Notes on Muscle Contraction and Structure

Introduction to Muscle Contraction

• Famous star-crossed lovers: Romeo and Juliet, Helen and Paris, Tristan and Isolde represent longing and tragic separation.
• Similar relationship exists in muscle cells between actin and myosin.
• Actin and myosin are responsible for all bodily motions, including involuntary actions like keeping posture against gravity.

Types of Muscle Tissue

1. Smooth Muscle Tissue

   • Found in walls of hollow organs (e.g., stomach, airways, blood vessels).
   • Functions involuntarily to move fluids and materials.

2. Cardiac Muscle Tissue

   • Specialized for the heart; looks striped (striated).
   • Functions involuntarily to pump blood.

3. Skeletal Muscle Tissue

   • Comprises 640 muscles that are striated and mostly voluntary.
   • Attached to the skeleton, allowing for movement by pulling on bones.
   • Each skeletal muscle is an organ made up of muscle tissue, connective tissue, blood vessels, and nerve fibers.

Anatomy of Skeletal Muscle

• Composed of fibers within fibers:
  • Myofibrils: Parallel threads forming muscle fibers.
  • Muscle Fibers: Muscle cells with mitochondria, multiple nuclei, and a sarcolemma (cell membrane).
  • Fascicles: Bundles of muscle fibers that form the muscle organ.
• Each muscle has connective tissue sheaths for support (reinforcements).

Rules of Muscle Function

1. Proteins change shape when substances bind to them.
2. Changing shapes allows proteins to bind or unbind with other substances.

Sliding Filament Model of Muscle Contraction

• Muscle fibers contain sarcomeres, the functional units divided into:
  • Actin: Thin filaments.
  • Myosin: Thick filaments.
• Z Line: Borders of each sarcomere.
• Muscle contraction involves:
  • Actin and myosin binding together, shrinking sarcomeres, and contracting muscles.

Resting State of Muscle Cells

• Actin & myosin do not touch when muscles are at rest due to blocking proteins (tropomyosin and troponin).
• ATP and calcium are required to facilitate binding.
• Muscle cells are powered by ATP produced in mitochondria.

Mechanism of Contraction

1. Action Potential: Triggered by brain signals, travels along motor neurons to muscle cells.
2. Sodium Channels: Ligand-gated sodium channels open, leading to an influx of sodium, creating a graded potential.
3. Calcium Release: Action potential travels down T-tubules, triggering voltage-sensitive proteins to open calcium channels in the sarcoplasmic reticulum.
4. Troponin and Tropomyosin Interaction: Calcium binds to troponin, removing tropomyosin from actin binding sites, allowing myosin to bind.
5. Power Stroke: Myosin pulls actin (like pulling a rope), contracting the muscle.
6. Resetting Cycle: Myosin releases actin after binding new ATP, allowing for the next cycle of contraction.
7. Calcium Resorption: Calcium is pumped back into the sarcoplasmic reticulum, re-establishing muscle cell resting state.

Conclusion

• Muscle contraction is an ongoing cycle of binding and unbinding facilitated by calcium and ATP.
• The continuous drama of muscle contraction occurs as we perform various activities.
• Understanding muscle structure and function is essential for grasping how movement is achieved in the body.