Lecture Notes: Sliding Filament Theory and Muscle Contraction

Overview

Muscle Contraction: Shortening of the muscle.
Key Bands and Zones:
- A Band: Entire length of thick filaments.
- H Zone: Area with only thick filaments.
- I Band: Area with only thin filaments.
- Overlap: Where actin and myosin (thin and thick filaments) overlap.
Relaxed Muscle:
- Actin and myosin are not bound.
Partially Contracted Muscle:
- Myosin attaches to actin, moving it towards the M line.
- H Zone and I Band decrease; A Band remains constant.
Fully Contracted Muscle:
- H Zone and I Band disappear; A Band remains unchanged.
Key Concept: Thick and thin filaments do not shorten; they slide past each other.

Triggered by: Action potential traveling down the sarcolemma and into T tubules.
Calcium Release:
- Calcium gates in the sarcoplasmic reticulum (SR) open.
- Calcium binds to troponin, causing tropomyosin to expose myosin binding sites on actin.
Contraction Cycle:
- ATP Hydrolysis: ATP is broken down, storing energy in myosin head (high energy state).
- Crossbridge Formation: Myosin attaches to actin.
- Power Stroke: Myosin head swivels, pulling actin towards the M line (low energy state).
- Detachment: ATP binds to myosin, causing it to release actin and reset.

Limiting Factor: Removal of calcium, not ATP.
Calcium Reabsorption:
- Active transport pumps use ATP to move calcium back into the SR.
- Calsequestrin protein helps retain calcium in the SR.
Troponin and Tropomyosin: Return to their original positions, blocking myosin binding and causing relaxation.

Muscle contraction and relaxation require ATP.
Calcium plays a crucial role in both contraction and relaxation.
The sliding filament theory explains how muscle length changes without changing the length of actin or myosin filaments.