ATP and Muscle Contraction

• Adenosine triphosphate (ATP) is the sole energy source used directly by muscles for contractile activities.
• ATP production's requirement and mechanism depend on the kind of work performed.
• Resting muscle cells have a small ATP reserve, but it can't depend on it once contraction starts, because after 3 seconds it will finish quickly
• Muscle cells must prepare for ATP production to keep up with the rising rate of use.

ATP Regeneration

• Stored ATP is used nearly instantly once exercise starts, lasting for only a few seconds.
• ATP is regenerated from ADP through the following methods:
  • Direct phosphorylation of ADP by creatine phosphate (CP)
  • Anaerobic pathway (glycolysis leading to lactic acid)
  • Aerobic respiration of fatty acids in the mitochondria

Phosphocreatine-Creatine System

• This system is formed from combined amounts of cell ATP + CP.
• Together providing maximal muscle power for 8-10 seconds, which is enough for a 100 meter run.
• The phosphagen system is useful for maximal short bursts of muscle power lasting 8-10 seconds.
• Muscle CP energy is immediately available for contraction, stored as ATP energy.
• Direct phosphorylation involves the coupled reaction of creatine phosphate (CP) and ADP which requires creatine kinase.
• The products from this system is 1 ATP per CP, creatine and uses no oxygen.
• The duration of energy provided is 15 seconds.

Anaerobic Glycolysis (Glycogen-Lactic acid system) Without Oxygen

• The primary energy source for peak muscular activity, lasts for 1.3-1.6 minutes of maximal muscle activity, and anaerobic metabolism maintains ATP supply for about 45-60s.
• Energy source includes Carbohydrate (glycolysis), Lactate and ATP
• The process involves Glycolysis and lactic acid formation.
• Glucose is derived from glycogen breakdown or delivered from blood.
• Producing 2 ATP molecules per molecule of glucose + 2 NADH.
• Lactic acid diffuses out of muscles towards the blood.
• The liver takes the lactic acid to convert into Glucose (by gluconeogenesis) and goes back to the blood to be taken by the muscle again.
• It produces 2 ATP per glucose, lactic acid and needs no oxygen..
• Duration of energy provided: 30-40 seconds, or slightly more

Anaerobic Glycolysis Inefficiency

• Large amounts of glucose are used for small ATP returns.
• The presence of lactic acid contributes to muscle fatigue.
• Sports requiring bursts of speed and activity, such as basketball and tennis, use anaerobic metabolism.

Aerobic Metabolism (With Oxygen)

• Primary energy source of resting muscles.
• Used to convert glucose into glycogen and create energy storage compounds such as CP.
• During rest and light to moderate exercise, aerobic metabolism provides 95% of the necessary ATP.
• Breaks down fatty acids, pyruvic acid (made via glycolysis), and amino acids.
• Produces 36 ATP molecules per glucose molecule and needs oxygen.
• The source of energy is mainly fatty acids, then carbs and amino acids.

Oxygen Debt

• The amount of extra O2 required after exercise to restore muscles to resting conditions.
• Excess oxygen intake serves tasks:
  • Replenish oxygen stored by myoglobin and hemoglobin
  • Convert remaining lactic acid back into glucose
  • Aerobic metabolism to make ATP by replenishing the phosphagen system.
  • Replenishing of glycogen stores.
  • Power the Na+/K+ pump to restore resting ionic conditions within the cell.

Muscle Fatigue

• Physiological inability to contract due to a relative ATP deficit.
• Factors contributing include decreased sarcoplasmic pH, increased sarcoplasmic [ADP], and ionic imbalances.
• Occurs due to prolonged strong contractions, and the inability of the contractile & metabolic process to continue supplying the same work output.
• Interrupted blood flow or decreased oxygen supply causes rapid fatigue

Causes of Muscle Fatigue

• Depletion of muscle glycogen or ATP.
• Depletion of acetyl choline stores at the nerve terminal.
• Accumulation of metabolites such as lactic acid.
• Results of Muscle Fatigue include:
  • Depletion of metabolic reserves
  • Damage to sarcolemma and sarcoplasmic reticulum
  • Low pH (lactic acid)
  • Muscle exhaustion and pain

Central Fatigue

• Describes the uncomfortable feelings that come from being tired, described as "psychological fatigue."
• Arises from factors released by the muscle during exercise that signal the brain to "feel" tired.
• Psychological fatigue precedes peripheral fatigue and occurs well before the muscle fiber can no longer contract.
• Training results in learning how to overcome psychological fatigue.

