Chapter 6 - The Musculoskeletal System

Question 1

From what embryonic tissue type does the skeleton derive?

Answer 1

The skeleton derives from the mesoderm (“means-o-derm”)

Question 2

What are the two types of skeletal systems? Provide examples of organisms that have each type.

Answer 2

Exoskeletons: skeletal systems that encase the whole organism. Arthropods such as shellfish and insects have exoskeletons.

Endoskeletons: skeletal systems that are internal. Vertebrates like mammals and birds have endoskeletons.

Question 3

What is the axial skeleton?

Answer 3

The axial skeleton consists of the skull, vertebral column, and ribcaage; it provides the basic framework for the body.

Question 4

What is the appendicular skeleton?

Answer 4

These are the bones of the appendages; it consists of the bones of the pelvis, the pectoral girdles, and the limbs. The bones of the appendicular skeleton are attached to the axial skeleton for stability.

Question 5

What two major components comprise the skeleton?

Answer 5

The skeleton is created from cartilage and bone

Question 6

What is cartilage and what is its origin?

Answer 6

Produced by cells called chondrocytes, cartilage is a firm, elastic, translucent connective tissue consisting of collagenous fibers embedded in a matrix of chondrin. It is the principle component of embryonic skeletons and can harden and calcify into bone. Of note, cartilage is relatively avascular and is not innervated.

Question 7

What does the root chondro- refer to?

Answer 7

chondro- relates to cartilage. It is the root for both chondrin, the firm but elastic matrix that comprises cartilage, as well as chondrocytes, the cells that secrete it.

Question 8

What are some advantages of the fetal skeleton?

Answer 8

The fetal skeleton is largely comprised of cartilage. This substance is firm and flexible, which provides support but also allows the fetus to grow in a cramped environment and pass lithely through the birth canal.

Question 9

In adults, where does one find cartilage? How do the unique structural properties of cartilage relate to the specialized functions of these areas.

Answer 9

Adults have cartilage only in areas that need a little extra flexibility, such as the joints, external ear, nose, and walls of the trachea and larynx. The firm but elastic properties of cartilage allow for this flexibility while still providing structure.

Question 10

What are the two types of bone? Briefly compare their macroscopic structures.

Answer 10

1. Compact bone: much more dense than spongy bone, it consists of harversian systems (osteons), which are structural units of bone matrix.
2. Spongy bone: lighter and less dense than compact bone, it consists of an interconnecting lattice of bony spicules (trabeculae). The cavities between the spicules contain bone marrow.

Question 11

What are the two types of bone marrow? Briefly define each.

Answer 11

Red marrow - filled with hematopoetic stem cells, which are responsible for generation of all blood cells.

Yellow marrow - composed primarily of fat, this form of bone marrow is relatively inactive.

Question 12

Know the structure of long bones

Answer 12

Question 13

From what structure does bone strength arise? Describe its composition.

Answer 13

Bone strength derives from the bone matrix, which has both organic and inorganic components. The organic components include collagen, glycoproteins and other peptides. The inorganic components include calcium, phosphate, and hydroxide ions.

Question 14

Describe the microscopic structure of bone.

Answer 14

The bone matrix is ordered into structural units known as osteons or haversian systems. Each osteon consists of concentric circles of bony matrix called lamellae that surround a central, microscopic channel known as a Haversian canal. The Haversian canal contains blood vessels, nerve fibers, and lymph. Interspersed within the matrix are spaces called lacunae, which house mature bone cells known as osteocytes.

Question 15

What is the function of osteocytes?

Answer 15

Osteocytes are the primary cell type involved in bone mainenance.

Question 16

What are the two types of osteocytes? What are their functions?

Answer 16

The two types of osteocytes and their functions are:

osteoclasts - destroy or resorb bone

osteoblasts - build bone

Question 17

What key chemicals are involved in bone re-formation and bone resorbtion (breakdown), and where do they come from?

Answer 17

During bone re-formation essential ingredients like calcium and phosphate are obtained from the blood. During bone resorbtion (breakdown), these ions are released into the blood stream.

Question 18

What is endochondrial ossification?

Answer 18

The process through which bones are created by the hardening of cartilage.

Question 19

What is intramembranous ossification?

Answer 19

The process in which undifferentiated embryonic connective tissue (mesenchymal tissue) is transformed into, and replaced by, bone.

Question 20

What are the two types of joints?

Answer 20

The two types of joints are movable and immovable joints.

Question 21

What is the difference between a tendon and a ligament?

Answer 21

A tendon connects muscle to bone, whereas a ligament connects bone to bone.

Question 22

What is the structure of a movable joint?

