Muscle and Skeletal System

Question 1

What main functions are served by muscle?

Answer 1

The muscular system is involved in four main functions:

1. movement
2. support
3. thermoregulation
4. circulation

Question 2

Briefly explain the role of the muscular system in thermoregulation.

Answer 2

In cold conditions, skeletal muscles are involved in shivering, in which they contract and convert energy to heat. Smooth muscle also facilitates vasoconstriction and piloerection (goosebumps), both of which reduce heat loss.

In warm conditions, smooth muscle facilitates vasodilation, which promotes the dissipation of body heat to the external environment.

Question 3

Briefly explain the role of skeletal muscle in circulation.

Answer 3

Skeletal muscles in the legs contract to compress nearby veins, assisting venous blood as it returns to the heart.

Specifically, blood returning from the lower regions of the body must counteract gravity. Since venous pressure is relatively low, the function of skeletal muscle is vital.

Question 4

Briefly explain the role of smooth muscle in circulation.

Answer 4

Smooth muscle lines arteries, arterioles, and veins. In response to a variety of factors, this muscle can contract (promoting vasoconstriction) or relax (promoting vasodilation).

Nervous system signaling and temperature changes, among other factors, are involved in promoting these responses.

Question 5

Name the three major types of muscle.

Answer 5

The three types of muscle are cardiac, skeletal, and smooth.

Question 6

What features characterize smooth muscle?

Answer 6

Smooth muscle is:

• mononucleated, meaning that a single cell contains only one nucleus
• rounded and irregular in appearance, not striated
• generally controlled involuntarily

Question 7

What features characterize skeletal muscle?

Answer 7

Skeletal muscle is:

• multinucleated, meaning that a single cell contains more than one nucleus
• striated, or striped in appearance
• generally controlled voluntarily

Question 8

In what way(s) does cardiac muscle resemble smooth muscle?

Answer 8

Cardiac muscle is controlled involuntarily. In general, its cells are also mononucleated.

(However, a small proportion of cardiac muscle cells do contain two nuclei.)

Question 9

In what way(s) does cardiac muscle resemble skeletal muscle?

Answer 9

Like skeletal muscle, cardiac muscle appears striated due to the presence of sarcomeres.

However, the two muscle types differ in the shapes of their cells. Striated muscle contains ordered, cylindrical cells, while cardiac muscle cells are more branched and irregular.

Question 10

Which muscle types require calcium for proper contraction?

Answer 10

All three muscle types—skeletal, cardiac, and smooth—require calcium ions to contract.

Note that these types use different mechanisms of contraction. In short, skeletal and cardiac muscle require calcium to bind to troponin, while smooth muscle needs the ion to trigger a signaling cascade.

Question 11

How does the role of calcium in skeletal and cardiac muscle contraction differ from its function in smooth muscle?

Answer 11

Skeletal and cardiac muscle require calcium to bind to troponin, exposing the myosin binding site.

Smooth muscle contains no troponin, but still relies on calcium for a signaling cascade that promotes contraction.

Question 12

A certain tissue sample displays three nuclei in a single cell. This sample is likely which type of muscle?

Answer 12

The sample is probably skeletal muscle.

In humans, skeletal muscle is the only one of the three types that is generally multinucleated.

Question 13

A certain tissue sample displays numerous mitochondria and large amounts of microfilament-based structures. This sample is likely which type of muscle?

Answer 13

This information is inconclusive.

All three muscle types contain mitochondria and substantial amounts of actin, a motor protein composed of microfilaments. While skeletal muscle does tend to possess more mitochondria than the other types, we do not know enough to answer this question.

Question 14

Which division of the nervous system is involved in the control of involuntary muscles?

Answer 14

The autonomic nervous system controls muscles that are not under voluntary influence. These include both smooth and cardiac muscle.

The autonomic nervous system is a division of the peripheral nervous system, or PNS.

Question 15

Which division of the nervous system is involved in the control of voluntary muscles?

