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).