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.

Muscle contraction is very tightly regulated. Tropomyosin binds actin filaments and covers the myosin binding sites. Troponin interacts with tropomyosin. When stimulated by calcium, troponin undergoes a conformational change that moves the tropomyosin and exposes the myosin binding sites on the actin protein. When the binding sites are exposed myosin can bind actin and muscle contraction can occur. Troponin does not bind myosin or actin directly, and it does not get ubiquinated and degraded.

Muscle contraction is regulated by blocking the myosin binding sites of actin and selectively exposing them when contraction is supposed to occur. Tropomyosin binds actin fibers and recruits troponin to perform this blocking function. Troponin will bind calcium ions, change conformation, and move tropomyosin out of the way of the myosin binding sites to allow contraction to occur. The most likely mutation described in the question is one that causes troponin to have a decreased affinity for calcium, thus never allowing the myosin binding sites to be exposed under normal physiological concentrations.

The question states that muscle cramps occur because myosin heads remain attached to the active site on actin; therefore, you are looking for a molecule that is responsible for the detachment of the myosin head from actin. Recall that binding of ATP to the myosin head releases the myosin head from the actin binding site. This allows tropomyosin to re-attach to actin and causes the muscle to relax. The ATP that is bound to the myosin head dissociates into ADP and inorganic phosphate, allowing the myosin head to enter its high-energy state and prepare for another contractile stroke. Once tropomyosin is released again from the actin filament, the ADP and inorganic phosphate on the myosin head are released, the myosin head attaches to actin, and the cycle continues.

Calcium is essential for muscle contraction because it allows for the removal of tropomyosin from the actin binding sites. Depletion of calcium, however, would cause the actin sites to be blocked, preventing contraction from occurring (as opposed to the sustained contraction of a muscle cramp). Depleted sodium may result in fewer action potentials at the neuromuscular junction. This would also inhibit muscle contraction, rather than sustain it. Muscular microtears can occur during exercise, but are unrelated to muscle cramps.

Troponin and tropomyosin are both proteins that play a big role in muscle contraction. Since they are proteins, troponin and tropomyosin are made up of amino acids. Recall that fatty acids make up lipids, not proteins. 

During a typical muscle contraction, tropomyosin is bound to actin. This blocks the binding of the myosin head to actin and prevents muscle contraction. When calcium ions are released from the sarcoplasmic reticulum, troponin interacts with actin and removes tropomyosin. Neither protein directly interacts with myosin. Tropomyosin is very important because it prevents prolonged muscle contraction which can lead to several muscle disorders.

ATP has a huge role in muscle contraction; however, it never interacts with tropomyosin. ATP is important to remove the myosin head from the active site on actin. Upon ATP binding, the myosin head detaches itself from actin, which allows tropomyosin to re-attach to the active site on actin. Without ATP, myosin heads can’t detach themselves from actin and prolonged muscle contraction occurs. After death a person stops producing ATP; therefore, contracted muscles can’t relax and rigor mortis ensues.