Answer 15

The somatic nervous system controls muscles that are influenced consciously, or voluntarily. This category includes skeletal muscles.

The somatic nervous system is a division of the peripheral nervous system, or PNS.

Question 16

A biopsy is taken from the lining of an artery wall. Which type(s) of muscle might be found in this process?

Answer 16

Smooth muscle would likely be found.

Arteries, veins, arterioles, and larger venules contain smooth muscle in addition to endothelium and connective tissue. Note that capillary walls contain only a single layer of endothelial cells.

Question 17

A tissue sample is taken from the diaphragm. Which type(s) of muscle could this sample contain?

Answer 17

The sample would likely contain skeletal muscle.

The diaphragm is the main muscle involved in respiration. It is composed of skeletal muscle, but can be controlled either voluntarily or involuntarily.

Question 18

Muscle cells are likely to have large amounts of which eukaryotic organelle?

Answer 18

In general, muscle cells are likely to contain high numbers of mitochondria. These organelles provide the ATP needed for contraction.

Some types of muscle fiber, generally those specialized for aerobic respiration, are higher in mitochondria than others. For example, slow-twitch or red muscle fibers contain extremely high numbers of the structures.

Question 19

Define:

sarcomere

Answer 19

A sarcomere is a highly organized unit within striated muscle. Sarcomeres are composed of alternating thick filaments (myosin) and thin filaments (actin).

Together, many sarcomeres make up a myofibril, and many myofibrils form a muscle cell.

Question 20

Label the following parts of a sarcomere on the diagram below: M line, Z line, H zone, I band, and A band.

Answer 20

Question 21

Which protein or proteins are present in the thick filaments of skeletal muscle?

Answer 21

Thick filaments are composed of myosin.

Each myosin molecule consists of a rounded head and a long tail. The myosin head can function as an ATPase, a role that is vital during a contraction cycle.

Question 22

Which protein or proteins are present in the thin filaments of skeletal muscle?

Answer 22

Thin filaments are composed of actin, tropomyosin, and troponin.

Microfilaments are composed of two long chains of actin monomers.

Question 23

Muscle cells can also be called by which alternative term?

Answer 23

Myocytes

While this term can refer to a muscle cell of any type, it is generally used when referring to the cells of the heart: cardiac myocytes.

Question 24

Order the following terms from largest to smallest: myofibril, sarcomere, muscle, myocyte.

Answer 24

Muscle > myocyte > myofibril > sarcomere

The sarcomere is the functional unit of striated muscle. Together, many sarcomeres form a myofibril, and many myofibrils form a myocyte. Myocytes, or muscle cells, then make up larger muscles.

Question 25

Define:

myoglobin

Answer 25

Myoglobin is the iron-containing protein that binds oxygen in skeletal muscle cells.

Because it picks up the oxygen that hemoglobin releases in the tissues, myoglobin has a higher O₂ affinity than hemoglobin.

Question 26

Describe the structural difference between myoglobin and hemoglobin.

Answer 26

While myoglobin is very similar to hemoglobin in its tertiary structure, it contains only one monomer instead of four.

For this reason, hemoglobin can undergo cooperative binding, while myoglobin cannot.

Question 27

Skeletal muscle fibers can be characterized by their color into which two groups?

Answer 27

Muscle fibers can be grouped into either red or white fibers.

Red, or “slow-twitch,” fibers gain their color from large amounts of myoglobin; they also contain many mitochondria. White, or “fast-twitch,” fibers contain less myoglobin and fewer mitochondria.

Question 28

What basic features characterize red muscle fibers?

Answer 28

Red fibers gain their color from large amounts of myoglobin. They also contain many mitochondria and are thinner than white fibers.

As components of “slow-twitch” muscle, red fibers are specialized for sustained aerobic activity.

Question 29

What basic features characterize white muscle fibers?

Answer 29

White fibers are thicker than red fibers, but contain less myoglobin and fewer mitochondria.

