Recall that sarcomeres are functional units of muscles that facilitate muscle contraction. Myosin heads bind to actin filaments and cause the filaments to overlap, shortening the sarcomere and, subsequently, the muscle.

Inside a sarcomere there are several regions. One such region is the I band, which consists of the actin filaments in the region where they are not superimposed by the myosin filaments. This means that the I band consists only of actin filaments; however, actin filaments aren’t exclusive to the I band. They are also found in other regions, such as A band. The A band is the region of the sarcomere that contains the myosin (thick) filaments, regardless of overlap. This means that myosin is exclusive to the A band, but that this region contains both actin and myosin due to overlap.

Sarcomeres are functional units of muscles, but they are only found in skeletal and cardiac muscle cells; smooth muscle cells do not contain sarcomeres. Actin and myosin filaments still cause the contraction seen in smooth muscle, but are not organized into alignment. This means that smooth muscle cells do not contract linearly and can essentially shrink in size during contraction, which can allow for things like constriction around organs and vessels.

To answer this question you need to understand the structural regions of the sarcomere. The I band, A band, and H zone are regions in a sarcomere that constitute of actin (thin) and myosin (thick) filaments. I band is the region of actin filaments that are not superimposed by myosin filaments. The H zone is the region of myosin filaments that are not superimposed by actin filaments. To calculate the length of the I band, you need the length of the myosin filament, the actin filament, and the H zone. Since we don’t have the length of H zone, we can’t solve for the length of I band. Essentially, without knowing the degree of overlap, we cannot determine the length of un-overlapped actin.

The A band is the region of the sarcomere that consists of the entire length of the myosin filament. The question states that the length of the myosin filaments is  micrometers; therefore, the length of the A band is  micrometers.

The only choice that is actually true is that the degree of overlap of myosin and actin plays a role in contractile strength. If there is little to no overlap, contractile strength is low; however, if there is too much overlap then contractile strength is also low. This trend can be represented in a force-tension curve, which demonstrates that maximum force generation occurs when the sarcomere begins at equilibrium.

In a normal sarcomere there is always a small area of overlap of myosin and actin prior to contraction. Myosin appears thicker than actin, and is considered the "thick filament."

Actin and myosin filaments are essential contractile elements found in muscle cells. They are essential because they conduct muscle contraction. A molecule of actin is made up of small microfilaments, which give them a very thin appearance. Myosin is made up of long polypeptide chains that join together to form a thick filament; therefore, actin molecules are classified as thin filaments, whereas myosin molecules are classified as thick filaments.

All muscle cells, regardless of type, contain both actin and myosin filaments. Muscle contraction is not possible without the presence of both contractile elements. Organization of these molecules can vary, as smooth muscle does not contain striations, but the molecules are still responsible for contractile actions. Troponin, calcium, and tropomyosin are all required to initiate the contact between myosin and actin. Calcium binds to troponin, which subsequently removes tropomyosin from actin (thin filaments). None of these interact with myosin, the thick filaments.

When a muscle is shortened, the force decreases as the filaments slide past one another and lose room to form new cross-bridges. When a muscle is lengthened, there is less filament overlap which leads to fewer cross-bridges. Stretching a muscle before contracting it will not affect the force produced, nor will shortening a muscle before lengthening it. Titin is a protein responsible for some of the elastic properties of muscle, but is not involved in force production.

At rest, the muscle has the potential to form the maximum number of cross-bridges, resulting in the maximum amount of force production. For further review, go over the length-tension curve for a muscle fiber.

A sarcomere is one contractile unit of a muscle fiber, and contains two half-filaments of actin and a full filament of myosin. The ends of the sarcomere are the Z-discs and the center is the center is the M-line (the middle of the myosin filament). The H-band lies between the two half-actin filaments where there is only myosin; however, it does not correspond to the full myosin filament. The I band corresponds to the region where only actin is present and the A-band correspond to the full length of the myosin filament.

The A band is the only element that remains constant during a muscle contraction. It represents the segment of the sarcomere that contains the length of the thick filament. The H zone refers to the part of the sarcomere where there are only thick filaments, and no superimposing thin filaments. Conversely, the I band refers to the area where there are only thin filaments and no superimposing thick filaments. As filaments overlap, both the H zone and I band will shorten. The N line does not exist in musculoskeletal physiology.

A sarcomere is the functional unit of the skeletal or cardiac muscle cell, and is made of interlocking myofibrils. A sarcomere is the smallest unit in the muscle cell to contract and relax.

Note that smooth muscle cells still contract using actin and myosin filaments, but do not organize these filaments into sarcomeres as skeletal and cardiac muscle do. This is why smooth muscle is not striated.

The sarcoplasmic reticulum, an organelle unique to muscle cells, sequesters calcium when the muscle is at rest. This calcium is released into the cytosol during stimulation, and is an integral part of contraction. The affected individual probably has a leaky sarcoplasmic reticulum, allowing the release of calcium into the cytosol and resulting in abnormally high levels of intracellular ion.

Ribosomes are used during protein synthesis and not related to muscle contraction. The nucleus also is not involved in muscle contraction. The Golgi body is involved in modification and packaging of proteins, and also not involved in muscle contraction. Mitochondria are responsible for producing ATP. While ATP is an important part of the contraction process, and mitochondria are abundant in muscle cells, a defect in the mitochondria would not directly cause an increase in intracellular calcium.

Tropomyosin is interwoven with actin and serves to cover the myosin-binding site in the absence of calcium. Once calcium enters the cell it interacts with troponin, which in turn causes a conformational change in tropomyosin leading to the interaction of myosin with actin and a resulting muscle contraction. Without tropomyosin in place spontaneous cross-bridges could form, leading to inappropriate muscle contraction in the absence of action potentials.