Lecture Notes: Muscle Twitch and Phases

Definition of Muscle Twitch

• A muscle twitch is defined as:
  • A single electrical event from a motor neuron.
  • The ensuing mechanical event (contraction in skeletal muscle).
• Variability:
  • Length of a muscle twitch varies among different skeletal muscles.
  • Example:
    • Lateral rectus (eye muscle): Very short contraction period for rapid eye movement.
    • Gastrocnemius and Soleus (leg muscles): More protracted contraction period.

Electrical Events in Muscle Fibers

• Similarities to Motor Neurons:
  • Propagation of the action potential via voltage-gated sodium and potassium channels.
• Differences:
  • Muscle fibers are not insulated (no saltatory conduction).
  • Despite this, propagation velocity is approximately 2 ms.

Phases of Muscle Twitch

1. Excitation Phase

   • Conversion of End-Plate Potential (EPP) into an action potential.
   • Involves voltage-gated sodium channels reaching threshold.
   • Propagation along T-tubules to the DHP protein.
   • DHP (voltage-sensitive) changes conformation, affecting RYR protein.
   • Calcium diffuses into cytosol, marking the latent period (no tension developed yet).

2. Contraction Phase

   • Calcium binds to troponin, shifting tropomyosin.
   • Actin and myosin interaction occurs because the binding site is exposed.
   • As long as calcium is in the cytosol, calcium-troponin complexes keep tropomyosin shifted.

3. Relaxation Phase

   • Focus on removal of cytosolic calcium.
   • Tropomyosin shifts back to block actin-myosin interaction.
   • Strong cross-bridge ends, stopping tension development.

Load Dependency of Phases

• Latent Period:
  • Short when carrying light loads.
  • Extends with heavier loads as more tension is required to counteract the load.
  • Few cross-bridges form but work against the load is insufficient to be considered part of the contraction.

Duration of Events

• Electrical Events: Take milliseconds.
• Contraction and Relaxation: Prolonged, lasting over 100 ms, varying in different muscles.

Upcoming Topics

• Next focus: Contractile Unit of the Muscle
  • Focus will be on the sarcomere, enabling greater tension generation by adding more sarcomeres.
  • Future focus on the motor unit to understand muscle as an organ.