Overview of Muscle Contraction and Cell Structure

Muscle Contraction Basics

  • Contractile Organelle:
    • Definition: Organelle involved in contraction (e.g., muscle fibers).
    • Found in: Cells, particularly muscle cells.

Muscle Fiber Structure

  • Muscle Fiber:
    • Defined as a single muscle cell (sarcomere).
    • Characteristics: Thin, long, contains multiple myofibrils that are aligned in the direction of muscle contraction.

Myofilaments

  • Myofilaments:
    • Types:
    • Thick Filaments (composed of myosin)
    • Thin Filaments (composed of actin, troponin, tropomyosin)
    • Elastic Filaments
  • Thick Filament:
    • Made of Myosin.
    • Myosin Molecule Structure:
    • Features two heads and an intertwined tail.

Thin Filaments

  • Composition:
    • Actin (two types: globular and fibrous), Troponin, and Tropomyosin.
  • Active Site on Actin:
    • Function: Provides a binding site for myosin heads during contraction; forms a strong bond known as a cross bridge.

Regulatory Proteins

  • Tropomyosin Function:
    • Blocks binding of myosin to the active site on actin.
  • Troponin Function:
    • Binds calcium ions, causing a conformational change that moves tropomyosin away from the active site, allowing myosin to bind.

Muscle Cell Membrane and Components

  • Sarcolemma:
    • The cell membrane of the skeletal muscle cell.
  • T Tubules:
    • Transmit the electrical impulse (action potential) into the muscle fiber.
  • Sarcoplasmic Reticulum (SR):
    • Organelles that wrap around myofibrils and store calcium ions; intracellular calcium levels are low until stimulated.

Sarcomere and Muscle Striation

  • Sarcomere:
    • Defined as the functional and structural unit of striated muscle (between adjacent Z discs).
  • Striations:
    • Defined by A bands (dark stripes, thick filaments) and I bands (light stripes, thin filaments).
    • A Band: Runs the entire length of thick filaments.
    • I Band: Region without thick filaments.
    • H Zone: Part within the A band where there are no overlapping thin filaments.

Muscle Contraction Process

  1. Activation Phase:
    • Involves a neuron supplying the impulse to the muscle fibers.
  2. Excitation-Contraction Coupling:
    • The action potential leads to the release of calcium ions from the sarcoplasmic reticulum, facilitating the contraction process.
  3. Cross Bridge Cycle:
    • Repeated formation and breaking of bonds between myosin heads and actin during contraction.

Nerve Impulse and Muscle Contraction

  • Nerve Impulse Definition:
    • Proper term: Action Potential.
    • It involves a change in membrane charge and is how signals are transmitted from the nervous system to muscles.
  • Neuromuscular Junction:
    • Connection point between the motor neurons and muscle fibers where motor impulses are transmitted.
    • Acetylcholine: The neurotransmitter released at the junction to initiate muscle contraction.

Important Terms to Remember

  • Cross Bridge: The bond formed between myosin and actin during contraction.
  • Neurotransmitter: Chemicals released by neurons to communicate with target cells (e.g., muscles).