EXAM 3: BIO&251 (Chapter 10: Part 2)

• All-or-none Principle: For a given "dose" of Ca²⁺, all sarcomeres within a muscle fiber contract together.
• More active cross-bridges → more tension produced

• 1. Fiber length at time of stimulation (Affects overlap between actin and myosin)
• 2. Frequency of stimulation (Affects duration of high [Ca²⁺])
• 3. Fiber diameter (Affects amount of actin and myosin available)

More tension produced by individual fibers and more fibers contracting → stronger whole muscle contraction

• Factors
• 1. Internal and External Tension (Series elastic component [stretch in CT])
• 2. Rectruiment (Number of active motor units)
• 3. Muscle Size (Number of muscle fibers present)

• Fiber length affects overlap between actin and myosin
• There is an optimal length for a fiber at which force generation is maximal.

• 1. Single Stimulus (Simple Twitch)
• 2. Multiple Stimuli (Treppe, Summation, Incomplete Tetanus, Complete Tetanus)

• Latent Period: Ca²⁺ release from SR, "Slack" taken out of system called Series Elastic Component (Contractile elements, tendons in whole muscle)
• Contraction Period: Tension Increases
• Relaxation Period: Tension decreases, passive process follows pumping of Ca²⁺ back into SR, passive process

• 
• Also called "warm up effect"
• Gradual increase in strength of contraction with the same stimulus

• Factors:
• 1. Increased Ca²⁺ available (No time to pump it all back into SR)
• 2. Muscle fiber warms up (Enzymes more efficient at higher temp)

Repeated stimulation before relaxation phase has been completed → stronger contraction

• Wave Summation: one twitch is added to another
• Incomplete Tetanus: muscle doesn't relax completely
• Complete Tetanus: relaxation phase is eliminated

Stronger contractions (summation) probably due to:

• Prolonged presence of Ca²⁺
• Action Potential (AP) duration vs. contraction duration:
• *AP is short
• *Contraction is longer
• *So another AP can stimulate the muscle before the contraction phase has ended

• 
• Occurs when successive stimuli arrive before the relaxation phase has been completed.

• 
• Occurs if the stimuli frequency increases further. Tension production rises to a peak, and the periods of relaxation are very brief.

• 
• The stimulus frequency is so high that the relaxation phase is eliminated. Tension plateaus at maximum levels.
• STRONGEST

• Internal Tension
• *Tension generated inside contracting muscle
• *Myosin pulling on actin in sarcomere

• External Tension
• *Tension generated on extracellular fibers
• *Endo-, peri-, and epimysium form tendons
• *Tendons stretch (Series Elastic Component)

• 
• Simple Twitch and Tetanus
• SEC = Series Elastic Component

At least some of the motor units of a muscle are active at any one time, even at rest.

• Which motor units are active varies
• Does not produce movement, but generates muscle tone

• Muscle Tone:
• Stabilizes bones and joints
• Maintains body position
• Allows more rapid activation of whole muscle

• 1. Isotonic: Means "same tension"
• Whole muscle's length changes
• a. Concentration Contraction (Tension>Load, Muscles shortens)
• 
• b. Eccentric Contraction (Tension<Load, muscle lengthens [stretches] while contracting)
• 

• 2. Isometric: Means "same length"
• Muscle's length does not change, but individual fiber lengths do shorten
• Tension is not greater than the muscle load
• 

• 
• Contraction velocity is inversely proportional to resistance.
• High resistance (heavy weight) leads to slow contraction speed.
• Fairly obvious from everyday life!
• Each muscle has optimum combination of speed and load.

• Energy-producing systems in a muscle:
• *Phosphagen system (ATP and CP reserves)
• *Glycolysis (Glycogen-lactic acid system)
• *Aerobic system

• Found only in muscle cells
• Involves creatine phophate (CP) and ATP
• A FAST, SHORT-TERM method of ATP gerneration
• *Involves only 1 enzyme (creatine phosphokinase), not a long pathway

CP+ADP+H⁺↔ creatine+ATP

• ATP is used for muscle contraction
• Creatine is only in muscle cells
• Total is enough for maximal burst of about 15 sec.

Allows time for other systems to "kick in."

