Recall that red muscle cells primarily use aerobic respiration and white muscle cells primarily use anaerobic respiration to produce ATP. Aerobic respiration, which involves the Krebs cycle and oxidative phosphorylation, occurs in the mitochondria. On the other hand, anaerobic respiration (glycolysis) occurs in the cytoplasm.

Type I muscle fibers, or red muscle, will use the mitochondria from ATP production and contain large amounts of myoglobin to help supply oxygen for these aerobic processes. In contrast, type II muscle fibers, or white muscle, will use glycolysis and lactic acid fermentation in the cytoplasm and contain very little myogloblin.

To answer this question you must know that running for long periods of time, such as running a marathon, requires aerobic respiration and sustained muscle contraction over a longer period of time. Type I muscle cells are most suited for these exercises because they utilize aerobic respiration and are slow to fatigue. In contrast, type II muscle fibers can function on anaerobic respiration and fatigue very quickly.

Muscle cells do not undergo mitosis, and therefore cannot increase in number due to training. Instead, the size of the muscle fibers will grow. In a marathon runner, we would expect there to be very large type II muscle fibers in the muscles of the legs.

Skeletal muscle is under voluntary control, and are innervated by the somatic nervous system. Skeletal muscle is responsible for skeletal movement, such as swinging the arms or lifting the legs.

Cardiac and smooth muscle are under the control of the autonomic nervous system. Cardiac muscle contracts the heart autonomously, without additional neuronal input.

The parasympathetic nervous system is more simply characterized as the “rest and digest” system, as it is activated during times of rest and helps in the digestion of food. Visceral smooth muscle tissue is mostly involved in digestion, and helps move food through the gastrointestinal tract.

In contrast, vascular smooth muscle is incorporated into the vascular tissues and plays a key role in vasoconstriction. Vasoconstriction helps increase blood pressure, increasing cardiac output during times of stress. Cardiac muscle is more active when the heart beats more rapidly, which would not occur while a person is at rest. Increased activity in vascular smooth muscle, cardiac muscle, and skeletal muscle occurs at times when the sympathetic nervous system ("fight or flight") is activated.

The extraocular muscles have small motor units to allow for very precise movements. These muscles are also non-fatigable because the muscles do not become weak after continual use throughout the day.

Large motor units would result in less precise control, affecting the ability of the eye to focus on a given target. Fatigable motor units would result in an inablility to keep the eye open and directed after sustained use.

Involuntary smooth muscle is used in many parts of the digestive tract. Smooth muscle moves food through the digestive tract, while also methodically spreading out the food's surface area, maximizing exposure to the nutrient-absorbing layers of the intestines. 

There are three types of muscle fibers: type I (slow twitch muscle fibers), type IIA, and type IIB (both fast twitch muscle fibers). Type I muscle fibers are very slow to fatigue, so we would expect to find them in athletes that specialize in endurance. Conversely, type IIA and B fibers are used for short lived, anaerobic activities. These fibers are typically seen in weight lifters. As a result, we would expect the weight lifter to have more type IIA and B muscle fibers than the runner.

Skeletal muscle and smooth muscle both require input from the brain in order to contract. Although smooth muscle is not under voluntary control, it is still controlled by the autonomic nervous system, which gets signals from the brain. Cardiac muscle has its own pacemaker cells and thus, even without any input from the brain, can keep the heart continuously beating.

We know that the "innervation of the muscle is unaffected" in muscular dystrophy patients, so we are looking for the answer choice that is true of healthy muscle. A neuromuscular junction consists of one neuron and all the muscle fibers it innervates (not just one). The basal lamina does not interfere with action potentials; it just helps the fibers contract together by forming physical connections between fibers. The sarcoplasmic reticulum releases Ca²⁺, not Na⁺. Finally, epinephrine is not the neurotransmitter used at neuromuscular junctions.

All neuron-muscle interactions use acetylcholine as the neruotransmitter, thus vesicles at the neuromuscular junction will contain acetylcholine.

The muscle fiber cytoplasm is the sarcoplasm. The outer covering of the muscle body is the epimysium. The region of the muscle closest to the tendon and the region of sarcolemma near the synapse do not have technical names that you need to know for the MCAT, nor do they make sense as answer choices; we are looking for something that the sarcolemma must be anchored to. The basal lamina is the extracellular matrix surrounding the muscle fiber, made up largely of collagen, that connects each muscle fiber to its neighbors and helps them contract in unison.