19. Physiology of skeletal muscles – functional morphology, mechanism, and energetics of muscle contraction. Types of muscle contractions. Types of muscle fibres. Muscle work and muscle fatigue. Electromyography.

1. Functional Morphology

• 19. Physiology of Skeletal Muscles

• 1. Functional Morphology

  • Skeletal muscle = striated muscle composed of thousands of cylindrical muscle fibers

  • Fibers are bound by connective tissue containing blood vessels and nerves

  • Fixed number of fibers; muscle strength & mass increase by fiber thickening & connective tissue growth

• Muscle fiber structure:

  • Fusion of many myoblasts

  • Contains:

    • Sarcolemma (plasma membrane)

    • Sarcoplasmic reticulum (ER storing Ca²⁺)

    • Sarcosome (mitochondrion)

    • Sarcoplasm (cytoplasm)

  • only skeletal muscles have t tubules. Without T-tubules, the electrical signal from the surface wouldn't reach the deeper parts of the cell fast enough.

• Functional unit: Myofibril composed of repeating sarcomeres

  • Thick filaments = myosin (A-band)

  • Thin filaments = actin + troponin + tropomyosin

  • Z-lines define sarcomere boundaries

• Contraction changes:

  • Z-lines move closer

  • I band narrows

  • H zone narrows

  • A band width unchanged

2. Mechanism of Muscle Contraction

• Controlled by nervous system

• Steps:

  1. Action potential travels down motor neuron to neuromuscular junction

  2. Acetylcholine released into synaptic cleft

  3. Acetylcholine binds muscle receptors → triggers muscle action potential

  4. Ca²⁺ channels open; Ca²⁺ released from sarcoplasmic reticulum into cytosol

  5. Ca²⁺ binds troponin → causes tropomyosin to move, exposing myosin binding sites on actin

  6. Myosin heads bind actin forming cross-bridges; ATP hydrolysis provides energy

  7. Thin filaments slide over thick filaments → muscle shortens (contracts)

3. Energetics of Muscle Contraction

• ATP = immediate energy source for contraction

• Three ATP regeneration sources:

  1. Creatine phosphate system:

     • Creatine phosphate + ADP → creatine + ATP

  2. Glycolysis of glycogen:

     • Produces 2 ATP + lactic acid molecules (anaerobic)

  3. Cellular respiration in mitochondria:

     • Uses lactic acid to resynthesize glycogen (aerobic)

4. Types of Muscle Contractions

• Isotonic contractions: muscle changes length, tension constant

  • Concentric: muscle shortens while generating force

  • Eccentric: muscle lengthens while generating force

• Isometric contractions: muscle generates force without length change; no joint movement

5. Types of Muscle Fibres

• Slow twitch (Type 1, slow oxidative):

  • Fatigue resistant, endurance muscles (postural), red color

• Fast twitch:

  • Type 2A (fast oxidative-glycolytic): fast strong contractions, relatively fatigue resistant, uses aerobic & anaerobic ATP, red

  • Type 2B (fast glycolytic): rapid fatigue, anaerobic ATP use, white

6. Muscle Work and Muscle Fatigue

• Muscle work:

  • Different muscle activities produce variable power output (gradation of power)

• Muscle fatigue:

  • Inability to maintain tension due to previous activity

  • Recovery depends on intensity/duration of activity and muscle fiber type

7. Electromyography (EMG)

• When muscle fiber action potentials occur, electrical currents spread to skin surface

• EMG records this electrical activity using:

  • Surface electrodes placed on skin

  • Needle electrodes inserted into muscle

• Used to measure muscle electrical activity during stimulation