As components of “fast-twitch” muscle, white fibers are specialized for short bursts of intense contraction.

Question 30

Near the end of an intense workout, an athlete’s muscles display a lowered amount of ATP, a buildup of lactic acid, and a decrease in the frequency of neuron signaling. What term is given to this condition?

Answer 30

This condition is known as fatigue.

Fatigue is generally accompanied by muscle soreness, caused by high lactic acid levels in the cells. Lactic acid is a product of anaerobic respiration.

Question 31

What structure is shown in the image below?

Answer 31

The structure shown is the neuromuscular junction, or NMJ.

The NMJ is the synapse that separates a motor neuron and a skeletal muscle. Here, vesicles containing neurotransmitters are released from the neuron, promoting contraction in the muscle.

Question 32

Name the main neurotransmitter at the neuromuscular junction.

Answer 32

The neurotransmitter released at the NMJ is acetylcholine.

For the MCAT, remember that acetylcholine promotes multiple responses. At the neuromuscular junction, it acts on a muscle to promote contraction. As part of the autonomic nervous system, however, it also triggers parasympathetic responses in effector organs.

Question 33

Curare, a plant-based toxin, blocks nicotinic acetylcholine receptors at the neuromuscular junction. What symptom might be seen in a person poisoned with curare?

Answer 33

The person would likely be paralyzed.

For skeletal muscle contraction to be initiated, acetylcholine must bind to receptors at the NMJ. If these receptors are blocked, no contraction can occur.

Question 34

A certain disorder causes a person to overproduce acetylcholinesterase (AChE) at the neuromuscular junction. What symptom might be seen in such an individual?

Answer 34

The person would likely be paralyzed or at least experience weakened muscle contraction.

The primary enzymatic function of AChE is to break down acetylcholine at the synapse or NMJ. Normally, this serves to prevent extended muscle contraction. However, overly high amounts of AChE would hydrolyze acetylcholine before it could reach its receptors, inhibiting contraction.

Question 35

Define:

motor unit

Answer 35

A motor unit consists of a single motor neuron and the muscle fibers upon which it synapses. Note that some neurons innervate only a few muscle fibers, while others innervate dozens.

Question 36

What term is given to the specialized endoplasmic reticulum found in muscle cells?

Answer 36

This organelle is the sarcoplasmic reticulum, or SR. Specifically, the SR is a type of smooth, not rough, ER.

The SR plays a crucial role in the contraction of skeletal and cardiac muscle. It stores calcium ions and releases them in response to an action potential, which then allows contraction to occur.

Question 37

In muscle cells, which specific term is given to the cytoplasm?

Answer 37

The cytoplasm of a muscle cell is called the sarcoplasm.

In general, cytoplasm contains both a cell’s cytosol and its non-nuclear organelles. In particular, sarcoplasm generally contains large amounts of myoglobin, myofibrils, and stored glycogen.

Question 38

In muscle cells, which specific term is given to the plasma membrane?

Answer 38

The plasma membrane of a muscle cell is called the sarcolemma.

The sarcolemma resembles a normal plasma membrane, but is specially suited to receive an action potential from adjacent motor neurons. It also contains structural adaptations.

Question 39

In skeletal muscle, which ion can be found in high concentrations in the sarcoplasmic reticulum?

Answer 39

The sarcoplasmic reticulum contains large amounts of calcium; in fact, it functions as a storage center for Ca²⁺ ions.

Question 40

Briefly describe the process that triggers calcium release from the sarcoplasmic reticulum.

Answer 40

1. Acetylcholine is released from an adjacent motor neuron and binds to receptors on the muscle cell membrane.
2. Ion channels open, causing a large influx of sodium ions and a depolarization of the cell.
3. Calcium release channels in the SR membrane are activated and open; Ca²⁺ flows down its gradient into the sarcoplasm.

Question 41

How do Ca²⁺ ions facilitate actin-myosin binding?

