What is the primary difference between the A-band and the I-band during muscle contraction?

The A-band remains a constant length because it corresponds to the full length of the myosin filaments. In contrast, the I-band shortens because it represents the region of actin that does not overlap with myosin, and this overlap increases during contraction.

How does the role of actin differ from the role of myosin in the sliding filament theory?

Actin acts as a passive track containing binding sites that are pulled toward the center of the sarcomere. Myosin is the active motor protein that uses its globular heads to bind to actin and exert the physical force required for movement.

Compare the roles of ATP and Phosphocreatine in muscle cells.

ATP is the immediate source of energy used for myosin head detachment and calcium pumping. Phosphocreatine acts as a rapidly accessible reserve that regenerates ATP by donating a phosphate group to ADP when ATP levels drop during intense activity.

What is a common misconception regarding the length of actin and myosin filaments during contraction?

A frequent error is believing that the protein filaments themselves shrink or contract. In reality, the filaments maintain a constant length; the muscle shortens only because the filaments slide past each other, increasing their degree of overlap.

Why is it incorrect to say that ATP hydrolysis powers the power stroke?

The power stroke (the bending of the myosin head) occurs spontaneously upon the release of ADP and inorganic phosphate. ATP hydrolysis actually occurs after detachment to provide the energy needed to reset the myosin head to its 'cocked' position for the next cycle.

What error occurs if one assumes muscle relaxation is a passive process?

Assuming relaxation is passive ignores the fact that calcium ions must be actively transported back into the sarcoplasmic reticulum. This process requires ATP, meaning energy is consumed even as the muscle stops exerting force.

Define the term 'Sarcomere'.

A sarcomere is the functional unit of a myofibril, defined as the section between two adjacent Z-lines. It contains the organized arrangement of actin and myosin filaments necessary for muscle contraction.

What is the Sarcoplasmic Reticulum?

The sarcoplasmic reticulum is a specialized type of endoplasmic reticulum found in muscle cells that stores high concentrations of calcium ions. It releases these ions into the sarcoplasm in response to an action potential to initiate contraction.

What is the function of ATP hydrolase in muscle contraction?

ATP hydrolase is an enzyme located on the myosin head that catalyzes the breakdown of ATP into ADP and inorganic phosphate. This reaction releases the energy required to return the myosin head to its original position after it has detached from actin.

Define 'Tropomyosin' and its role in a relaxed muscle.

Tropomyosin is a regulatory protein that wraps around actin filaments. In a relaxed muscle, it covers the myosin-binding sites on the actin, preventing myosin heads from attaching and initiating contraction.

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Muscle Contraction

Summary

Muscle contraction is explained by the sliding filament theory, where actin and myosin filaments slide past one another to shorten the sarcomere without changing their individual lengths. This process is regulated by calcium ions and powered by ATP, involving a cyclic formation and breakage of cross-bridges between protein filaments.

1. Definition & Core Concepts

Skeletal Muscle Structure: Muscles are composed of bundles of muscle fibres, which contain rod-like structures called myofibrils. These myofibrils are made of repeating functional units known as sarcomeres, which are the basic units of contraction.
Protein Filaments: Contraction relies on two main proteins: actin (thin filaments) and myosin (thick filaments). Actin filaments possess binding sites for myosin, while myosin filaments feature globular heads that can bind to actin to form cross-bridges.
Regulatory Proteins: Tropomyosin is a fibrous protein that wraps around actin. In a relaxed state, it physically blocks the myosin-binding sites, preventing the formation of cross-bridges and thus preventing contraction.

Diagram of the sliding filament model showing actin and myosin filaments with a myosin head forming a cross-bridge.

2. The Sliding Filament Mechanism

The Process of Sliding: During contraction, myosin heads attach to actin and pull the actin filaments toward the center of the sarcomere (the M-line). This action increases the overlap between the two types of filaments, effectively shortening the sarcomere.
Sarcomere Changes: As the filaments slide, the distance between Z-lines decreases. While the A-band (total length of myosin) remains constant, the I-band (actin only) and the H-zone (myosin only) both shrink as the degree of overlap increases.
The Power Stroke: The actual movement is triggered by the release of ADP and inorganic phosphate ( $\text{P}_i$ ) from the myosin head. This causes the head to bend sharply, pulling the attached actin filament along with it.

3. The Role of Calcium and ATP

4. Energy Sourcing: Phosphocreatine

5. Key Distinctions: Contraction vs. Relaxation

6. Exam Strategy & Tips