Answer 22

Movable joints are strengthened by ligaments, which are pieces of fibrous tissue that connect bones to one another, and consist of a synovial capsule, which encloses the actual joint (articular) cavity. Articular cartilage coats the surfaces of bones and restricts impact to the joint cartilage rather than the bones themselveds. Synovial fluid acts as a lubricant to ease the movement of one structure over the ther.

Question 23

How do muscles cause movements at joints?

Answer 23

They decrease in length, bringing the muscle’s origin and insertion closer together.

Question 24

What are immovable joints?

Answer 24

Immovable joints (e.g. in the skull) consist of bones that do not move relative to eachother. These joints are fused and ensure that the connected bones are held in a fixed position.

Question 25

What nervous system innervates skeletal muscle?

Answer 25

The somatic (voluntary) nervous system innervates the skeletal muscle.

Question 26

What are the three types of muscle? Provide a brief description of each.

Answer 26

The three types of muscle tissue are

1) Skeletal – striated, multinucleate cells that are controlled by the voluntary/somatic nervous system. Provide strong forceful contractions.
2) Smooth – responsible for involuntary action and controlled by the autonomous nervous system. Smooth muscle is uninucleated, nonstriated, and provides smooth, continuous contractions.
3) Cardiac – an intermediary form of muscle that is uninucleate and involuntary like smooth muscle, but striated like skeletal muscle. Cardiac muscle provides strong forceful contractions.

Question 27

What is the Sarcoplasmic Reticulum?

Answer 27

Specialized endoplasmic reticulum that surrounds each myofibril. Lumen is filled with Ca²⁺ ions for use in contraction.

Question 28

What is the sarcomere?

Answer 28

The sarcomere is the basic contractile unit of a muscle. Sarcomeres are composed of long, fibrous proteins that slide past each other when the muscles contract and relax.

Question 29

Define the hierarchy, in terms of composition, of muscle tissue.

Answer 29

Muscles are composed of a parallel arrangement of tubular muscle cells (myocytes or myofibers). These muscle cells contain many tubular myofibrils, which are composed of repeating sections of sarcomeres. Sarcomeres are composed of long, fibrous proteins (myosins) that slide past each other when the muscles contract and relax.

Question 30

Describe the structure of a myocyte

Answer 30

Myocytes contain many end-to-end connected sarcomeres, which comprise myofibrils. These myofibrils are surounded by a covering known as the sarcoplasmic reticulum, a specialized endoplasmic reticulum in muscle cells. The sarcoplasm (a modified cytoplasm) surrounds the sarcoplasmic reticulum, and the sarcolemma functions as the plasma membrane and is capable of propagating an action potential.

Question 31

What are the properties of skeletal muscle?

Answer 31

Voluntary; consciously controlled. Connects one bone to another (via a tendon). Allows for movement, posture, circulation (blood and lymph movement). Contraction produces heat. Striated (composed of sarcomeres), multinucleate.

Question 32

Define Sarcolemma

Answer 32

A modified membrane that wraps several myofibrils together into a (multinucleate) muscle cell.

Question 33

What is a myofibril?

Answer 33

Myofibrils are end-to-end connected sarcomeres, and are a basic rod-like unit of a muscle. Myocytes, or muscle fibers, contain many chains of myofibrils. They are created during embryo development in a process known as myogenesis.

Question 34

Know the skeletal muscle diagram

Answer 34

Question 35

What are the two categories of fibers that comprise skeletal muscle? Provide a brief description of each.

Answer 35

Traditionally, fibers were categorized depending on their varying color, which is a reflection of myoglobin content.

• Red muscle fibers (slow twitch) - have a high myoglobin content (and so appear red) and primarily derive their energy aerobically.
• White muscle fibers (fast twitch) - have a low myoglobin content (and so appear white) and primarily derive their energy through glycolysis.

Question 36

Myoglobin

Answer 36

Protein similar to Hb, but can only hold one O₂ molecule.

Question 37

Compare the different muscle fiber types regarding their mpode of energy generation and how this impacts fatigue.

Answer 37

Red muscle fibers (slow twitch) - derive their energy aerobically through the electron transport chain and have a high number of mitochondria. These fibers are more suited for endurance and are slow to fatigue.

White muscle fibers (fast twitch) - derive their energy anaerobically through glycolysis and have a low number of mitochondria. These fibers can contract more rapidly but are also easier to fatigue​.

Question 38

Define T-tubules

Answer 38

Small channels in the sarcolemma (muscle cell membrane) that allow for uniform muscle contraction because action potential spreads more quickly

Question 39

Describe the filaments of the sarcomere. What proteins comprise them?

Answer 39

Sarcomeres are made up of thick and thin filaments. The thick filaments are bundles of myosin, whereas the thin filaments are made up of actin (as well as troponin and tropomyosin).

Mnemonic: Myosin is the “thicker” word and makes up the thick filaments. *Actin *is the “thinner” word and makes up the thin filaments. Troponin and tropomyosin both start with T, and are associated with acTin.