• Used after phosphagen system during burst activity
• *Is the first part of aerobic pathway, BUT does not require oxygen (anaerobic)

• Anaerobic metabolism leads to H⁺ buildup
• *Decreases muscle cell pH
• *Affects enzyme function

Can provide maximal burst of energy for about 2 minutes

• REQUIRES oxygen delivery to mitochondria
• *Krebs Cycle/Electron Transport Chain
• *Inolves a multi-enzyme pathway

• Resting Muscle
• *Use fatty acid as a substrate (ATP)

• Active Muscle
• *Use pyruvate from glyocolysis as substrate
• *Glycogen→glucose→(glycolysis)→pyuvate
• *Pyruvate-(mitochondria)→Aerobic ATP generation
• *Provides energy for long-term exercise (Marathon run=almost all aerobic)

Resting muscle 

• Low ATP demand
• Lots of Oxygen available to mitochondria
• *Surplus ATP→CP RESERVES
• *Surplus glucose→glycogen RESERVES
• Use fatty acids from blood for energy production

• Increased demand for ATP
• Increased demand for O₂, but O₂ delivery still matching O₂ demand by mitochondria.
• Aerobic metabolism still active
• Muscle glycogen→glucose→pyruvate→ATP

OR

• Also use fatty acids (and amino acids)
• No surplus ATP so no new CP is produced (not enough left over).

O₂ deliver cannot meet O₂ demand

• Anaerobic Metabolism
• Mitochondrial ATP production very low (about 1/3 of total needed) due to low oxygen levels
• ATP production is via glycolysis (decreased cellular pH helps limit exercise duration)

• Normal Function Requires:
• 1. Energy Reserves
• 2. Blood Supply (Deliver O₂, Nutrients/Carry away wastes [CO₂, heat, etc.])

• Factors leading to muscle fatigue:
• 1. Low energy reserves (Low substrate)
• 2. Low pH, which effects
• *Changes in enzyme acitivty
• *H⁺ displaces Ca²⁺ from troponin
• *H⁺ interferes with hemoglobin reoxygenation
• 3. Central Fatigue 
• *pH effects on brain
• *Pain effects on brain
• *Recover faster when performing a "diverting" mental activity
• 4. Other factors
• *Sarcolemma, SR damage
• *Increase in ADP

• Return muscle cell conditions to resting levels
• Return oxygen consumption rate to resting levels

• Resting mechanism include:
• 1. Lactic acid removal/recycling
• 2. Excess postexercise oxygen consumption (EPOC)
• 3. Heat Loss (sweating, vasodilation)

Fate of Lactic Acid depends upon severity of exercise

• 1. Moderate Exercise
• *Glycogen stores and blood glucose not severely depleted
• *Lactate→Pyruvate→Mitochondria→energy for recovery from exercise
• *This is the usual fate of lactate

• 2. Severe, prolonged exercise
• *Glycogen and glucose depleted
• *Lactate to liver, converted to glucose→muscle cells→Glycogen (Cori cycle)

Oxygen consumption does not return to resting levels immediately after exercise. Some reasons:

• 1. ATP is required to restore CP levels, and go from lactate to glucose to glycogen
• 2. Sweat glands active (using ATP)
• 3. Effects of exercise on the mitochondria
• *Mitochondria less efficient at using O₂ for ATP production
• *Ca²⁺ leaks into mitochondria, must be pumped out
• 4. Myoglobin must be reoxygenated
• 5. Epinephrine causes leakage of Na⁺ into and K⁺ out of cells
• *Must be pumped back out or in
• 6. Increased body temperature→increased reaction rates→increased ATP consumption

THESE ALL CONTRIBUTE TO WHY WE BREATHE HARDER

• Slow Fibers: "dark meat", Type l, red, slow twitch, slow oxidative
• Intermmediate Fibers: Type ll-A, FR (fast resistant, fast twitch oxidative)
• Fast Fibers: "white meat", Type ll-B, white, FF (fast fatigue), fast-twitch glycolytic.

• Percentages of Fast and Slow fibers are genetically determined. Training can cause:
• *Fast Fibers↔Intermmediate Fibers
• *Slow Fibers↔Intermmediate Fibers

**KNOW THE DIFFERENC BETWEEN THESE TWO FROM THE CHART**

• Anaerobic, limited by:
• *ATP/CP availabibity
• *Glycogen availability
• *Tolerance for acidosis

• Aerobic, limited by:
• *Oxygen delivery to mitochondria (Cardiac Output, Capillary Density)
• *Number of mitochondria
• *Aerobic substrate availablilty