Muscle and Skeletal System Flashcards

From the neuromuscular junction to the role of calcium in smooth muscle contraction, use these cards to master the muscular and skeletal systems as they appear on the MCAT.

1
Q

What main functions are served by muscle?

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

Briefly explain the role of the muscular system in thermoregulation.

A

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.

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3
Q

Briefly explain the role of skeletal muscle in circulation.

A

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.

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4
Q

Briefly explain the role of smooth muscle in circulation.

A

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.

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5
Q

Name the three major types of muscle.

A
  • cardiac
  • skeletal
  • smooth
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6
Q

What features characterize smooth muscle?

A
  • mononucleated, meaning that a single cell contains only one nucleus
  • rounded and irregular in appearance and not striated
  • generally controlled involuntarily
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7
Q

What features characterize skeletal muscle?

A
  • multinucleated, meaning that a single cell contains more than one nucleus
  • striated, or striped in appearance
  • generally controlled voluntarily
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8
Q

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

A

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

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

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9
Q

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

A

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 orderly, cylindrical cells, while cardiac muscle cells are more branched and irregular.

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10
Q

Which muscle types require calcium for proper contraction?

A

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.

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11
Q

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

A
  • 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.
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12
Q

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

A

skeletal muscle

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

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13
Q

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

A

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.

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14
Q

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

A

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.

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15
Q

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

A

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.

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16
Q

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

A

smooth muscle

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.

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17
Q

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

A

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.

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18
Q

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

A

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.

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19
Q

Define:

sarcomere

A

A highly organized unit within striated muscle. These 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.

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20
Q

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

A
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21
Q

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

A

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.

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22
Q

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

A
  • actin
  • tropomyosin
  • troponin

Microfilaments are composed of two long chains of actin monomers.

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23
Q

Muscle cells can also be called by which alternative term?

A

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.

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24
Q

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

A

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.

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25
Q

Define:

myoglobin

A

It is an 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.

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26
Q

Describe the structural difference between myoglobin and hemoglobin.

A

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.

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27
Q

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

A
  • red fibers
  • 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.

28
Q

What basic features characterize red muscle fibers?

A

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.

29
Q

What basic features characterize white muscle fibers?

A

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.

30
Q

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?

A

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.

31
Q

What structure is shown in the image below?

A

neuromuscular junction

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.

32
Q

Name the main neurotransmitter at the neuromuscular junction.

A

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.

33
Q

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

A

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.

34
Q

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

A

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.

35
Q

Define:

motor unit

A

It 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.

36
Q

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

A

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 muscles. It stores calcium ions and releases them in response to an action potential, which then allows contraction to occur.

37
Q

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

A

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.

38
Q

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

A

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.

39
Q

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

A

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

40
Q

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

A
  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.
41
Q

How do Ca2+ ions facilitate actin-myosin binding?

A

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.

42
Q

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

A

These 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.

43
Q

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

A

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.

44
Q

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

A

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.

45
Q

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

A

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.

46
Q

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

A
  • 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.
47
Q

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

A
  1. Ca2+ binds to 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.
48
Q

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

A

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.

49
Q

What main functions are served by the skeletal system?

A
  1. protection of internal organs
  2. structure and support
  3. calcium storage
  4. immune cell and erythrocyte production
50
Q

Name the two major types of bone.

A
  1. compact bone
  2. spongy bone

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

51
Q

What features characterize compact bone?

A

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

Compact bone is comprised of cylindrical subunits called osteons.

52
Q

What features characterize spongy bone?

A

It 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.

53
Q

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

A
54
Q

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

A
55
Q

Define:

tendon

A

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

Tendons generally function to facilitate movement.

56
Q

Define:

ligament

A

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.

57
Q

What is cartilage, and what substances does it include?

A

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.

58
Q

What functions does cartilage serve in the human body?

A

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.

59
Q

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

A

endoskeletons

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

60
Q

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

A

exoskeletons

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

61
Q

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

A
  1. axial skeleton
  2. 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.

62
Q

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

A

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.

63
Q

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

A
  • 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.
64
Q

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

A

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.

65
Q

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

A

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.

66
Q

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

A

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.

67
Q

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

A

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.