Sliding Filament Theory

Sliding Filament Theory Overview

• Definition: Describes the interaction between myofilaments leading to muscle contraction.

Muscle Function

• Muscles can only pull, not push; their contraction results from microscopic parts pulling along each other through the sliding filament theory.

Muscle Structure

• Key components to label in a skeletal muscle:
  • Perimysium
  • Endomysium
  • Nucleus
  • Muscle Fibre
  • Fascicle
  • Epimysium
  • Muscle Tendon
  • Sarcolemma
  • Myofibril
  • Sarcoplasm
  • Sarcomere

Myofibrils & Sarcomeres

• Myofibrils: Show a striped appearance due to actin (thin) and myosin (thick) filaments.
• Sarcomere: Basic contractile unit of a muscle fiber; composed of:
  • Actin (thin filaments)
  • Myosin (thick filaments)

Mechanism of Contraction

• Process: Sarcomeres shorten when actin and myosin slide past each other, without changing their lengths.
• This contraction occurs simultaneously across the muscle fiber, resulting in overall muscle contraction.

Actin and Myosin Interaction

1. Actin: Has tropomyosin, troponin, and binding sites for myosin.
2. Myosin at rest: contains ADP and phosphate (P) molecules.

Initiation of Muscle Contraction

1. Action potential reaches the neuromuscular junction, leading to:
   • Depolarization of the membrane.
   • Release of acetylcholine (ACh), causing sodium ions (Na+) to enter the cell.
2. Calcium ions release from the sarcoplasmic reticulum once muscle activation begins.
   • Calcium binds to troponin, shifting tropomyosin to reveal myosin binding sites on actin filaments.

Steps of the Sliding Filament Mechanism

Step 1: Cross-Bridge Formation

• Myosin head forms a cross-bridge with actin; binding remains until ATP releases it.
• Continuous cross-bridge formation leads to maximum contraction if calcium is present.

Step 2: Power Stroke and Recovery Stroke

1. Myosin head pivots (power stroke) once ADP is released, pulling actin toward the sarcomere center.
2. ATP binds to myosin heads, leading to their release from actin (recovery stroke). Breaks down into ADP and phosphate as heads return to position.
3. Cycle continues while calcium is present, pulling Z lines toward the H zone.

Step 3: Termination of Contraction

• Cross-bridge terminations occur when calcium is actively transported back to the sarcoplasmic reticulum, allowing muscle relaxation.
• Tropomyosin covers the myosin binding sites on actin once again.

Key Terms in the Sliding Filament Theory

• Myofibrils: Contractile elements of muscle.
• Z discs: Separate sarcomeres.
• A band: Region of thick filaments; includes overlapping thin and thick filaments.
• I band: Region of thin filaments only.
• H zone: Region of thick filaments only.
• M line: Middle line of sarcomere; holds thick filaments together.

Controlling Muscle Force

• The Central Nervous System (CNS) regulates muscle contraction based on required force:
  • Smaller motor units are recruited first (size principle).
  • For larger forces, larger motor units are activated.
  • Frequency of motor unit activation (rate coding) affects contraction strength.

Exam Question Practice

• DP Question: Explain sliding filament theory after acetylcholine increases muscle membrane permeability.
  • Answer Elements:
  • Sodium ions enter, changing polarization
  • SR releases calcium ions
  • Calcium binds to troponin
  • Tropomyosin shifts, revealing actin binding sites
  • Myosin cross-bridges form with actin
  • Power strokes occur
  • Z lines approach, H zone decreases
  • Myosin releases actin upon ATP attachment
  • Cycle repeats until ACh is broken down.