physiology 9a

• Activation and mechanical properties of skeletal muscle

  • Describe the nature of the length-tension relationship for individual sarcomeres and for whole muscle
  • Indicate what influences force summation
  • Differentiate between muscle types on basis of fibre length, diameter and energetics of contraction
• Sarcomere length-tension relationship


  • Passive tension results from muscle being stretched
  • Active tension represents force developed during cross-bridge cycling
  • Total tension is the sum of these
  • Many joints are limited to keep muscles in the middle range
• Whole muscle contraction

  • Recruitment of different (numbers of) motor units
  • Summation of action potentials
  • Different ways to release ATP for contraction
• Motor units in the muscles

  • One motor neuron → many muscle fibres
  • One muscle fibre ← only one motor neuron
  • One motor neuron + all fibres it innervates = motor unit
  • Size of motor unit depends on muscle type and required force range
• Motor unit examples


  • Laryngeal muscle (2-3 fibres)
  • Soleus muscle (100s of fibres)
• Spatial summation


  Temporal summation
• Muscle fibre differences


  • Length can simply be measured
  • Diameter can simply be measured
  • Energetics can be implied by colouration (more mitochondria = more aerobic)
• Muscle fibre types


  Slow vs fast fibres
• Energy sources for muscle contraction

  1. Creatine phosphate

  2. Anaerobic glycolysis

  3. Oxidative phosphorylation
• Whole muscle contraction

  1. Isolate a muscle fibre from an animal

  2. Attach weight to it, apply electrical current

  3. Repeat the experiment in presence of apyrase, an enzyme that depletes ATP from the medium
• Schematic structure of the sarcomere


  Sarcomere length-tension relationship
• Sarcomere length-tension relationship

  • Passive tension results from muscle being stretched
  • Active tension represents force developed during cross-bridge cycling
  • Total tension is the sum of these
  • Many joints are limited to keep your muscles in the middle range
• Passive force

  Due mostly to elastic proteins known as titin, which stretch like springs, so passive tension increases with sarcomere length
• Titin is the 3rd most abundant protein in your muscles and is also the longest human protein (it has over 34,000 amino acids!)
• Dystrophin


  Another very long protein (3685 amino acids)
• Duchenne muscular dystrophy is an X-linked genetic disease that affects 1/5000 males, with most mutations in exons 43-55, and currently no therapy exists
• Motor units in the muscles

  • One motor neuron → many muscle fibres
  • One muscle fibre ← only one motor neuron
  • One motor neuron + all fibres it innervates = motor unit
• Motor neurons cause muscle contraction
• Muscle types


  • Length can simply be measured
  • Diameter can simply be measured
  • Energetics can be implied by the colouration of the fibres, as darker fibres have more mitochondria and rely more on aerobic respiration
  • Slow vs fast fibres
• Muscle energetics


  • Creatine phosphate (fastest)
  • Anaerobic glycolysis (fast but inefficient)
  • Oxidative phosphorylation in mitochondria (slow but ATP-efficient)
• Type I fibres


  Endurance activity, long periods of activity, low levels of tension, fatigue resistant, examples: postural muscles, soleus (used in walking, long distance running, functional activities)
• Type II fibres


  Burst activity, fast, powerful explosive contractions, fibres fatigue easily, examples: sprinting, weight lifting, e.g. gastrocnemius
• Fibre ratios vary based on muscle type, training regime, inheritance
• Fatigue


  Central (CNS), Muscular, Physiological causes: Local increase in phosphate, Ca2+ leakage through SR, Quick depletion of glycogen