Skeletal Muscle Function and Contraction

Jun 9, 2024

Lecture on Skeletal Muscle Function and Contraction

Types and Characteristics of Skeletal Muscles

  • Number of skeletal muscles: 640.
  • Shapes and sizes: Varies from the longest (sartorius) to the biggest (gluteus maximus) to the smallest (stapedius).
  • Range of functions: Can perform both delicate and powerful tasks.

Mechanics of Muscle Movement

  • Muscles never push, they pull:
    • Muscles extend over joints to connect bones.
    • Insertion point: Moving bone.
    • Origin: Less or non-moving bone.
    • Example: Pushups - Pectoralis major pulls the humerus ( insertion) toward the sternum (origin).
  • Principle: Muscles pull insertions towards origins, creating movement.

Functional Groups of Muscles

  1. Prime Movers (Agonists): Main muscles producing movement.
    • Example: Pectorals in jumping jacks.
  2. Antagonists: Muscles working in reverse of a prime mover.
    • Example: Deltoids in jumping jacks.
  3. Synergists: Assist prime movers by adding force or stabilizing joints.
    • Example: Rotator cuff muscles.

Muscle Contraction Mechanics

  • Motor Units: Group of muscle fibers that get signals from one motor neuron.
    • Large motor units: Several fibers per neuron, used for large movements.
    • Small motor units: Few fibers per neuron, used for precise movements.
  • Twitch: Contract/relax cycle due to a single action potential.
    • Three phases: Latent period, Contraction period, Relaxation period.

Graded Muscle Responses

  • Affected by:
    • Frequency of stimulation: Higher frequency increases force.
    • Strength of stimulus: More motor units activated, greater force.
  • Temporal summation: Successive twitches add up to increase force.
  • Tetanus: Maximum tension when twitches fuse into one continuous contraction.
  • Motor Unit Recruitment: Increasing the number of motor units to increase force.
    • Size principle: Smallest to largest motor units get recruited.

Types of Muscle Contractions

  1. Isotonic: Muscle changes length to move a load (e.g., lifting a mug).
  2. Isometric: Muscle tension without changing length (e.g., trying to lift something immovable).

Summary

  • Skeletal muscles create and reverse movements through coordinated actions of prime movers, antagonists, and synergists.
  • Motor units' size and activation affect contraction force and duration.
  • Graded responses enable controlled and powerful movements.
  • Two main contraction types: isotonic and isometric.
  • Muscle fatigue and twitch summation are key in understanding muscle performance limits.

Note: No corgis were harmed in this presentation.