Question 40

Know the structure of the sarcomere.

Answer 40

• Z-lines define the boundaries of each sarcomere
• The M-line runs down the center of the sarcomere
• The I-band is the region containing exclusively thin filaments
• The H-zone is the region containing exclusively thick filaments
• The A-band contains the thick filaments in their entirety, including any overlap with thin filaments.

Question 41

During contraction, which distances of the sarcomere become smaller? which stay the same?

Answer 41

Z-lines, I-band, and H-zone all get smaller or closer together during contraction because they are defined relative to one another. The A-band remains constant because it is defined as the total length of the thick fibers, regardless of state of contraction.

Question 42

What are the three major steps of muscle contraction?

Answer 42

1. Initiation - neuronal signaling culminates in the release of neurotransmitters (acetylcholine) at the synapse between the motor neuron and the muscle (neuromuscular junction). If enough acetylcholine binds the receptor, the muscle will be depolarized.
2. Sarcomere shortening - Depolarization of the sarcolemma yields an action potential and the release of Ca²⁺ ions from the sarcoplasmic reticulum. Ca^2+​ binds troponin, shifting tropomyosin to expose binding site on actin; myosin binds actin. ATP is hydrolyzed to ADP by myosin, powerstroke occurs and the sarcomere contracts.
3. Relaxation - Ca²⁺​ levels fall, tropomyosin covers myosin binding sites on actin, preventing further contraction. New ATP binds myosin and myosin head dissociates from actin. Sarcomere relaxes.

Question 43

What is the latent phase of muscle contraction?

Answer 43

The latent phase is the interval from the stimulus application until the muscle begins to contract (shorten). Note that there is no traced activity during this phase, but there are some electrical and chemical changes taking place.

Question 44

What are the steps of muscle contraction?

Answer 44

1. Nerve impulse arrives at motor neuron; acetylcholine is released
2. Acetylcholine diffuses across synaptic cleft; binds to motor end plate and causes action potential
3. Action potential traveling along T tubule opens Ca²⁺ channels in sarcoplasmic reticulum; calcium from blood enters sarcoplasm
4. Ca²⁺ binds to troponin (on thin/actin filament), exposing binding site for myosin (active site is normally blocked by tropomyosin). Myosin has ADP + Pi attached to it at this point.
5. Myosin head binds to actin, expels ADP + Pi, myosin head swivels and actin is pulled toward center of sarcomere (“power stroke”; contraction)
6. ATP attaches to myosin head, releasing it from active site (and tropomyosin covers site again)
7. ATP splits into ADP + Pi again on the myosin, repositioning the myosin for another “power stroke”
8. Ca²⁺ actively pumped back into SR after each cycle

Question 45

What are the possible functions of muscle contraction?

Answer 45

1. Body movement
2. Stabilization of body position
3. Movement of substances through body
4. Generating heat to maintain body temperature

Question 46

Skeletal muscle

Answer 46

Voluntary; consciously controlled. Connects one bone to another (via a tendon). Allows for movement, posture, circulation (blood and lymph movement). Contraction produces heat. Striated (composed of sarcomeres), multinucleate.

Question 47

What is a neuromuscular synapse?

Answer 47

A neuron attached to muscle cell

Question 48

What are the properties of cardiac muscle

Answer 48

Striated (composed of sarcomeres), single nucleus. Larger and more numerous mitochondria. Involuntary. Cells separated by intercalated disc. Cardiac muscle forms a net which contracts in on itself like a squeezing fist. Growth: hypertrophy (no mitosis).

Question 49

Smooth muscle: characteristics

Answer 49

Involuntary, innervated by autonomic nervous system. Single nucleus, NO sarcomeres (no striations). Contain thin and thick filaments; also intermediate filaments, which are attached to dense bodies throughout the cell.

Question 50

Intermediate filaments (muscle)

Answer 50

Smooth muscle only. Attached to both thin/thick filaments and dense bodies throughout muscle cell. Intermediate filaments pull dense bodies together during contraction. (Smooth muscle shrinks lengthwise.)

Question 51

Single-unit smooth muscle

Answer 51

Aka visceral. Most common. Cells connected by gap junctions that spread action potential from a neuron through many cells so cells contract as a unit. Found in: small arteries and veins, stomach, intestines, uterus, bladder

Question 52

Multi-unit smooth muscle

Answer 52

Each muscle fiber attached directly to a neuron. So can contract independently. Found in: large arteries, bronchioles, pili muscles (hair follicles), iris.

Question 53

Bone: functions

Answer 53

1. Support of soft tissue
2. Protection of internal organs
3. Help with movement of body
4. Mineral storage
5. Blood cell production
6. Energy storage (adipose cells in bone marrow)

Question 54

Spongy bone

Answer 54

Contains red bone marrow, where RBCs formed.

