Lecture Notes: Actin and Myosin in Muscle Contraction

Introduction

• Famous couples: Romeo & Juliet, Helen & Paris, Tristan & Isolde.
• Actin and myosin are like the famous star-crossed lovers in muscle cells.
• Muscle movement is fueled by the interaction of actin and myosin.

Muscle Tissue Types

• Smooth Muscle: Found in visceral organs (stomach, airways, blood vessels); involuntary.
• Cardiac Muscle: Found in the heart; striated and involuntary.
• Skeletal Muscle: Striated, voluntary; associated with visible body muscles.

Structure of Skeletal Muscles

• Skeletal muscles resemble ropes made of bundles of fibers.
• Myofibrils: Tiny threads forming muscle fibers.
• Muscle Fibers: Muscle cells with multiple nuclei and a sarcolemma (cell membrane).
• Bundles of muscle fibers form fascicles, which make up larger muscle organs.
• Muscles have connective tissue sheaths for protection.

Muscle Contraction Process

• Proteins Rules:
  1. Proteins change shape when stuff binds to them.
  2. Shape change allows proteins to bind or unbind.
• Sarcomeres: Segments in myofibrils containing actin & myosin.
  • Actin: Thin, light, twisty protein strands.
  • Myosin: Thick, lumpy protein strands.
• Sliding Filament Model: Actin & myosin interaction causes muscle contraction.

Contraction Mechanism

• Resting Muscle Cells:
  • Actin & myosin do not touch.
  • Myosin wants to bind to actin but is blocked by tropomyosin and troponin.
• Calcium and ATP Role:
  • Calcium binds to troponin, changing its shape, moving tropomyosin, and exposing actin.
  • ATP is used by myosin heads to change shape and bind to actin.
• Myosin heads powered by ATP lead to contraction by pulling actin (like pulling a rope).
• Cycle involves ATP breakdown, myosin shape change, and actin-myosin binding/unbinding.
• Calcium is recycled back to the sarcoplasmic reticulum, resetting the cycle.

Summary

• Muscle movement is through the cycle of contractions in the sliding filament model.
• Skeletal muscles are structured with bundles and supported by a network of connective tissues.
• Use of calcium and ATP is crucial for muscle contraction cycles.

Closing Remarks

• Acknowledgment of contributors and production team.
• Reference to supporting platforms like Patreon for educational series like Crash Course.