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.

The sarcoplasmic reticulum is the specialized organelle of the muscle cells that allows for calcium to be sequestered. Once calcium is released into the cytoplasm it interacts with troponin and tropomyosin, allowing myosin and actin to bind and cause contraction. Calcium must be sequestrated to allow for the myosin-actin bridges to be broken and reset for future contraction.

The sarcolemma is the muscle cell membrane. The T-tubules permeate the muscle cell to allow for propagation of the stimulating action potential to all parts to the cell. The sarcomeres are the contractile units of the muscle cell.

Keep in mind that muscle cells do not undergo mitosis. Instead, the increase in muscle size is due to increased muscle fiber diameter. The number of sarcomeres and mitochondria in each muscle fiber will also increase over time. This increase in size, but not cell number, is called hypertrophy.

The proteins troponin and tropomyosin are attached to the actin filaments in sarcomeres. These proteins function to block the myosin-binding site on the actin protein, preventing unnecessary contraction. When calcium is released from the sarcoplasmic reticulum, it will attach to troponin. The troponin will then pull tropomyosin away from the actin filament, which allows myosin heads to attach and cause a contraction.

ATP binds myosin to release it from the actin binding site and is converted to ADP in order to adjust the myosin head to a high-energy position.