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.

In a typical muscle cell, tropomyosin is bound to an active site on actin. This prevents muscle contraction because the myosin head cannot bind to actin active site. Muscle contraction is initiated when the sarcoplasmic reticulum releases calcium ions into the cytoplasm of the muscle cell. Calcium ions bind to and activate troponin. Activated troponin molecules subsequently remove tropomyosin from the active site on actin. This allows muscle contraction to occur because the myosin head can now bind to the active site on actin and initiate a power stroke to shorten the sarcomere.

An individual with depleted calcium ions in his sarcoplasmic reticulum will not activate troponin and, therefore, will have reduced muscle tone and strength.

The sarcoplasmic reticulum is responsible for the storage of calcium ions that are used in muscle contraction. Prior to a contraction, an action potential will reach the sarcoplasmic reticulum, making it permeable to calcium ions. At the end of the contraction, the sarcoplasmic reticulum will actively pump calcium ions back into its lumen.

The sarcoplasmic reticulum is not involved in protein synthesis.

Calcium plays a huge role in the regulation of muscle contraction. Without the presence of calcium, the myosin binding sites on the actin filaments are blocked by tropomyosin and muscle contraction cannot occur. Stimulation from the nervous system causes a chain reaction that releases large stores of calcium from the sarcoplasmic reticulum to regulate muscle contraction.

Sodium ions play an essential role in initiating the chain reaction that eventually leads to calcium release, but is not stored in the sarcoplasmic reticulum. Potassium plays a role in regulating membrane potential, but also is not stored in the sarcoplasmic reticulum. Protons are essential to mitochondrial function and play a crucial role in myocyte metabolism, but are not linked to the sarcoplasmic reticulum or contractile function.

The question states that the sarcoplasmic reticulum is a specialized endoplasmic reticulum. This means that the structures that make up the sarcoplasmic reticulum must be similar to the endoplasmic reticulum. Recall that both endoplasmic reticulum (rough and smooth) are made up of a network of tubules. Similarly, both structures contain vesicles that transport processed molecules to a target location (proteins in rough endoplasmic reticulum and lipids in smooth endoplasmic reticulum); therefore, the sarcoplasmic reticulum must contain vesicles and a network of tubules.

The sarcoplasmic reticulum does not contain digestive enzymes, nor does the endoplasmic reticulum. Digestive enzymes are usually found in degradative organelles, such as lysosomes.

The main function of the sarcoplasmic reticulum is to store and release calcium ions. Calcium ions released into the cytoplasm of a muscle cell activate troponin. Activated troponin removes tropomyosin from actin, which opens up the myosin active site on actin. Myosin head binds to actin, which causes muscle contraction. 

The sarcoplasmic reticulum does not play a role in releasing calcium ions into synapses during action potential propagation. The calcium ions in synapses are released via vesicles in the presynaptic neuron; therefore, an abnormal sarcoplasmic reticulum will not stop the release of calcium ions in neuronal synapses.

The sarcoplasmic reticulum is an organelle that closely resembles the smooth endoplasmic reticulum. They are structurally similar, but have very different functions. The main function of the sarcoplasmic reticulum is to store and release calcium, whereas the smooth endoplasmic reticulum functions in lipid synthesis.

Muscle contraction is usually initiated by an action potential. The first step in muscle contraction is the release of calcium ions from the sarcoplasmic reticulum; therefore, a change in membrane potential (action potential) will stimulate the sarcoplasmic reticulum to become more permeable to calcium ions. Once stimulated, the sarcoplasmic reticulum releases calcium ions into the cytoplasm, where the calcium ions interact with troponin and remove tropomyosin from actin. This sequence allows for myosin to bind actin and shorten the sarcomere, resulting in contraction.

The sarcoplasmic reticulum is actually found in all three types of muscle cells: smooth muscle, cardiac muscle, and skeletal muscle. Recall that skeletal muscles are voluntary, whereas smooth and cardiac muscles are involuntary; therefore, sarcoplasmic reticulum is found in both voluntary and involuntary muscles.