Muscle Fiber Response to Stimulus
Key Concepts
- Tension Development: Illustrated on the y-axis against time on the x-axis.
- Stimulus: Action potential triggers changes within a muscle fiber.
Phases of Muscle Fiber Contraction
- Latent Period: No tension develops right after stimulation.
- Contraction Phase:
- Calcium binds to troponin, causing tropomyosin to shift.
- Binding sites on actin exposed; activated myosin forms cross-bridges.
- Sarcomeres shorten, tension develops.
- Relaxation Phase:
- Calcium sequestered back into smooth ER.
- Binding sites covered, tension decreases.
- Results in a muscle twitch.
Action Potential and Twitch
- Action potential: ~1-2 milliseconds.
- Muscle twitch: ~100 milliseconds.
- Summation: Increased tension from repeated stimuli before relaxation.
- Tetanus: Maximal tension reached when all binding sites are exposed.
Factors Influencing Muscle Fiber Contraction
- Frequency of Stimulation: Summation increases tension.
- Fiber Thickness: More actin and myosin = more potential cross-bridges = more tension.
- Sarcomere Length:
- Optimal overlap allows maximal tension.
- Too short or too stretched sarcomeres decrease tension.
Motor Units
- Definition: A single motor neuron and all muscle fibers it innervates.
- Activation leads to all fibers reaching tension or tetanus.
- Muscles consist of multiple motor units, not just one.
Muscle Contraction as a Whole
- Recruitment of Motor Units: More motor units lead to greater tension.
- Sub-Maximal Load: Lifting less than maximum capacity.
- Asynchronous Recruitment: Rotating motor units to prevent fatigue.
- Maximal Load: Activating all motor units, leading to eventual fatigue.
Movement Precision
- Motor Unit Size: Affects control precision.
- Smaller motor units allow for more precise movements.
- Larger motor units contain more fibers, less precision.
These notes summarize the muscle fiber response to stimuli, the phases of contraction, factors influencing tension, and the concept of motor units. Understanding these concepts is crucial for comprehending muscle physiology and mechanics.