Muscle Contraction: Twitch, Summation, and Tetanus

Key Concepts

• Muscle Fiber: Basic unit of muscle contraction.
• Twitch: Contraction of a single muscle fiber by a single stimulus; smallest unit of muscle contraction.

Summation of Force

Muscle contraction force can be increased by adding multiple twitches together through:

1. Frequency Summation

   • Stimulating the same muscle fiber at higher frequencies.
   • Graph Explanation:
     • X-axis: Time
     • Y-axis: Tension developed by a muscle fiber
   • Twitch Duration: 25 to 200 milliseconds, longer than the action potential.
   • Mechanism:
     • Stimulate before the first twitch ends to add tension.
     • Increased frequency leads to increased total tension.
     • Tetanus:
       • High-frequency stimulation leads to fused twitches.
       • Tension reaches a steady plateau.
       • Caused by insufficient time to pump calcium back, preventing relaxation.
       • Above this frequency, tension increases minimally.

2. Multiple Fiber Summation

   • Stimulating multiple muscle fibers simultaneously.
   • Motor Unit: Group of muscle fibers supplied by one motor neuron.
   • Schematic: Shows multiple motor units.
   • Force Generation: Stimulate more motor neurons for greater force.
   • Size Principle:
     • Motor units vary in size.
     • Weak stimuli activate small motor units first, making force increase in small steps.
     • Stronger stimuli activate larger motor units, leading to larger force increments.
     • Allows fine adjustment in force generation during weak contractions.
   • Asynchronous Activation:
     • Motor units alternate during sustained contractions (e.g., holding a cup).
     • Smooth alternation maintains constant muscle force.

Summary

• Twitch: Smallest muscle contraction unit by single stimulus.
• Frequency Summation: Adds twitches of the same fibers via high-frequency stimulation.
• Tetanus: Fusion of twitches with undistinguishable individual twitches.
• Multiple Fiber Summation: Simultaneous stimulation of multiple fibers.
• Motor Unit Recruitment:
  • Small units for weak stimuli.
  • Larger units with stronger stimuli.
  • Asynchronous activation maintains muscle force.

Conclusion

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