Muscle Contraction Overview

Overview of Muscle Types

• Three main types of muscles: skeletal, smooth, and cardiac.
  • Cardiac Muscle: Located in the heart; involuntary control.
  • Smooth Muscle: Surrounds hollow organs (like intestines, esophagus); involuntary control.
  • Skeletal Muscle: Attached to bones; voluntary control.

Muscle Structure and Terminology

• Muscle Characteristics:
  • Each muscle is an organ made of bundles of muscle fibers, which are individual cells.
• Cellular Components:
  • Sarcoplasm: Cytoplasm of a muscle cell.
  • Sarcolemma: Plasma membrane of a muscle cell.
  • Myofibrils: Bundles of filaments within a muscle fiber. Comprised of thin and thick filaments.
• Two important prefixes:
  • Sarco-: Greek for muscle.
  • Myo-: Latin for muscle.

Mechanism of Muscle Contraction

• Contraction Process:
  • Muscle contraction occurs when myofibrils shorten due to sliding of thin and thick filaments.
• Filament Types:
  • Thin Filaments: Composed primarily of actin.
  • Thick Filaments: Consist mainly of myosin.
• Sarcomeres:
  • The repeating unit of myofibrils; composed of pairs of thin filaments with thick filaments in between.
  • Contraction decreases the distance between sarcomeres, pulling thin filaments toward the center of the sarcomere.

Sliding Filament Theory

• During contraction:
  • Myosin heads bind to actin, undergo a conformational change, pulling actin filaments toward the sarcomere center.
• Importance of Calcium:
  • Calcium binds to troponin on the thin filament. This causes tropomyosin to move and expose the binding sites for myosin on actin.
• ATP's Role:
  • ATP is required for releasing myosin heads from actin after a power stroke.

Muscle Cell Contraction Regulation

• Calcium must be present for muscle contraction; stored in the sarcoplasmic reticulum.
• Troponin and Tropomyosin:
  • Tropomyosin blocks myosin binding sites on actin in a relaxed state.
  • Troponin changes shape when calcium binds, displacing tropomyosin, allowing myosin to bind to actin.
• Control Mechanisms:
  • Action potentials from motor neurons trigger the release of calcium from the sarcoplasmic reticulum.

Motor Units and Muscle Control

• Motor Unit:
  • One motor neuron and all muscle fibers it innervates.
  • Action potentials in the motor neuron result in simultaneous contraction of all muscle fibers in that unit.
• Strength of Muscle Contraction:
  • Controlled by the number of motor units activated.
• Fine Motor Control:
  • Muscles requiring fine control (like fingers) have smaller motor units compared to muscles that require less precise control (like the legs).

Additional Features of Muscle Function

• Muscles can only contract; they must work in opposition to return to their original length.
• Action of muscles against resistance provides movement in conjunction with skeletal structures.
• Importance of continued neural stimulation to sustain contraction; without action potentials, calcium is pumped back, causing relaxation.