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.

An antagonist is defined as the muscle that strecthes when another muscle (the agonist) is contracting. When the antagonist contracts, it will stretch the agonist and move the bone in the opposite direction.

The biceps brachii is responsible for flexion of the forearm, while the triceps brachii is responsible for the extension of the forearm. As a result, we say that the triceps brachii is the antagonist of the biceps brachii.

Synergistic muscles assist agonists by properly positioning the insertion bone or stabilizing the origin bone. This cooperation by multiple muscles allows for better movement and posture. Since the temporalis assists in raising the jaw with the masseter, we say that the temporalis is the synergist of the masseter.

During muscle contraction, the H zone and I band contract, decreasing in length. The A band never changes in length. Also, while actin and myosin myofilaments slide over each other, their length does not change either.

There are three primary types of muscle tissue: skeletal, smooth, and cardiac.

Skeletal muscle and cardiac muscle are highly organized, with their contractile filaments arranged into sarcomeres. This organization results in a "striped" look on the cells, known as striations. Smooth muscle contains the same contractile filaments, but lacks the sort of highly organized structure found in the other two muscle types. As a result, smooth muscle is not striated.

Smooth muscle and cardiac muscle are generally mononucleate, meaning that each cell has only on nucleus. In contrast, skeletal muscle cells contain multiple nuclei.

The correct answer is that skeletal muscle is striated and contains multiple nuclei per cell.