Answer 41

Each calcium ion binds to troponin, causing tropomyosin to change its shape. This frees the myosin binding site on the relevant actin molecule.

When calcium is not present, tropomyosin is bound to actin in a position that blocks the myosin binding site. For contraction to occur, this site must be open for the head of a myosin molecule to attach.

Question 42

What is the function of the T-tubule system in skeletal muscle?

Answer 42

T tubules are deep folds in the sarcolemma of muscle cells. Since they bring the cell membrane closer to the central regions of the cell, they allow action potentials to quickly propagate to the sarcomeres.

Question 43

What name is given to the current model of skeletal muscle contraction?

Answer 43

Skeletal muscle is thought to contract according to the sliding filament model.

According to this theory, myosin heads bind to actin molecules, then undergo a quick stroke to shorten the sarcomere. The filaments themselves do not change in length during this process; they simply overlap.

Question 44

Which of the following does not change in length during contraction: the A band, the I band, or the H zone?

Answer 44

The length of the A band remains unchanged.

In a sarcomere, the A band refers to the length of an entire myosin fiber. Since actin and myosin do not actually shorten during contraction, the A band’s length is always constant.

Question 45

Which of the following does change in length during contraction: the A band, the I band, or the H zone?

Answer 45

Both the I band and the H zone change in length during contraction.

In a sarcomere, the I band refers to the unoverlapped actin region, while the H zone refers to the region that contains myosin alone. Contraction involves the overlapping of filaments, so both of these regions shorten as the actin and myosin pull together.

Question 46

In skeletal muscle, how does the role of tropomyosin differ from that of troponin?

Answer 46

Tropomyosin directly attaches to actin. In the absence of calcium, it physically blocks the myosin binding site.

Troponin binds to tropomyosin. It binds to calcium (when the ion is present) and triggers a conformational change in the tropomyosin molecule.

Question 47

Briefly list the steps involved in skeletal muscle contraction, beginning with the influx of Ca²⁺ into the sarcoplasm.

Answer 47

1. Ca²⁺ binds troponin on actin molecules; on each one, tropomyosin is moved away from the myosin binding site.
2. Myosin heads have already bound ATP and are situated in a high-energy position. Now, they can freely bind actin, forming cross-bridges.
3. ATP is hydrolyzed; the myosin pulls back in a “power stroke,” shortening the sarcomere.
4. A new ATP molecule is required to remove myosin from its bound position and reset the cycle.

Question 48

After death, the cells of the body no longer synthesize ATP. What effect does this have on the condition of skeletal muscle?

Answer 48

Skeletal muscle becomes permanently contracted in a condition known as rigor mortis.

A common misconception is that ATP is necessary for the initiation of muscle contraction. Technically, ATP is required for the termination of contraction, where myosin dissociates from actin. Deceased individuals would be unable to perform this process, so their muscles would remain contracted.

Question 49

What main functions are served by the skeletal system?

Answer 49

The skeletal system is involved in four main functions:

1. protection of internal organs
2. structure and support
3. calcium storage
4. immune cell and erythrocyte production

Question 50

Name the two major types of bone.

Answer 50

The two main types are compact and spongy bone.

Compact bone is also called cortical bone; spongy bone is also called cancellous bone.

Question 51

What features characterize compact bone?

Answer 51

Compact bone is hard and dense. It is mainly composed of the minerals, salts, and collagen that form the bony matrix.

Compact bone is comprised of cylindrical subunits called osteons.

Question 52

What features characterize spongy bone?

Answer 52

Spongy bone is softer and less dense than compact bone. It contains many blood vessels, as well as gaps in its structure that are filled with bone marrow.

Spongy bone is comprised of thin, sharp subunits called trabeculae.

Question 53

Label the following parts of a long bone on the diagram below: epiphysis, diaphysis, compact bone, spongy bone, periosteum.