Question 55

Lamellae

Answer 55

Concentric circles of new bone matrix (around Haversian canal walls) created by osteoblasts.

Question 56

Haversian (central) canals

Answer 56

Tunnels through compact bone created by osteoclasts.

Question 57

Canaliculi

Answer 57

Passageway for osteocytes to exchange nutrients between bone and blood.

Question 58

Why do muscle fibers contract in an all-or-none fashion?

Answer 58

Muscle fibers are innervated by neurons whose basic signal is an action potential, which is an all-or-none phenomenon. For muscle cells to respond, stimuli must reach a threshold value. Once reached, depolarization occurs and an action potential arises.

Question 59

How is the overall force of a muscle reaction regulated?

Answer 59

Since muscle contraction is an all-or-none phenomenon, the strength of any individual fiber’s response cannot be adjusted. Rather, muscles control overall force by the number of fibers they recruit to respond. Maximal response occurs when all fibers are stimulated to contract simultaneously.

Question 60

Define tonus

Answer 60

The constant state of low-level contraction that occurs in certain muscles in their passive states. Tonus is essential for many normal body functions, such as holding the spine erect, the eyes open, and the jaw closed.

Question 61

What is myogenic activity (myogenic contraction)?

Answer 61

Myogenic activity (myogenic contraction) refers to a contraction initiated by the myocyte cell itself, instead of an outside occurrence or stimulus such as nerve innervation. Note that myogenic activity is muscular contraction without nervous system input. Such contractions only occur in smooth muscle. Thus while smooth muscle will respond to nervous input, it does not require external stimuli in order to contract.

Question 62

What nervous system is responsible for smooth muscle contraction?

Answer 62

Smooth muscle is responsible for invuluntary actionand is controlled by the autonomic nervous system.

Question 63

What role does creatine phosphate play in muscle contraction?

Answer 63

Creatine phosphate is a high energy compound that functions as an energy reserve. During times of excess energy creatine phosphate is generated through the transfer of a phosphate from ATP to creatine. This process can be reversed during muscle use to rapidly generate ATP from ADP. It thus allows for the immediate creation of ATP that would otherwise need to be formed through glycolysis or the TCA cycle.

Question 64

What are some differences between hemoglobin and myoglobin?

Answer 64

• Hemoglobin (Hb) binds four O₂ molecules, while myoglobin only binds one.
• Myoglobin is only found in muscle tissue
• Myoglobin binds oxygen more tightly than hemoglobin (and so has a dissociation curve that is shifted left compared to hemoglobin)
• Myoglobin serves as an oxygen reserve to keep aerobic respiration going during excercise

Question 65

What three proteinaceous fibers found throughout the body comprise loose connective tissue?

Answer 65

loose connective tissues are used to attach epithelium to underlying tissues and hold organs in place. They are comprised of:

1. Collagenous fibers - composed of collagen and have great tensile strength
2. elastic fibers - composed of elastin and endow connective tissue with resilience
3. reticular fibers - tightly woven fibers that join connective tissue to adjoining tissue

Question 66

What are the two major cell types in loose connective tissue?

Answer 66

1. Fibroblasts - secrete substances that are components of extracellular fibers
2. Macrophages - engulf bacteria and dead cells via phagocytosis

Question 67

What is dense connective tissue?

Answer 67

Connective tissue with a high proportion of collagenous fibers. Dense connective tissue forms tendons, which attach muscle to bone, and ligaments, which attach bones at the joints.

Question 68

Frequency summation

Answer 68

The strengthening of contraction that results when the stimuli are so frequent that muscle cannot fully relax. The stronger contraction is due t the incorporation of more muscle fibers.

Question 69

threshold value

Answer 69

The minimal value that must be reached in order for the system to respond. Muscle fibers ad neurons exhibit an all-or-none response, where the system initiates an action potential only if the threshold value is met.

Question 70

Refractory period

Answer 70

A short period of time immediately following an action potential in which neurons or muscle cells are unresponsive to stimulus. In some cases, a stimulus that is much larger than the threshold causes an action potential in a cell in a refractory period (relative refractory period).

Question 71

Describe the the various phases of an electrophysiological recording of an action potential

Answer 71

Question 72

If a given muscle is attached to two bones, how does contraction of the muscle cause the bones to move?

Answer 72

Only one of the two bones will move upon muscular contraction. The end of the muscle attached tot he staionary bone is called the origin, which corresponds to the proximal end in limb muscles. The end of the muscle attached to the bone that moves during contraction is called the insertion, which corresponds to the distal end in limb muscles

Question 73

What are antagonistic muscle pairs, and how do they work?

Answer 73

Many muscles work in pairs where one muscle relaxes while the other contracts. Such pairs are said to work antagonistically.