without breathing any new oxygen.

• This stored oxygen consists of the following: (1) 0.5 liter in the air of the lungs, (2) 0.25 liter dissolved in the body fluids, (3) 1 liter combined with the hemoglobin of the blood, and (4) 0.3 liter stored in the muscle fibers them-selves, combined mainly with myoglobin

• After the exercise is over, this stored oxygen must be replenished by breathing extra amounts of oxygen over and above the normal requirements.

• In addition, about 9 liters more oxygen must be consumed to provide for reconstituting both the phosphagen system and the lactic acid system.

• All this extra oxygen that must be “repaid,” about 11.5 liters, is called the oxygen debt.

Measuring Maximal Oxygen Uptake Is the most Common Method of Quantifying Dynamic Exercise

Oxygen debtIf the energy demands of exercise cannot be met by oxidative phosphorylation, an oxygen debt is incurred. After completion of exercise, respiration remains above the resting level in order to "repay" this oxygen debt. The extra oxygen consumption during this recovery phase is used to restore metabolite levels (such as creatine phosphate and ATP) and to metabolize the lactate generated by glycolysis. The increased cardiac and respiratory work during recovery also contributes to the increased oxygen consumption seen at this time and explains why more O2 has to be "repaid" than was "borrowed."

TYPES OF SKELETALMUSCLE FIBERS

Slow oxidative fibers Fast glycolytic fibers

Myoglobin content Large Small

Mitochondria Many Few

Capillaries Many Few

Colour Red White (Pale)

Capacity for generating ATP and method used

High capacity, aerobic respiration

Low capacity, anaerobic respiration

Glycogen stores Low High

Creatine Kinase Lowest amount Highest amount

Contraction velocity Slow Fast

Fatigue resistance High Low

Location where fibers are abundant

Postural muscles such as those of the neck

Upper limb muscles

Primary function of the fibers

Maintaining posture and aerobic endurance activities

Rapid intense movements of short duration

Classification of Fiber Types in Skeletal Muscles

Examples:• Type I Red fibers: in postural muscles– Large myoglobin content and many mitochondria

• Type IIa Red fibers: in muscles needed for activities like middle distance running, swimming, etc.– Large myoglobin content and many mitochondria

• Type IIb White fibers: needed for activities like sprinting• Low myoglobin content and few mitochondria

Muscle fatigue.

• The inability of a muscle to maintain force of contraction after prolonged activity .

• Causes-inadequate release of calcium ions from the SR.Depletion of creatine phosphate insufficient oxygen, depletion of glycogen and other nutrients, build up of lactic acid and ADP,failure of action potentials in the motor neuron

to release enough acetylcholine.

Q. Fast twitch muscle fiber differ from slow twitch muscles in that former:a. have low ATPase activityb. are adapted for posture maintaining

contractionc. contains more sarcoplasmic reticulum with

high glycogen contentd. resistant to fatigue