Types of Contractions

• Contractions can be Isometric and Isotonic
  • Isometric - Iso = same, Metr = measure
  • Isotonic - Iso = same, Ton = tension

Isotonic Contraction

• Requires enough tension of cross-bridges to overcome resistance, internal tension rises until external tension exceeds the amount of resistance.
• As the muscle shortens, internal and external tensions remain constant slightly exceeding resistance.
• Isotonic Contraction example is lifting dumbbells, where tension increases and muscle fibers shorten and lengthen.
• Concentric Contraction requires tension effort to exceeds resistance, causing the muscle shortens.
• The is contraction where Resistance exceeds tension during the eccentric contraction, where the muscle lengthens due to gravity.

Isometric Contractions

• The muscle's length does not change/shorten, and the produce tension never exceeds the resistance.
• Example is pushing against a wall, in which Tension (effort) never exceeds the resistance
• Example is attaching a 6kg weight where Muscle contracts (isometric contraction) and tension develped in kg.
• Although cross-bridges form and tension the muscle cannot overcome the resistance of the weight and cannot shorten.
• The individual fibers shorten until the tendons are taut and the external tension equals the internal tension.

Comparison of Isotonic Contraction vs Isometric Contraction

• Isotonic contraction- Isotonic

  • Tension is constant
  • Length Decreases
    • Work done is present
  • Duration is Longer
  • Energy needed More
  • converted to external work and waste heat
  • Example Contraction of biceps to lift an object

• Isometric contraction- Isometric contraction

  • Tension Rises markedly
  • Length Constant
  • Work done -No
  • Duration-Shorter
  • Energy needed-Less
  • converted to waste heat
  • Example-Contraction of quadriceps to stiff the knee

Muscle fibers

• There are two main types - Slow fibers
  • Fast fibers

Slow Fibers Red Fibers

• Contract slowly because myosin works slowly.
• Depend on oxygen delivery and aerobic metabolism.
• Fatigue resistant and has high endurance.
• Thin in diameter, large amt of cytoplasm impedes O2 & nutrient diffusion.
• Cannot develop high tension – small diameter means few myofibrils.
• Has rich capillary supply and lots of mitochondria.
• Contains lots of the O2-storing protein, myoglobin which gives it a red color.
• Uses lipids, carbs, and amino acids as substrates for it aerobic metabolism, best suited for endurance type activities.

Fast Fibers

• Can contract in 0.01 seconds or less after stimulation.
• Large in diameter; they contain densely packed myofibrils, large glycogen reserves, and relatively few mitochondria.
• Able to develop a great tension (they contain a large number of sarcomeres).
• Use ATP in massive amounts, supported by anaerobic metabolism and Fatigue rapidly.
• Describes fast fatigue (FF) fibers, fast glycolytic (FG) fibers, white fibers, best suited for short term, power activities.
• Contain densely-packed myofibrils-
• Have large glycogen reserves & few mitochondria
• Produce powerful contractions-Fatigue rapidly-“white muscle fibers”

Slow Twitch fibers:

• Half diameter
• Specialize to continue contracting for extended periods and use higher oxygen supply.
• Are very red due to Myoglobin and more mitochondria than fast fibers.
• Long contractions that continue
• Take 3x longer to contract

Comparison

• Fast Fibers-White fibers -Larger -Larger Nerve Fiber -Larger amount Glycolytic enzymes -S. Reticulum Extensive (Rapid calcium release) -Blood supply Less extensive -Mitochondria Fewer -Myoglobin Small amount -Contraction Adapted for very rapid intense contraction for short period using: anaerobic metabolism -Fatigue: rapid -Example:Ocular muscle

• Slow Fibers-Red Fibers -Smaller -Smaller Nerve Fiber -Smaller amount Glycolytic enzymes -S. Reticulum Less extensive -Blood supply- more Extensive -Mitochondria Increased number -Myoglobin Large amount -Contraction adapted for slow continuous muscle activity using: aerobic metabolism -Fatigue Delayed -Example Soleus muscle

Cramp

• Prolonged spasm causes the muscle to become taut and painful with motor neurons firing.
• Found commonly in muscles of the leg, especially the lower leg and foot and has two theories:
  • Central theories- changes in the brain or spinal cord
  • Peripheral theories- there is some change around peripheral motor neurons discharges spontaneously.

Other Important Terms

• Atrophy: Reduction in size of a cell, tissue, or organ often caused by disuse.
• Hypertrophy: Increase in size of a cell, tissue or an organ due to cellular hypertrophy rather than cellular hyperplasia
• Fibrosis: Replacement of normal tissue with heavy fibrous connective tissue (scar tissue).

Tetanus

• Toxins, drugs & diseases interfere with events at the neuromuscular junction
• Tetanus: infection of Nervous System from potentially deadly bacteria Clostridium tetani
• Bacteria spreads & makes poison called tetanospasmin
• Results in release of inhibitory transmitters from nerve terminals is blocked, thereby causing unopposed muscle stimulation by acetylcholine and generalized tonic spasticity
• Spasms tear the muscles or case fractures of the spine.