Lecture Notes: Muscle Tissue and the Sliding Filament Model

Introduction

• Famous couples of proteins: Actin and Myosin
• Role: Cause all muscle contractions, voluntary and involuntary
• Muscle tissues turn chemical energy into mechanical energy (movement)

Types of Muscle Tissue

1. Smooth Muscle Tissue

   • Found in hollow visceral organs
   • Involuntary control (e.g., stomach, airways)
   • Function: Push fluids/materials by contracting and relaxing

2. Cardiac Muscle Tissue

   • Striated, found in the heart
   • Involuntary control
   • Function: Pumps blood

3. Skeletal Muscle Tissue

   • Striated, mostly voluntary
   • Attach to skeleton, create movement by pulling bones
   • Composed of muscle fibers, connective tissue, blood vessels, and nerves
   • Example: Biceps brachii

Skeletal Muscle Anatomy

• Constructed like a rope with fibers within fibers

• Muscle Fibers:

  • Made up of myofibrils (tiny parallel threads)
  • Have mitochondria, multiple nuclei, and a sarcolemma (membrane)

• Fascicles:

  • Bundles of muscle fibers forming larger muscle organs

• Connective Tissue Sheaths:

  • Provide reinforcement and protection

Muscle Contraction Mechanics

• Rules of Proteins:

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

• Sliding Filament Model:

  • Myofibrils divided into segments called sarcomeres
  • Sarcomeres contain actin (thin filaments) and myosin (thick filaments)
  • Z-lines mark the borders of sarcomeres

Muscle Contraction Process

• At rest, actin and myosin do not touch

• Actin blocked by tropomyosin and troponin

• Calcium and ATP remove these blocks

• Role of ATP:

  • Provides energy for myosin to bind with actin
  • Myosin heads break down ATP to ADP + phosphate, moving into an extended position

• Action Potential:

  • Triggered by acetylcholine at the motor neuron synapse
  • Travels along sarcolemma and T-tubules
  • Activates calcium channels in sarcoplasmic reticulum

• Binding Sequence:

  • Calcium binds to troponin, changes shape, removes tropomyosin block
  • Myosin binds to actin, releases energy, contracts sarcomere

Relaxation Process

• Myosin unbinds, new ATP binds, cycle repeats
• Calcium pumped back into sarcoplasmic reticulum
• Troponin and tropomyosin reset

Recap and Credits

• Sliding filament model: contraction and relaxation of muscles
• Special thanks to contributors and supporters of Crash Course