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 signal for muscle contraction causes the release of calcium from the sarcoplasmic reticulum. This calcium floods the cell and is necessary for causing muscle contraction. Parvalbumin, a protein in the cytoplasm, binds to calcium and acts as a slow-releaser of calcium. This binding reaction of calcium with parvalbumin causes the release of heat, which is termed as 'unexplained heat.' The 'unexplained heat' is also known as 'labile heat.'

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.

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.

All of the choices indicate muscles of the arm and hand, but they have different functions. The brachialis is involved in forearm flexion, along with muscles such as the biceps brachii and pronator teres. The triceps are involved with forearm extension, which is the opposite of flexion. The extensor carpi radialis longus is involved in extension and abduction of the hand. Lastly, the extensor carpi ulnaris is involved with extension and adduction of the hand. We can tell that the extensor carpi ulnaris and extensor carpi radialis longus are not involved in flexion because they are named "extensors," and, as stated, extension is the opposite of flexion.

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 muscle length-tension relationship measures muscle tension developed during isometric contractions (the muscle is set to fixed lengths and length is held constant). Active tension is the difference between total tension and passive tension. Active tension is the active force created when the muscle contracts. Passive tension is created by stretching the muscle to different lengths. Total tension is the tension developed when the muscle is stimulated to contract at different lengths.

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.

Glucose is transported into skeletal muscles via insulin dependent facilitated diffusion. This type of diffusion requires a specific trans-membrane protein to allow for the passage of glucose. In the presence of insulin, these membrane channels allow glucose to move from outside the cell to inside the cell in an effort to lower blood glucose levels.

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.