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.

The masseter muscle is a facial muscle that plays a major role in the chewing of solid foods (it is one of the muscles of mastication: masseter, temporalis, medial pterygoid, lateral pterygoid). The action of the muscle is to elevate the mandible (raise the lower jaw) which results in closing of the jaws. It connects to the mandible (lower jawbone) and the maxilla/zygomatic arch. 

The levator scapulae is a long skeletal muscle of the shoulder girdle. It originates in the transverse processes of the atlas and axis as well the posterior tubercles of the 3rd and 4th cervical vertebrae (or in simple terms: it originates at the first four cervical vertebrae, and runs along the back and side of the neck). Its function is to elevate the scapula. 

The correct answer is contraction. Muscle cells shorten or contract to move body parts. These contractions can be under voluntary or involuntary control. The primary function of muscle cell contraction is movement, but contraction is responsible for many other important functions like posture, joint stability and heat production. Muscles contract to allow maintenance of postures like sitting and help stabilize joints. Heat is also generated by contraction.

The other answer choices are functions performed by epithelial cells. Epithelial cells contain neuron endings (nerves) that perceive external stimuli and function as sensory receptors. Epithelial cells form coverings over body parts, including the skin, which prevents infection from microbes. They can form taste buds, line the nose and are located in the ear and eye. Epithelial cells serve an absorptive function through active-transport systems to absorb filtered material and transport to the rest of the body. Epithelial cells also secrete fluids necessary for other functions. For example, some epithelial cells secrete mucus, which lubricates body cavities and the passageways they line.

The force velocity relationship measures the velocity of shortening of isotonic contractions when the muscle is challenged with different afterloads (the load against which the muscle must contract.) The velocity of shortening decreases as the afterload increases.