When the muscle is at rest, myosine heads are not interacting with actin. Stage 1 in the image represents a muscle that is fully at rest (Option B).
What are the components of the Sarcomere?
In the sarcomere, we can identify the following components,
• A-band. This band reflects the length of the thick filament, including a small portion of the thin filament.
• I-band. This is the area located between the ends of the adjacent thick filaments. It is composed only of thin filaments.
• H-zone. This is the area located between the ends of the thin filaments. It is only composed of a portion of the thick filaments.
• Z-band is the vertical line placed at the end of each sarcomere. In adjacent sarcomeres, Z-band can be found in the middle of the I band.
How does muscle contraction occur?
At rest, tropomyosin is inhibiting the attraction strengths between myosin and actin filaments.
Tropomyosin is obstructing binding sites for myosin on the thin filament.
During contraction, binding sites between myosine and actin become unblocked.
Myosin heads join to the uncovered actin-binding points forming cross-bridges. A power stroke initiates when this binding occurs. While the power stroke happens, myofilaments slide impulsed by chemical energy collected in myosin heads.
The muscular fiber gets shorter because the sarcomere reduces in length. The H line and the I band get shorter. Z-bands are pulled toward each other, shortening the sarcomere and the I-band, producing muscle fiber contraction. A band keeps constant in length.
Stage 1 in the image represents a muscle that is fully at rest (Option B). Binding sites between myosin and actin are blocked.
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