Muscle Contraction and Fiber Types: A Cellular Process

Posted on Jan 17, 2025 in Human Biology

Phases of Neuromuscular Transmission

  1. The action potential originates in the axonal cone and travels along the axon, specifically through the nodes of Ranvier. Upon reaching the presynaptic membrane, it triggers the opening of voltage-gated calcium (Ca) channels.
  2. Calcium ions enter the neuron, causing the presynaptic membrane to attract vesicles containing acetylcholine (ACh) located in the cytosol of the axon.
  3. Acetylcholine is released into the synapse.
  4. Chemical-gated channels open when coupled with ACh, allowing sodium (Na) to enter the cell.
  5. An action potential occurs in the muscle cell membrane and spreads throughout the plasmalemma.
  6. In the triad, the dihydropyridine receptor (DHPR), located near the terminal cisternae, is stimulated by the action potential. This, in turn, stimulates ryanodine receptors.
  7. Ryanodine receptors, which are calcium channels, open, releasing calcium into the sarcoplasm.
  8. Once in the sarcoplasm, calcium binds to troponin C.

Stepper Theory of Muscle Contraction

  1. Initially, ATP is not present. The myosin head and actin are linked.
  2. ATP appears and binds to the myosin head, detaching it from actin.
  3. ATP is hydrolyzed, releasing energy and inorganic phosphate (P). This energy causes a structural change in the myosin head. ADP and P remain bound to the myosin head.
  4. ADP and P are released, allowing the myosin head to bind to another actin site.
  5. Through a passive mechanism, the myosin head returns to its initial position, pulling the actin filaments and causing contraction.

Relaxation of Muscle Contraction

Muscle relaxation occurs through two mechanisms:

  • Withdrawal of cytosolic calcium.
  • Action of acetylcholinesterase (AChE).
  1. The myosin head is coupled to the active site of actin. ATP is separated from actin.
  2. ADP and Pi are held together to myosin.
  3. The myosin head rotates, reaching the height of actin.
  4. The release of Pi initiates the recovery movement. The head rotates on its hinge to drive the actin filament.
  5. ADP is released, and the myosin head resumes its original position.
  6. Calcium pumps in the sarcoplasmic reticulum (SR) remove calcium from the sarcoplasm. Troponin covers the active sites on actin again.

Muscle Fiber Types

Myosin varies with the nuclear domain, depending on the characteristics of the muscle cell. Muscle fibers are classified based on their myosin function:

  • Type I Fibers (Slow-Twitch or Red)

    • Capillaries per mm2 section: Highly vascularized
    • Myosin ATPase activity: Low
    • Glycolytic capacity: Low
    • Oxidative capacity: High
    • Predominant energy system: Aerobic

  • Type II Fibers (Fast-Twitch)

    Some books differentiate between type IIA and IIB, but in humans, only type IIX exists.

    • Type IIA

      • Capillaries per mm2 section: Many
      • Myosin ATPase activity: High
      • Glycolytic capacity: Intermediate
      • Oxidative capacity: Intermediate
      • Predominant energy system: Combined

    • Type IIX

      • Capillaries per mm2 section: Less vascularized
      • Myosin ATPase activity: High
      • Glycolytic capacity: High
      • Oxidative capacity: Low
      • Predominant energy system: Anaerobic

Smooth Muscle vs. Skeletal Muscle

Unitary Smooth Muscle (Visceral or Syncytial)

  • Location: Part of the structure of vessels or hollow viscera
  • Function: Contraction-vessel dilation and movement inside hollow viscera
  • Appearance: Smooth
  • Action potential: Spontaneous
  • Duration of electrical activity: 300 milliseconds
  • Contractile duration: Long (minutes to hours)
  • Shortening speed: Very slow

Skeletal Muscle

  • Location: Attached to bones
  • Function: Movement
  • Appearance: Striated
  • Action potential: Motor neuron action
  • Duration of electrical activity: 0.5 milliseconds
  • Contractile duration: Short
  • Shortening speed: Fast