Answer 53

Question 54

The diagram below shows the microscopic anatomy of the bony matrix. Label the following structures: osteon, trabeculae, Haversian canal, lamellae.

Answer 54

Question 55

Define:

tendon

Answer 55

A tendon is a strip of tough connective tissue that connects a muscle to a bone. Tendons are composed of collagen.

Tendons generally function to facilitate movement.

Question 56

Define:

ligament

Answer 56

A ligament is a strip of tough connective tissue that connects a bone to another bone. Ligaments are composed of collagen.

Ligaments generally function to stabilize joints. For example, the anterior cruciate ligament, or ACL, stabilizes the knee.

Question 57

What is cartilage, and what substances does it include?

Answer 57

Cartilage is a form of connective tissue that is less rigid than bone. It includes elastin protein fibers, proteoglycan, and collagen.

The collagen in cartilage is produced by specialized cells called chondrocytes.

Question 58

What functions does cartilage serve in the human body?

Answer 58

During development, cartilage solidifies into bone in a process known as ossification. It also provides soft, flexible protection for certain parts of the anatomy.

For example, the trachea, nose, and ears contain cartilage.

Question 59

What broad term can be used to describe the skeleton of a horse?

Answer 59

Horses, like humans and other vertebrates, have endoskeletons.

An endoskeleton is an internal rigid structure. It can provide support to the organism, but does not encase the entire body.

Question 60

What broad term can be used to describe the skeleton of a lobster?

Answer 60

Lobsters, like many other invertebrates, have exoskeletons.

An exoskeleton is an external rigid encasement. It protects the entirety of the body, but must be replaced as the organism grows.

Question 61

The human skeleton can be split into which two major divisions?

Answer 61

The skeletal system is generally divided into the axial skeleton and the appendicular skeleton.

The axial skeleton is relatively central; it contains the vertebrae, ribcage, and skull. The appendicular skeleton is more peripheral and includes the bones of the appendages and the pelvis.

Question 62

Regulation of which ion involves both the skeletal and endocrine systems?

Answer 62

These systems are involved in the regulation of calcium.

Plasma Ca²⁺ concentration is controlled by two hormones: calcitonin and parathyroid hormone (PTH). Calcitonin lowers plasma calcium by promoting its storage in bone, while PTH has the opposite effect.

Question 63

How does the role of an osteoclast differ from that of an osteoblast?

Answer 63

Osteoclasts break down bone and release its calcium content into the plasma. As a result, osteoclast activity increases blood calcium levels.

Osteoblasts promote the synthesis of bone, removing calcium from the plasma in the process. As a result, osteoblast activity decreases blood calcium levels.

Question 64

What effect would excess production of calcitonin have on plasma calcium levels?

Answer 64

Plasma calcium levels would decrease.

Calcitonin is released from the thyroid when plasma calcium levels are high. It stimulates osteoblasts to increase bone formation, taking calcium out of the blood in the process. In contrast, parathyroid hormone stimulates osteoclasts.

Question 65

When a bone sample from a certain patient was analyzed, unusually high osteoblast activity was observed. In which hormone might this patient be deficient?

Answer 65

The patient might be deficient in parathyroid hormone (PTH).

Abnormally high osteoblast activity means that excess calcium is being taken out of the plasma and stored as bone. This can result from high levels of calcitonin or low levels of PTH.

Question 66

What is the main mineral component of bone, and what is its chemical formula?

Answer 66

Bone is composed of hydroxyapatite (Ca₅(PO₄)₃OH).

While you likely won’t be asked about this chemical formula on the MCAT, it is important to know that bone contains calcium, phosphate, and hydroxide.

Question 67

What name classifies movable joints that contain closed regions filled with lubricating fluid?

Answer 67

These structures are known as synovial joints.

The most commonly mentioned type of joint on the MCAT, synovial joints allow for flexibility at the junctions between bones. The rigid bones are separated by synovial fluid. Ball-and-socket joints are an example of this type of structure.