Whole Muscle Mechanics

Muscle twitch

• Contraction of one muscle fiber in response to a single stimulus
• Twitches in different muscles can vary in duration<
• Myogram - record of muscle fiber contraction
  • Latent period - time between stimulus and when contraction begins, Ca⁺² release (2 msec)
  • Contraction period - shortening, produces maximum tension (10-100 msec)
  • Relaxation period - lengthening, active transport of Ca⁺²   (10-100 msec)

Figure in class

All or None Principle

A muscle fiber at a given resting length, when stimulated to contract, always produces the same amount of tension.

Variation in the amount of tension produced in a whole muscle is determined by:

1. Frequency of stimulation - internal and external tension
2. Number of muscle fibers stimulated or recruitment

Stimulation Frequency (figures in lecture)

Normal activities require more tension than is produced by single fiber twitch, they involve sustained muscle contractions within the whole muscle

• Wave summation - if 2^nd stimulus is applied before relaxation is complete, the second contraction is greater
• Tetanus - at higher frequency of stimulation muscle relaxation between contractions is reduced
  • Incomplete tetanus - producing peak tension during rapidly alternating cycles of contraction and partial relaxation
  • Complete tetanus - sustained maximal contraction at peak tension, typical of normal muscle contraction
• Treppe - relaxation is complete before next stimulus occurs, each contraction is a little stronger than previous.  Physiological efficiency improves because of temperature rise and gradual increase in Ca⁺² in sarcoplasm.  Basis for warm-up.

Effects of internal tension and external tension during muscle contraction

• Internal (active) tension - force generated in individual muscle fibers by myofibrils
• External (passive) tension - force generated in the connective tissue and tendons of the whole muscle
  • Muscle fibers are surrounded by connective tissue layers which are continuous with the tendons that attach to bones.  Connective tissues are flexible and elastic, called series elastic elements
• Process:  Internal tension is applied to the series elastic elements, which stretch, stiffen and then transfer tension to the resistance.  During summation external tension gradually climbs to level of internal tension.  (Demonstration in class)

Muscle fiber recruitment

The amount of tension produced in a whole muscle is also determined by the number of muscle fibers stimulated, called recruitment.  Muscle is composed of many muscle fibers each belonging to a motor unit.

1. Motor unit - all muscle fibers that are innervated by a single motor neuron

• When stimulated muscle fibers contract maximally for conditions (All or None Principle)
• Amount of whole muscle tension is determined by number of muscle fibers contracting, this is controlled by number and size of motor units functioning at one time
• Small fast-reacting fine-control muscles have few muscle fibers per motor unit, large slower muscles have many fibers per motor unit
• Gradation of muscle contraction is achieved by multiple motor unit summation, begins with small motor units and then adds larger motor units
• Muscle fibers of a motor unit are typically distributed throughout the fascicles of a muscle producing uniform contraction whether weak or strong
• Smoothness of contraction is a function of motor units in the muscle firing asynchronously to prevent fatigue of fibers or jerky action
• Tone is the involuntary activation of a small number of motor units that give firmness to relaxed muscle

Characteristics of contraction

• Isotonic contraction - when a muscle contracts the external tension gradually increases to peak level, slightly exceeding the resistance and causing a change in muscle length. Causes changes in shape of muscle
  • Concentric contraction - tension increases as muscle shortens
  • Eccentric contraction - tension is maintained as muscle lengthens
• Isometric contraction - when a muscle contracts the peak tension produced is less than resistance and no significant muscle shortening occurs.  Contraction and stretching (gravity) are equal and opposing forces that create tension but no muscle shortening.  Produces no movement but stabilizes joints and maintains posture, uses energy.  ex. holding book
• Speed of muscle contraction - the heavier the resistance, the longer the muscle takes to reach peak tension and start to shorten, and the less the muscle will shorten.  For any given resistance there is an optimal combination of tension and speed for each muscle.
• Fatigue - nerve impulses arrive but muscle contractions become weaker because of ACh depletion or buildup of metabolic wastes and depletion of ATP

Muscle Energetics

1. Resting muscle
   1. ATP is produced by aerobic metabolism of fatty acids faster than needed
   2. Excess is used to produce creatine phosphate
2. Contracting muscle
   1. Normal stores of ATP last a few seconds (each thick filament uses about 2500 ATP molecules/sec)
   2. Then muscle switches to aerobic metabolism of pyruvic acid from glucose, stored glycogen, and fatty acids
3. Peak muscle activity
   1. There is insufficient O₂ available and muscle switches to anaerobic metabolism of pyruvic acid from glucose and stored creatine phosphate
   2. Creatine phosphate can transfer a high energy P to ADP to form ATP, extending activity for 15 sec

Types of Cellular Respiration:

1. Aerobic respiration
   1. For moderate activity longer than 30 sec (marathon is 100% aerobic)
   2. Fatty acids, glucose or glycogen is broken down into pyruvic acid that is used by mitochondria to form ATPs
   3. Slower than glycolysis but yields more ATP (36 net/glucose molecule)
   4. Monosaccharides preferred but can also break down fatty acids and amino acids
   5. Aerobic requires oxygen and is limited by oxygen availability, oxygen in blood is bound to Hb and in muscle is bound to muscle myoglobin
2. Anaerobic respiration
   1. Up to 3 min supply at maximal muscle activity (aerobic respiration continues to produce about 30% of ATP consumed)
   2. Glucose or glycogen is broken down into pyruvic acid in a series of reactions called glycolysis in the muscle cytoplasm (2 ATP net/glucose molecule).  Pyruvic acid is formed too rapidly for mitochondria to use and is converted into lactic acid which diffuses out of the muscle
   3. Anaerobic requires no oxygen
   4. Cori cycle
      1. During maximal activity lactic acid accumulates in blood and muscle (oxygen debt)
      2. When oxygen is available lactic acid is converted back into pyruvic acid in the liver and then into glucose and is returned to skeletal muscle to be stored as glycogen

Skeletal muscle fiber types:

Type I, Red slow-twitch fibers - contract slowly and fatigue slowly

• small diameter cells
• aerobic with many mitochondria, low glycolytic enzymes
• very vascular (O₂), high myoglobin levels (binds O₂)
• metabolism of lipids and carbohydrates, some amino acids
• Endurance activities, marathon, swimming, jogging

Type IIx, White fast-twitch - contract fast and fatigue quickly

• large diameter cells, most common
• anaerobic with few mitochondria, high glycolytic enzymes
• less vascularized, low myoglobin levels
• store glycogen, metabolism of carbohydrates
• Dashes and sprints, weight lifting, throwing ball

Type IIa, Intermediate fibers - contract fast and fatigue relatively slowly

• intermediate diameter cells
• anaerobic with intermediate number of mitochondria, high glycolytic enzymes
• intermediate vascularization, low myoglobin
• store glycogen, metabolism mainly carbohydrates
• Increase favors endurance activities

Smooth Muscle Contraction

• No T-tubules, gap junctions are present
• Sarcoplasmic retiiculum is a loose network. Calcium enters sarcoplasm from SR and ECF and binds with calmodulin protein, no troponin.
• Actin and myosin myofilaments overlap forming scattered bundles. Calmodulin-Ca activates myosin kinase, which transfers a phosphate from ATP to myosin and crossbridges form. Contraction produces a cellular twisting motion.
• Relaxation: myosin phosphatase removes phophate from myosin