The ryanodine receptor is a calcium channel that is responsible for the release of calcium ions. It is located in the sarcoplasmic reticulum. The release of calcium is a required step in muscle contraction. The ryanodine receptor pumps calcium ions from the intracellular fluid into the interior of the sarcoplasmic reticulum, this process keeps the intracellular calcium ions low and creates a concentration gradient. 

Each myofibril contains interdigitating thick and thin filaments arranged longitudinally in sarcomeres. Repeating units of sarcomeres account for the unique banding pattern in striated muscle. A sarcomere runs from Z line to Z line.

Troponin is a complex of three globular regulatory proteins (T, C, and I). Troponin is essential to the contraction of skeletal and cardiac muscle, but is not involved in the contraction of smooth muscle. Troponin T (think of: T for tropomyosin) binds to tropomyosin creating the troponin-tropomyosin complex, which helps to position tropomyosin onto actin. Troponin I (think of I for inhibition) binds actin, holding the actin-tropomyosin complex in place. This inhibits the myosin heads from binding their binding sites on actin. Lastly, troponin C (think of C for calcium) is responsible for binding calcium. This step is allows for the interaction of actin and myosin, thus activating the muscle for contraction.

Type 2 muscle fibers are fast twitch, and grossly appear white due to decreased mitochondria and myoglobin concentration. Because of the decreased number of mitochondria, sustained activity of type 2 muscle fibers results in lactic acid accumulation due to anaerobic glycolysis.

Type 1 muscle fibers are slow twitch, and grossly appear red due to increased mitochondria and myoglobin concentration, which allows for oxidative phosphorylation. Type 1 muscle fibers allow for low, sustained muscle contraction.

The sarcoplasmic reticulum of the muscle fiber is used to store calcium. When an action potential causes depolarization of the T-tubules, adjacent proteins cause the calcium channels of the sarcoplasmic reticulum to open. The released calcium binds to troponin, influencing a change in tropomyosin and allowing myosin to bind the active sites of actin. Without the presence of calcium, tropomyosin remains in place to block myosin binding and contraction cannot occur.

This process, however, does not occur in smooth muscle. Smooth muscle lacks both troponin and tropomyosin, and is thus not reliant on calcium for contraction.

The sarcoplasmic reticulum is a specialized cell structure, characteristic of skeletal muscle cells, that is used to store calcium ions.

Upon neural stimulation, depolarization of the T-tubules causes a cellular reaction to open ion channels in the membrane of the sarcoplasmic reticulum. Calcium is released into the cell, where it can bind to troponin and allow for muscle contraction.

The endoplasmic reticulum is found in most eukaryotic cells, and is used for lipid synthesis, detoxification, and several other functions. Sodium and potassium play significant role in regulating membrane potential, but are not stored in the muscle cell in large amounts as calcium is.

During fasting, glycogen production is reduced and glycogen catabolism (glycogenolysis) is increased. Glycogen stored within muscle is broken down into glucose by circulating catecholamines (epinephrine, not glucagon) during prolonged fasting.

Prolonged fasting also causes glycogen breakdown in the liver. This process is mediated by glucagon.

GLUT4 glucose transporters are insulin-mediated, and found on the apical membrane of certain cells. Insulin release follows a heavy meal, and thus we would expect to find increased levels of GLUT4 transporters on muscle cells following the meal, with the absorbed glucose being used to produce intra-muscular glycogen.

The remaining glucose transporters are located as follows:

GLUT1: Erythrocytes, constitutively expressed

GLUT2: Pancreatic islet cells monitoring serum glucose levels, constitutively expressed

GLUT3: Brain tissue, highest glucose affinity & constitutively expressed

GLUT5: Present on the basolateral side of enterocytes to allow movement of absorbed carbohydrate from the enterocyte into the circulation, constitutively expressed

Note that GLUT4, in contrast to other subtypes, is inducible by insulin release.

Of the three primary muscle types, only skeletal muscle is under voluntary control. The upper third of the esophagus is under voluntary control by the cerebral cortex, and thus relies on skeletal muscle to contract. This can be appreciated when one initiates a swallow. The swallow is started deliberately, and then continues down deeper into the esophagus without conscious or voluntary effort.

Striations are characteristic of skeletal muscle, eliminating the option of muscle cell 3. Muscle cell 2 is said to contain gap junctions, which are characteristic of cardiac muscle, but not skeletal muscle. We can conclude that muscle cell 1 corresponds to skeletal muscle, and must be the only answer.

In order for the myosin head to perform a power stroke, an ATP must be cleaved to form ADP and a phosphate. This places the head in a cocked, high energy position. When the phosphate and ADP are expelled from the head, the myosin moves to a low energy position and drags the actin, causing the sarcomere to shorten. Once a new ATP is attached to the myosin, it detaches from the actin filament to begin the process again.

Lack of ATP causes myosin to remain attached to actin. This is the cause of rigor mortis.