Thick and thin myofilaments have different compositions. For each descriptive phrase, indicate whether the filament is, a. thick or b. thin. 
_____ (1) contains actin 
_____ (2) contains ATPases 
_____ (3) attaches to the Z disc 
_____ (4) contains myosin 
_____ (5) contains troponin 
_____ (6) does not lie in the I band

Verified step by step guidance

1. Step 1: Understand the composition of thick and thin myofilaments. Thick filaments are primarily composed of myosin, while thin filaments are mainly composed of actin, along with regulatory proteins like troponin and tropomyosin.
2. Step 2: For the phrase 'contains actin,' identify which filament has actin as its main protein. Since actin is the primary component of thin filaments, this corresponds to the thin filament.
3. Step 3: For 'contains ATPases,' recall that myosin heads have ATPase activity to hydrolyze ATP during muscle contraction. Therefore, ATPases are found in thick filaments.
4. Step 4: For 'attaches to the Z disc,' remember that thin filaments are anchored to the Z disc in the sarcomere structure, while thick filaments are located more centrally and do not attach to the Z disc.
5. Step 5: For 'contains myosin,' identify that myosin is the main protein in thick filaments, so this phrase refers to thick filaments.
6. Step 6: For 'contains troponin,' know that troponin is a regulatory protein bound to thin filaments, so this phrase corresponds to thin filaments.
7. Step 7: For 'does not lie in the I band,' recall that the I band contains only thin filaments, so thick filaments do not lie in the I band; they are located in the A band.

Key Concepts

Structure and Composition of Thick Myofilaments

Thick myofilaments are primarily composed of myosin proteins, which have ATPase activity essential for muscle contraction. These filaments do not attach to the Z disc and are located mainly in the A band of the sarcomere. Their ATPase enzymes hydrolyze ATP to provide energy for the power stroke during contraction.

Structure and Composition of Thin Myofilaments

Thin myofilaments consist mainly of actin, along with regulatory proteins troponin and tropomyosin. They attach to the Z disc and extend through the I band into the A band. Troponin plays a key role in calcium binding, which regulates muscle contraction by controlling the interaction between actin and myosin.

Sarcomere Bands and Filament Localization

The sarcomere is divided into bands: the I band contains only thin filaments and is lighter, while the A band contains thick filaments and overlapping thin filaments, appearing darker. Thick filaments do not lie in the I band, whereas thin filaments span from the Z disc through the I band and partially into the A band.