Download ANPS 019 Beneyto-Santonja 10-12

ANPS 019 Beneyto-Santonja 10/12/12
Muscle Metabolism: Energy Use and Muscle Contraction
Learning Objectives
 Describe how muscle fibers obtain energy for contraction
 Distinguish between aerobic and anaerobic contraction, muscle fiber types, and muscle
performance.
Muscle Contraction requires large amount of energy
 Creatine phosphate releases stored energy to convert ADP to ATP
 Aerobic metabolism provides most ATP needed for contraction
 At peak activity, anaerobic glycolysis needed to generate ATP
 Creatine phosphate; ATP and Glycogen; 15 billion thick/fiber; each one 2500 molecules
of ATP/second; thousands of muscle fibers per muscle
 Cell 3.75 10(16) power ATP molecules required.
Muscle Metabolism
 Resting muscle: fatty acids are catabolized; the ATP produced is used to build energy
reserves of ATP, CP, and glycogen
 Moderate activity: Glucose and fatty acids are catabolized; the ATP produced is used to
power contraction
 Peak activity: Most ATP is produced through glycolysis, with lactic acid as a by-product.
Mitochondria activity (not shown) now provides only about one-third of the ATP
consumed.
Energy use and level of muscular activity
 Energy production and use patterns mirror muscle activity
 Fatigued muscle no contracts
o Build up of lactic acid
o Exhaustion of energy resources
Recovery Period
 Begins immediately after activity ends
 Oxygen debt (excess post-exercise oxygen consumption)
o Amount of oxygen required during resting period to restore muscle to normal
conditions
Muscle Performance
 Types of skeletal muscle fibers
o Fast Fibers  larger diameter, paler color; easily fatigued
o Slow Fibers  smaller diameter, darker color due to myoglobin; fatigue
resistance
o Intermediate fibers  Similar to fast fibers; greater resistance to fatigue
Fast Fibers
 Large in diameter
 Contain densely packed myofibrils
 Large glycogen reserves
 Relatively few mitochondria
 Produce rapid, powerful contractions of short duration
Slow Fibers
 Half the diameter of fast fibers
 Take three times as long to contract after stimulation
 Abundant mitochondria
 Extensive capillary supply
 High concentrations of myoglobin
 Can contract for long periods of time
Muscle performance and the distribution of muscle fibers
 Pale muscles dominated by fast fibers are called white muscles
 Dark muscles dominated by slow fibers and myoglobin are called red muscles
 Training can lead to hypertrophy of stimulated
Physical conditioning
 Anaerobic endurance  time over which muscular contractions are sustained by
glycolysis and ATP/CP reserves
 Aerobic endurance  time over which muscle can continue to contract while supported
by mitochondrial activities
Physiological profiles of motor units: all fibers in a motor unit are of the same fiber type
 Slow motor units contain slow fibers:
o Myosin with long cycle time and therefore uses ATP at a slow rate
o Many mitochondria, so large capacity to replenish ATP
o Economical maintenance of force during isometric contractions and efficient
performance of repetitive slow isotonic contractions
 Fast motor units contain fast fibers:
o Myosin with rapid cycling rates
o For higher power or when isometric force produced by slow motor units is
insufficient.
o Type 2A fibers are fast and adapted for producing sustained power
Muscle is adaptable!
 Muscle “adapts” to meet the habitual level of demand placed on it, i.e. level of physical
activity
 Level of physical activity determined by the frequency of recruitment and the load
 Increase muscle use
o Endurance training
o Strength training (cannot be optimally trained for both strength and endurance)
 Decrease muscle use
o Prolonged bed rest
o Limb casting
o Denervation
o Space flight
Endurance training
 Little hypertrophy but major biochemical adaptations within muscle fibers
 Increased numbers of mitochondria; concentration and activities of oxidative enzymes
(e.g. succinate dehydrogenase, see below)
Increased use: Strength training
 Early gains in strength appear to be predominantly due to neural factors… optimizing
recruitment patterns
 Long term gains almost solely the result of hypertrophy i.e. increased size
Disuse causes atrophy – USE IT OR LOSE IT!
 Individual fiber atrophy (loss of myofibrils) with no loss in fibers.
 Effect more pronounced in Type II fibers (Fast, white fibers)
 “Completely reversible” (in young healthy individuals)
 ATPase activity:
o Type I fibers light
o Type II fibers dark
Motor unit remodeling with aging  Fewer motor units; more fibers/motor unit
Muscle injury may play a role in the development of atrophy with aging
 Muscles in old animals are more susceptible to contraction-induced injury than those in
young or adult animals
 Muscles in old animals show delayed and impaired recovery following contractioninduced injury
 Following severe injury, muscles in old animals display prolonged, possibly irreversible,
structural and functional deficits.
You should now be comfortable with the following concepts!
 The organization of muscle and the unique characteristics of skeletal muscle cells
 The structural components of the sarcomere
 The events at the neuromuscular junction
 The key concepts involved in skeletal muscle contraction and tension production
 How muscle fibers obtain energy for contraction
 Aerobic and anaerobic contraction, muscle fiber types, and muscle performance.