The Sliding Filament Theory of Muscle Contraction

Basic Structure Review

Before diving into how muscles contract, let's quickly review the key players:

• Myofibrils contain sarcomeres (the basic functional unit of muscle contraction)
• Sarcomeres contain two types of protein filaments:
  • Thick filaments (made of myosin)
  • Thin filaments (made of actin)

The arrangement of these filaments gives skeletal muscle its characteristic striped (striated) appearance under a microscope.

The Sliding Filament Theory Explained

1. The Resting State

In a relaxed muscle:

• Myosin heads are bound to ATP
• Tropomyosin covers the binding sites on actin
• No cross-bridges are formed
• Calcium ions are stored in the sarcoplasmic reticulum

2. The Contraction Process

Step 1: Neural Stimulation

• A nerve impulse arrives at the neuromuscular junction
• Acetylcholine is released
• This triggers an action potential in the muscle fiber

Step 2: Calcium Release

• The action potential travels along the sarcolemma
• T-tubules carry the signal deep into the muscle fiber
• Calcium ions are released from the sarcoplasmic reticulum

Think of calcium as the "key" that unlocks the contraction process!

Step 3: Cross-bridge Formation

• Calcium binds to troponin
• Troponin changes shape
• Tropomyosin moves away from binding sites on actin
• Myosin heads can now attach to actin

Step 4: The Power Stroke

1. ATP bound to myosin is broken down to ADP + Pi
2. This provides energy for the power stroke
3. Myosin head pivots and pulls the actin filament
4. The filaments slide past each other
5. Sarcomere shortens

Students often think the filaments themselves change length. They don't - they slide past each other!

3. The Recovery Phase

• A new ATP molecule attaches to myosin
• This breaks the cross-bridge
• Myosin head returns to its original position
• Ready for another power stroke if calcium is still present

The Role of ATP

ATP is crucial for muscle contraction in three ways:

1. Powers the calcium pumps in the sarcoplasmic reticulum
2. Provides energy for the power stroke
3. Detaches myosin from actin after the power stroke

Without ATP, muscles remain contracted - this is why rigor mortis occurs after death.

When studying the sliding filament theory, focus on understanding the sequence of events rather than memorizing every detail. The process follows a logical chain of cause and effect.

Think of it like rowing a boat:

• The myosin head is like the oar
• The actin filament is like the water
• The power stroke is like pulling the oar through the water
• ATP is like the energy from your food that powers the whole process