Musculoskeletal System and Muscle Tissue - MCAT Biological and Biochemical Foundations of Living Systems

Skeletal muscle is the only type of muscle that can be controlled voluntarily. Skeletal muscle is the muscle type used to control the movement of our bones (and therefore our bodies). An exception to this rule is that smooth muscle can be controlled by some individuals, but to a extremely limited capacity.

White fibers have less myoglobin than red fibers. White fibers have less mitochondria than red fibers. White fibers fatigue more quickly than red fibers.

White fibers primarily use anaerobic respiration (red primarily uses aerobic). Remember that red fibers are active over long periods of sustained stress, while white fibers are more explosive. Red fibers require oxygen from respiration and myoglobin to maintain activity.

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.

The only type of muscle tissue that can be controlled voluntarily is skeletal muscle tissue. Both skeletal and cardiac muscle tissue have the sarcomere as the functional unit, and only skeletal muscle has multinucleated cells. Smooth muscle does not have troponin, and therefore does not require calcium in order to contract.

The two main types of muscle fibers are red and white fibers. Also known as slow twitch and fast twitch fibers, respectively, red fibers are used for high endurance activities, such as long distance running, while white fibers are used for quick movements, such as jumping. We expect red fibers to have more mitochondria and myoglobin, as these are essential to extended aerobic respiration and fatigue prevention.

The first requirement of this question is that the muscle type be striated. Striated muscle can be identified by the presence of alternating dark and light bands, caused by the repetition of sarcomeres, the basic muscle units composed of actin and myosin. Both cardiac and skeletal muscle types are striated.

The second requirement is that the muscle type must be multinucleate, meaning that individual cells have fused to form larger units with more than one nucleus. The only muscles with multinucleate cells are skeletal muscles, so that is our solution.

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.

Both cardiac and skeletal muscle is striated, however, cardiac muscle is mononucleated, whereas skeletal muscle in multinucleated. Smooth muscle is not striated. The student is observing muscle that is both striated and mononucleated, meaning it must be cardiac muscle.

Smooth muscle: mononucleated with no striations

Skeletal muscle: multinucleated with striations

Cardiac muscle: mononucleated with striations

Cardiac muscle is organized into sarcomeres, giving it a striated appearance, and cardiac contraction is strongly dependent on calcium concentrations and release. While the heart is capable of beating without nervous innervation, autonomic regulation via the parasympathetic nervous system mediates the frequency of cardiac stimulation from the sinoatrial (SA) node.

Signals from the parasympathetic nervous system travel to the cardiac muscle via chemical synapses, by use of a neurotransmitter. Signals from one cardiac fiber to another are transmitted via electrical synapses, or gap junctions, the allow proliferation of the signal without a chemical intermediate.

Cardiac muscle and skeletal muscle are both composed of sarcomeres. This layout gives both muscle types a striated appearance, alternating dark bands of myosin with lighter bands of actin. Only cardiac muscle has intercalated discs and skeletal muscle is the only type that is multinucleated. No muscle type is attached directly to bone, but skeletal muscle is linked to bone via tendons.

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.

Smooth muscle is involuntarily controlled, and is used to regulate the autonomic functions in the body. When stimulated by the sympathetic nervous system, contracting smooth muscle cells will shrink the diameter of arterioles and arteries, causing vasoconstriction.

All muscle types play some role in helping to move blood through the body. The heart actively pumps blood through the circulatory system. Smooth muscle in the arteries and arterioles works to constrict or dilate various shunts and pathways to direct blood flow. Skeletal muscle contractions alter the shape of the muscle, pushing blood in various directions to affect its flow.

You can test this principle by making a tight fist with one hand; when you release the fist, look at the color of your palm compared to your hand that remained open. The hand that made a fist pushed blood away from the surface due to the change in shape, causing your palm to look white.

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.

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.

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.

Type I muscle cells, or red muscle cells, use aerobic respiration, whereas type II muscle cells, or white muscle cells, use anaerobic respiration. Aerobic respiration requires oxygen; therefore, myoglobin must be present to carry oxygen within type I muscle cells. Aerobic respiration occurs in mitochondria. This means that type I cells must have more mitochondria and myoglobin to facilitate aerobic respiration.

Calcium ions from the sarcoplasmic reticulum are essential for muscle contraction, and are found in equal amounts in both cell types.

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.

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.